Deep Seismic Reflection Profiles Research Articles

Deep seismic reflection profiling across the Ou Backbone range, northern Honshu Island, Japan

Knowledge of the crustal structure, especially the geometry of seismogenic faults, is key to understanding active tectonic processes and assessing the size and frequency of future earthquakes. To reveal the relationship between crustal structure and earthquake activity in northern Honshu Island, common midpoint (CMP) deep reflection profiling and earthquake observations by densely deployed seismic stations were carried out across the active reverse faults that bound the Ou Backbone range. The 40-km-long CMP profiles portray a relatively simple fault geometry within the seismogenic layer. The reverse faults merge at a midcrustal detachment just below the base of the seismogenic layer, producing a pop-up structure that forms the Ou Backbone range. The top of the reflective middle to lower crust (4.5 s in travel time (TWT)) nearly coincides with the bottom of seismogenic layer. The P-wave velocity structure and surface geology suggest that the bounding faults are Miocene normal faults that have been reactivated as reverse faults.

Depth migration of deep seismic reflection profiles: crustal thickness variations in Alberta

Variations in crustal thickness provide important clues as to the formation of the crust, present-day isostatic equilibrium, and crustal stress. The map of the topography of the Mohorovicic discontinuity in Alberta is revised using 1900 km of reanalysed seismic reflection profiles acquired as part of the Lithoprobe Alberta Basement Transect. Time sections were depth migrated using a parallelized algorithm that accounts for steeply dipping structures. The migration process employed a geologically consistent velocity model of the metamorphic crust derived from earlier refraction experiments and constrained by compilations of high-pressure rock velocity measurements. We found that knowledge of the velocities in the sedimentary column strongly influenced the quality of the migration calculations. The Mohorovičić discontinuity is generally distinguished in these profiles, on the basis of sharp changes in reflectivity, at depths of 35–48 km. Sharp reflections from this boundary are rare. A number of geologic features are of note in these lines. A localized (~50 km extent) crustal thinning is observed in the Peace River region; this thinning is consistent with an adjacent oxygen isotopic anomaly indicative of crustal extension. In central Alberta, the Mohorovičić discontinuity topography is suggestive of a sharp jump of 10 km indicative of mantle faulting associated with the Snowbird tectonic zone. In southern Alberta, the crust thickens substantially across the Vulcan structure, with the greatest thickness correlating with the Vulcan structure itself indicating a collisional origin as noted by other authors.

Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada

Research Article| August 01, 2002 Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada D.J. White; D.J. White 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar S.B. Lucas; S.B. Lucas 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar W. Bleeker; W. Bleeker 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar Z. Hajnal; Z. Hajnal 2University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0, Canada Search for other works by this author on: GSW Google Scholar J.F. Lewry; J.F. Lewry 3University of Regina, Regina, Saskatchewan S4P 4V4, Canada Search for other works by this author on: GSW Google Scholar H.V. Zwanzig H.V. Zwanzig 4Manitoba Industry, Trade and Mines, 1395 Ellice Avenue, Winnipeg, Manitoba R3G 3P2, Canada Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (8): 735–738. https://doi.org/10.1130/0091-7613(2002)030<0735:SZGAAI>2.0.CO;2 Article history received: 27 Dec 2001 rev-recd: 18 Apr 2002 accepted: 19 Apr 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D.J. White, S.B. Lucas, W. Bleeker, Z. Hajnal, J.F. Lewry, H.V. Zwanzig; Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada. Geology 2002;; 30 (8): 735–738. doi: https://doi.org/10.1130/0091-7613(2002)030<0735:SZGAAI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Lithoprobe acquired a 190-km-long deep seismic reflection profile across the Superior boundary zone at the eastern margin of the Paleoproterozoic Trans-Hudson orogen. The profile is located ∼250 km south of a major promontory along the ancient margin of the Superior craton and complements earlier profiles closer to the promontory. The new data image the remnants of a craton-verging thrust belt, younger than 1864 Ma, within the Superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal décollement. Across the surface suture zone, a dip reversal to east-dipping reflections occurs within the adjacent Reindeer zone, where a crustal-scale imbricate stack is imaged. Reindeer zone lower crust and upper mantle extend eastward beneath the Superior craton margin for 40–100 km. Preservation of a foreland thrust belt within the Superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the Thompson promontory. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic heterogeneity and anisotropy in the Western Superior Province, Canada: insights into the evolution of an Archaean craton

