IGCP Project 474
Images of the Earth's Crust & Upper Mantle
Updated: Oct 13, 2010
2nd International Symposium on Deep Seismic Profiling of the Continents and Their Margins
Cambridge, United Kingdom, 15-18 July 1986
from Geophysical Journal of the Royal Astronomical Society, Volume 89, Number 1, Pages 1-448 (April 1987)
Edited by: Simon Klemperer, Nigel Holmes, Paula Aarons, Sandie Adam and the BIRPS core group of conference organisers.
(Full copies of these papers may be obtained by consulting Geophysical Journal Volume 89, Number 1
Coincident seismic reflection/refraction studies of the continental lithosphere: a global review
Walter D. Mooney and Thomas M. Brocher,
Vertical-incidence reflection profiling has identified several characteristic features of the continental lithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling.
MT and reflection: an essential combination
Alan G. Jones,
At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/ tectonic interpretations of the seismic sections.
Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.
Imaging deep reflectors in the presence of signal -generated noise
B.S. Gibson and A.R. Levander,
In contrast to the continuous character of reflectors seen in seismic data from the sedimentary column, a pervasive feature of deep crustal reflections is their laterally discontinuous nature. Deep reflections often appear on the seismic section as groups of flat-lying or dipping segments. The length of the individual segments may vary by an order of magnitude. Moreover, the segments are often shorter than the Fresnel zone for a specular reflection. The appearance of deep flat-lying reflectors of this type has motivated geologic hypotheses of a layered and heterogeneous lower crust. Dipping bands of discontinuous reflectors are interpreted variously as decollement surfaces, sutures, or faults penetrating to midcrustal or deeper levels. A significant geophysical concern is whether truly discontinuous reflectors in the deep crust can be distinguished from an inaccurate or incoherent image. Incoherence can result from the presence of signal-generated noise created at the earth's surface or along the transmission path. Given the types of signal-generated noise likely to be present in deep crustal data, we wish to know how well we can resolve both laterally continuous and discontinuous reflectors using standard reflection data acquisition and processing.
To test the effects of such noise on the CMP stack we have conducted finite-difference synthetic seismogram experiments for two similar layered models each having two specular reflectors. One model contained an irregular surface layer, the other contained a zone of random velocity fluctuations between the two reflectors. We processed the data normally, from shot gather to CMP stack and migrated section. The two numerical experiments produced CMP sections with substantially different character. Processing the surface model data produced a clear image of the two deep reflectors as the CMP stack suppressed noise generated in the irregular surface layer. The surface generated noise limited the vertical resolution as it degraded the action of predictive deconvolution applied before stack. Signal-generated noise in the second model did not affect the deconvolution process because the scatterers were located below the zone of surface layer reverberations. The laterally inhomogeneous zone degraded the image of the deepest reflector. The signal-generated noise in the second model may be viewed as signal reflected from the random zone. The dip-filtering action of the CMP stack resulted in reflection events from the random zone having greater horizontal continuity than is present in the velocity model, causing further smearing by a standard time-migration technique.
Migration - why doesn't it work for deep continental data?
Mike Wamer, BIRPS,
Conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free. Surprisingly, deep data often subjectively appear to be best migrated at velocities which are up to 50% less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities. The explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections. Since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce. During the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a "smiley" section which appears overmigrated. Since the lateral extent of individual smiles increases with increasing migration velocity, increasing two-way-time, and increasing seismic wavelength, the effect is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data.
Seismic reflection studies using local earthquake sources
Robert P. Meyer,
David E. James,Carnegie Institute ofWashington, Department of Terestrial Magnetism, 5241 Broad Branch Road, N.W. Washington, D.C. 20015
A method is presented for processing three-component digital recordings of micro-earthquakes to obtain near-vertical reflection profiles in regions of shallow seismicity. The processing includes magnitude and focal-depth normalization and event stacking, where stacking is by small localized groups, with ray theoretical time and distance corrections applied to compensate for varying focal depths. In areas with high seismicity, this procedure allows earthquakes to be treated as "controlled" sources to probe layered structures of the deep crust and upper mantle. The validity of our approach is demonstrated using S-waves from aftershocks of the Borah Peak, Idaho, earthquake (Ms = 7.3) of 1983.
Improved resolution of reflections from the crystalline upper crust
Kabir Roy-Chowdhury and Robert A. Phinney,
Use of higher frequencies and a careful selection of the other data acquisition parameters can significantly enhance the resolution obtained in reflection seismic CDP profiling in hard rock terranes. Experiments with a 20-80 Hz non-linear vibroseis sweep near the projected ADCOH site reveal that the wave-field reflected from the upper crust contains frequencies spanning the entire source spectrum even in the presence of a weathered layer of saprolite. Attenuation of higher frequencies and the severe distortion produced by the 'notch' filter for line-frequency necessitate some pre-processing spectral shaping. The operations are linear and yield, as a by-product, an estimate of the average 'Q for the section - about 250 in the present case.
New possibilities in controlled-source seismology with a magnetic levitation vibrator
R. Unger, W.A. van Kampen, A.J. Berkhout, R.G. Boiten, and A.M. Ziolkowski,
A. R. Ritsema,
D. Ph. Schmidt,
A new-technology, broad-band seismic land source based on the principle of magnetic levitation is under development. The new source is designed to overcome limitations experienced with controlled-signal seismic sources at the present time, and to create new possibilities. In particular, the new source extends the constant peak force frequency band down to 2 Hz, and is capable of at least several weeks continuous, coherent transmission. It will enable three-dimensional long-range (>200 km) controlled-signal seismology with the potential for active earth tomography down into the mantle and systematic earthquake precursor measurement, and should improve shallow and deep seismic profiling technology. The optimum transmission frequencies depend on Q, transmission range, and noise spectrum.
