IGCP Project 474
Images of the Earth's Crust & Upper Mantle
Updated: Oct 13, 2010
1st International Symposium on Deep Seismic Profiling of the Continents and Their Margins
Cornell University, New York, USA, 26-28 June 1984
from American Geophysical Union (AGU) publication, Geodynamics Series Volume 13, Pages 1-311 & Volume 14, Pages 1-339 (1986)
(Full copies of these papers may be obtained by purchasing the AGU Geodynamics Series Volumes 13 and 14. Contact AGU via www.agu.org for details.)
A Global Perspective on Seismic Reflection Profiling of the Continental Crust
Pages vol.13, 1-3
It is the time in history for thorough exploration of the entire continental crust. The seismic reflection profiling technique, developed by industry for petroleum exploration, will almost certainly be the principal tool for probing the deep continental basement. It has already been clearly demonstrated by studies in many countries that this method will be productive of basic information and important discoveries. One can safely predict an era in which the deep crustal features of all continents are discovered, mapped, named, studied, understood, and made familiar to all earth scientists, as has happened for features of the deep sea floor within the past few decades. As they have for earlier phases, major practical benefits are certain to accompany and follow this phase of earth exploration.
Earth scientists working in this field have both the opportunity to participate in exploration of this great frontier, and the responsibility to carry out this exploration effectively and expeditiously so as to provide prompt and optimum benefit for society.
Some goals for this branch of science include:
Deep Reflections from the Caledonides and Variscides West of Britain and Comparison with the Himalayas
Pages vol.13, 5-19
Drummond H. Matthews & Michael J. Cheadle
A cartoon is used to summarise the results of 3000 km of reflection profiles to 15 seconds echo time obtained by BIRPS north, west, and south of Britain. It shows a Mesozoic sedimentary basin, few other reflections from the crystalline upper crust, and numerous short reflections in the lower crust with a sharp downwards cutoff at the Moho. Dipping reflections in the upper crust are interpreted as from low angle normal faults, probably reactivated Palaeozoic thrusts. These dipping reflections appear to terminate amongst the lower crustal reflections, as do dipping events in the upper mantle which are also interpreted as fault zones. Examples are quoted in the first part of the paper to justify the cartoon. The reflections in the lower crust may be from intrusive sills, from flatlying foliation in essentially dry rocks or from foliation containing fluids. We suggest that flatlying foliation is related to Mesozoic extension of the lower crust preceding the opening of the Atlantic. We compare our cartoon with results of French-Chinese profiling across the Himalayas and argue that both represent imbricate faulting of two brittle layers in the lithosphere, one in the uppermost crust and the other in the uppermost mantle and we argue that BIRPS results support the idea of a ductile lower crust which has been advocated by Meissner and others.
Deep Seismic Profiling of the Crust in Northern France: The ECORS Project
Pages vol.13, 21-29
B. Damottel, A. Galdgano and A. Hirn
J. F. Raoult
The goal of the ECORS project was to investigate France's main geological features bydeep seismic profiling of the crust, and to improve current seismic methodology both in field work and in data processing. The first ECORS work, a 230-km line studying the Variscan in northern France, was completed in 1984. The originality of the ECORS field procedure lies in the combination of reflection, refraction and wide-angle reflection seismic surveying on the same continuous profile. A preliminary cross-section shows the root of the Variscan front, located 130 km south of the Midi fault, above the lower crust. The Moho is found at depths of 36 and 40 km and at even greater depths in the north and south. The Bray fault cuts across the whole crust, downthrowing the Moho by 1 s TWT in the north. The Somme fault forms the boundary between two crustal areas having different facies.
Nature and Development of the Crust According to Deep Reflection Data from the German Variscides
Pages vol.13, 31-42
Rolf Meissner and Thomas Wever
At present data sets from seven near-vertical reflection profiles in the Variscides of Europe are available. These data, together with data from a dense refraction - wide angle network, were used to study the deep structure of the Variscan area by analyzing the fine structure, and reflectivity. Near vertical reflection profiles were concentrated on rather short segments of specific geologic interest. In to revealing a fine structure with regard zones and reflecting boundaries, the velocity also could be mapped with an accuracy up to 1%by using long spread length and explosives. Portable refraction stations were generally employed in the wide-angle area both in-line and off-line of the reflection profile in order to gather information about the lateral extension of anomalous structures.
The reflectivity of the Variscan crust is analysed by special histograms and compared with data from other areas, as well as data collected by other techniques, e.g., those of COCORP and BIRPS. The thin low-velocity crust of Variscan (and Caledonian) areas with its high reflective lower crust is thought to have originated by a strong syn- and post-orogenic melting with a subsequent cooling under quiet tectonic conditions. The strong reflectivity in the Variscan lower crust is attributed to a lamination considered to be relics of the melting and differentiation process. This picture od a horizontally stratified lower crust is compatible with rheoloqical studies showing the lowercrust to be a low-viscosity channel imbedded between the rigid upper crust and uppermost mantle. In contrast to the low viscous and reflectophile lower crust, the rigid upper crust with its vertically dominated plutons and melt injections is reflectophobe.
Detailed Crustal Structure from a Seismic Reflection Survey in Northern Switzerland
Pages vol.13, 43-54
P. Finckh and W. Frei
B. Fuller and R. Johnson
S. Smithson Departement of Geology and Geophysics, University of Wyoming
In 1982 a total of 180 km Vibroseis reflection profiles were recorded in northern Switzerlans to investigate the suitability of the crystalline basement as host rock for the of highly radioactive waste. The recording configuration was selected for resolving in great detail the structures in the uppermost 4 s. Night time recording considerably improved the signal-to-noise ration and high-precision static corrections, as well as the survey's high coverage were found essential to the data quality. Portions of a running from west to east and of a crossline running from south to north were selected for a reprocessing procedure based on correlation of the field data with only the first 10 s of the up sweep, thus extending the correlated record length to a maximum of 14 s.
Both seismic sections clearly show returns of energy from all levels of the crust, and they can be traced across almost the entire sections. Upper crustal reflections are evident between 2.0 and 3.5 s and are interpreted as the result of pronounced differentiation in the upper crust. Several major reflections are observed between 5.8 and 7.2 s, and their upper occurence can be correlated with the Conrad discontinuity. Strong energy returns between 8.4 and 9.5 s are interpreted as reflections from the Moho-discontinuity. Their layered layered configuration indicates a possibly laminated just above the Moho. The trend to the south-east of the dip of these lowermost reflections is in good agreement with the Moho mapped from numerous refraction measurements.
These main features of the reflection survey are compatible with the velocity-depth function in the lower crust from refraction surveys in the Rhinegraben rift system, where a distinct velocity increase of about 0.5 km/s at a depth between 15 and 20 km was found. The sloping reflector between 1.5 and 3.0 s at the northern end of the south-to-north seismic line can be correlated to energy returns from the vertical Vorwald fault in the Black Forest crystalline massif. Geometric considerations of the wave propagation, as well as velocity information from a nearby small refraction survey, indicate an almost horizontal raypath for these reflections and near-surface velocities,of the crystalline rocks between 3.9 and 4.3 km/s.
Characteristics of the Reflecting Layers in the Earth's Crust and Upper Mantle in Hungary
Pages vol.13, 55-65
Karoly Posgay, Albu Istvan, Raner Geza and Varga Geza
This paper reviews lithospheric studies carried out by the "Ebtvos Lorand" Geophysical Institute using the seismic reflection method. At: some places where detailed structure of the lithosphere can be seen on the seismic section, comparisons are made with geothermal and magnetotelluric data, and conclusions are drawn on the structure and the physical and chemical nature of the lithosphere and its temporal changes.
Deep seismic sounding (DSS) measurements along the Hungarian sections of the International Network (Sollogub et al, 1973) and on complementary profiles over Hungarian territory were essentially completed by the end of the sixties. The results were based mainly on the observation of wide-angle reflection and refraction seismic arrivals. In the seventies we turned to the system atic use of the classical reflection seismic method. In these measurements near vertical reflections are used.
Observations on the shallow crust (along the socalled "geological basic profiles") were carried out like those along the seismic prospecting profiles but by extended recording time (to 10 15 s). Number of channels was 48 or 120, length of the spread 2350 m or 2975 m, the CDP system used was of 12 or 24 fold. Charge weight amounted to 10-20 kg. The processing sequence consisted of demultiplexing, dynamic amplitude equalization, predictive deconvolution, band pass filtering, static correction, mute, sorting, constant velocity analysis, NMO correction, automatic residual statics, velocity spectra, shape- and phase correction, 12 or 24-fold CDP stack, coherence enhancement, wave equation migrat ion, time-va rying filtering.
In the special seismic measurements for probing the deeper crust and upper mantle charges of 200 300 kg were used. Ground roll and waves from the sedimentary cover appeared with a significant amplitude. Thus the shotpoint-spread spacing (of about 6 km) was selected so as to yield a maximum signal/noise ratio. Duration of recording was 25 30 s. To determine velocities in the crust and upper mantle, observations in a CDP system were performed up to a distance of 25 30 km (Posgay 1975).
A Review of Continental Reflection Profiling in Australia
Pages vol.13, 67-76
F. J. Moss and S. P. Mathur
Deep seismic reflection recordings have been made in 14 sedimentary basins and 3 cratonic areas of Australia by the Bureau of Mineral Resources, Geology and Geophysics (BMR). Early analog recordings made since 1957 were mainly from special large shots with recording times extended to 16 s or more. Important basic information on optimum recording parameters was obtained from experimental surveys in 1968-1969. This information provided the basis for digital recordings on short continuous profiles obtained by extending the record times on normal CDP surveys in sedimentary basins in the late 1970s. The success of these experiments has led the BMR to make CDP reflection recordings to 20 s or more on all seismic surveys since 1980. During 1980 1982 a total of 1400 km of six fold CDP recordings were made along regional traverses up to 270 km long. In 1984 one of these traverses was extended to a total distance of 1200 km across a number of geological provinces in eastern Australia.
