34th International
Geological Congress
|
Updated:Jun 02, 2016
5th International Symposium on Deep Seismic Reflection Probing of the Continents and Their Margins
Banff, Canada, 16-12 September 1992
from TECTONOPHYSICS Volume 323, Issues 1-4, Pages 1-450 (1994)
Edited by Ron M. Clowes and Alan G. Green
Abstracts of papers presented for publication in a special issue of Tectonophysics
(Full copies of these papers may be purchased for about US$30 each from Elsevier through their web sitehttp://www.elsevier.com and follow the "journals" link toTectonophysics)
Review of recent results from continental deep seismic profiling in Australia
Pages 1-12
Bruce R. Goleby, Barry J. Drummond, Russell J. Korsch, J. Barry Willcox, Geoffrey W. O'Brien and Kevin D. Wake-Dyster
The Australian Geological Survey Organisation regularly collects 450-500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in Australia. These recordings are made in a wide range of tectonic provinces, including, in the last few years, late Palaeozoic-Mesozoic intracontinental and Palaeozoic-Mesozoic-Cenozoic continental margin extensional basins, moderately deformed Palaeozoic transtensional basins and compressional fold belts, and Archaean greenstone terranes. Several of these provinces are major petroleum exploration provinces, whereas others contain significant mineral deposits. The primary purpose of the deep seismic profiling program is to resolve the tectonic history of the Australian continent, and thereby to encourage exploration for hydrocarbons and mineral resources in Australia.
On the northwest Australian continental margin, major basin systems including the Bonaparte Basin, formed as a result of complex interactions since the Carboniferous, involving episodes of extension followed by strike-slip movements and inversion, which reactivated both the primary extensional and ancient basement structures. Off southeastern Australia, basins such as the Gippsland Basin formed as part of a linked transtensional system related to movement on a common mid-crustal detachment complex.
On continental Australia, the Bowen Basin, in the northeast, was deformed by thrust faults that root in a major E-dipping detachment that flattens in the middle crust. The Cobar Basin, in the southeast, is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin. The greenstone terranes within the Eastern Goldfields region of Western Australia show crustal-scale fault systems that are planar and steep dipping, more in keeping with those interpreted in data from other Precambrian provinces rather than those of the Palaeozoic provinces.
Elsevier online abstract
Seismic images of the Brooks Range fold and thrust belt, Arctic Alaska, from an integrated seismic reflection/refraction experiment
Pages 13-30
A. Levander, G. S. Fuis, E. S. Wissinger, W. J. Lutter, J. S. Oldow and T. E. Moore
We describe results of an integrated seismic reflection/refraction experiment across the Brooks Range and flanking geologic provinces in Arctic Alaska. The seismic acquisition was unusual in that reflection and refraction data were collected simultaneously with a 700 channel seismograph system deployed numerous times along a 315 km profile. Shot records show continuous Moho reflections from 0-180 km offset, as well as numerous upper- and mid-crustal wide-angle events. Single and low-fold near-vertical incidence common midpoint (CMP) reflection images show complex upper- and middle-crustal structure across the range from the unmetamorphosed Endicott Mountains allochthon (EMA) in the north, to the metamorphic belts in the south. Lower-crustal and Moho reflections are visible across the entire reflection profile. Travel-time inversion of PmP arrivals shows that the Moho, at 33 km depth beneath the North Slope foothills, deepens abruptly beneath the EMA to a maximum of 46 km, and then shallows southward to 35 km at the southern edge of the range.
Two zones of upper- and middle-crustal reflections underlie the northern Brooks Range above ~ 12-15 km depth. The upper zone, interpreted as the base of the EMA, lies at a maximum depth of 6 km and extends over 50 km from the range front to the north central Brooks Range where the base of the EMA outcrops above the metasedimentary rocks exposed in the Doonerak window. We interpret the base of the lower zone, at ~ 12 km depth, to be from carbonate rocks above the master detachment upon which the Brooks Range formed. The seismic data suggest that the master detachment is connected to the faults in the EMA by several ramps. In the highly metamorphosed terranes south of the Doonerak window, the CMP section shows numerous south-dipping events which we interpret as a crustal scale duplex involving the Doonerak window rocks. The basal detachment reflections can be traced approximately 100 km, and dip southward from about 10-12 km near the range front, to 14-18 km beneath the Doonerak window, to 26-28 km beneath the metamorphic belts in the central Brooks Range. The section documents middle- and lower-crustal involvement in the formation of the Brooks Range.