COCORP: new perspectives on the deep crust
L. Brown, D. Wille, L. Zheng, B. DeVoogd, J. Mayer, T. Hearn, W. Sanford, C. Caruso, T.-F. Zhu, D. Nelson, C. Potter, E. Hauser, S. Klemperer, S. Kaufman and J. Oliver
Relict sutures from colliding continents, regions characterized by a "young" Moho, layering and faulting throughout the crust, mid-crustal magma traps, and seismic "bright spots" which suggest deep crustal fluids are among recent COCORP findings. In addition, new studies of signal penetration, noise mitigation, recording geometry, and coherency filtering have yielded better understanding of, and substantial improvements in, data quality. Amplitude anomalies, or "bright spots", in the Basin and Range may be due to magma at mid-crustal levels; in one case, a normal fault appears to link the deep magma with young surface volcanics. Another bright spot, 15 km deep in southeastern Georgia, has a flat geometry that suggests a gas/liquid interface, perhaps within fluids underthrust along an Appalachian suture. The Moho continues to appear relatively undisturbed in many regions of past deformation, suggesting that its formation post-dates these major tectonic episodes. The diversity of reflection patterns from the U.S. Cordillera casts further doubt on the generality of the common model of a reflective, layered lower crust underlying a transparent upper crust.
Deep seismic reflection evidence for the role of extension in the evolution of continental crust
Beatrice de Voogd,
Deep seismic reflection profiling, as well as geologic studies, indicate that extensional basins are a common feature of continental crust. The wide-spread occurrence of extensional basins, combined with published models on the effects of extension on lower crustal rocks, suggests that extensional processes play an important role in the evolution of continental crust. Extension is now recognized to have been the last major tectonic event to affect approximately 50% of the United States. This observation and the preservation of extensional features in areas which have experienced subsequent episodes of compression, suggests that extensional processes may lead to a strengthening of continental crust. In areas of active extension such as the western United States, seismic and petrologic data, as well as theoretical modeling of heat flow data, suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present Mohorovicic discontinuity formed during extension. Although the interpretation of the various data is somewhat speculative, we suggest that the volume of continental material in some areas has doubled as a result of extension. Thus, extension may result in the addition of a significant amount of new material to the continents.
Seismic reflection models of continental crust based on metamorphic terrains
David M. Fountain, Daniel T. McDonough and Julie M. Gorham,
Laboratory seismic velocity measurements on rock samples from metamorphic terrains, coupled with geologic cross sections, provide the basis for synthetic seismic reflection profiles for various types of continental crust. Results from greenstone belts, mylonite zones and partial cross sections of continental crust indicate that lithologic heterogeneity and geometrical factors control crustal reflection characteristics.
Deep crustal structure and genesis from contrasting reflection patterns: an integrated approach
S.B. Smithson, and R.A. Johnson,
C.A. Hurich, P.A. Valasek and C. Branch,
An integrated approach including technique development into data acquisition, processing and modeling is used to aid the final interpretation of crustal reflection data by minimizing the many unknowns. Thus, a project may start out with a series of experiments involving data acquisition designed to attack a particular problem and conclude with modeling as a test to the interpretation of a problem.
Crustal structure beneath exposed accreted terranes of Southern Alaska
G.S. Fuis, E.L. Ambos, W.D. Mooney, R.A. Page, M.A. Fisher and T.M. Brocher,
The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8-9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.
Reflection mapping across the convergent margin of western Canada
C.J. Yorath and R.D. Hyndman,
Five marine multichannel seismic reflection profiles totalling 520 km were recorded across the western Canada convergent margin where the Juan de Fuca plate is subducting beneath North America. The data extend the results of LITHOPROBE on Vancouver Island. The primary objectives are definition of the offshore accretionary structures and clarification of the convergent interaction between the two plates. The main features of this preliminary interpretation are: (1) the subduction deformation front is complex with evidence of sediments being accreted and subducted; (2) the top of the oceanic crust and the Moho are imaged below the deep water sedimentary basin; (3) the top of the subducting plate is clearly imaged below the shelf; (4) beneath the inner shelf, one band of high reflectivity underlain by a zone of lesser reflectivity lies above the plate; (5) alternative interpretations place the present zone of decoupling at the base of the reflective band or the top of the plate; (6) the San Juan and Leech River faults that bound small accreted terranes are imaged as thrusts that merge at depth.
Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing
A.G. Green, B. Milkereit, L. Mayrand, C. Spencer, R. Kurtz,
R. M. Clowes, Department of Geophysics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1W5
Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of 'deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high density/high velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from > 60 deg. near the surface to 20 deg. at 3 km depth and ~38 deg. at 6 km depth.
Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex
F.A. Cook, P.S. Simony, K.C. Coflin,
A.G. Green, B. Milkereit,
R.A. Price, R. Parrish, C. Patenaude,
LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera. These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3-4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9-10 s (27-30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.
COCORP deep seismic reflection traverses of the U.S. Cordillera
Christopher J. Potter, Richard W. Allmendinger*, Ernest C. Hauser, and Jack E. Oliver*,
COCORP seismic reflection traverses of the U.S. Cordillera at 40'N and 48.5'N latitude reveal some fundamental similarities as well as significant differences in reflection patterns. On both traverses, autochthonous crust beneath thin-skinned thrust belts of the eastern part of the Cordillera is unreflective; immediately to the west the Cordilleran interior is very reflective above a flat, prominent reflection Moho. Mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns, in constrast to more easily deciphered patterns beneath areas of Cenozoic accretion. The prominent reflection Moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during Cenozoic extension. The main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the Cordilleran interior; these differences are probably related to the way Cenozoic extension was accommodated at depth. Laminated middle and lower crust above the reflection Moho in the western Basin and Range (40'N) may be related to magmatism, ductile pure shear and large-scale transposition during Cenozoic extension. By contrast, beneath the eastern Basin and Range (40'N), and the orogenic interior in the NW United States (48.5'N), Cenozoic extension was probably accommodated along dipping deformation zones throughout the crust.
A transect across the Mesozoic accretionary margin of central California
Carl M. Wentworth,
Mark D. Zoback,
Andrew Griscorn, Robert C. Jachens, and Walter D. Mooney,
The Franciscan assemblage in the Coast Ranges and metamorphic rocks of the foothills of the Sierra Nevada in central California have long been considered to be type examples of subductive accretion, formed along the Pacific margin of North America in the Mesozoic. We are exploring deep structure across this accreted margin using coordinated reflection and detailed refraction profiling; gravity and magnetic interpretation; together with surface and well geology. Although none of these techniques alone is sufficient to define the critical elements of structure, in complement they place important constraints on geologic interpretation.
For both the foothill rocks and, particularly, the Franciscan assemblage, we find evidence of obductive emplacement (overthrusting from the west), in contrast to the eastward subduction of most previous views. These surprises emphasize the importance of such geophysical studies to geologic analysis.