The quality of deep reflections in Australia generally fair-to-good. The results from early (1957 1972) isolated analog recordings assisted in the interpretation of regional :gravity and refraction data in terms of :generalised crustal structure. The digital recordings made on short traverses during 1976-78 provided information on the thickness and the reflective nature of the crust in different domains. The recent recordings on long traverses are being used to determine the structure of the deep crust as well as of the near-surface geological features and the relation between them.
An Australian Continental Reflection Profiling (ACORP) Program has recently been initiated to record long continuous profiles of 500 km over the boundaries of most of Australia's major tectonic provinces, and to investigate major geological structures within these provinces which are relevant to the occurrence of minerals and hydrocarbons.
Recent Reflection Seismic Developments in the Witwatersrand Basin
Pages vol.13, 77-83
Reflection seismology has recently been applied to the exploration for gold and uranium in the Witwatersrand basin of South Africa. This early Proterozoic basin contains a thick succession of metamorphosed sediments and lavas, and is structurally complex. Results from two test seismic surveys that demonstrate the success with which the structure of the supracrustal rocks can be mapped are presented. The boundary between upper and middle crust observed in the Vredefort dome has also been recognised seismically, and appears to be a relatively sharp transition.
Gold was discovered in early Proterozoic rocks of the Witwatersrand basin near Johannesburg, South Africa in 1886. Since then the basin has produced more than half the gold ever mined, worldwide [Pretorius, 1976]. The gold-bearing strata outcrop along a 200 km arc on the north-western margin of the basin. The remainder of the mineralised strata are concealed beneath Proterozoic and Palaeozoic rocks reaching thicknesses in excess of 4000 m. Gravity and magnetics proved very successful in locating new goldfields beneath the cover rocks during the 1930's. However, the resolution of these methods is inadequate for detailed structural mapping of deeply buried strata. Drilling of deep exploration boreholes is both expensive and slow. The introduction of reflection seismology in 1982 has initiated a new phase in the exploration of the Witwatersrand basin. It enables the deeply buried parts of the basin and the structurally complex basin edges to be mapped, as well as the underlying Basement Complex.
Recent Seismic Reflection Studies in Canada
Pages vol.13, 85-97
A.G. Green, M.J. Berry, C.P. Spencer
E.R. Kanasewich, S.Chiu
D.B. Stewart, J.D. Unger
Multichannel seismic reflection data have been collected across the northern Appalachian orogen of Quebec and Maine and across the exotic terrains/active subduction zone of Vancouver Island, and a novel seismic refraction/wide angle reflection survey has been conducted across the Williston basin. The northern Appalachian sections suggest that practically all of the Paleozoic units between the St. Lawrence River and the Quebec-Maine border are allochthonous and possibly overlie a continuous but faulted Grenville basement.
Seismic sections from Vancouver Island have imaged two slabs of oceanic lithosphere at depth, one tectonically underplated and one that is currently being subducted. Also observed on the Vancouver Island data are moderately dipping reflections (30 to 45 deg.) that originate from major structures that have previously been interpreted as transform faults. The study of the Williston basin has revealed substantial relief on the underlying Moho and in particular has mapped a large offset that lies close to the axis of the North American Central Plains electrical conductivity anomaly.
Seismic Reflection Studies by the U.S. Geological Survey
Pages vol.13, 99-106
Robert M. Hamilton
Exploration of the continental crust using seismic reflection profiling is a major activity of the U.S. Geological Survey (USGS). Reflection studies have been carried out during the last decade in several programs: 1) Continental Margin Studies - offshore marine seismic surveys to determine the geologic framework and petroleum potential of the continental margin. 2) Earthquake Studies - profiling in the New Madrid, Mo., Charleston, S.C., northern New Jersey, and Utah and Nevada regions to evaluate earthquake hazards. 3) Central and Southern Appalachian Studies - Reflection surveys to examine thin-skinned tectonics and the existence of a master decollement, with application to petroleum potential evaluation. 4) Arctic National Wildlife Refuge Processing and analysis of reflection data to evaluate petroleum potential.
In 1982, the USGS received funding for a new program specifically designed to study deep continental structure. Under that program seismic reflection profiles have been acquired in central California from the Coast Ranges to the Sierran Foothills, and in Maine across the northern Appalachians. Projects are underway in Alaska along the pipeline route and in southwestern Arizona across the transition zone between the Colorado Plateau and the Basin and Range provinces.
The First Decade of COCORP: 1974 - 1984
Pages vol.13, 107-120
Larry Brown, Muawia Barazangi, Sidney Kaufman, and Jack Oliver
During the ten years following its first field experiment in Hardeman County, Texas, the Consortium for Continental Reflection Profiling (COCORP) has compiled an extensive record of new discoveries about the structure and evolution of the continental crust. COCORP results, individually and cumulatively, have contributed substantially on issues of continental accretion, rifting, and deformation. In the process COCORP has demonstrated the viability of applying the existing technology of the hydrocarbon exploration industry to a major program of continuing scientific exploration. This paper briefly outlines the structure and history of the COCORP organization, reviews some of the key findings to date, and contemplates some of the outstanding problems of continental geology that remain to be fully addressed by future deep seismic reflection profiling.
Crustal Structure Studies in New Zealand
Pages vol.13, 121-132
T.A. Stern, F.J. Davey and E.G.C. Smith
New Zealand is an area of continental lithosphere that has suffered appreciable deformation due to the presence of the Indian Pacific plate boundary. The relative plate motion is everywhere one of oblique convergence so that the mode of deformation and consequent crustal structure are determined by the nature of the lithosphere on each side of the boundary. Beneath the North Island, oceanic lithosphere of the Pacific Plate is being subducted, thus giving rise to a configuration of accretionary prism, fore-arc basin, volcanic arc and back-arc spreading basin. In central South Island there is continental lithosphere on both sides of the boundary and mountain building and shear are the mechanisms of deformation. The diversity of dynamic conditions imposed upon the lithosphere within the New Zealand region has led to the development of a wide range of observed crustal and upper mantle structures. In particular, within the North Island crustal thicknesses vary between 15 and 45 km and seismic velocities within the upper mantle range from 7.4 to 8.5 km/s.
Tectonic Framework of Narmada-Son Lineament - A Continental Rift System in Central India from Deep Seismic Soundings
Pages vol.13, 133-150
Three Deep Seismic Sounding (DSS) profiles: (1) Mehmadabad Billimora, (2) Ujjain Mahan and (3) Khajuria Kalan - Pulgaon, each about 250 km long were shot by the National Geophysical Research Institute, Hyderabad, India, across the Narmada Son lineament. This lineament cuts across the whole of central India in a NNE-SSW direction. Interpretation of both shallow refraction and the deeper reflection data along these profiles has revealed between Dorwa and Mahan, a large WNW ESE graben, here called the Tapti graben, under a thin Deccan Trap cover about 400 meters thick (velocity 5.0 km/sec). The Tapti graben is about 200 km long by about 100 km wide and is filled with Mesozoic sediments to a maximum thickness of about 1700 meters (velocity 3.2 km/sec). This graben under the Deccan Trap cover may be connected right up to Anklesvar-Surat area on the west coast of India where Mesozoic sediments about 1.2 km thick have been revealed under a Deccan Trap layer about 1.1 km thick. These inferred basins under Deccan Traps appear to be connected through a widespread Mesozoic sea which extended from Dorwa-Mahan region in an arcuate fashion encompassing the Mesozoic basins in Saurashtra, Kutch, Sind, Rajasthan and Salt Range. Quite separated from the Tapti graben lies a small Gondwana graben (100 km x 100 km) in the Multai-Pulgaon region that has a maximum Gondwana sedimentary thickness of about 400 meters (velocity 3.2 km/sec) underlying Deccan Traps about 100 meter thick. This Gondwana graben most probably is the extension under Deccan Traps of the exposed Godavari graben running NW SE from the east coast of India.
Between the Narmada and Tapti rivers, the Moho shows a big depression with a depth of about 42 km near Dorwa and 39 km near Chipaner and Rahatgaon. The Moho is at a depth of 36 km near Sanwer and south of Sehore. There is an uplift of the Moho to a depth of 34 km near Multai. Near Bawanbir the Moho has a depth of 38 km increasing towards the northeast in the form of a nose. Near Mahan it has a depth of 38 km and near Pulgaon 36 km. The interval velocity functions along the Ujjain-Mahan profile and the Mehmadabad Billimora profile in the Cambay basin reveal that the top of the crustal basaltic layer (velocity 6.9 km/sec) lies at a shallow depth of about 10 km with a reduced granitic layer under a major portion of the Deccan Trap covered area west of Dorwa-Mahan region.. The crustal thickness along the Mehmadabad-Billimora profile decreases to about 18 km under Billimora which lies over the northern part of the large Bouguer gravity high extending in a north south direction from Bombay. Therefore, it is inferred that this gravity high is due to large scale upwarping of the Moho, with the top of the basaltic layer lying here at a depth of about 6 km from the ground surface. Thus the crustal structure in this region is of a transition type, which was subjected to rifting during Late Cretaceous period resulting in extensive lava flows spreading in all directions.
A Geophysical Investigation of Deep Structure in China
Pages vol.13, 151-160
Shi Wang(2), Li Li
Geophysical study of deep crustal structure in China started in 1958, with seismic sounding profiles that now total 25000 km. The north south earthquake belt, oriented 100 deg. 105 deg.E, divides the whole country into two parts. The eastern part is the Circum-Pacific structural unit, which is affected mainly by the evolution of the Pacific, whereas the western part is the Tethys-Himalayas structural unit, which is affected mainly by the evolution of the Tethys. In the Tethys-Himalayas structural unit, isostasy has not been reached. The Tibetan plate and the Indian Plate have collided successively with Eurasian old land since the Mesozoic, giving rise to a series of overthrusts converging 20 30 km deep into the low velocity - low resistivity layer of the earth's crust. As a result, the lower crust has crept and thickened under the propulsive force of the Indian plate. On the top of the upper mantle in northern Tibet, an anomalous layer has formed. In the Circum Pacific structural unit, the isostatic anomaly approaches zero. In North China, many dustpan-like block basins were formed due to the spreading of the continent toward the ocean. The crust became heavier with cooling and intrusion of basalt, leading to continual subsidence of the North China region since the Tertiary.