Abstract The Superior Province, which forms the nucleus of North America, is the largest preserved Archaean crustal block in the world and may have originated as the result of a widespread crustal accretion event ( c. 2.7 Ga) manifested in Archaean cratons worldwide. An understanding of the accretionary evolution of this craton is the objective of the continuing Lithoprobe transect in the Western Superior Province, Canada. The geophysical components of the transect include seismic reflection and refraction, magnetotellurics and teleseismic experiments. The Teleseismic Western Superior Transect (TWiST) was designed to explore the structural and physical properties of the subcrustal lithosphere and their implications for proposed accretionary models. A north-south-trending array of 17 broadband three-component seismometers was deployed between May and November 1997. Surface-wave analyses, SKS-splitting studies and travel-time tomography show variations in the velocity structure and anisotropy between the southern end of the transect, a region affected by Keweenawan rifting, and the northern part, which lies in the Proterozoic Trans-Hudson shear zone. Surface waves reveal evidence for a thin high-velocity layer, 5–20 km thick, beneath a 37–43 km thick crust and above c. 250 km of high-velocity continental root. This thin layer is also visible in wide-angle refraction data from the southern end of the line and may be evidence of underplating during terrane accretion. Discrepancies in the Love and Rayleigh waves and surface-wave particle motions show evidence for an anisotropic mantle. SKS analysis shows large amounts (up to 2 s) of shear-wave splitting with a roughly eastwest trend in the fast-shear-wave polarization direction for most stations. This conforms with crustal deformation trends. Stations in the younger Trans-Hudson orogen show much less splitting. Detailed analysis at a permanent station in the Western Superior shows evidence for two layers of anisotropy. A thinner upper layer is aligned with the surface geology, indicating crust-mantle coupling during craton formation, whereas a thicker lower layer is aligned with the direction of absolute plate motion. Tomographic results show a featureless mantle beneath the Sachigo proto-craton and more heterogeneity towards the south end of the line. A steeply dipping slab-like feature in the lithosphere correlates with wide-angle refraction and deep-reflection seismic profiles. A similar high-velocity feature continues well into the transition zone, but its origin remains to be understood. Towards the southern end of the line there is a deep-seated low-velocity anomaly, which may be associated with Keweenawan plume activity. As a whole, the seismic results show many features that support ideas of subduction-related accretion of a thick stable Archaean tectosphere. There are, however, interesting details that are to date unique to the Western Superior Province. These include thicker than normal Archaean crust, a slab-like velocity anomaly in the mantle transition zone, and large SKS splitting in the Archaean Superior Province but little splitting in the surrounding Trans-Hudson Proterozoic shear zone.

Fine structure of the continental reflection Moho

Comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the Earth's history lead to a threefold categorization of reflection patterns near the crust-mantle transition: type I patterns have no distinct reflections (including no reflection Moho at all); type II patterns have one or more subhorizontal reflections, and type III patterns have reflections that project from the crust into the mantle. Subdivisions within each of these are based upon the geometric relationships between the reflection Moho and reflections immediately above and, in some cases, below. Hence, type I examples may have essentially no reflectivity near the crust-mantle transition, or they may be characterized by crustal reflectivity that fades downward. Type II examples may have a single prominent reflection, or they may have several subhorizontal reflections. Type II patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity. Type III examples may have a single prominent reflection that projects below the Moho (as delineated by seismic refraction data), or they may have a series of subparallel dipping reflections. More than one type of reflection Moho may be visible along a single profile, sometimes changing within distances of only a few tens of kilometers. This diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection Moho is a consequence of variations in the thermal and structural evolution of the crust-mantle transition at many scales from a few tens to many hundreds of kilometers.