CALCRUST (1985) seismic reflection survey, Whipple Mountains detachment terrane, California: An overview
T.E. Henyey and D.A. Okaya,
CALCRUST is a new consortium of six academic institutions from California engaged in seismic reflection studies of the continental crust. CALCRUST activities are concentrated in the southwestern U.S. where a diversity of geotectonic styles occur which are fundamental to the evolution of continental margins. The good surface geologic control provided by excellent outcrop exposure and more than two decades of intensive mapping adds to the attractiveness of the region for deep-crustal investigation.
Current CALCRUST efforts are directed in large part, toward development of a crustal transect from the Colorado Plateau (stable North American craton) to the Salton Trough/Gulf of California (continental rifting/transform). The transect crosses a variety of tectonic elements central to the Mesozoic and Cenozoic evolution of the continental margin of western North America (e.g. Coney 198 1; Dickinson 198 1).
The line of transect is shown schematically in Fig. IA; also shown are the various major tectonic elements to be crossed. Both compressional and extensional structures are involved. CALCRUST is not only addressing the seismic character and geometry of the individual elements or structures, but also relationships between them. The first work on the transect began during CALCRUST's pilot program from 1984 to 1986. It focussed on the detachment fault terrane of the Whipple Mountains in southeastern California (Fig. 113). The Whipple Mountains are part of the "metamorphic core complex" of the North American Cordillera (Davis et al. 1979, 1980; Crittenden et al. 1980).
Well exposed in the Whipple Mountains and common to the core complexes of Arizona and southeastern California is a series of low-angle normal, or detachment faults of mid-Tertiary age, indicating appreciable crustal extension. Also exposed in the range are zones of pronounced mylonitic foliation, which are seen on a regional basis throughout large parts of the southwestern U.S. (Crittenden et al. 1980). Finally, immediately south of the Whipple Mountains, Mesozoic thrusts and nappes (Hamilton 1982) are exposed in the Riverside and Big Maria Mountains (Fig. IB).
Two important questions are posed by the presence of these three structural elements (detachments, mylonites, and thrusts) in the vicinity of the Whipple Mountains:
Crustal structure of the Whipple Mountains, southeastern California: a refraction study across a region of large continental extension
Jill McCarthy*, Gary Fuis, and Jeff Wilson,
Closely spaced refraction profiling across the Whipple Mountains metamorphic core complex in southeastern California yields a complex picture of crustal structure in this region of large continental extension. A NE-directed profile, parallel to the extension direction, reveals a high-velocity mid-crustal layer (6.6-6.8 km/s) at 16-18 km depth, bounded above and below by laterally discontinuous low-velocity zones (<6.0 km/s). In marked contrast, a NW-directed profile shows a more uniform 6.0 km/s crust down to the crust-mantle boundary. The apparent contrast between these two perpendicular profiles may be related not only to a more complex geologic structure in the NW-SE direction, but also to velocity anisotropy associated with mid-crustal mylonites. Despite the differences between the two refraction profiles, both define a flat Moho at 26-27 km depth with an associated upper mantle velocity of 7.8 km/s. This observation is significant as it suggests that, although the amount of extension has been highly variable regionally, the crust is no thinner beneath the Whipple Mountains (where extension has been extreme) than the surrounding mountain ranges. Such an observation requires either that the crust was considerably thicker prior to extension, or that lateral flow in the lower crust and/or inflation of the crust via magmatism occurred contemporaneous with extension.
The nature of deep crustal structures in the Mojave Desert, California
John N. Louie and Robert W. Clayton,
The character of multi-offset reflections from the deep crust in the Mojave Desert are examined to reveal the physical nature of the reflecting structures. We focus on distinguishing classical abrupt discontinuities, such as traditional models of the Conrad and Moho boundaries, from more unusual structures. Finite-difference modeling and simple interference relations show that pre-critical reflections exhibiting an increase in peak frequency with offset arise from thinly-layered horizontal structures, while reflections from step discontinuities show no change in frequency with offset. In the deep crust thin layers may result from sill intrusion or fault motion.
The sense of changes in Poisson's ratio and the relative strength of density changes determine whether reflection amplitudes will increase or decrease with offset. A simple linear regression on pre-critical reflection amplitudes against offset is adequate to separate reflections arising from increases in Poisson's ratio from those arising from decreases in Poisson's ratio and/or density changes. The latter condition may be the result of strong anisotropy or the presence of pore fluid. Comparisons of the properties of major deep reflectors across the Mojave Desert suggest that the effects of tectonic motion and fluid injection have penetrated all levels of the crust.
Deep seismic reflectors in the Campos basin, offshore Brazil
W.U. Mohriak and J. F. Dewey
Some deep crustal features underlying the Campos basin are best recognized in a few reflection seismic sections that have been reprocessed recently to 10 s two-way traveltime. A prominent climbing -to-the-basin reflector is interpreted as the Moho, and a relatively steep fracture zone is, probably, the first example so far of an extensional fault crossing the whole crust and offsetting the Moho. Further constraints on the deep structure of the basin are provided by estimating the thinning of the crust from shallow seismic data and gravity modelling, and by cross-plotting backstripped subsidence curves against curves predicted by the lithospheric stretching model.
Results of recent COCORP profiling in the southeastern United States
K.D. Nelson, J.H. McBride, J.A. Arnow, D.M. Wille, L.D. Brown, J.E. Oliver, S. Kaufman
Recently acquired COCORP profiles in the southeastern United States show that: 1) Reflections associated with the Appalachian detachment are prominent beneath the Inner Piedmont of western Georgia, but do not extend further southeast beneath the Pine Mountain belt. 2) The Brunswick magnetic low is associated with a broad zone of crustal-penetrating dipping reflections that probably marks the Alleghanian suture in the southeastern U.S. 3) The South Georgia basin is a composite feature consisting of several half-graben, locally containing >5 km of Triassic - Early Jurassic basin fill. These basins occur within the interior of the Alleghanian orogen, but are not specifically associated with Alleghanian suture. 4) Across-strike crustal thickness variation, and distribution and character of lower-crust and Moho reflections in the Southern Appalachians is grossly similar to that observed in other parts of the Appalachian/Hercynian orogenic belt. Global comparisons suggest that these regional variations are a consequence of post-collisional extensional tectonics, rather than a primary (Palaeozoic or older) feature of the orogenic belt.
Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study
C. Coruh, J.K. Costain,
R.D. Hatcher, Jr.,
In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians.
The major points in the interpretation are: 1) The Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) The Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures ("duplex tuning wedges") connected by other thrust faults that duplicate shelf strata to a thickness of 4-5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged.
The ADCOH data were originally recorded with 14-56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.
Tectonic implications of new Appalachian Ultradeep Core Hole (ADCOH) seismic reflection data from the crystalline southern Appalachians
Robert D. Hatcher, Jr.,
John K. Costain, and Cahit Coruh,
Robert A. Phinney,
Richard T. Williams,
Some 180 km of new VIBROSEIS profiles have been acquired in the southern Appalachian Inner Piedmont, Brevard fault zone and eastern Blue Ridge as part of the ADCOH Project site investigation. These data are of the highest quality yet obtained in a crystalline terrane in the US, perhaps in the world, and reveal several conclusions that should have a direct bearing upon the world-wide nature of composite crystalline thrust sheets and their modes of interaction with the platform rocks beneath. Strong reflections previously interpreted as the base of the crystalline sheet are clearly part of the platform sedimentary (clastic rocks) sequence resting upon the autochthonous basement and early Palaeozoic rift basins. This reflection package and related transparent zones are clearly repeated beneath the crystalline sheet indicating a complex of thrusts repeating units within the platform succession. Reflectors (granitoid-amplibolite contacts) in the crystalline sheet in the Inner Piedmont represent recumbent folds of similar wavelengths and amplitudes to folds mappable on the surface. Duplexing of platform rocks beneath the crystalline sheet appears to have resulted in doming of the crystalline sheet. Similarly, duplex formation in the platform was probably controlled by both the thickness of the crystalline sheet and the rheological properties of the platform succession.
Lower crustal reflections in central Virginia, USA
T.L. Pratt*, C. Coruh, and J.K. Costain,
Vibroseis reflection data across the Blue Ridge and Piedmont provinces in Virginia, acquired in 1981 by the United States Geological Survey, have been reprocessed at Virginia Tech. The 12-fold, 14-56 Hz vibrator data were originally 8 s in length, but were extended to 14 s during correlation. Interpretation of the data was further improved by using a well-defined crustal velocity model derived by previous workers from earthquake and blast analyses in the area.
Low reflectivity areas on the seismic section are interpreted to represent Grenville-aged crust on the section, in contrast to highly reflective allochthonous units. These regions of reflectivity allow for mapping of the gross crustal structure.
Strong, subhorizontal arrivals at 9 to 12 s in the reflection data are interpreted to be from lower crustal layering just above the Mohorovicic Discontinuity (Moho) defined by earlier refraction work. This layering may be reflective only beneath regions of deformed crust and not at the base of the undisturbed craton. The depth to these reflectors is approximately 35 km beneath Richmond, Virginia and increases gently westward until the reflections disappear beneath the Blue Ridge Mountains about 70 km west of Richmond. These thicknesses are in agreement with earlier refraction work which also indicated greater Moho depths to the west. A rethickening of the crust near the Adantic coast is also interpreted from the refraction data but is not evident on the reflection line, probably due to an acquisition or energy penetration problem.
The deep reflections consist of a subhorizontally layered package about 5 km in thickness, thinning slightly toward the craton. Their base coincides with the crust-mantle boundary determined from refraction data, indicating that the reflections lie within the lower crust. Lower crustal reflections were not recorded west of the Piedmont on the multichannel data. PmP arrivals recorded by earlier workers from beneath the Blue Ridge, however, are consistent with a Moho reflector about 8 km shallower than indicated by the refraction data. If the crust-mantle boundary is a second-order discontinuity (smooth transition zone) there, the PmP arrivals may have been reflected from near the upper boundary of the transition zone and the refracted waves may have travelled along the base. The discrepancy in depths may therefore be a measure of the transition zone thickness.
Interpretation of migrated seismic reflection profiles across the northern Appalachians in Maine
J. D. Unger, D. B. Stewart and J. D. Phillips,
A simple line-migration technique has been developed and applied to deep crustal reflection data collected in the northern Appalachians of Quebec and central Maine. Preliminary interpretation of these data, combined with the results of gravity and refraction studies, shows thinning of the crust in two distinct steps beneath the Silurian-Devonian Merrimack synclinorium and shows evidence for Mesozoic crustal stretching and dike intrusion.
More seismic evidence on the location of Grenville basement beneath the Appalachians of Quebec-Maine
Carl Spencer, and Alan Green,
High-quality multichannel seismic (~133 fold) and refraction/ wide-angle reflection (1 to 3 km receiver spacing and 30 to 60 km shot spacing) data have been collected across the northern Appalachians in Quebec and Maine. An integrated interpretation of the seismic data from the southeastern Quebec-westem Maine region provides strong evidence that the rocks of the predominantly oceanic "Dunnage" zone are allochthonous having been thrust westwards over Precambrian Grenville basement during and subsequent to the closing of the Iapetus Ocean.
Wide-angle deep crustal reflections in the northern Appalachians
James H. Luetgert,
Carol E. Mann,
Simon L. Klemperer,
Seismic refraction data collected in the northern Appalachians provide an unusual opportunity to use wide-angle reflections to examine the lower crust and upper mantle. The PmP phase, clearly identified on several hundred records, has been used to construct an isopach map of the crust which shows a stepwise regional thickening of the crust beneath the axis of the Appalachians. The data have been examined to times of up to 40 s two-way travel time to investigate the possibility of coherent upper-mantle reflections such as those recently observed by BIRPS on near-vertical data northwest of Britain. Although substantial coherent energy appears after the PmP phase, synthetic seismogram modelling shows that all of these arrivals are explicable by S-phases, converted phases and multiples from within the crust. We conclude that in this region of the northern Appalachians we have not detected any significant regionally extensive reflecting horizons within the upper mantle.
Structure of the lower crust beneath the Gulf of Maine
D.R. Hutchinson, K. D. Klitgord, and A. M. Trehu,
The variability of deep crustal reflections in USGS line IA across the offshore New England Appalachians shows the differing influence of Paleozoic and Mesozoic tectonic events. Mesozoic extension has not significantly modified Paleozoic thrust faults penetrating the lower crust in the northern Gulf of Maine. Mesozoic extension or Paleozoic crustal melting could explain the lower crustal character in the central Gulf.