Since the first shot for deep seismic sounding in Qaidam basin in 1958, a total length 25000 km of seismic sounding profiling has been carried out in China by the State Bureau of Seismology, Academia Sinica, and the Ministry of Geollogy and Mineral Resources.
Deep Crustal Knowledge in Italy
Pages vol.13, 161-165
From 1956 to 1982 extended deep seismic surveys have been performed in Italy under the auspices of the Consiglio Nazionale delle Ricerche (CNR) and the European Seismological Commission (ESC, an IASPEI Commission). The surveys were carried out on a number of different tectonic domains, both on land and on sea (orogens: Alps and Apennines; island arc: Calabria; thick sedimentary basins: Po Plain and peri-Apenninic basins; rifts: Sicily Channel; paleo-rifts: Ligurian Sea, Sardinia; oceanic crust: Tyrrhenian Sea). In particular, the Apennines domain is the area where in the 1930's geologists discovered that overthrusts are the dominant feature.
The main results are: (1) On both the continental and the oceanic domains different types of crustal structure have been revealed. (2) A double crust probably exists all around the Adriatic microplate. (3) Extended Moho faulting along the axis of the Apennines, from the Po Plain to Calabria, was delineated and found to be correlated with the strongest seismicity in Italy. (4) A deep vertical Moho fault with about10 km throw was found in the Larderello geothermal area.
Long Range Seismic Refraction Profiles in Europe
Pages vol.13, 167-182
St. Mueller and J. Ansorge
Long range seismic explosion observations have provided the means to elucidate fine details of the velocity-depth structure in theuppermost part of the mantle. There are now several profiles available in Europe, each about 1000 km in length with station spacings of 4 to 10 km. One long range profile traversing the Baltic Shield was nearly 2000 km long. Good crustal control along all the profiles made it possible to resolve even minor changes in structures at depth.
A very consistent pattern of consecutive travel-time branches was found for all the profiles. It consists of 3 to 4 separate pairs of prograde retrograde phases which can be correlated in addition to the Pn PMP system. An iterative inversion scheme was used to deduce velocity-depth structures compatible with the observed travel-times and amplitudes.
The lower lithosphere down to a depth of 140 km consists of four alternating high and low-velocity layers with pronounced contrasts in velocity and strongly varying layer thicknesses. At depths below 140 km the velocity structure becomes much smoother. The top of the mantle transition zone under Scandinavia was found at a depth of 440 km, with an average P-velocity of 8.75 km/s in the depth range from 140 to 440 km. Only two less pronounced high-velocity zones (with a maximum velocity of 9.1 km/s) could be identified in that depth range. The presently available data do not allow to delineate structural features of less than about 5 km in thick.
Crustal Studies in Central California using as 800 Channel Seismic Reflection Recording System
Pages vol.13, 183-196
Mark D. Zoback* and Carl M. Wentworth
Seismic reflection studies in central California have been carried out to study crustural relations between 1) the Mesozoic Franciscan assemblage, Great Valley sequence, and basement rocks along the western Great Valley and eastern Coast Ranges, 2) the Jurassic island-arc (?') sequence and the Sierra Nevada batholith of the Sierran foothills, and 3) the presumed suture between continental and oceanic basement rocks beneath the Great Valley. Because of the diverse nature of these targets, an innovative seismic reflection method was employed to record an east-west profile from the Coast Ranges to the Sierran foothills at the latitude of Merced (37.25 deg. N). Field recording employed 800 independent channels. This allowed data to be collected with high fold for signal enhancement and noise cancellation, long offsets for optimal velocity control and refracted wave analysis, and close group spacing for optimal near-surface resolution. Examples are given to illustrate the advantages of this method over conventional methods for this type of study.
Interpretive Processing of Crustal Seismic Reflection Data: Examples from Laramie Range COCORP Data
Pages vol.13, 197-208
Roy A. Johnson and Scott B. Smithson
Because crustal reflection studies are new and the targets are complex, special processing is often necessary to improve data quality. Detailed analysis, modelling, migration, and the use of advanced processing techniques can provide insights crucial to accurate interpretations of seismic reflection data from the crust. Reanalysis of COCORP deep-crustal reflection data from the Laramie Range, Wyoming shows that processing artifacts may confuse interpretation of the original data. Subsequent reprocessing enhanced data quality and enabled imaging a near-surface fault zone reflection which constrains the near surface dip of the fault to be 30 deg. W-35 deg. W. Reprocessing, migration, and modelling of deep events has possibly revealed a large recumbent fold on Minnesota COCORP data and a layered mafic intrusion on Laramie Range COCORP data. Synthetic seismic data generated from a well log enabled selection of an effective deconvolution operator for Laramie Range COCORP data, and tau-p filtering demonstrates the potential for improvements in deep crustal reflection data quality. Together these examples emphasize the interdependence of processing and interpretation which is especially critical in deep-crustal reflection profiling.
Aspects of COCORP Deep Seismic Profiling
Pages vol.13, 209-222
Larry D. Brown
Seismic exploration of the continents on land using near vertical reflection techniques represents a complex interlay of geological objectives and geophysical technique. Numerous approaches to such exploration are possible: COCORP has thus far pursued the reconnaissance strategy, emphasizing long, regional transects to identify and solve first-order problems of crustal structure and evolution. COCORP has exploited the operational advantages of existing oil industry technology and expertise by using a professional contractor for field work, and has enjoyed the economic benefts of a continuous, large-scale effort. This paper briefly overviews COCORP's application of the reflection method to crustal exploration during the past decade, with special attention to technique. Though technical issues continue to warrant attention, the experience of COCORP and other research groups have demonstrated that reflection profiling is a robust, effective, and often critical tool in deep crustal exploration.
Enhanced Imaging of the COCORP Seismic Line, Wind River Mountains
Pages vol.13, 223-236
J. Sharry, R. T. Langan, D. B. Jovanovich, G. M. Jones, N. R. Hill, and T. M. Guidish
We have reprocessed the COCORP seismic line from the Wind River Mountains using an integrated geophysical and geological approach. We have made corrections for problems arising from crooked-line geometry and line orientation. Distortions of subthrust geometry which are due to large lateral velocity gradients were corrected using wave-equation datuming. Refraction analysis and ray tracing suggest the presence of a normal fault antithetic to the previously mapped Continental fault. The migrated section shows the subthrust sediments of the Green River basin curving upward slightly into the thrust. The Wind River thrust dips at 30 deg. from the surface to 6.5 km depth, where the begins to flatten, soling out at 22 km depth. We interpret the crust below the Wind River thrust to be thrust faulted into lens-shaped packages which give rise to a duplex structure at 17 to 26 km depth. Two additional detachment zones at depths of 10 to 13 km and 33 to 34 km, in addition to the Wind River thrust, exist. Faulting on all the detachment zones was preceded by the formation of a flat seismic fabric.
An Expanding Spread Experiment During COCORP'S Field Operation in Utah
Pages vol.13, 237-246
Char Shine Liu(1) , Tianfei Zhu, Harlow Farmer(2), and Larry Brown
A limited expanding spread profile (ESP) was recorded in the Sevier Desert, west-central Utah, by the Consortium for Continental Reflection Profiling (COCORP) using relatively unmodified VIBROSEIS (registered trademark of the Continental Oil Company) reflection profiling equipment. A maximum offset of 32 km was attained, much larger than the 10 km spread length used in COCORP's normal Common Depth Point (CDP) profiling. Traveltimes picked from five distinct reflections were used to derive velocity structure. Based on the hyperbolic traveltime-distance relationship, the error of the stacking velocity for Moho reflections estimated from the ESP should be 15 times smaller than that estimayed from a typical COCORP CDP. However, since the hyperbolic traveltime curve assumption is inappropriate for large offset reflection data, the actual improvement in velocity estimation is model dependent. We have used Al-Chalabi's shifting stack method and a constrained linear least squares inversion method to analyze the ESP traveltimes. We found that the velocities estimated by assuming a 1 D multi layered earth were unsatisfactory, as the deepest reflection indicates lateral velocity variation in the middle crust. Results from a 2 D inversion puts the Sevier Desert Detachment at depth of 4.8 km, and the Moho at 32.1 km. This exercise shows that the usefulness of sn ESP depends on its location, the nature of the geological problems involved, and the velocity analysis procedures employed.
Crustal Reflection and Refraction Velocities: A Comparison
Pages vol.13, 247-256
Four reversed crustal refraction and four expanding spread reflection profiles were analyzed in the Canadian portion of the Williston basin. At the eastern margin in the Superior tectonic province, a simple three layer crust was found with a minimum and maximum crustal velocities of 5.95 - 6.55 km/sec. and with a thickness of slightly more than 40 km. To the west toward the central region of the basin, the upper crust thickness and heterogeneity increases reaching maximum velocity of 6.48 km/sec. On the top of the Moho a thick lower crustal layer became recognizable with velocity of higher than 7.00 km/sec beyond the margins of the craton. The crust extends anomalously to a depth of 52 km in the middle of the basin. The upper mantle velocity increases to 8.20 km/sec. Several structural disturbances can be observed along two east-west profiles of the crust. The location of these features is correlatable to margins of lithostructural domains of the Canadian shield. Crustal velocities and structural settings derived from independent reflection and refraction data are highly comparable, revealing a highly disturbed three dimensional crustal model of the region.
The Continental Mohorovicic Discontinuity: Results from Near Vertical and Wide Angle Seismic Reflection Studies
Pages vol.13, 257-272
L.W. Bralle and C.S. Chiang(1)
(1) Now at Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882
The continental Mohorovicic discontinuity (Moho) has been recognized worldwide from refracted (Pn) and wide-angle reflected (PmP) phases from earthquake and explosive sources. Studies utilizing Pn and PmP arrivals have generally interpreted the Moho to be a simple discontinuity in velocity between lower crustal and upper mantle rocks which is laterally continuous and which exists virtually everywhere at the base of the continental crust. Recently, deep seismic reflection profiling data have shown that the near-vertical reflection signature of the Moho is more complex. The reflection data have been interpreted to indicate that the Moho consists of a transition zone of several km thickness and that it is laterally variable and even discontinuous or non existent in places.