Moho depth structure of the eastern part of the Pyrenean belt derived from gravity data

This paper proposes a new and updated interpretation of the gravity map of the eastern part of the Pyrenean belt based on the well-established correlation existing between gravity anomalies and crustal structure. A new set of gravity data, collected over the past few years by the University of Barcelona both for academic purposes and hydrocarbon exploration, have been merged with the data previously available. First a quantitative interpretation of the crust’s thickness has been estimated by establishing a local empirical relationship between Bouguer anomaly and the Moho depth calculated from deep seismic refraction and reflection profiles. Then, a spectral analysis of the radial wavenumber of the Bouguer anomaly values using the FFT and a 3D inversion has been performed. Finally, a 212 D density model from a SE–NW gravity profile linking the southern edge of the coastline with the northern part of the ECORS–Pyrenean profile was also established.

Crustal structure in northern margin of Tianshan mountains and seismotectonics of the 1906 manas earthquake

The thin-skinned structure in the crust of the northern Tianshan piedmont is explored by an 86-km-long, NS-trending deep seismic reflection profile through the Urumqi depression of the north margin of the Tianshan mountains. On the CDP stacking section, the first- and second-row parallel to anticlines in the north margin of the Tianshan mountains are shown on the segment to the south of the Shihezi. The detachments, corresponding to the reflection events at TWT 2.5–3.0 s and 5.5–6.0 s respectively, join the crustal deep structure to the reverse fault-fold zone. The Manas thrust extends downwards in listric shape, merges into the detachment at about TWT 2.5 s and joins to the Qingshuihe thrust. The reflection events on 5.5–6.0 s are corresponding to the main detachment, which joins the lower Manas anticline, and finally converge to the Junggar Southern Marginal Fault. A 12–14 km-thick sedimentary basin exists on the region in the north to the Shihezi. The depth of the Moho discontinuity beneath the Junggar basin is about 45 km, and increases southwards to 50 km. The crustal structure inferred from the deep seismic sounding profile and the Bouguer anomaly in the same region is consistent with the image from the deep seismic reflection profile. The seismogenic model of the 1906 Manas earthquake is related to the fault system, which consists of the Qingshuihe thrust, the detachments and the shallow Manas ramp.

Collision Tectonics between the Tarim Block (Basin) and the Northwestern Tibet Plateau: New Observations from a Multidisciplinary Geoscientific Investigation in the Western Kunlun Mountains

AbstractNew results from deep seismic reflection profiling, wide‐angle reflection‐refraction profiling and broadband seismic experiments reveal that a series of south‐dipping reflectors occur on the southern margin of the Tarim block (basin). However, it is these south‐dipping structures that are intercepted by another series of north‐dipping reflectors at depths from 30 to about 150 km beneath the foreland of the W Kunlun Mountains. No evidence from the above geophysical data as well as geochemical and surface geological data indicate the southward subduction of the Tarim block beneath the W Kunlun Mountains (NW Tibet plateau), forming the so‐called “two‐sided subduction” model for the Tibet plateau as proposed by previous studies. So the authors infer that the tectonic interaction between the Tarim block and the W Kunlun block was chiefly affected by a “horizontal compression in opposite directions”, which brought about “face‐to‐face contact” between these two lithospheric blocks and led to the thickening, shortening and densifying of the lithosphere. Hence a “delamination” was formed due to the gravitational instability created by the thickening and densifying; then alkaline basic volcanic rocks (mainly shoshonite series) was erupted along the northern margin of the Tibet plateau owing to the delamination. This inference for the formation of the alkaline basic volcanics has been confirmed by recent geochemical and petrological studies in Tibet, indicating that different contacts control different magmatic activities: the alkali basalts are always developed in the “horizontal shortening boundary (contact)” on the northern margin of the Tibet plateau, while the muscovite granite and two‐mica granite (leucogranite) in the “subductional contact” on the southern margin of the Tibet plateau.