Lithoprobe east: results from reflection profiling of the continental margin: Grand Banks Region.
Beatrice de Voogd and Charlotte E. Keen,
The 1985 Lithoprobe East profiles resolve deep crustal structure of the Grand Banks off eastern Canada. Basins are 7 to 20 km deep, and bounded by major faults traceable to Moho depth. The lower crust is reflective along much of the survey, and the top of this reflective layer has a variable depth. Prominent and often surprisingly flat Moho reflections are observed. Puzzling rotated fault blocks are imaged at the continent-ocean transition.
Reflection experiment on a floating ice platform
Preliminary analysis of a multifold seismic survey on a floating ice platform, northwest of Axel Heiberg Island, along the Canadian portion of the continental margin of the Arctic Ocean revealed a structurally complex basement at a depth beyond 4000 m. The basement is covered with high velocity (5000 m/s) rock types. The results conform to the anticipated geologic model for regions of passive margins.
Some unsolved BIRPS problems
Drummond Matthews and the BIRPS group*,
R. Hobbs, S.L. Klemperer, S. McGeary, C.P. Peddy, M.R. Warner and C.A. Smith
BIRPS has 8000 km of records around Britain that are relevant to the rheology of continents and to faulting of the lithosphere. Problems include the need to measure seismic velocities with better resolution and to eliminate multiples in deep water, the question "Are blank bits of records due to noise increase or to geology?" and the need to know the physics of the origin of deep reflections in order to answer geologists' questions.
Reflectivity of the crystalline crust: hypotheses and tests
Simon L. Klemperer and the BIRPS group,
The nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop. Geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms, but most geological information still comes from speculative interpretations of seismic experiments run in different regimes. Pronounced lower-crustal reflectivity, detected worldwide, is ascribed in various geological hypotheses to primary lithologic layering, to ductile strain banding, or to trapped fluids.
A BIRPS deep crustal profile across the Atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone. Study of worldwide crustal profiles shows that exposed high-pressure terranes are not as reflective as in situ lower crust at high pressure, suggesting either that these granulite terranes are not representative of the lower crust or that physical properties, possibly the presence of fluids or thermally controlled ductile strain banding, are more likely responsible for observed reflectivity than are simple lithologic boundaries. The argument for the importance of physical properties in causing observed lower-crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat-flow.
Seismic reflection coefficients from mantle fault zones
Mike Wamer & Susan McGeary,
Several bright reflections from structures within the mantle can be seen on BIRPS' deep seismic reflection profiles. We have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0.1. It is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle. These large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite. The brightest reflections seem to be best explained as major faults or shear zones within the mantle.
Nontypical BIRPS on the margin of the northern North Sea: The SHET Survey
Striking similarities in the reflectivity of the crust and upper mantle on BIRPS profiles has led to the development of the "typical BIRP", a model seismic section for the British continental lithosphere. The SHET survey, collected in the region of the Shetland Islands and the northern North Sea, fits the general pattern to a certain extent. Caledonian structures and Devonian or younger basins are imaged in the otherwise acoustically transparent upper crust. An unexpected and exciting feature imaged on SHET is a short wavelength structure on the Moho or abrupt Moho offset beneath the strike-slip Walls Boundary Fault. SHET differs markedly from the SWAT typical BIRP, however, by showing a poorly reflective lower crust. Only a narrow zone (~1 s) at the base of the crust contains high-amplitude reflections. The SHET survey therefore highlights the wide variation in lower crustal reflectivity within the total BIRPS data set rather than the similarities.
Is lower crustal layering related to extension?
Richard W. Hobbs, Caxolyn Peddy and the BIRPS
The Western Approaches Margin (WAM) profile was shot to test the hypothesis that the reflectivity observed in the lower crust is related to extensional processes. The preliminary results of the experiment show that the reflectivity in the lower crust appears to become weaker on the continental shelf near the slope break. Detailed examination of the data however, show a significant increase in noise in the region where the layering appears to fade. The noise may be of sufficient amplitude to obscure any coherent lower crustal events present. Therefore, the only conclusion that can presently be drawn from the dataset is that the layering does not become more pronounced in the region of maximum extension.
Deep events in U.K. South Western Approaches
R.R. Hillis* and G.A. Day,
Apparently planar dipping events are observed in seismic data off south-west Britain within otherwise essentially transparent upper and middle crust. These are believed to represent Variscan thrusts, some of which were re-activated during the post-Carboniferous phase of extension that affected the southern U.K. They can be seen in two extensive commercial seismic surveys recorded to 6 s two-way-time (TWT) and, where laterally persistent, they have been mapped to reveal their essentially planar nature. Commonly these dipping events are associated with deeper, near-horizontal, or gently convex upwards events with which they often appear to converge. Where 'real', these are thought to indicate a complex fault system or possibly the top of the reflective lower crust. The thrusts are seen over the whole area except where granite is known to occur, and commonly exert a major control on the position and subsequent deformation of overlying sedimentary basins.
Possible mid-crustal shears at the edge of the London Platform
T.J. Reston and D.J. Blundell,
A reflection survey across part of the southern North Sea has revealed SW-dipping bands of reflection segments in the midcrust under the edge of the London Platform. The upper limit of each group of reflection segments has been contoured in TWT to give a three dimensional impression of the shape of the features. The shape, position and orientation of these groups, together with the reflection character within each group, suggest that they represent large-scale extensional, probably dilatant shear zones. It is proposed that they developed at the edge of the North Sea Basin due to relative movement between an undeformed brittle upper crust and a more ductile lower crust which has been stretched towards the basin to the NE. The shears are thus caused by heterogeneous crustal stretching.
The PUMA experiment west of Lewis, U.K.
C.M.R. Powell and M.C. Sinha
In August-September 1984 a 165 km long, reversed, wide-angle, seismic line was shot along part of the BIRPS WINCH deep seismic reflection profile west of Lewis in the Outer Hebrides. The experiment was recorded using a new type of sea-bottom receiver, the PUMA (Pull-Up Multichannel Array). This consists of an 1100 m long array of hydrophones connected to a sea-bottom recording package. Airgun and explosive shots recorded by the PUMA provide a densely sampled record section, allowing even low amplitude arrivals to be traced across the section. An initial, 1-D interpretation of the data reveals a crustal thickness of 27 km, and confirms that a band of reflectors seen at around 8.6 s TWT on the normal incidence data, and interpreted as the 'reflection Moho', coincide with the Moho determined by wide-angle reflections and refractions.