We have examined near-vertical and wide-angle seismic reflection record sections and performed model studies in an attempt to understand this disparity in interpretation and to try to resolve the actual seismic structure of the Moho. Part of the discrepancy in interpretation is due to distinct differences in characteristics and information content of near-vertical and wide-angle reflection data. Differences in frequency content, angle of incidence, station spacing and processing methods can lead to disparate interpretations from the two data sets even for identical Moho models. We have utilized near-vertical and wide-angle reflection/refraction synthetic seismogram studies for one- and two-dimensional velocity models to investigate the seismic signature of a variety of Moho transition zone models. For all Moho velocity structures. the wide-angle reflection (PmP) and the refraction (Pn) arrivals have simple and short duration waveforms. Travel times and amplitudes of the wide-angle observations are relatively insensitive to details of the velocity structure of the Moho. However. the Pn and PmP arrivals are excellent indicators of the existence and depth of the Moho and its general lateral continuity. Furthermore, the amplitude-distance characteristics of the Pn and PmP phases can be used to estimate the thickness of the Moho transition zone.
Near vertical incidence reflections from Moho transition zones are primarily sensitive to fine structure of the transition. Smooth velocity transitions have low reflectivity for near-vertical data. Constructive and destructive interference of reflections from laminated Moho structures produces a Moho signature consisting of bands of strong reflections which vary laterally. Where observed, these laminated reflectors provide information on the nature of the transition zone and its approximate thickness. However. band-limited, near vertical reflection data do not always reliably indicate the existence or structure of the Moho.
Comparison of observed near-vertical and wide-angle reflection data with the results of synthetic seismogram model studies suggests that the continental Moho is normally a transition zone of about 0 to 5 km thickness in which the velocity generally increases with depth, but also may contain thin laminations of high and low velocity material. The gross structure of the Moho is laterally continuous and appears to exist virtually everywhere as the base of the continental crust. However, the fine structure of the Moho, where it exists as a transition zone of thin laminations, is laterally variable resulting in the discontinuous character of Moho reflections on near-vertical incidence record sections.
Reassessing Seismic Refraction on the Edwards Plateau
Pages vol.13, 273-280
H.J. Dorman, T.H. Crawford, J.W. Stelzig, and P.J. Tarantolo
A high-speed marker produces a reliable refracted arrival on the Edwards Plateau of West Texas, a well-known problem area for seismic reflection exploration. The refracted arrival can be displayed in an intercept time section which has many of the useful properties of a CDP reflection section. It appears that seismic refraction data in this format might be useful for detailed exploration of the M discontinuity and other deep features.
Deep Seismic Reflaction Profiling the Continental Crust at Sea
Pages vol.13, 281-286
M. R. Warner
BIRPS (British Institutions Reflection Profiling Syndicate) have collected and processed some 5600 km of marine, multichannel, deep seismic reflection data on the continental shelf around Britain. The data are of high quality and we believe that this is a result of working at sea, where a repeatable, well coupled, high energy seismic source can be used. Marine acquisition is about one tenth the cost of acquisition on land, and at sea it is possible to shoot very long straight lines with minimal statics problems.
The data are collected and processed for BIRPS by commercial contractors using conventional techniques with minor modifications designed to enhance deep reflections. Modifications to the acquisition system include the use of very large volume, areally extensive airgun arrays. The processing sequence is fairly conventional but extensive testing is required to determine the often unconventional processing parameters. Processing is designed to increase the signal to noise rather than enhance resolution. Migration of deep data is difficult because the data are often very noisy. Both signal and noise are migrated to give a final section which is obscured by smiles and random noise becomes organised and looks like signal. This problem can be overcome by producing detailed line drawings from the unmigrated data and using simple inverse ray tracing to migrate these line segments.
Modeling Lower Crust Reflections Observed on BIRPS Profiles
Pages vol.13, 287-295
D.J.Blundell and B.Raynaud
BIRPS seismic reflection profiles such as SWAT 8, recently acquired in collaboration with ECORS, across the western English Channel typically show an upper crust largely devoid of reflections below the sedimentary cover whilst the lower crust has prolific bands of reflections, as observed in similar experiments elsewhere. These bands are mostly sub horizontal reflection segments that extend laterally from 1 to 10 km. Since they have not been found in the upper crust and are not accessible to direct observation, it is left to modelling to constrain speculation as to their nature and cause.
First consideration must be to the geometry of the reflecting interfaces. Two-dimensional depth migration of a line drawing of the SWAT 8 CDP stack record section noticeably improves the clarity. Three dimensional modelling of an undulating surface, however, indicates that reflections from out of the vertical plane of section can produce a multiplicity of reflection segments spread over a range exceeding 1 s twoway time (TWT).
Possible causes of lower crustal reflections all require constructive interference of seismic signals from interfaces between laminae of alternating acoustic impedence contrasts. So each reflecting surface may represent a number of laminae. It is shown that laminae only a few meters thick could be responsible for deep crustal reflections. It is argued that the relative lack of reflections in the upper crust even where lithological contrasts are known to occur suggests that lower crustal reflecting interfaces arise from physical rather than compositional contrasts. Around Britain these could be deformational fabrics or even thin fluid-filled fissures in a lower crustal ductile zone beneath a brittle upper crust within a modern, predominantly extensional, tectonic regime.
Comparison of Deep Reflection and Refraction Structures in the North Sea
Pages vol.13, 297-300
P. J. Barton
Deep reflection profiles collected on the continental shelf around Britain show a characteristic pattern of reflections, including a distinct band of short reflecting segments which occurs at lower crustal levels and has a well-defined base. In this paper the BIRPS deep normal incidence profiles SALT from the North Sea are compared with the results from a coincident long-range wide-angle seismic experiment. This seismic refraction experiment was interpreted using computer modelling of the amplitudes and travel times of the observed seismograms and a laterally varying model. Significantly the Moho, defined by seismic refraction as the step in the velocity of horizontally travelling p waves from about 7 km/s to about 8.1 km/s, is shown to fall at the base of the layering seen in the lower part of the crust on the deep reflection record.
Interpreting the Deep Structure of Rifts with Synthetic Seismic Sections
Pages vol.13, 301-311
Carolyn Peddy(1) , Larry D. Brown, and Simon L. Klemperer(1)
Low velocity sedimentary rocks in rift basins commonly result in the distortion of reflections from underlying features. This distortion, known as velocity pull down, is not corrected by conventional time migration. Seismic modelling can provide the most effective means of recognizing pull down effects, and thus modelling facilitates more accurate interpretations. In this paper, we use synthetic seismic sections to identify interpretational difficulties caused by pull down effects, such as flattening and dismemberment of fault plane reflections and the degradation and smearing of sub basin reflections. Modelling BIRPS data from the Scottish Caledonides has led to the recognition of a Moho-offsetting fault, and modelling of results from the Bay of Biscay has led to the tentative identification of severely distorted reflections coming from listric normal faults unrecognized in previous interpretations.
Implications of Deep Crustal Evolution for Seismic Reflection Interpretation
Pages vol.14, 1-7
David M. Fountain
Inference of the nature and evolution of the lower continental crust from seismic reflection profiling requires calibration of reflection events in terms of lithology and structure. One approach to provide this calibration is to construct synthetic reflection seismograms of known crustal cross-sections. Synthetic records coupled with geologic data for the well-known Ivrea and Strona-Ceneri zones of northern Italy indicate that highly reflective lower crust can result from complex lithologic layering generated by underplating of mafic and ultramafic magmas, high grade metamorphism and partial melting of metasedimentary rocks, and deformation.. Upper crustal levels appear transparent although structural and lithologic complexity is prevalent. The similarity of the synthetic records to the COCORP Kansas line suggests this approach has great potential in assisting in interpretation of lower crustal reflection data.
Interpretation of Seismic Reflection Data in Complexly Deformed Terranes: A Geologist's Perspective
Pages vol.14, 9-12
Robert D. Hatcher, Jr
Horizontal seismic reflectors have been used recently to speculate on the existence and continuity of major tectonic features such as thrust faults in crystalline rocks, or as boundaries between crystalline basement and cover sedimentary rocks. A large body of published seismic reflection profiles, in areas where structure can be verified using drilling and/or downplunge projection, supports this interpretation. However, it has been shown with the Arizona A-1 hole and elsewhere that prominent, continuous horizontal reflectors in crystalline rocks do not necessarily prove a major break is present. indicating a great deal of work remains before the nature of seismic reflection events in the deeper crust is understood.
The seismic reflection method commonly detects gently dipping layers having contrasting acoustic properties, although it may be theoretically possible to detect steeply dipping layers. Correct interpretation of recurrent reflection patterns could provide enormous insight into crustal structure and evolution. However, geologic interpretations from the same sets of reflectors are numerous. Single layered sub-horizontal reflectors may be unconformities, mylonite/cataclasite zones, stratigraphic contacts, or facies boundaries. Inclined reflector packages may be interpreted as imbricate thrusts or normal faults, duplexes, root zones or ramps. Curved reflectors may indicate the tops of plutons, broad open folds or refolded folds. Transparent zones are the most difficult to interpret and may indicate zones of structural complexity, structurally simple areas or rocks that contain no acoustic contrast.
The interiors of mountain chains contain a complex assemblage of rocks of low to high metamorphic grade which have markedly different mechanical and probably acoustic properties. Reflection coefficients occur at interfaces between rocks of different densities, and anisotropies or compositions without the presence of tectonic discontinuities. Sub-horizontal tectonic discontinuities such as thrust faults, may provide additional acoustic contrasts, provided there is a difference in properties of the rocks on either side of the discontinuity. Later thrusts in orogenic belts are less deformed and, if they juxtapose rocks of contrasting acoustic properties, should provide excellent reflectors. However, unless independently verifiable data exist, continuous reflectors in orogenic terranes may not prove to be tectonic or even lithologic boundaries.
Models of crustal structure should, of necessity, include geologic, seismic reflection,. and data from other geophysical techniques, such as potential fields. These should increase understanding of the nature of the deeper continental crust.