Estructura cortical de la cordillera y margen continental cantábricos: perfiles ESCI-N

The aim of this study is the N-S crustal structure of the Cantabrian Mountains and the North Iberian margin along a transect close to the 5°W meridian. The geological transect is based on all geological and geophysical data available, especially the onshore and offshore deep seismic reflection profiles ESCIN-2 and ESCIN-4. The structure of the orogen results from the superposition of the Alpine deformation, related to the partial closure of the Bay of Biscay during the Tertiary, on the Variscan crust. The N-S structure is that of an orogenic prism, with opposite vergences at both ends where two foreland basins developed: the Duero Basin in the S and the North Iberian platform basin in the N. The lower crust from the North Iberian margin slided southwards along a detachment and indented between the upper and middle Iberian crust. This forced the delamination and northwards underthusting of the middle and lower Iberian crust. As a result, the crust thickened under the Cantabrian Mountains and a 55 km thick crustal root developed. Simultaneously, the Cantabrian Mountains were uplifted by several north vergent crustal thrusts rooted at 14-15 km deep. Shortening of the margin during the Tertiary has been estimated in 96 km.

Deep seismic reflection profile across the juncture zone between the Tarim Basin and the West Kunlun Mountains

Fine structures of the crust and upper mantle of the basin-and-range juncture on the northwestern margin of the Qinghai-Tibetan Plateau are first delineated by the deep seismic reflection profile across the juncture zone between the Tarim Basin and the West Kunlun Mountains. Evidence is found for the northward subduction of the northwest marginal lithosphere of the Qinghai-Tibetan Plateau and its collision with Tarim lithosphere beneath the West Kunlun Mountains. The lithosphere image of the face-to-face subduction and collision determines the coupling relationship between the Tarim Basin and the West Kunlun Mountains at the lithosphere scale and reflects the process of continent-continent collision.

Crustal geometry and tectonic evolution of the Archean crystalline basement beneath the southern Alberta Plains, from new seismic reflection and potential-field studies

Deep seismic reflection profiling of the southern Alberta crystalline basement, namely the Archean Medicine Hat block (MHB), has revealed that the crust is characterized by west- and southwest-dipping reflection fabrics, a layered lower crust, and geometric cutoffs that are interpreted to be remnants of a complex tectonic history. Upper to middle crustal dipping reflections are interpreted to be associated with Archean rocks drilled below the Western Canada Sedimentary Basin and their geometry is consistent with east- and northeast-verging imbrication. Layered lower crustal reflections of the MHB are interpreted as Early Proterozoic delaminated Loverna block crust and injected mafic material, based on geometric relationships, xenolith dating, and gravity modelling. The reflection Moho is diffuse below the MHB at ~15 s two-way time (~47 km). This is similar to estimates based on adjacent seismic refraction data and is up to ~8 km deeper than regions north of the MHB. The MHB crust is interpreted to have undergone two episodes of crustal deformation in the Precambrian: (1) Late Archean formation of the MHB crust by continental collision of two Archean domains along a crustal-scale ramp; and (2) Early Proterozoic crustal shortening associated with the formation of the Vulcan structure, the delamination of the Loverna block crust, and postcollisional injection of mafic melts beneath the MHB crust. Radiometric dating of lower crustal xenoliths suggests a significant Early Proterozoic thermal event in the deep crust beneath the MHB that is coeval with orogenic activity in the Trans-Hudson Orogen and the East Alberta Orogen.

Introduction to special issue of Canadian Journal of Earth Sciences: The Alberta Basement Transect of Lithoprobe