Elsevier online abstract
A collisional crustal fabric pattern recognised from seismic reflection profiles of the Appalachian/ Caledonide orogen
Pages 31-42
Jeremy Hall and Garry Quinlan
Deep seismic reflection profiles across the Appalachians, and across the once contiguous Caledonides around Britain and Ireland, show a broadly similar pattern of crustal reflection fabrics. The pattern is of two adjacent fabric "domains", each characterized by dips towards the other, bounded by an interface which dips in conformity with one of the fabrics. The domain interface strikes parallel to the orogen, suggesting a genetic relationship between the fabrics and the orogen. From the North Sea to the Long Island platform, the domain boundary dips to the northwest. In the southern Appalachians, the pattern appears with less consistency. Between the Charlotte and Carolina slate belts the pattern appears with the same polarity as further north, but below the coastal plain it occurs with opposite polarity-the interface dips to the southeast. An attractive, but not exclusive, hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis.
Elsevier online abstract
LITHOPROBE East onshore-offshore seismic refraction survey -constraints on interpretation of reflection data in the Newfoundland Appalachians
Pages 43-58
F. Marillier, J. Hall, S. Hughes, K. Louden, I. Reid, B. Roberts, R. Clowes, T. Coté, J. Fowler, S. Guest et al.
Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust.
Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle.
Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.
Elsevier online abstract
Lateral variations in the deep crustal structure at the Iberian margin of the Valencia trough imaged from seismic reflection methods
Pages 59-75
The ESCI-Valencia Trough Working Group, J. Gallart, N. Vidal and J. J. Danobeitia
New seismic images of crustal structure have been obtained in an area of western Europe where attenuation of the continental crust by a factor of two has been reported previously. The deep lateral variations in velocity and reflectivity at the transition between the northeastern Iberian Peninsula and the Valencia trough are investigated with three seismic reflection data sets. First results of the Spanish deep vertical reflection ESCI-profile on the Iberian mainland extend the crustal image of the -Pyrenees profile to the Mediterranean coast. A highly-reflective zone about 15 km thick is imaged in the lower crust beneath the Ebro basin and the Catalan coastal ranges, where the Moho is located at 29 km depth. This deep reflective pattern appears to be disrupted by the effects of low-velocity sediments in the Neogene Reus basin, but continuity of the principal horizons is inferred from illuminations of the area using sources and receivers at wide-angle offsets. High-quality recordings up to 50 km inland of air-gun shots from the marine ESCI-profile provide a complementary image of the structure on land and show that the major crustal thinning toward the Valencia trough starts near the shoreline. The lateral variability of the crust is investigated using a second profile displaced 30 km to the northeast. Independent refraction and reflection interpretation techniques applied to this profile, which consisted of densely-spaced shots, resulted in consistent depth sections with crustal thinning beginning further inland. Thinning of 10 km takes place within 60 km at the Catalan margin. Moho-reflected waves (PMP) have been processed as far traces of a deep reflection profile. The processing included common midpoint (CMP) sorting, stacking and migration. The inferred lateral variations in deep crustal structure near the Catalan coast are consistent with the Bouguer gravity field. They may be related to episodes of extensional block faulting that have affected the Catalan-Valencian domain since Paleogene times.
Elsevier online abstract
A deep seismic reflection survey across the Betic Chain (southern Spain): first results
Pages 77-89
ESCI-Béticas Working Group, V. García-Dueñas, E. Banda, M. Torné and D. Córdoba
Two land seismic reflection profiles across the Betic Chain have imaged the deep structure of the crust belonging to two different crustal domains. To the north, one profile samples the crust of the Variscan Iberian Massif that underlies the sedimentary cover of both the Guadalquivir foreland basin and the South Iberian crustal domain. The upper crust is non-reflective along the profile, but the lower crust is reflective in the northern half of the profile between 7 and 12 s two-way travel time. In this segment of the profile the Mono is also well imaged. The quality of the data diminishes significantly as the profile enters the Neogene Guadix-Baza basin. A second profile crossing the Alpine metamorphic complexes of the Betics reveals a reflective lower crust and a conspicuous reflection in the upper crust that is continuous for about 10 km. At 11 s, a distinct reflection has been interpreted as the reflection Moho. Deeper reflections are also seen in the central part of the profile in a segment about 10 km long. Comparison with available refraction and wide-angle reflection data shows some differences in both the crustal configuration and in the depth to the Moho.