Hatton Bank (northwest U.K.) continental margin structure
G. K. Westhrook,
S.R. Fowler, G.D. Spence, P.J. Barton, M. Joppen and J. Morgan,
A.N. Bowen, C. Prestcott and M.H.P. Bott,
The continent-ocean transition near Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic (mcs) profiles. Extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin. At the landward (shallowest) end the dipping reflectors overlie continental crust, while at the seaward end they are formed above oceanic crust. Beneath the central and lower part of the margin is a mid-crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence. Beneath this mid-crustal layer and above a well defined seismic Moho which rises from 27 km (continental end) to 15 km (oceanic end) across the margin, the present lower crust comprises a 10-15 km thick lens of material with a seismic velocity of 7.3 to 7.4 km/s. We interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas. A considerable quantity of new material has been added to the crust under the rifted margin. The present Moho is a new boundary formed during creation of the margin and cannot. therefore be used to determine the amount of thinning.
Deep seismic reflection profiles across the western Barents Sea
S.T. Gudlaugsson, J. I. Faleide,
The continental crust beneath the western Barents Sea has been acoustically imaged down to Moho depths in a large scale deep seismic reflection experiment. A first-order pattern of crustal reflectivity has been established and the thickness of the crust determined. A number of features with important implications for the tectonics of the area have been discovered. The results are presented in the form of two transects.
Operations and main results of the ECORS project in France
C. Bois, B. Damotte, and A. Mascle,
The ECORS project was launched in 1983 with the aim of studying fundamental mechanisms of geodynamics in France. The ECORS deep seismic profiles have concentrated on a few structures of major significance: outer zone of the Variscan orogen in northern France, the basin formed during the Bay of Biscay's opening and transects of recent orogenic ranges (Pyrenees and Alps). The seismic profiles have been carried out with all the available modem techniques of industry and completed wherever possible by additional geophysical surveys (magnetism, gravity, MT, wide-angle and refraction seismics) and geological surveys. The first results already shed new light on major geodynamic phenomena such as variations in the frontal Variscan detachment, lower crust formation, crustal behaviour during orogenesis and variations in the formation of cratonic basins.
Crustal reflection seismics: the contributions of oblique, low frequency and shear wave illuminations
A. Hirn, B. Damotte, G. Torreilles and ECORS
Continuous vertical seismic reflection profiling, the use of which has been extended by COCORP, BIRPS, ECORS, DEKORP and other programmes (Barazangi & Brown 1986) from stratified sedimentary basins to the entire crust, provides a high resolution cross section of reflectivity. A similar extension of oblique or variable offset sounding would be expected only to provide complementary velocity information. However, combined use of the two methods at crustal scale is new, and we show that refractions and wide angle reflections do contribute original and relevant information, but generally in areas other than velocities - the reason being that interfaces and layers may have a different nature in the crystalline crust from that in sediments.
Crustal laminations in deep seismic profiles in France and neighbouring areas
A. Masele, L. Montadert and B. Pinet,
Remarkable crustal features appear on the ECORS profiles carried out in northern France and the Bay of Biscay as well as on the SWAT profiles shot in the western Channel and the Celtic Sea. The most striking one is the occurrence of flat laminations in the lower crust. Dipping events and laminations are also present in the upper and lower crust, especially in the SWAT profiles. They can readily be related to tectonic events, Variscan in age, some of them identified in the field. The flat laminations in the lower crust are at first interpreted as resulting from delamination, shearing, magmatism and metamorphism at the crust-mantle transition during the Variscan orogeny. This interpretation raises some difficulty concerning the space and time correlation of the laminations with the Variscan orogeny. They seem to have been emplaced after the Permian-Triassic infilling of the Plymouth Bay basin and before the early Cretaceous opening of the Bay of Biscay. An early to middle Jurassic age is suggested, a period when large cratonic basins were formed without noticeable extension. Heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations. Choosing between orogenic and non-orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination.
Deep seismic reflection and refraction profiling along the Aquitaine shelf (Bay of Biscay)
Bertrand Pinet, Lucien Montadert and the ECORS Scientific Party,
The 300 km ECORS - Bay of Biscay profile was carried out along the Aquitaine shelf and comprised a complete set of experiments including zero-offset and 7.5 km constant offset vertical seismic reflection and six expanding spread profiles (ESP). Large offset recordings were fundamental for the definition of the layered lower crust and the Moho, while ESPs provided decisive complementary information for the geological interpretation. These data show a strong variation in crustal thickness from about 20 km beneath the rifted Parentis basin, a failed arm of the oceanic Bay of Biscay, up to 35 km to the north below the Armorican shelf, in the Hercynian domain, and to the south below the Cantabria shelf, in the vicinity of the Pyrenean deformation front. The results have important implications for the behaviour of the crust during the formation of rifted sedimentary basins and during continental collision.
Wide-angle vibroseis test across the Rhine graben
K. Fuchs, E. Lüschen, and F. Wenzel
R. Schlich, Institute of Geophysics, Strasbourg;
As a joint operation of ECORS and DEKORP, a deep wide-angle seismic experiment using vibrators was carried out in the autumn of 1984. The object was to get information on the deep crust under the Rhine graben without crossing through sedimentary layers. Offsets were in the range of 50 to 89 km. In the first phase, two vibration points were executed in the Vosges mountains. A signal was received in the Black Forest from solely the farthest VP. In the second phase, fourteen VPs were executed in the Black Forest. No stacking or correlation was performed in the field in France. The quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre.
Results of DEKORP 2-S and other reflection profiles through the Variscides
R. Meissner, Th. Wever, and R. Bittner,
The first DEKORP profile, DEKORP 2-S, a 250 km long line perpendicular to the Variscan strike direction, has provided evidence of major crustal shortening during the Variscan orogeny. Sporadic dipping events in a generally transparent upper crust are interpreted as thrust faults, while the highly reflective lower crust fits into the general picture of Palaeozoic provinces. Correlations are established between certain reflectivity patterns and rheology. Moho depths and reflecting lamellae are considered to be post-Variscan.
Pre-drilling reflection survey of the Black Forest, SW Germany
KTB-Research Group Black Forest,
The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection measurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25-27 km, above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.