From outcrops in northeastern Alberta to exposures within structural culminations in the Canadian Cordillera, the crystalline basement that lies buried beneath the Phanerozoic Alberta Basin of western Canada represents a significant part of the Canadian Shield, ca. 750 000 km2. The advent of modern high resolution aeromagnetic data and U–Pb geochronology have opened the doors for the next era of investigation of the crystalline basement of the Alberta Basin. Aeromagnetic data and geochronology provided important physical and temporal links to the exposed Canadian Shield, which allowed its structural framework to be extrapolated into the subsurface. The most recent chapter in this evolving story has been the Alberta Basement Transects of Lithoprobe, a 12-year program, which has used multi disciplinary geoscience studies to further elucidate the nature, history, and geometric characteristics of basement domains beneath the Alberta Basin. This special issue is the second in a three issue set of publications that synthesize the results of over a decade of Lithoprobe research in the Alberta Basin. The first issue, published in the Bulletin of Canadian Petroleum Geology, concentrated on the shallow sedimentary record and new insights gained from linking deep seismic reflection profiles with the evolution of the sedimentary succession. The present issue presents a crustal perspective and concentrates on the results from mapping, geochronology, igneous petrology, and seismic investigation of the crystalline basement of the Alberta Basin. The diversity of disciplines covered in this issue address questions about the evolution of the crust and mantle of western Canada and offer unique perspectives on the insight gained from the merger of reflection seismology, magnetotellurics, potential field studies, and field based observations. New breakthroughs in mantle dynamics and its role in sedimentary basin formation, as well as our ability to image mantle structure and relate it to surface tectonic processes, are important steps forward.

Internal configuration of the Levantine Basin from seismic reflection data (eastern Mediterranean)

In the easternmost Mediterranean, the Levantine Basin is filled by up to 10 km of Mesozoic to Recent sediments next to the Alpine deformation front of the Cyprean Arc, near the junction between Africa and Arabia. Away from the deformation front the basin has undergone little deformation for the past 100 Ma as evidenced by deep seismic reflection profiles. Mostly subhorizontal reflections image the undisturbed sedimentation above a possibly Paleozoic basement. Two marked reflections are identified within the sedimentary fill (M and K). They correspond to unconformities near the deformation front and their laterally equivalent concordant surfaces basinward. In the deepest part of the basin there are more than 4 km of Tertiary to Recent sediments, within which the M-reflection is well defined above the 1.5 km thick Messinian evaporites and beneath the 500 m thick Plio–Quaternary sedimentary sequence. Pre-Tertiary sediments reach up to 6 km thick beneath the K-reflection and above the top basement reflection. They are little deformed and possibly correspond to Late Jurassic–Cretaceous age. The limit with the Cyprean Arc deformation front is a sharp tectonic boundary south of the Hecateaus Rise, where the whole sedimentary fill terminates against a subvertical fault zone that can be followed down to more than 14 km depth. To the east, this limit changes into a 30 km wide zone of strike-slip deformation that includes flower structures progressively buried basinward by Tertiary to Recent syntectonic sediments. Possibly since Paleocene times, the basin corresponds to the almost undisturbed part of a larger basin whose northern side has been incorporated within the Alpine deformation of the Cyprean Arc, and limited to the east by the Dead Sea transform system.

Crustal structure across the Vulcan Sub-basin from seismic refraction and gravity data

The attenuated continental crust between the Kimberley Block and the Timor Trough hosts some of the major oil and gas fields in Western Australia. The crustal structure has been interpreted from wide-angle seismic data acquired by the Australian Geological Survey Organisation during 1995/96. Ten ocean-bottom seismometers were deployed along a deep seismic reflection profile aligning NW with exploration wells Fagin and Crane. Data were recorded to offsets of 300 km. A total profile 335 km long and 48 km deep has been developed, by ray-tracing and inversion modelling of the refraction data, and verified by modelling gravity data. The model differentiates 7 crustal and 2 upper mantle layers. The crustal thickness varies between 25 to 30 km, being greatest beneath the Kimberley Block and Vulcan Sub-basin. The velocities within the upper mantle are ~8.1 km/s, while in the lower crust they vary from 5.9-6.2 km/s beneath the Kimberley Block, to 6.4-7.0 km/s elsewhere. Mid-crustal velocities are 5.5-6.2 km/s and 2.0-5.0 km/s in the overlying sedimentary section. Significant lateral variations exist in the lower crust and can be used to infer the limit of the Kimberley Block. A prominent reflector at 6s twt, which is widespread in the region, is not a significant velocity discontinuity. The Moho is not seen on the reflection records, but is a prominent feature in the refraction data. Iso-velocity lines do not necessarily coincide with reflection boundaries, possibly due to decreasing porosity with depth, especially beneath the Cartier Trough.