Elsevier online abstract
Crustal structure of the external variscides in northwest Spain from deep seismic reflection profiling
Pages 91-112
ESCI-N Research Group, A. Pérez-Estaún, J. A. Pulgar, E. Banda and J. Alvarez-Marrón
Recent deep-crustal seismic reflection profiling provides a detailed image of the external parts of the Variscan Belt in northwest Spain. These seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt (Cantabrian Zone) and the transition to the hinterland areas (Narcea Antiform area).
The image in the Cantabrian Zone includes strong inclined reflections in the upper 5 s that correspond to mappable Paleozoic rocks at the surface which have been deformed by thrusting and folding during the Carboniferous. Some of the major thrust faults imaged show a ramp-flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the Cantabrian Zone detachment. The Precambrian basement is almost transparent underneath the Cantabrian Zone detachment. There are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection Moho.
At the transition to the hinterland, the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s. These correspond to the Narcea Antiform, which comprises at the surface a stack of thrust sheets containing Precambrian rocks. The Narcea Antiform is placed above a crustal-scale ramp imaged by strong W-dipping reflections between 6 and 9 s. These reflections merge upwards into the Cantabrian Zone detachment at 6 s, and downwards into a reflective lower-crustal zone located between 9 and 12 s. These midcrustal reflectors are interpreted as a major Variscan shear zone that marks the transition between the thin-skinned tectonics of the external areas and the thick-skinned tectonics of the hinterland areas.
The lower crust is highly reflective below the Narcea Antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt. The different seismic responses at middle- and lower-crustal levels in the foreland and hinterland areas suggest that Variscan tectonics strongly affected deep-crustal levels in the hinterland areas, whereas they remained undeformed beneath the foreland.
Elsevier online abstract
Styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere
Pages 119-132
Christopher Beaumont, Philippe Fullsack and Juliet Hamilton
Crustal-scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere. The plane-strain finite-element model results provide indications of the styles of deformation to be expected in small compressional orogens. In particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other. The crust is modelled using Coulomb plastic (frictional) and thermally-activated power-law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included. Results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary, surface denudation, partial and total subduction of the crust, and compositional layering are examined.
The results show the development of inclined step-up shear zones, which are a consequence of conjugate thrusting in regions of Coulomb-controlled rheology. These zones link to sub-horizontal shear zones which occur where composition and temperature render the crust viscously weak. The model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation. Finally, we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens.
Elsevier online abstract
Simple-shear deformation of the Skagerrak lithosphere during the formation of the Oslo Rift
Pages 133-141
J. E. Lie and E. S. Husebye
Simple-shear deformation of the entire lithosphere has been postulated by Wernicke (1985) and others, but up to now unequivocal seismic evidence in support of this hypotheses has been lacking. Here we describe well-defined seismic reflectors below the Skagerrak Sea, one of which is interpreted as a low-angle fault underlying the Skagerrak Graben segment of the Permian Oslo Rift. This reflective lineament can be traced from the mid-crust through the lower crust, offsetting Moho and continuing downwards to ca. 50 km depth (16 s). A separate mantle reflection beneath the graben may be associated with an earlier period of thrusting. The 1730 km of deep seismic reflection data in Skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the Proterozoic Grenvillian-Sveconorwegian orogeny. During formation of the Oslo Rift, reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non-magmatic Skagerrak Graben.
Elsevier online abstract
Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada
Pages 143-160
J. F. Lewry, Z. Hajnal, A. Green, S. B. Lucas, D. White, M. R. Stauffer, K. E. Ashton, W. Weber and R. Clowes
An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.
Elsevier online abstract
Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada
Pages 161-178
S. B. Lucas, D. White, Z. Hajnal, J. Lewry, A. Green, R. Clowes, H. Zwanzig, K. Ashton, D. Schledewitz, M. Stauffer et al.
The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of décollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10-15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.