Continental Evolution by Lithospheric Shingling
Frederick A. Cook
Crustal geometries observed on seismic reflection data indicate that the boundaries of terranes accreted to the margins of continental cratons often have a low dip angles (30 deg. or less). Where terranes have encroached upon a subducted passive margin, the boundaries are typically listric into upper or mid-crustal detachments and rarely, if ever, penetrate the entire crust. Outboard of the subducted margin, terrane boundaries may penetrate the lower crust and upper mantle. Examples of the former type include the west margin of the Bronson Hill arc in New England, the Rheno-Hercynian zone in France and the Piedmont Carolina slate belt in the southern Appalachians. Examples of the latter type may include the Brunswick terrane in the southern Appalachians and the Gander terrane in the northern Appalachians. Such observations suggest a model in which the areal extent of continental cratons is increased in collisional orogens by a process of lithospheric "shingling".
Crustal Reflections and Crustal Structure
Scott B. Smithson, Roy A. Johnson and Charles A. Hurich
The nature of crustal reflectors is still a major question, especially since typical structures in the crystalline crust are so complex that they would generate short, possibly arcuate events. Evidence starting with the Wind River thrust suggests that crustal fault zones (mylonites) may be good reflectors, and recent profiling over mylonites and accompanying detachment faults in the Kettle Dome and the Ruby Mountains, metamorphic core complexes, shows consistent subhorizonta1 reflections that can be correlated with the lithology of the mylonite zones. Reflections are so abundant within these sillimanite-grade mylonite zones as to resemble a sedimentary terrain. These specially designed reflection experiments conclusively demonstrate for the first time that mylonites are good reflectors. On the other hand, the flanking thrust fault on the Laramie Range only generates a minimal reflection in the relatively shallow zone where brittle deformation predominates. These results show that my1onite zones may be the best reflectors in the crust because of their layered, planar geometry that commonly includes low dip, and crustal scale deformation may thus be mapped seismically. This means that such major feature as thrusting, crustal doubling and crustal extension may be recognized through reflection profiling. Crustal-scale duplexes may exist and give the appearance of dipping sedimentary reflections generated from their numerous mylonite zones. The best crustal reflections in the U.S. seem to be related to crustal extension in the western U.S.
Fluids in Deep Continental Crust
Pages vol.14, 33-39
W. S. Fyfe
The production of high grade metamorphic rocks of the amphibolite-qranulite facies in continental crust requires an increase in temperature associated with magma underplatinq or tectonic thickening. Underplating of continental crust by dense mantle magmas may lead to assimilation of wet, heavy crustal components and their degassing (H20-CO2-S). Deep subduction of ocean spillites, serpentinites and sediments rich in H2O-CO2-S may also cause fluid release into the basal crust. Major continental collisions and minor overthrust events associated with transform faults can lead to massive degassing. The fluid mobilisation in the Himalayan event may involve a fluid mass similar to the ice caps. In addition, such a large scale event may cause mobilization of significant quantities of C02 and salt depending on the litholoqy of the thickened crust. Whenever large volumes of fluids move, there is ore potential. In general, the environmental consequences of such tectonic events are little understood.
Tectonic Escapes in the Evolution of the Continental Crust
Pages vol.14, 41-53
Kevin Burke(1) and Celal Sengor(1)
(1) Also at Department of Geosciences, University of Houston, Texas
The continental crust originated by processes similar to those operating today and continents consist of material most of which originated long ago in arc-systems that have later been modified, especially at Andean margins and in continental collisions where crustal thickening is common. Collision-related strike-slip motion is a general process in continental evolution. Because buoyant continental (or arc) material generally moves during collision toward a nearby oceanic margin where less buoyant lithosphere crops out, we call the process of major strike-slip dominated motion toward a 'free face' "Tectonic escape".
Tectonic escape is and has been an element in continental evolution throughout earth-history. It promotes: (1) rifting and the formation of rift-basins with thinning of thickened crust; (2) pervasive strike-slip faulting late in orogenic history which breaks up mountain belts across strike and may juxtapose unrelated sectors in cross section; (3) localised compressional mountains and related foreland-trough basins.
Modern Analogs for some Midcrustal Reflections Observed Beneath Collisional Mountain Belts
Pages vol.14, 55-65
Robert J. Lillie
Seismic reflection data across collisional mountain belts often show prominent events at midcrustal levels. Through an example in the southern Appalachian mountains, three types of sequences are identified and compared to reflection data from modern compressional and extensional settings. Beneath foreland areas, dipping events which lie entirely beneath underthrusted shelf strata are interpreted as late-Precambrian rift graben fill, analogous to Mesozoic rift grabens observed on seismic profiles from the east coast of the United States. More typical features of the foreland, however, are small normal fault offsets of the underthrusted shelf strata. These normal faults commonly result in thrust ramping but apparently are not associated with continental rifting processes. Rather, the faults are similar to features contemporaneous with thrust faulting which have been observed on seismic profiles across areas of modern collisional deformation, such as the Himalayan foreland in Pakistan. The third type of reflection sequence consists of eastward dipping events which lie along the Appalachian gravity and magnetic gradients associated with the edge of pre-collisional continental basement. Although these dipping sequences may be a product of collisional deformation, it is possible that they represent original volcanic stratigraphy at the early Paleozoic continent/ocean boundary.
Reflections from the Subcrustal Lithosphere
Pages vol.14, 67-76
Today there is a wide consensus that near-vertical seismic reflections out of the crust and from the crust-mantle boundary are not generated by isolated first-order discontinuities but rather by laminated zones which increase the reflectivity by constructive interference of multiple internal reflections in certain frequency bands. Although at present there are only a few observations of near-vertical reflections from the subcrustal lithosphere, in this region of the upper mantle the conditions appear to be favourable for the generation of such reflections. Probing of the upper mantle on long range seismic profiles revealed properties of the subcrustal lithosphere which make it very likely that the same reflection mechanism could be effective here. The observation of unexpectedly high P-wave velocities of up to 8.5 8.6 km/s as shallow as 10-30 km below the crust-mantle boundary in layered zones embedded in regions of "normal" mantle material of around 8.0 8.2 km/s indicates that velocity contrasts in the upper mantle may become as large as at the crust-mantle boundary. A second property of the upper mantle recently discovered is an azimuth-dependent velocity distribution. This anisotropy starting at the Moho reaches into the upper mantle to a depth of about 100 km or even deeper. It may be generated by a preferred orientation of olivine in a more or less horizontal flow pattern. The same flow in a stress field orienting the crystals will also transform lateral heterogeneities with non- horizontal boundaries into flat horizontal layers in which the velocities depend mainly on depth. Such shear-flow flattening is most likely the mechanism which produces laminated zones forming the reflectors within the crystalline earth.
From these properties of the subcrustal lithosphere there is good reason to expect that reflections from the upper mantle exist and can be found if a search is made for them as systematically and thoroughly as has been done during the last decade in the crust. In some cases there is already observational evidence for the existence of upper mantle reflections.
Deep Crustal Signatures in India and Contiguous Regions from Satellite and Ground Geophysical Data
Pages vol.14, 77-94
M. N. Qureshy and R. K. Midha
Topography and Bouguer anomalies show an inverse relationship in the Indo-Ganga-Brahmaputra basin and Himalaya-Hindu Kush region. The Indo-Ganga-Brahmaputra basin, devoid of any prominent closure on the Bouguer anomaly map, exhibits a -100 mgal closure on the Airy- Heiskanan map. The positive isostatic anomaly over the Himalaya can be explained by two possible intracrustal mass distributions that could represent either a remnant of the Neo-Tethys floor or large-scale igneous intrusions into the crust from the mantle. We thus suggest that the isostatic compensation in the Himalaya and the Indian shield is nearly complete. The correlation of Moho depths derived from the empirical relationship between crustal thickness and elevation with the depths determined from Deep Seismic Sounding (DSS) provides support for this contention. The steep gradient on the regionalized isostatic anomaly map of the area indicates that the Main Mantle Thrust (MMT), which is located west of the Nanga Parbat, may represent a direct continuation to the Main Central Thrust (MCT), which is located farther east. Likewise, the northern gradient of the isostatic anomaly map is correlated with the Northern Main Suture (NMS) in the Kohistan and Himalaya-Tibet region.
That prominent lineaments, some dating back to the Precambrian, such as the Narmada-Son rift, Godavari graben, and Gandak-Karakorum, show up conspicuously on both ground and satellite-based gravity and magnetic maps suggests a genetic relationship between the near-surface and deep controlling structures. The Magsat vertical field (Z) map shows a positive anomaly (4 to 8 nT) over the southern Indian shield and a negative ENE-trending anomaly (-4 to -8 nT) over the northern shield, with the Narmada-Son line marking the transition zone. The negative magnetic anomaly over the Ganga-Himalaya area suggests a possible rise of geotherms in this region. There is growing evidence to show that some blocks in the Indian continent, such as the Aravali ranges, Narmada rift, Shillong plateau, and Godavari graben, are associated with recent tectonic activity, suggesting that the Indian shield is not as stable as normally thought and that it may be passing through a phase of rejuvenation.
Seismic Reflection Profiles of Precambrian Crust: A Qualitative Assessment
Pages vol.14, 95-106
Allan K. Gibbs
Seismic reflection profiles of Precambrian terranes reveal some structural features that were probably developed in Precambrian times. Seismically transparent zones are associated with granitoid rocks, anorthosites, and gneisses; zones of stratified, relatively continuous upper crustal reflections are typical of Precambrian continental sedimentary basins; relatively continuous, coherent, moderately-dipping zones are associated with faults at Precambrian terrane boundaries; and zones of complex reflections and diffractions occur in gneiss terranes and basement to continental sedimentary basins. Features at Moho depths are typically weak or barely perceptible, discontinuous, and in some instances appear to have moderate dips. The base of the crust is revealed elsewhere by decrease in the abundance of reflections. The similarity of structures in Archean, Proterozoic, and Phanerozoic crust may be due to qualitatively similar tectonic processes in all eras. The retention of Precambrian structures in the middle and lower crust implies that such crust has remained coherent through subsequent time.