Combined seismic reflection and refraction profiling in southwest Germany - detailed velocity mapping by the refraction survey
D. Gajewski, W.S. Holbrook and C. Prodehl
Within the framework of site survey studies of the Deep Drilling Program of the Federal Republic of Germany (KTB), coincident deep-seismic reflection and refraction experiments in the Black Forest, southwest Germany, were carried out. The simultaneous interpretation of the reflection and the refraction data reveals in particular both a strong velocity reduction in the upper crust and a laterally varying laminated structure of the lower crust. Additional refraction lines result in a three-dimensional crustal model which shows two distinct crustal types of different seismic properties. These crustal types seem to correlate with the major geologic units of Southwest Germany. Variations of Poisson's ratio derived from clearly recorded shear wave data show a similar trend.
Physical properties and structure of the lower crust revealed by one- and two-dimensional modelling
K.-J. Sandmeier, W. Wäde and F. Wenzel,
Data from a refraction and a reflection seismic survey in the Black Forest, southwest Germany, are used for extensive one- and two-dimensional modelling. The data are available along approximately the same line, and therefore the same piece of crust is probed by two seismic methods. We utilize this favourable circumstance for detailed model calculations concerning both data sets. Lower crustal properties vary on the scale of a wavelength and thus full solutions of the elastic equations are required: the Reflectivity Method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response. Details of the geometry and physical properties of the lamination are derived. Vertical layering on a scale of 100 m is found; horizontal extent of reflecting elements is in the range of a few hundreds of meters; rocks with velocities between 5.6 and 7.2 km/s constitute the lower crust.
Synoptic interpretation of seismic reflection and refraction data
St. Mueller, J. Ansorge, N. Sierro, and P. Finckh,
The structure of the upper lithosphere beneath southern Germany, northern Switzerland and west-central Utah (U.S.A.) has been investigated in detail by various geophysical methods. A synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide-angle reflection surveys, combined with near-normal incidence reflection observations, now permits the elucidation of the fine structure in a more quantitative and unified manner. With this scheme it is possible to unambiguously identify low-velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process.
Results of deep reflection seismic profiling in the Oberpfalz (Bavaria)
DEKORP Research Group,
In order to investigate the target area of the Continental Deep Drilling (KTB) in the Oberpfalz a network of six seismic reflection lines was acquired in 1985 using the Vibroseis technique. The average length of these lines was 50 km. In addition, the 185 km long NW/SE striking line DEKORP 4 with its short appendix line 4-Q of 40 km length was acquired with the same technique. The results reveal a strongly structured upper crust. This is in contrast with previous surveys in the German Variscides which show a poorly reflective upper crust and a strongly reflective lower crust. Except for the S part of DEKORP 4 in the Oberpfalz area the Moho is only weakly reflective. In addition to the Vibroseis survey 96 shots along line DEKORP 4 were recorded by conventional reflection techniques and by portable reflection and refraction stations from university institutes and geological surveys in order to obtain wide-angle reflection and expanding spread data.
Reflection measurements along the EGT POLAR-profile, northern Baltic Shield
The EGT POLAR-profile runs entirely within the Archeaen part of the northern Baltic Shield. The fieldwork was done in August 1985. Signals from 30 explosions fired at 6 shotpoints along the 440 km profile were recorded by 44 three-component stations at 205 sites, giving an average distance between the recording sites of 2.1 km. In addition along the central part of the refraction profile two densely spaced reflection profiles, of lengths 36 km and 16.4 km were recorded, using Sercel 348 equipment from the University of Uppsala and a 48 channel Texas Instrument DSF-V from the University of Hamburg respectively. All shots from the main refraction profile were recorded, as well as 16 additional smaller shots (80 kg TNT) fired at 3 shotpoints within the total reflection spread. The two reflection profiles were placed in a granulite belt crossing the profile.
The use of land recorded long-range marine airgun data in crustal reflection-refraction investigations
C-E. Lund, R.G. Roberts, C. Juhlin, R. Bodvarson, and H. Palm,
During the EUGENO-S field campaign in 1984 a large number of airgun shots were fired at sea in the Skagerrak and Kattegat and in Lake Vanern in southwestern Sweden. The signals were recorded on land by analogue "MARS" and digital "SN-PCM-80" three channel stations, by a digital 48 channel "SERCEL SN348" reflection instrument, and by "Geostore" stations. The airguns were shot about every 300 m along profiles up to 100 km in length. Clear reflected and refracted arrivals were observed from 5 km to 250 km shot-receiver offset. The field and data processing techniques used are briefly described, and two examples of data are discussed.
Deep reflection seismics in the Precambrian of Sweden
T. Dahl-Jensen, D. Dyrelius, C. Juhlin, H. Palm, and L.B. Pedersen,
A total of 161 km of deep seismic profiles have been shot in the region. One profile crosses the Protogine zone in SW Sweden. Over most of the profile short, weak reflectors are seen. The only area with a concentration of reflectors is in the upper two seconds between the two tectonic zones. A nearly transparent area east of the Protogine zone is interperted as a deep granite intrusion. In the Si1jan impact structure where four profiles were shot, the NE part of the structure is dominated by upper crustal high amplitude reflectors. Possible causes are discussed.
Geological structure of the Earth's crust above the Moho discontinuity in Yugoslavia
D. Skoko, E. Prelogovic and B. Afinovic,
The Moho discontinuity was modelled on the basis of the 6 DSS profiles across the Yugoslav area, by the use of regression analysis and expressed by fitting surfaces from the 1st up to the 4th degree. Their characteristics were correlated to the geological structure of the Earth's crust, with intention to point out their connection with the deformations of the Moho discontinuity.
Crustal profiles of active continental collisional belt: Czechoslovak deep seismic reflection profiling in the West Carpathians
Cestmir Tomek, Libuse Dvorakova and Ivan lbrmajer,
Czechoslovak deep seismic reflection profiles across the West Carpathians, the first in the Alpine-Himalayan belt, and surface geological data, suggest that the passive margin of the Eurasian plate was obliquely overriden by the upper Carpatho-Parmonian plate during the end of the Krosno sea subduction some 17-14 Ma ago. The following period was dominated by slight oblique continental collision (transpression and transtension) of the West Carpathian-East Alpine continental material escaping from the East Alpine collision zone and Eurasian Brunovistulic passive margin. Crustal shortening in the North was accommodated by significant northerly dipping backthrusting and crustal thickening. Backthrusting is clearly observable on deep seismic lines 2T and 3T. Different subsidence features are present on the deep seismic line 3T. There are active pull-apart graben in the Vienna basin, mid-Miocene (16-10 Ma) low-angle normal faulting in the Danube basin, and there is a normal simple shear zone offsetting the Moho boundary beneath the Danube basin.