Elsevier online abstract
Pre-critical wide-angle reflections from the Baltic shield: evidence for a 1.8 Ga subduction complex
Pages 179-194
Gillian Lindsey and Dave Snyder
During the collaborative BABEL experiment in 1989, over 30,000 airgun shots were fired along reflection profiles in the Gulf of Bothnia and Baltic Sea and recorded at 62 onshore seismic stations, giving densely sampled wide-angle data of many forms. At one such station, in-line and three-dimensional wide-angle arrivals from 14-40 km offset were recorded on a linear geophone array placed off-end to marine reflection line 2. The multifold data from this station have been CDP-sorted and stacked, and also displayed as a single fold, high-resolution time-distance record section with an average trace spacing of 4 m. Prominent wide-angle reflections observed on both wide-angle stacks and receiver gathers can be correlated with steep northward dipping reflectors observed in the lower crust on coincident near-normal incidence reflection (marine) profiles. High-resolution ray tracing through a three-dimensional model shows that the observed reflections likely occur in a reflective zone, 8 km thick, composed of individual reflectors 100-300 m thick. The necessary impedance contrasts can be produced by alternating high and low velocities or densities. These reflectors parallel the trend of the Skellefteå mineral district and its associated conductivity anomaly, and are here interpreted as metamorphosed remnants of subducted oceanic-type crust and its overlying sediments.
Elsevier online abstract
Shear-wave splitting in the lower crust beneath the Archean crust of southwest Greenland
Pages 195-210
William P. Clement, Ramon Carbonell and Scott B. Smithson
Slant stacks of seismic data from rifted ancient Archean crust along the southwest coast of Greenland indicate that the lower crust is strongly seismically anisotropic. The slant stack data show that a lower-crustal, wide-angle S-wave reflection has a different intercept time and ray parameter on the radial and transverse components from a receiver gather recorded on three-component seismometers. From the S-wave analysis, the continental crust is clearly seismically anisotropic above a high-velocity wedge in the lower crust. Possibly, magmatic underplating during Late Cretaceous Labrador Sea rifting heated the pre-existing lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy.
Elsevier online abstract
Seismic profiling of Archean crust: Crustal structure in the Morton block, Minnesota River Valley subprovince
Pages 211-224
Nicholas K. Boyd and Scott B. Smithson
A 58 km seismic reflection profile recorded by the University of Wyoming over the early Archean gneisses (3.6-3.8 Ga) of the Minnesota River Valley subprovince (Superior Province) shows more complicated structure and rock variation with depth than the limited surface geology suggests. Wide-angle data show a high average velocity of 6.8 ± 0.1 km/s and a crustal thickness of 49 ± 1 km from a zero-offset time of 14.2 ± 0.2 s. Upper-crustal (0-10 km) reflections are consistent with the gentle dips from surface folding, while middle-crustal (10-24 km) reflections are interpreted as a 40 km long SE dipping thrust or fold structure, 6 km thick. Lower-crustal (24-48 km) reflections are short and low amplitude, consistent with small reflection coefficients and short reflectors. Modeling suggests that this reflectivity is consistent with small (< 2 km horizontal length) lenticular, boudin-like bodies or heterogeneities within the lower crust, and/or the lower crust may be complexly deformed. At 14 s (49 km) the Moho occurs as a locally layered, 10 km long structure at the depth determined for Moho from the wide-angle data. This vertical-incidence Moho reflection demonstrates the occurrence of lateral changes in Moho structure. The seismic profile suggests a model that incorporates a thrust sheet in the middle crust, emplaced by horizontal tectonics in the Archean, and restite bodies in the deep crust, formed from the production of the presently exposed gneisses.
Elsevier online abstract
High-resolution images of the lower crust: deep seismic reflections from 15 to 180 Hz
Pages 225-237
Mike Warner, Bastiaan Spaargaren, Rob Jones, Doyle Watts and Jonathan Doody
We report the results of an unconventional, broad-band deep-seismic reflection survey undertaken on land in southwest England. Previous marine seismic lines in this area have shown strong layered lower-crustal reflectivity. The broad-band survey shows near-normal-incidence P-wave reflections from the lower continental crust and from the Moho with frequencies that range from 15 to 180 Hz. High-frequency lower-crustal S-wave reflections were also observed. These high frequencies were obtained by placing both sources and receivers in boreholes up to 2 km deep, drilled within a large homogeneous granite pluton. Recording down-hole avoids loss of high frequency energy during transmission through heterogeneous and highly attenuative near-surface layers. The high frequencies were generated using explosive sources, and were recorded on sub-surface accelerometers and hydrophones, and on a conventional array of surface geophones. Seismic attenuation within the granite is low; the quality factor, Q, lies above 2000.