Composition, Structure and Evolution of the Early Precambrian Lower Continental Crust: Constraints from Geological Observations and Age Relationships
Pages vol.14, 107-119
Ancient granulite terranes have been interpreted as segments of juvenile crust, added to continents during accretion along active plate margins. I review the geology and age relationships in the major early Precambrian granulite provinces in the light of this model and conclude that the majority of high-grade terranes display rock assemblages and structures that are unlike those found in modern accretion belts. In Archean terranes age data and isotopic systematics also that granulite formation preceded greenstone belt formation significantly in the majority of cases examined and that many high-grade complexes have long deformational histories extending over hundreds of Ma. Shallow-water metasediments of continental character predominate in many granulite terranes and display a surprising continuity of lithological layering as exemplified by the supracrustal assemblages in Sri Lanka. I suggest that many of these layers and associated recumbent structures and thrusts constitute near-horizontal reflectors as seen in lower crustal seismic profiles. Granulites may form in a variety of tectonic settings that are all compatible with the plate tectonic concept. However, oversimplification and gross generalization of granulite genesis do not contribute to a better understanding of the of the ancient lower continental crust.
Precambrian Crustal Structure of the Northern Baltic Shield from the FENNOLORA Profile: Evidence for Upper Crustal Anisotropic Laminations'
Pages vol.14, 121-126
Kenneth H. Olsen
Carl Erik Lund
Because Archean heat generation was two to four times its present value, the rates and style of crustal evolution and global tectonic mechanisms during the Archean and Proterozoic (3900 600 Ma) were possibly quite different than those familiar from Phanerozoic plate tectonics. In particular, the lateral scales and depths of convection and lithospheric subduction elements may have been smaller than contemporary analogs. Structures preserved in the upper and midcrustal levels of the cratonic area of the northern Baltic Shield therefore may be very useful in formulating more detailed models of Precambrian lithospheric tectonics. The northern part of the NNE-SSW-trending FENNOLORA profile traverses Precambrian basement complexes ranging in age from 1800-2800 Ma, which are adjacent to the 2800 Ma Kola nucleus. We use reflectivity method synthetic seismogram modelling to assist interpretation of a 700-km-long segment of the FENNOLORA line running from Northcape, across portions of Norway, Finland, and Sweden to about the Arctic circle in the south. Our Finnish colleagues also provided, for comparison, a record section approximately perpendicular to FENNOLORA running southeastward for approximately 300 km across Finnish Lappland (FINLAP). FENNLORA record stations reveal an en echelon pattern of P-wave first arrivals with apparent velocities between 6.0 and 6.8 km/s. The en echelon pattern is observable in both north- and south-trending directions from shotpoint G. This suggests a fine structure of the upper crust to depths of approximately 20 km consisting of several alternating high- and low-velocity layers, each about 1 or 2 km thick. On the other hand, the en echelon pattern cannot be clearly seen on the perpendicular FINLAP profile from shotpoint G, which implies some of the laminations are anisotropic with the high speed axis trending approximately north-south. One speculative interpretation is that the anisotropic layers are basaltic fragments of Archean or Proterozoic oceanic crust that were "stranded" beneath thin sialic crust by very shallow angle subduction zones.
Evidence for an Inactive Rift in the Precambrian From a Wide Angle Reflection Survey Across the Ottawa-Bonnechere Graben
Pages vol.14, 127-134
Robert Mereu, Dapeng Wang(l), and Oliver Kuhn(2)
(1) Now at Department of Geology and Geophysics, University of Wyoming
During the summer of 1982 the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long range seismic refraction and wide-angle reflection experiment across the Grenville Province of the Canadian Shield. One of the main aims of the experiment was to investigate the structure and origin of the Ottawa-Bonnechere graben from a set of in-line and fan-type profiles recorded both along the length of the graben and in directions perpendicular to it. Wide-angle reflection observations from the Central Gneiss Belt to the north of the graben revealed a very simple one layered crust with a sharp Moho. Large amplitude wide-angle PmP reflected waves were clearly identified. This was in sharp contrast to the poor or non existent PmP signals associated with the profiles obtained along the graben. The data supports the theory that the graben was part of the St. Lawrence rift system and that the Moho was disrupted with the entry of upper mantle material into the crust. The results of the first arrival direct wave observations showed that the near-surface velocities of rocks of the Grenville province varied from 5.8 to 6.4 km/s. There was no evidence for any intermediate discontinuity, however regional differences in velocity gradients within the upper crust were pronounced and had a major influence on the appearance of the record sections. The complexity of the energy in the coda supported geological observations that the rocks of the Central Gneiss belt are more homogeneous than those associated with the Central Metasedimentary belt south of the graben. Amplitude and direct wave arrival time fluctuations all indicated that the seismic velocity function is a fractal quantity. Seismic waves travelling through the crust smooth out the small scale variations to create regions of high and low velocity each separated from the other by both lateral and vertical velocity gradients.
A Possible Exposed Conrad Discontinuity in the Kapuskasing Uplift, Ontario
Pages vol.14, 135-141
John A. Percival
Mid-crustal seismic velocity discontinuities have been recorded at several localities in the Archean Superior Province of the Canadian Shield. In the central part of the province, the Kapuskasing uplift exposes a relatively complete oblique section through the upper two thirds of the crust. At a structural depth within the uplift of approximately 20 km, based on geobarometry of metamorphic assemblages, a complex transition occurs between an upper region consisting dominantly of tonalitic gneiss in the amphibolite facies (average density about 2.70 g/cub.cm) and a lower level of interlayered tonalite, diorite, anorthosite, paragneiss and mafic gneiss in the upper amphibolite and granulite facies (average density about 2.82 g/cub.cm). The transition zone, thought to be analogous to a mid-crustal velocity discontinuity, separates upper crust, in which felsic plutonic compositions predominate, from lower crust, where intrusive rocks include mafic tonalite, diorite and anorthosite.
Seismic Crustal Structure Northwest of Thunder Bay, Ontario
Pages vol.14, 143-155
Roger A. Young(1), Jeffrey Wright(2), G.F. West
(1) Now at Phillips Research Centre, 169 GB, Bartlesville, OK 74004
A seismic refraction wide angle reflection survey of limited scope was carried out in the Shebandowan-Atikokan-Savant Lake region of western Ontario using open pit mine blasts as the principal energy sources. Conclusions drawn from the data are: 1) The seismic crustal structure varies laterally from one part of the area to another, but the variations are not substantial, nor is the crustal structure anywhere very unusual. The interpreted depth of the M discontinuity lies everywhere in the range 37-44 km. P-wave velocity everywhere rises in the lower crust reaching at least 7 km/s near the M discontinuity, whereas the upper crust displays a more uniform velocity of less than 6.3 km/s. In three of the four interpreted sections, velocity jumps rapidly at an intermediate depth to define a distinct lower crustal layer. The depth to the top of this lower crustal layer, is quite variable (13 21 km). 2) An 8 km thick, near-surface capping of slightly higher velocity is interpreted for the more northerly profile which passes through the Sturgeon Lake-Savant Lake greenstone belt. It appears to be a manifestation of the more frequent occurrence of mafic metavolcanic material in crust which otherwise exhibits granitoid velocities (6.0 km/s). The result lends support to the view that greenstone belts of the Superior Province are generally of limited depth extent and`are underlain by granitoids. 3) The M discontinuity at least beneath the northerly profile is a complex zone about 5 km thick, seemingly with a lamellar structure. Upper mantle velocity is not well defined by the surveys, but is consistent with previous regional estimates of about 8.1 km/s. On the northerly profile, there is evidence for a rise in mantle velocity to 8.3 km/s at about 50 km depth.
A Seismic Cross Section of the New England Appalachians: The Orocen Exposed
Pages vol.14, 157-172
Robert A. Phinney
Several marine multichannel seismic lines collected by the U.S. Geological in its assessment of the Eastern U.S. continental margin for oil and gas are found to constitute a high quality deep reflection profile of the continental crust on the Long Island platform. In the time range 4-11 seconds (10-35 km), the CDP section shows nearly continuous, dense, well correlated reflections. The systematic variation in local dip angle of these reflections defines tectonic packets which constitute the bulk of the crystalline crust in this area. Most conspicuous is a single large packet in the eastern end of the line which dips to the W at 25 degrees and extends from the sediment-basement boundary to the lower crustal boundary layer (Moho) at 25 30 km. This packet is interpreted as an accretionary structure formed and thickened to continental thickness during the late Paleozoic accretion of Avalonia and the Appalachian margin of North America. In the center and western portion of the line a low angle complex packet dipping east at depth is interpreted as the pre-Acadian margin of North America. A "keystone" packet lying between these two bodies, and forming most of the basement subcrop under the central portion of the line, appears to correlate with the high grade medial zone of south central New England, and appears to be the strongly compressed, thickened, and uplifted remains of the oceanic basin, volcanic islands, and marginal sedimentary wedges which separated Avalonia from North America before their collision. The lowest 1-2 seconds of the crust appears as a nearly horizontal layered complex, with at least two pronounced local structural breaks. I suggest that this "Moho" forms the lower carapace of a compressional orogen which was thickened to at least 45km during Paleozoic collision(s). It is interpreted as the highly strained tectonic boundary layer established during the subduction of oceanic crust at an active continental margin.
Moho Reflections from the Long Island Platform, Eastern United States
Pages vol.14, 173-187
J.A. Gr ow
Strong reflections from 9.5 12 s depth (two-way travel time), which were recorded on a grid of seismic-reflection profiles on the Long Island platform of the U.S. Atlantic continental margin, are interpreted as reflections from the Mohorovicic discontinuity. The character of the reflection is generally sharp although some laminations occur to the east. The southerly dip of the Moho surface is explained by the effects of thickening low-velocity sediment and water layers. A region of lower travel times east of the Long Island rift basin may be crust that was thinned during Mesozoic rifting. A region of apparent crustal thickening in the central platform may have been relatively unaffected by Mesozoic extension. Travel times through the crust (excluding sediments and water) decrease towards the basement hinge zone, in agreement with theories of crustal thinning across passive continental margins. The velocity of the crust beneath the platform is not well enough known to permit accurate conversion of the time-sections to depth or migration of the deep events.