Deep seismic reflection studies in Israel - an update
Y. Rotstein, Z. Yuval and P. Trachtman,
The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.
Seismic reflection and refraction studies of the deep structure of the Agulhas Bank
A 12 s two-way time seismic reflection profile, 46 km in length and straddling the Cape Seal Arch, was surveyed on the Agulhas Bank during 1985. The contact between the marine sediments and the pre-Mesozoic basement produces a strong reflection at 2 s. The folded Cape and Kaaimans sediments give rise to occasional strong reflections from 2-6 s. Strongly reflecting segments occur between 9 and 10 s, and with a time-to-depth conversion made using refraction velocities, this zone of occasional strong reflections is identified as the Moho. The section from 6-9 s does not give rise to significant coherent reflections, and is considered to represent the Archaean crust. An analysis of the faults active during Gondwana break-up, revealed by reflection seismology, show the Agulhas Fracture Zone to be a divergent wrench fault system.
Deep seismic soundings along Hirapur-Mandla profile, central India
K.L. Kaila, P.R.K. Murty, D.M. Mall, M.M. Dixit and D. Sarkar,
The crustal depth section along Hirapur-Mandla profile has been computed in two steps from Deep Seismic Sounding (DSS) data. The shallow section up to the crystalline basement is derived by inverting first arrival refraction travel times. The upper Vindhyan sediments (velocity 4.5 km/s) have a maximum thickness of about 1.5 km at Bakshaho. The lower Vindhyan sediments (velocity 5.4 km/s) were deposited north of Narmada-Son lineament between Katangi and Narsinghgarh in a graben developed in crystalline basement. The thickness of the lower Vindhyans increases from north to south towards Katangi and the depth to the basement reaches 5.5 km near Jabera. The depth to the Moho boundary varies from 39.5 km near Tikaria to 45 km at Narsinghgarh. The narrow block between Katangi and Jabalpur forms a horst feature which represents the Narmada-Son lineament forming the southern boundary of the Vindhyan basin. Two-dimensional ray tracing was performed generating travel time curves from various shot points which were matched with observed travel time data.
Explosion study of the structure and seismic velocity distribution of the crust and upper mantle under the Xizang (Tibet) Plateau
Editor's note. This paper has been extensively edited and several of the figures have been redrawn. In some places the sense may have been changed but there has not been time to consult the author. The paper is a review by a distinguished Chinese scientist and is not written in the Cartesian style customary in the West.
Foreword. The Qinghai-Xizang Plateau is a very active region, with its spectacular Himalayas. In order to investigate the uplift of this plateau and the formation of the extremely thick crust, as well as the collision between the Indian and Eurasian plates, explosion seismic sounding was carried out in the area from 1975 to 1977 and 1981 to 1982. Shots were fired in the lakes of Pumayan (Po Mo), Peiku (Pei Ku), Nam, Siling (Shi Lin) and Peng, and in wells at Dinggye, Ngamring and Yangamdo (Ya An Do). 160 tons of TNT were used in the period 1975-1982 and five seismic profiles were completed.
Lower crustal involvement in upper crustal thrusting
J.H. Leven and D.M. Finlayson,
Basement structures mapped in the Devonian Adavale Basin, eastern Australia, indicate two styles of lower-crustal involvement in the formation of upper-crustal structures. The first style is typified by thrust features in the upper-crustal sedimentary section and basement, a response to lower-crustal shortening over a wide area. The second style includes lower-crustal thrusting and thickening in a limited region, with associated uplift of the upper crust. These two styles suggest that the upper and lower crust were mechanically decoupled during Palaeozoic compressive episodes.
A deep seismic profile of 800 km length recorded in southern Queensland, Australia
K.D. Wake-Dyster, M.J. Sexton, D.W. Johnstone, C. Wright and D.M. Finlayson,
In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 kin. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.
The Central Australian seismic experiment, 1985: preliminary results
C. Wright, B.R. Goleby and C.D.N. Collins,
S. Sugiharto and S. Greenhalgh,
The major objective of the Central Australian seismic experiment is to investigate the structural evolution of the Arunta Block and the Ngalia and Amadeus Basins. A regional north-south reflection line of 420 km length from the Northern Arunta Province to the southern part of the Amadeus Basin was recorded in 1985. The most significant basement features are prominent bands of reflectors from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s that dip towards the north. Deep crustal features south of the Ngalia Basin are less clear except in the Redbank Zone. Bands of deep reflectors similar to those observed in the north occur at times of between 5 and 10 s beneath the southern part of the Amadeus Basin. Additional seismic profiling included a reflection line of 40 km length recorded across the northern margin of the Redbank Zone, three expanding spread reflection profiles and a tomographic experiment. An east-west seismic refraction profile of 400 kin length was recorded within the Arunta Block, and suggests an average crustal thickness of 55 km.
Preliminary deep reflection studies in the Arunta Block, Central Australia
B.R. Goleby and C. Wright,
In 1985, near-vertical incidence reflection profiling was carried out across the Arunta Block in Central Australia. This region consists of exposed Proterozoic metasediments, granites and granulites. There is usually a limited sedimentary coverage generated by deep weathering. The seismic sections for the deep crust are markedly different from those previously recorded in Eastern Australia where there is extensive sedimentary cover. One of the striking features is the presence of energy with frequencies as high as 100 Hz at two-way times of 5-6 s. Reflections are found throughout the crust, and there is no zone that can be characterised as non-reflective. The strongest reflectors commonly lie in the intervals around 4-6 s and 8-11 s and display significant dip. Individual shot records show fairly rapid variations in amplitude and waveform within a reflection band and the correlation between records from adjacent shots can also be somewhat limited. Such features are not well suited to the application of standard processing techniques designed for subhorizontal structures, and call into question the utility of conventional stacking. The character of the reflections changes markedly with varying frequency which suggests that they arise by interference phenomena, probably associated with laterally varying lamellar structures.
Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.
The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.
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