These new, high-frequency observations show that the lower crust does not preferentially reflect particular frequencies. The data are most readily explained as resulting from the combination of a frequency-independent Q in the upper crust with a white reflectivity response in the lower crust. The data are not consistent with crustal models which are highly tuned to reflect preferentially the frequencies employed in conventional deep seismic reflection surveys. The data require relatively sharp boundaries within the lower crust; significant variation in physical properties must occur over vertical distances of not more than about 10 m. Sharp boundaries are most easily explained if the reflections result from lithological contrasts.
Elsevier online abstract
The reflectivity of magmatic underplating using the layered mafic intrusion analog
Pages 239-255
S. J. Deemer and C. A. Hurich
The reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates. One-dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0.18. A seismic profile across the Bjerkreim intrusion in southern Norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0.05 produces detectable reflections. Two-dimensional models demonstrate that the subhorizontal cumulate layering in typical mid- to upper-crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image. Reflection coefficients predicted for granulite-facies mineral assemblages are similar to those predicted for the igneous assemblages. Reflection coefficients predicted for cumulates intruded or re-equilibrated at eclogite-facies conditions may be either decreased or enhanced with respect to the igneous assemblages, depending on the abundance of plagioclase in the cumulate layers and the pressure. Results suggest that magmatic underplating should not be invoked as an explanation for non-reflective crust except in cases where eclogitization has occurred.
Elsevier online abstract
Lower-crustal deformation during terrane dispersion along strike-slip faults
Pages 257-266
Bruce C. Beaudoin
After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60-90 mWm-2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.
Elsevier online abstract
Some attributes of wavefields scattered from Ivrea-type lower crust
Pages 267-279
Klaus Holliger, Alan Levander, Ramon Carbonell and Richard Hobbs
The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its "layered" self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data:
- Ivrea-type lower crust can explain the high observed reflection coefficients and bright reflectivity in the lower crust;
- observed lower-crustal Q-factors can be largely explained by scattering losses and intrinsic Q-factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles;
- in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and
- for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.
Elsevier online abstract
The crust as a heterogeneous "optical" medium, or "crocodiles in the mist"
Pages 281-297
A. Levander, R. W. Hobbs, S. K. Smith, R. W. England, D. B. Snyder and K. Holliger
Based on petrophysical data, geologic maps, and a well log, we present statistical descriptions of likely upper-, middle-, and lower-crustal rocks to characterize the fine-scale heterogeneity observed in crustal exposures and inferred from deep-crustal seismic data. The statistical models, developed for granitic and metamorphic upper crust, and for an extended metamorphic lower crust, are used to construct whole-crustal models of seismic velocity heterogenity. We present finite-difference synthetic CMP data from several models which compare favorably with field data. The statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime. The "optical", or scattering properties of importance for classification are the velocity fluctuation intensity, the horizontal and vertical correlation lengths of the medium, the correlation function of the medium, and the velocity population function. For the crustal properties we measured, the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime, with implications for crustal seismic data processing and imaging. Notably, deep-crustal signals are likely to have experienced multiple scattering, making common seismic imaging techniques of questionable value. Moreover, the details of the unmigrated CMP stacked section bears little resemblance to the underlying medium.
Elsevier online abstract
Moho reflectivity and seismic signal penetration
Pages 299-307
Arthur E. Barnes
Analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by LITHOPROBE and the Consortium for Continental Reflection Profiling (COCORP). Changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth. Results from Montana show pronounced increases in average frequency and slight drops in amplitude associated with a die-out of reflections at the Moho. Results from Quebec show amplitude and frequency change occurring through the Moho on lines with and without Moho reflections. These contrasting observations confirm both adequate signal penetration and that the differences in Moho reflectivity are representative of geologic differences.