The Quebec Western Maine Seismic Reflection Profile: Setting and First Year Results
Pages vol.14, 189-199
D. B. Stewart, J. D. Unger, J. D. Phillips, and R. Goldsmith
W. H. Poole
C. P. Spencer and A. G. Green
M. C. Loiselle
The Quebec Western Maine seismic reflection profile is part of a nearly continuous profile about 1000 km long across the Northern Appalachian orogen from the craton to the ocean basin. During 1983, 219 km of 800-channel sign-bit data were collected for 15 seconds two-way travel time using VIBROSEIS sources. Variable upsweeps from 7 to 45 Hz with a 12 km spread, 30 m group intervals, and 90 m vibration points were used. The profiles obtained have nominal 133 fold, and numerous reflectors can be seen at depths corresponding to two-way travel of 1 to 12 seconds (about 3-42 km). This paper summarizes the regional geology, gravity and magnetic fields, and velocity structure from seismic refraction.
Preliminary interpretations are given for parts of three seismic reflection profiles. Rocks of the Connecticut Valley-Gaspe synclinorium and Chain Lakes massif are allochthonous above a major regional decollement that dips south from a two way travel time of 3.5 seconds in Quebec to 7.8 seconds beneath the southern part of the massif. Profiles in central and coastal Maine image the shapes of plutons tobelow 3 seconds, offsets on steep faults to over 7 seconds, and many subhorizontal reflectors at various depths. The 800-channel sign-bit method gave high quality shallow and deep data that correlate well with geologic, gravity, and magnetic data.
Structural Interpretation of Multichannel Seismic Reflection Profiles Crossing the Southeastern United States and the Adjacent Continental Margin-Decollements, Faults, Triassic(?) Basins and Moho Reflections
Pages vol.14, 201-213
John C. Behrendt
In 1981 the U.S. Geological Survey (USGS) acquired 1350 km of 96-channel, 24 fold, multichannel seismic-reflection data along three profiles (S4, S6, and S8), recorded to 6 s and 8 s. extending across South Carolina and Georgia from the Appalachians to the Atlantic coast. Previously, in 1979, a 6-line grid (CH 1 CH 6) comprising 650 km of 64-channel, 32-fold data recorded to 12 s was surveyed over the continental shelf near Charleston, S. C. That offshore grid is tied to line S4 onshore and to the regional survey of the Atlantic continental margin. The result is a transect of four lines (including published COCORP data for Tennessee-Georgia) across the southeastern United States, extending, on a number of offshore deep-reflection lines, to oceanic crust.
The Appalachian decollement can be seen discontinuously on S6 and S8 from the Appalachian Mountains southeastward as far as the Carolina Slate Belt; it is not apparently continuous to the surface interpreted as the Charleston decollement offshore. A series of reflections on lines S4, S6 and S8 and on the COCORP line is interpreted as evidence of southeastward-dipping imbricate faults, from the Brevard fault on the northwest to the Augusta fault, which marks the southeastern extent of the Eastern Piedmont fault zone. The Carolina Slate Belt is characterized on the four seismic profiles by a complex series of diffractions and reflections extending from less than 1 s to 8 s. A number of Triassic(?) basins are apparent in the reflection data for the rifted Charleston terrane identified from low-gradient magnetic anomalies. These basins are bounded by normal faults reactivated in the meizoseismal area of the Charleston earthquake of 1886 and elsewhere, in a compressional reverse or strike-slip sense during Late Cretaceous and Cenozoic time It appears probable that the seismicity in the Charleston terrane is related to movement on these fault zones bounding the basins; movement on the faults identified at depth in the eastern Piedmont fault zone may be related to seismicity there. Good reflections from the Moho are observed in the 6 CH lines offshore of Charleston in the range of 8 11 s, which is consistent with COCORP reflection data for land surveys.
Crustal Thickness, Velocity Structure, and the Isostatic Response Function in the Southern Appalachians
Pages vol.14, 215-222
Leland T. Long and Jeih San Liow
The COCORP southern Appalachian traverse in eastern Tennessee shows relatively 1ittle evidence of differential vertical movement in the seismic reflectors associated with the lower portion of the column of Paleozoic sediments under the Valley and Ridge and Blue Ridge Provinces. On the other hand, estimates of crustal thickness from seismic refraction studies and gravity data analysis imply thickening of the crust in areas of significant topography. The crustal thickness from a time term analysis of Pn arrivals and an analysis of gravity anomalies varies from 33 km in the Georgia Piedmont and 35 km in central Alabama to greater than 50 km under the mountainous areas of eastern Tennessee and northern Georgia. The apparent association of the topographic load on the overthrust crust with isostatic compensation at the base of the lower crust is difficult to explain without evidence of differential vertical movement. We suggest that, in a thrust regime, the existing relief of the underlying plate was compensated at depth and the topography of the upper plate exists because the thrust sheets are draped over the lower plate bulge.
Nature of the Lower Continental Crust: Evidence from BIRPS Work on the Caledonides
Pages vol.14, 223-231
The Western Isles and North Channel ('WINCH') seismic reflection profile traverses the metamorphic Caledonides of northern Britain. The lower crust appears to be more reflective than the crust above or the mantle below. The base of the reflective sequence, assumed to be the base of the crust, can be drawn with confidence over most of the profile. The top of the reflective sequence can also be picked, but with greater uncertainty. Across the Caledonide Dalradian metasediments the profile indicates an antiformal Moho and a synformal top of the reflective sequence. There is little variation in the gravity field over this feature. In effect the Moho relief is compensated isostatically by necking of the lower crustal reflective layer. Modelling this using velocity-density systematics indicates that the lower crust here must have a density of around 3100 kg/cu.m, and a mean P-wave velocity of about 7.3 km/s. Along strike the lower crust has high electrical conductivity. It is suggested that the lower crust is of basic composition, of variable metamorphic grade, and containing free water trapped by contraction during cooling.
The Hercynian Evolution of the South West British Continental Margin
Pages vol.14, 233-241
G. A. Day
Motions of platelets in W. Europe during the Devonian are not well defined, but the evidence supports northward movement with the closure of oceanic basins. Geophysical data demonstrate that the major features of the crust in the western English Channel and the area to the west out to the shelf edge, all trend WSW ENE, and in addition that the whole southern flank of the Cornubian High is deformed by north-directed: thrusting active in Devonian times. The character of the crust changes to the east and it is proposed that a transform fault, along the line of the Bray Fault in northern France, offsets the zone of Hercynian deformation in SW Britain from a similar zone in N. Germany. Post-Hercynian adjustment produced further movement of the European blocks, firstly in a zone of dextral shear and finally during a period of extension, when Permian grabens appeared along the lines of the suture transform complex.
The Deep Crust in Convergent and Divergent Terranes: Laramide Uplifts and Basin Range Rifts
Pages vol.14, 243-256
George A. Thompson
Janice L. Hill
Seismic profiles across the Pacific Creek anticline, the Wind River Range, and the Casper arch in western Wyoming provide a coherent, nearly continuous section deep into the crystalline crust through these Laramide basement uplifts. Anticlines or monoclines in the sedimentary section overlie reflective thrust faults in the Precambrian crystalline basement. Structural relief on the folds and displacements on the faults are quantitatively coupled. The hrust faults tend to splay and/or decrease in dip with depth until they disappear as distinct reflectors at mid-crustal levels. The reflection data clearly reveal transitions downward from folding to brittle thrust faulting to distributed or ductile compression in the deep crust.
Seismic sections in the rifted Basin and Range Province (including the Rio Grande rift) also reveal striking transitions with depth. The near-surface, high-angle normal faults commonly merge with, and do not displace, subhorizontal detachment faults at depths of only a few kilometers. The abruptness and shallowness of the change suggest to us that it is not governed solely by rock softening due to increased temperature with depth. Instead we suggest that conditions of open hydrothermal circulation in the broken upper crust change abruptly at depth to conditions of high pore-water pressure in rocks self-sealed by mineral deposition or metamorphic processes.
Zones of subhorizontal reflectors at mid-crustal depths are much better developed in extensional regions such as the Basin and Range provinces than in the compressional Wyoming province. On reflection sections from both regions, however, the basal crust appears laminated. A tectonic explanation is suggested for the subhorizontal reflectors by the flattening of both thrusts and normal faults in the deep crust, but an origin coupled also to metamorphic and magmatic events is likely.
Phanerozoic Tectonics of the Basin and Range Colorado Plateau Transition from COCORP Data and Geologic Data: A Review
Pages vol.14, 257-267
Richard W. Allmendinger, Harlow Farmer (1), Ernest Hauser, James Sharp (2), Douglas Von Tish (3), Jack Oliver, and Sidney Kaufman
(1) Present address: Pecten International, Box 205, Houston, TX 77001
The COCORP 40*N transect in Utah and easternmost Nevada crosses the eastern Basin and Range Province and the northwest Colorado Plateau. This part of the Cordillera of western North America has been the site of late Precambrian rifting, Mesozoic and early Cenozoic thrust faulting and middle and late Cenozoic extension. The COCORP data, in combination with drilling and surface geologic data, suggest that a major change in seismic character and crustal structure occurs at the location of the autochthonous hingeline, which was formed during the Precambrian rifting .and subsequent passive margin stage. East of the hingeline, there is little evidence that neither thrust faults nor low-angle normal faults cut deeply into the continental basement. West of the hingeline, a prominent, west-dipping seismic fabric may correspond to Cenozoic and perhaps Mesozoic low-angle structures that cut down to middle and lower crustal levels. The deepest reflections in the Basin and Range (28-30 km) correspond in depth to a prominent mid-crustal horizon (about 27 km) in the Colorado Plateau; their relation is uncertain. The Colorado Plateau crust appears to be more than 15 km thicker than the Basin and Range crust. Also, the Colorado Plateau data are dominated by diffractions, where as the Basin and Range has a more persistent west-dipping or layered fabric. These contrasting seismic characters may be typical of cratonic versus orogenic crust, respectively.