Ringed structural zones with deep roots formed by the Chicxulub impact

The Chicxulub meteorite/comet impact of 65 Ma occurred on the present northeast coast of the Yucatan peninsula of Mexico and is considered by many to have caused a mass extinction at the end of the Cretaceous period. Impact craters are traditionally described in terms of ring structures. Deeply rooted faults underlying four previously recognized ring zones of the Chicxulub impact crater, as revealed on new deep seismic reflection profiles, demonstrate the catastrophic and diverse deformations that produced these zones. The innermost annular zone at Chicxulub coincides with a central peak ring. The seismic data show that the peak ring is composed of low‐velocity (&lt;5 km s−1), chaotically reflective material, interpreted as impact breccia, lying on top of blocks of pre‐impact strata that were downdropped 4–6 km. Normal faults within the Cretaceous strata and deep, inward dipping reflections characterize two deformation zones at radial distances of 55–65 and 85–98 km. These normal faults accommodated the collapse, during the modification stage of crater development, of large (radial width of 5–25 km) blocks of pre‐impact strata uplifted during the excavation stage. The structures root in the lower crust at depths of 15–30 km. Blind thrusts indicated by monoclines in pre‐impact sedimentary rocks and deep, inward dipping reflections within the crystalline basement mark the outermost ring of deformation. These monoclines correlate with small topographic changes on the Cretaceous‐Tertiary boundary layer at radial distances of 120–135 km. Palinspastic restoration of coherent blocks of downdropped pre‐impact strata enabled improved estimates of the size and shape of the hole formed by the impact in a prominent reflector at 3‐ to 4‐km depth. This hole is slightly elliptical along a SE‐NW major axis (radius of 68 versus 63 km).

Structure of the highly mineralised late-Archaean granitoid- greenstone terrain and the underlying crust in the Kambalda- Widgiemooltha area, Western Australia, from the integration of geophysical datasets.

The highly mineralised (Au, Ni) Kambaida-Widgie- mooltha area comprises typical late-Archaean granitoid- greenstone sequences, and is situated within the Kalgoorlie Terrane of the southern Norseman-Wiluna Belt in the Yilgarn Craton of Western Australia. In the Kambalda- Widgiemooltha area, high-resolution aeromagnetic data, two deep seismic reflection profiles and gravity data are available to examine the geophysical signature and three- dimensional structure of this important region of the Kalgoorlie Terrane.The Kambaida-Widgiemooltha area has previously been divided, from west to east, into three fault-bounded tectono-stratigraphic domains, the Coolgardie, Kambaida and Parker Domains. Lithological associations and structural features defined by the aeromagnetic data substantiate the division of the study area into these three north-northwesttrending domains, and indicate that each domain consists of two, approximately equal-width, sub-domains. A four-stage deformation sequence, which is recognised from the aeromagnetic data, includes early south-to-north directed recumbent folding and thrust repetition (Dl), regional north-northwest folding and imbrication (D2), transcurrent movement on major north-northwest shear zones (D3), and the formation of late north-northeast and northwest-trending lineaments (D4).Two deep (20s TWT) seismic reflection profiles across the Coolgardie and Kambaida Domains reveal a threelayered crust beneath the study area. The upper layer (0-10 km), which includes the greenstone belt, is generally unreflective in the upper 5 km. The lower part of this layer (7.5 to 10 km) is more reflective and comprises a felsic unit which could be a continuation of the greenstone belt stratigraphy, attenuated basement, or rift-related sedimentary rocks. Domain-bounding faults are interpreted as listric structures, which have allowed the partial imbrication of the domains during regional deformation.The middle layer (10-27 km) is characterised by moderate to strong, variably dipping reflections. It is interpreted as an imbricate sequence of granitoids and gneisses. A more homogeneous, but weakly reflective, character is indicative of the lower crustal layer (27 - 35 km). East-dipping truncating reflectors at the western margin of the study area suggest that this layer is younger than the upper and middle layers and may, therefore, represent the late-tectonic external granitoids. The Moho is not imaged, but a general decrease in the number of reflections indicates that the crust is about 35 km thick.