Elsevier online abstract
The nature of crustal boundaries: combined interpretation of wide-angle and normal-incidence seismic data
Pages 309-318
Roger E. Long, Patricia A. Matthews and Daniel P. Graham
After a few seconds two-way traveltime, normal-incidence seismic reflection sections are composed mainly of assemblages of short reflections. Very rarely are seen continuous reflections that might correspond to the Moho or a mid-crustal discontinuity. The inferred continuity of these boundaries has traditionally come from refraction seismology. There is now a body of high quality, coincident wide-angle and normal-incidence seismic data that have been recorded with 50-100 m shot spacing and with high frequency sources (e.g. Mobil, Babel). The complexity and characteristics of the wide-angle arrivals seen on these data suggest that they do not originate from continuous boundaries. It is suggested that these arrivals are reflections from the same assemblage of short length reflectors that are responsible for normal-incidence reflections. Seismic velocities below the middle crust may
- change corresponding to normal-incidence reflectivity, or
- generally increase with depth with localized sills or lens structures of different velocity accounting for the observed reflections. Wide-angle arrivals that have traditionally been identified as reflections from crustal boundaries (e.g. the mid-crust and Moho) and which were considered indicative of a sharp velocity discontinuity from continuous boundaries, may instead result from a concentration of lamellae.
Elsevier online abstract
The polarity of deep seismic reflections from the lithospheric mantle: Evidence for a relict subduction zone
Pages 319-328
J. V. Morgan, M. Hadwin, M. R. Warner, P. J. Barton and R. P. Ll. Morgan
BIRPS offshore deep seismic reflection profiles to the north of Scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle, the dipping Flannan and sub-horizontal W reflectors. The polarity of reflections from the Flannan has been determined from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calculated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the source wavelet.
Reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the Flannan reflection. In eight out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repeated for Moho reflections; in this case nine out of ten stacks were picked as positive. The modelling also demonstrated that the Flannan reflection is apparently from a simple interface; complex interlayering is not required to explain the waveforms, and they are not consistent with reflections from a single thin layer.
Seismic data acquired at wide-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. Only a high-velocity eclogitic layer, contained within normal mantle, with a sharp upper boundary and a diffuse base can explain all our observations. We suggest that the Flannan reflector represents the top of a relict oceanic and eclogitic component of a pre-Caledonian subduction zone within the lithospheric mantle.
Elsevier online abstract
Seismic anisotropy at the continental deep drilling site (Germany)
Pages 329-341
Wolfgang Rabbel
The existence of seismic anisotropy at the Continental Deep Drilling Site (KTB, Oberpfalz, Germany) has been established by the DEKORP ISO 89 borehole experiments. The anisotropy, quantified in-situ for a sequence of gneissic rocks, comprises two parts: a background component reflecting mineral composition and its state of ordering (foliation), and a superimposed component originating from fractures. The anisotropy values obtained in the direct vicinity of the KTB pilot hole, however, cannot be extrapolated into its surroundings without modification. Whereas metamorphic rocks in the immediate vicinity of the KTB exhibit a relatively strong anisotropy (> 10%) with an approximately NW-SE-striking axis of high velocity, the anisotropy of an adjacent granite area is considerably lower and of different strike direction (roughly W-E). The directions of high seismic velocities can be correlated with strike directions of geologically mapped fracture zones and rock units as well as to estimates of the direction of maximum horizontal tectonic stress.
Elsevier online abstract
Crustal "bright spots" and anisotropy from multi-component P- and S-wave measurements in southern Germany
Pages 343-354
Ewald Lüschen
Controlled shear-wave experiments have focused on deep crustal targets that were earlier recognized by DEKORP P-wave surveys in southern Germany (Black Forest, Upper Palatinate). A prominent P-wave reflection ("bright spot") at 8 km depth at the KTB deep drilling site is not seen on a comparable S-wave reflection section, in contrast to deeper reflections. This behaviour can be explained either by purely compositional effects (layer with anomalously high quartz content) or by fluid accumulation ("reservoir") in a fracture zone. A 20 km deep, 10 km long prominent P-wave reflection in the southwestern Black Forest is only partially accompanied by corresponding S-wave reflections. The influence of the physical state of the rock e.g. stress-related cracking, in addition to possible compositional variations is suggested to explain the contrasting P- and S-wave reflection strength. Seismic anisotropy of 10% magnitude, revealed by S-wave splitting in vertical seismic profiling data at the KTB to more than 4 km depth, is attributed to sub-vertically dipping and NW-striking foliation.