Seismic Profiling of the Lower Crust: Dixie Valley, Nevada
David A. Okayal
Shallow industry reflection profiles may be used to image the lower crust provided (1) a Vibroseis (registered trademark of CONOCO, Inc.) source was used, (2) the source signal was an upsweep, composed of low to high frequencies, and (3) the original uncorrelated field gathers are available for extended correlation. Extended correlation increases Vibroseis gather lengths by using a correlation sweep shorter in time than the original field sweep. The correlation sweep may either contain a fixed frequency bandwidth or be a "self-truncating" sweep whose frequency content diminishes with time. Recorrelation of three seismic lines in Dixie Valley using a "self-truncating sweep" converts profile travel time from 4 seconds to 12 seconds. Conventional CDP stacking of the recorrelated data reveals basin reflections and many short, sub horizontal refections present in the intermediate to deep crust.These reflections are present in adjacent recorrelated field gathers, suggesting they are not seismic artifacts. Reflections in the intermediate crust may be due to Mesozoic basinal shelf sediments or their metamorphic equivalent. Lower crustal reflections may be due to some combination of internally layered or extensionally elongated magmatic intrusions, banded or laminated schists or gneisses, or compositionally varied granulites derived from recrystallization of lower crustal mafic rocks. A zone of reflections at the base of the lower crust may be related to the Moho transition zone. Possibilities to account for such a thick or laminated zone of reflections include crystallized layering from multiple melt, layering of partial melt, mantle-derived intrusion, delamination of upper mantle material, cumulate layering. and metasedimentary layering. A sharp drop in the density of these reflections occurs below 10 seconds.
Reflection Profiles from the Snake Range Metamorphic Core Complex: A Window into the Mid Crust
Pages vol.14, 281-292
The northern Snake Range (NSR) metamphic core complex in eastern Nevada is characterised by a detached and distended cover of Paleozoic strata overlying ductilely-strained and metamorphosed Precambrian sediments and Mesozoic and Tertiary(?) plutons. A gently-dipping to subhorizontal zone of detachment (the northern Snake Range decollement (NSRD) separates these rheologically contrasting units. This decollement is best developed on the eastern flank the range where lower plate ductile strain is greatest. Seismic profiles from the Consortium for Continental Ref1ection Profiling (COCORP) and Sohio Petroleum Company have traced this shallowly-dipping (5-10 deg.) reflecting horizon over 10 km to the east beneath the Confusion Range, where it dies out at a two-way travel time of 3.0 seconds. Along the western flank of the range, however, a 128-fold sign bit seismic line shot across Spring Valley between the Snake and Schell Creek Ranges was unable to image the westward continuation of the NSRD for any appreciable distance.
The absence of ductile deformation exposed in the Schell Creek Range and the correlation of the NSRD with a purely brittle fault in this region suggests that the westward disappearance of the NSRD as a major reflecting horizon is due to a decrease in ductile strain to the west, away from the Snake Range. Although a westward transition of the rheological character of the NSRD from completely ductile deformation on the east flank of the NSR to completely brittle deformation on west flank is compatible with a low-angle zone of simple shear rooted to the east into the lower crust or upper mantle, a major shear zone with displacements the order of 60 100 km should be laterally more extensive and resolvable at greater crustal depths than those imaged on the seismic reflection profiles.
A highly-reflective middle and lower crust has also been imaged on COCORP and Sohio Petroleum seismic ref1ection profiles beneath the NSR. These strong, laminated reflections which extend from 4.0 seconds two-way travel time to the Moho are believed to represent a compositionally-layered and structurally-deformed fabric imposed on the middle and lower crust during Tertiary extension. If true, this implies that extension in the NSR has not been localized along the NSRD, but has been distributed throughout the entire crust down to .the Moho.
Shallow Structure of the Southern Albuquerque Basin (Rio Grande Rift), New Mexico, from COCORP Seismic Reflection Data
Pages vol.14, 293-304
Detailed examination of COCORP Line 1A across the Rio Grande rift suggests that in contrast to listric faults and close-spaced steep normal faults proposed by others, a folded detachment surface and a set of fan-shaped reverse faults exist beneath the Albuquerque Basin. The folded detachment is thought to be the subsurface projection of the curved, low-angle Jeter fault exposed on the north-eastern corner of the Ladron Mountains. Evidence indicates that both the folding of the detachment and thrusting along the reverse fault took place about 10 m.y. ago. This interpretation infers that at that time the area occupied by the present Albuquerque Basin was undergoing compression rather than extension. This compression might have resulted from the clockwise rotation of the Colorado Plateau relative to the Great Plains. Subsequently, compression apparently gave way to extension. This may have been caused by crustal relaxation as rotation of the plateau gradually ceased.
Geometries of Deep Crustal Faults: Evidence from the COCORP Mojave Survey
Pages vol.14, 305-312
M. J. Cheadle(1), B. L. Czuchra(2), C. J. Ando(3), T. Byrne(4), L. D. Brown, J. E. Oliver and S. Kaufman
(1) Present address: Dept. of Earth Sciences, Cambridge University, Cambridge, CB3 OEZ
Several reflecting horizons imaged during deep seismic reflection profiling in the western and northern Mojave Desert are interpreted as fault zones which penetrate the deep crust of that region. The most prominent is a complex, though laterally correlatable midcrustal horizon (9-20 km) which extends over the northern area of the Mojave Survey into the Basin and Range Province and is interpreted to be a major southwesterly dipping crustal fault zone. Its shape resembles ramp and flat geometry, which suggests that deep –faults– in crystalline terranes can have geometries similar to thrusts mapped in foreland thrust belts.
The crust mantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey, and at about 8-9 s (26-29 km) in the south. The change in two way travel time to this horizon, the base of which is interpreted to be the Moho, provides evidence for a fault which offsets the Moho.
The COCORP survey also traversed the two major strike-slip faults that bound the Mojave block. The San Andreas fault zone, though poorly constrained by the seismic data, appears to be a major vertical feature separating Mojave basement, with numerous discontinuous reflections down to 30 km depth, from basement to the south , which is devoid of such reflections. Conversely the Garlock fault appears to be a relatively shallow feature, extending to less than 9 km depth, because it does not offset an underlying reflecting horizon.
Structure of the Lithosphere in a Young Subduction Zone: Results from Reflection and Refraction Studies
Pages vol.14, 313-321
Ron M. Clowes and George D. Spence (1), Robert M. Ellis and David A. Waldron (2)
(1) Now at Bullard Laboratories, Department of Earth Sciences, University of Cambridge, England
Vancouver Island, on the west coast of Canada, is believed to have formed through the process of accretionary tectonics and represents a dispersed block of the Wrangellia terrane. Presently, the oceanic Juan de Fuca plate is subducting beneath the continental plate. A series of refraction and reflection experiments has been carried out to provide better delineation of lithospheric structure in this complex zone of convergence. An offshore-onshore refraction profile has enabled development of a lithospheric model from the deep ocean to near the volcanic arc. Seaward of the continental slope, the plate which includes a 9 km thick crust dips about 1 deg. landward; below the slope the angle of dip increases to about 3 deg; and below the continental shelf it increases substantially to about 15 deg. At depths of 30 to 40 km below the shelf, a landward-dipping upper mantle boundary has been inferred from wide-angle reflection phases, and is interpreted as the base of the subducting oceanic lithosphere. In the overlying continental crust a large block of high velocity (about 7.7 km/s) material is embedded and may represent a detached remnant of subducted slab. A test multichannel explosion profile shows reflections to two-way traveltimes of 11 s; four reflectors correspond closely to boundaries in the refraction model. A 1984 follow-up Vibroseis program has produced high quality data, which, combined with the refraction data, illustrate the complementary nature of the two data sets.
The Victoria Land Basin: Part of an Extended Crustal Complex Between East and West Antarctica
Pages vol.14, 323-330
Yeadong Kim, L. D. McGinnis, and R. H. Bowen
Seismic reflection soundings to 12 seconds two way time in the southern Victoria Land Basin of the western Ross Sea indicate the presence of a deep sedimentary basin overlying a thinned crust. In addition to reflection data, seismic refraction and gravity studies provide control on the configuration of crystalline basement and depth to the Mohorovicic discontinuity. Dipping reflectors suggest a basin depth of 13 km and a 200 km long reversed refraction profile provides a MOHO depth of 21 km below sea level. The basin contains undeformed sediments dipping seaward and is similar to continental margins which were formed by rifting. Since the only apparent periods of rifting occurred during the emplacement of the Ferrar dolerites and the McMurdo Volcanics, it is believed that the deep, layered strata are synrift sediments of Jurassic age and younger. Two areas of normal faulting bound the rifted basin on the east and west. Flat-lying glacial marine sediments with few internal reflections cover the basin. Present day high heat flow and active volcanism suggest that the basin beneath McMurdo Sound is undergoing a second phase of rifting. The Victoria Land Basin and the Wilkes subglacial basin lying west of the Transantarctic Mountains form part of an extensional complex along the boundary of East and West Antarctica.
Whole-Lithosphere Normal Simple Shear: An Interpretation of Deep Reflection Profiles in Great Britain
Pages vol.14, 331-339
Marine deep-seismic reflection profiles from the British Isles acquired by the British Institutions' Seismic Reflection Profiling Syndicate (BIRPS) provide some of the best images to date of the deep structure of a zone of intra-continental extension. Simple quantitative considerations of finite strain indicate that the reflection geometry on the eastern half of the MOIST profile is consistent with the concept that large, low-angle normal faults persist as single zones of displacement through the entire lithosphere. It is proposed that the lower crust absorbs displacement by the formation of a foliation parallel to the maximum elongation direction within finite width normal shear zones. In contrast, both up-dip and down-dip in stronger or more brittle layers, the shear zones not only narrow, causing foliation to become aligned with their boundaries, but they also absorb strain via a foliation comprised of discrete, boundary-parallel slip surfaces. Such a geometry of through-going zones of displacement predicts non-alignment of lower crustal reflections with those in the mantle and upper crust within the same displacement zone. This model runs counter to current interpretations of deep reflection data that ascribe a more fundamental role to rheological stratification for the localization and development of zones of displacement in the lithosphere.
|Disclaimer | Copyright|