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview

Abstract The Gulf of Lion margin is one of the Tertiary extensional basins of the western Mediterranean that opened during convergence of Africa and Europe. This Oligocene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of ‘Atlantic-type’ margins. However, when the Integrated Basin Study (IBS) project was initiated, several outstanding questions remained about the present structure and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based on the existing onshore and offshore, industrial and academic data, which were heterogeneous and unevenly distributed. Compilation of the stratigraphic correlations on a regional scale allowed precise calculation of the timing of rifting, and clarification of the relationships with Alpine and Mediterranean geodynamics. Reprocessing of the existing ECORS deep seismic reflection profiles shed new light on the extensional structure and mechanisms of extension of the continental margin. Structural and sedimentological studies onshore led to the definition of new tectonostratigraphic models for extensional basins. Results of structural analyses showed a partitioning of the extensional deformation processes across the continental margin. 3D gravity modelling of the margin and basin area led to the production of a new map of the Moho depth by inversion, and testing several hypotheses for the origin of the present day subsidence. Although the Gulf of Lion margin displays structural and stratigraphic features similar to ‘Atlantic-type’ margins, its structure and evolution corresponds to that of a rifted margin of a large continent formed during the opening of a marginal basin. Integration of the new results of IBS-Gulf of Lion within the geodynamic evolution of the western Mediterranean suggests that the Oligocene rifting of the Gulf of Lion represents the initial stage of a succession of rifting events and back-arc basin formation, due to continuously retreating subduction during convergence of Africa and Europe.

Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near‐crustal‐thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower‐crustal subduction beneath the Himalaya.

Crustal architecture and tectonic evolution of the Sergipe-Alagoas and Jacuípe basins, offshore northeastern Brazil

The Sergipe-Alagoas and the Jacuípe basins are divergent margin basins located in the South Atlantic Ocean, at the northeastern Brazilian margin. High-quality 18 s (two-way travel time) seismic reflection profiles, which extend from the coastline towards the boundary with the oceanic crust, were integrated with potential field data and calibrated with the results of several exploratory boreholes. When combined, these data provide evidence that the deep seismic reflectors in the deep-water region are related to the crustal architecture of the basin, rather than to sedimentary features. Arrays of high-impedance reflectors, previously interpreted as top of basement, probably correspond to detachment planes near the base of the crust, or to underplated igneous rocks above the Moho. The extensional processes that were active during rifting resulted in normal crustal faults that apparently detach near the seismic Moho in the deep-water region. These crustal faults control and rotate syn-rift sedimentary units along the margin. The seaward portion of the rift seems to have been uplifted and highly eroded during a post-break-up tectonic event. The deep seismic reflection profiles also indicate that the deep-water province is characterized by wedges of reflectors that dip seawards. These features probably correspond to seaward-dipping reflectors (SDR), which are composed of volcanic rocks marking the transition to oceanic crust. The SDR wedges are locally associated with volcanic intrusions. Several volcanic plugs in deep-water region are aligned with oceanic fracture zones that apparently penetrate through the whole crust and reach the upper mantle. On the other hand, there are some diapiric structures located near the boundary between continental and oceanic crust that bear evidence for deep-water salt tectonics.

Structure and tectonics of the Proterozoic Aravalli-Delhi Fold Belt in northwestern India from deep seismic reflection studies

Seismic imaging of the crust along a 400-km-long deep seismic reflection profile across the Palaeo/Mesoproterozoic Aravalli-Delhi Fold Belt, in the northwestern Indian Shield, brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt. The reflectivity pattern varies considerably for different crustal units along the profile.The deep-crustal reflection data image two sets of oppositely dipping strong reflection bands, from upper- to lower-crustal levels. These are identified as the signatures of the collision corresponding to Aravalli and Delhi orogeny. The data also exhibit a clear Moho and strong lower-crustal reflections near the collision boundaries. A stack of dipping reflections from the top of the Moho to the surface is identified as a major thrust fault indicating that the Proterozoic collision and deformation were primarily thick-skinned in nature