Elsevier online abstract
Seismic mapping of the marmousi data set with the common reflecting element method (CRE method)
Pages 355-363
H. Steentoft and W. Rabbel
The inversion of seismic reflection data collected across a laterally heterogeneous Earth is an important objective in reflection seismology. The purpose of this paper is to illustrate the potential of the common reflecting element (CRE) imaging method, a new topological technique that involves the stacking of multi-offset reflection seismic data. The CRE method has four advantages:
- it is based on ray theory and, thus, is an elementary geometrical method,
- it only requires knowledge of seismic velocities at a "reference level" (near-surface) as a-priori information,
- it delivers wavefront parameters (radius of curvature and angle of incidence) that can be used to formulate new inversion methods, and
- normal-moveout stretching is avoided.
The local dip of common subsurface reflecting elements (CRE's) is used as a criterion for choosing traces to be stacked in order to approximate "true" zero-offset sections. Since the method includes a prestack coherency analysis it may be regarded as a data-adaptive stacking technique. In the present study, the CRE method is applied to a complex synthetic data set (Marmousi model) to investigate its applicability under "ideal field conditions". Our results show that the CRE algorithm, which works automatically, is stable and yields reasonable images. Information on the kinematic properties of the stacked reflected wavefronts is also produced.
Elsevier online abstract
Three-dimensional imaging of the crust using a sparse land acquisition grid: The Eecors 2.5-D experiment
Pages 365-377
J. -M. Marthelot, R. Siliqi, A. Bitri, A. Paul, A. Hirn, M. Daignières, B. Damotte, M. Specht, E. De Bazelaire, A. Lortscher and D. Rappin
In order to test the feasibility of 3-D imaging of the deep crust at a reasonable cost, Ecors has designed and conducted an experiment within the western North Pyrenean thrust zone, using a sparse 3-D land acquisition grid. Each shot was recorded on 6 parallel geophone lines sampling an 8 × 12 km2 area, and 14 shot columns covered a total surface of 8 × 27 km2 in a single swath. Despite the complex structure of the zone, standard processing using a bin size of 50 × 200 m2 provided low-fold 3-D stacks on which coherent reflection surfaces could be picked interactively down to 5 s. These surfaces, which originate from the North Pyrenean frontal thrust, associated thrust sheets, and an uplifted basement wedge, have a clearly 3-D geometry. The internal structure of the thrust sheets appears to be particularly complex. A two-pass frequency-wavenumber 3-D migration, using a dip limitation of 45 and 15° for in- and crosslines, respectively, effectively focused the main reflections and attenuated 2-D migration smiles. Whereas processing provides a general outline of the complex 3-D structure of the thrust system, a detailed interpretation will require an interactive integration of the information present on both original and processed data.
Elsevier online abstract
Joint P- and S-wave velocity determination from reflected PP, SS and converted PS/SP phases from large aperture seismic reflection measurements
Pages 379-389
Ramon Carbonell, William P. Clement and Scott B. Smithson
As less aliased (better sampled) seismic studies are acquired more SS reflections and especially PS/ SP converted phases are recognized. We use a synthetic (t, p) data set to demonstrate the viability of using PP, SS reflections and PS/ SP conversions in a joint inversion scheme to obtain average, one-dimensional P- and S-wave velocity models. We achieve a coupling between the P- and S-wave velocity-depth functions by introducing all these wave types in a single joint inversion. The new information introduced by the converted phases provides additional constraints, and better resolved velocity-depth models can be estimated. In our synthetic data case the resolution kernels become sharper, decreasing the width of the main lobe by 10-50% and increasing their height by 20-40%. This increase in resolution can result in a significant decrease in the uncertainty estimates of Poisson's ratios. In our synthetic test the uncertainties decrease approximately 50%. We use this inversion scheme to invert (t, p) sections from horizontal components recorded in a seismic experiment along the southwest coast of Greenland. The joint inversion results resolve different levels of anisotropy within the crust. These anisotropic fabrics can be responsible for the time difference (delay) between the radial and transverse (t, p) sections, that causes a decrease in the amplitude of the S-wave field when the two (t, p) sections (radial and transverse) are stacked.
Elsevier online abstract
The cover-basement contact beneath the Rawil axial depression (western Alps): True amplitude seismic processing, petrophysical properties, and modeling
Pages 391-409
L. Levato, S. Sellami, J. -L. Epard, B. Pruniaux, R. Olivier, J. -J. Wagner and H. Masson
Since 1986, several near-vertical seismic reflection profiles have been recorded in Switzerland in order to map the deep geologic structure of the Alps. One objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystalline massifs, important elements for understanding the geodynamics of the Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil depression of the western Swiss Alps, provides important information on this subject. It extends northward from the "Penninic front" across the Helvetic nappes to the Prealps. The crystalline massifs do not outcrop along this profile. Thus, the interpretation of "near-basement" reflections has to be constrained by down-dip projections of surface geology, "true amplitude" processing, rock physical property studies and modelling.
3-D seismic modelling has been used to evaluate the seismic response of two alternative down-dip projection models. To constrain the interpretation in the southern part of the profile, "true amplitude" processing has provided information on the strength of the reflections. Density and velocity measurements on core samples collected up-dip from the region of the seismic line have been used to evaluate reflection coefficients of typical lithologic boundaries in the region. The cover-basement contact itself is not a source of strong reflections, but strong reflections arise from within the overlaying metasedimentary cover sequence, allowing the geometry of the top of the basement to be determined on the basis of "near-basement" reflections. The front of the external crystalline massifs is shown to extend beneath the Prealps, about 6 km north of the expected position. A 2-D model whose seismic response shows reflection patterns very similar to the observed is proposed.
Elsevier online abstract
Eclogite-facies shear zones-deep crustal reflectors?
Pages 411-424
David M. Fountain, Theresa M. Boundy, Håkon Austrheim and Patrice Rey
Strongly foliated eclogite-facies rocks in 30-150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway. The predominant eclogite-facies mineral assemblages in the shear zones are omphacite + garnet + zoisite + kyanite in gabbroic anorthosite and omphacite + garnet in gabbro. Eclogite-facies rocks in shear zones are generally fine-grained; alternating omphacite/garnet- and kyanite/clinozoisite-rich layers define gneissic layering. A strong shape preferred orientation of omphacite, kyanite, and white mica (phengitic muscovite and/or paragonite) define the foliation. The anorthositic eclogites show omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane. P-wave velocities (Vp) determined at confining pressures to 600 MPa for samples from eclogite-facies shear zones range from 8.3 to 8.5 km s-1 and anisotropy ranges from 1 to 7%. The few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation. The fast propagation direction lies within the c-axis girdles (parallel to foliation) and the slow propagation direction is parallel to the b-axis concentration (normal to foliation) in samples for which omphacite crystallographic preferred orientation was determined. Vp for the granulite-facies protoliths average about 7.5 km s-1. High calculated reflection coefficients for these shear zones, 0.04-0.14, indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high-pressure conditions but escaped thermal reactivation.
Elsevier online abstract
Seismic reflection profiling in deep water: avoiding spurious reflectivity at lower-crustal and upper-mantle traveltimes
Pages 425-435
J. H. McBride, T. J. Henstock, R. S. White and R. W. Hobbs
The acquisition of seismic reflection data in deep water can be complicated by persistent multiple reflections from the seabed and sediments that continue to contaminate subsequent records beyond their initial source point. These multiples may create both severe and subtle artifacts that could easily be misinterpreted as sub-horizontal primary reflections from the lower crust or uppermost mantle. Contamination is particularly severe for shot repetition rates less than about 40 s (i.e. for shot-points spaced less than 100 m) and in areas with a sedimentary cover that is thick and contains reflection coefficients that are large relative to the seabed. Under these circumstances, shots fired at constant time increments maximize the likelihood of contamination in the stacked section. In this study, we use seismic reflection data from three sites in the North Atlantic to show how, and under what conditions, such multiples are produced and how they may mislead the interpreter. We also outline a field-recording procedure to minimize their effect on the common mid-point stacked profile.
Elsevier online abstract
Residual statics estimation in crustal reflection profiling using the simulated annealing search technique-an ensemble approach
Pages 437-450
Kris Vasudevan and Frederick A. Cook
Stochastic methods such as simulated annealing search techniques have been shown to provide alternative means to estimate residual statics in structurally complex areas where the signal-to-noise ratio is poor and where large static shifts are necessary. Because of the stochastic nature of the simulated annealing method, several experiments are necessary to arrive at the final statics solution. An ensemble approach based on executing several copies of the data in parallel is used to solve the problem. The simulated annealing search technique considered here uses an objective function that enhances the coherency between common mid-point gathers and that includes the structure term. It employs a system-dependent cooling schedule. With this schedule, temperature updates are computed based on the ensemble average properties of the system. We demonstrate using data from the Rocky Mountain Trench that the new approach to statics estimation has aided crustal seismic imaging and interpretation.
Elsevier online abstract
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