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Updated: Jun 02, 2016

7th International Symposium on Deep Seismic Profiling of the Continents and their Margins, II: A global survey

Elsevier logo

Asilomar, CA, USA, 12 September - 15 September 1996

from TECTONOPHYSICS Volume 288, Issues 1-4, Pages 1-269 (30 March 1998)
Edited by Simon L. Klemperer and Walter D. Mooney

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 site and follow the "journals" link to Tectonophysics)

Seismic studies around the Kola Superdeep Borehole, Russia

Pages 1-16
Y. V. Ganchin, S. B. Smithson, I. B. Morozov, D. K. Smythe, V. Z. Garipov, N. A. Karaev and Y. Kristofferson

The Kola Superdeep Borehole (SG-3) provided an ideal opportunity to test hypotheses on the origins of crustal reflections and on the presence and seismic expression of fluids in the upper crust. The alternative sources of crustal reflections include compositional changes, shear zones, fluids, and metamorphic facies changes, all of which are represented at the well. Both the 38-km-long CDP section and the borehole VSPs in the range 2.2-6.0 km demonstrate the presence of reflections from dipping compositional layering, shear zones, and fluid-filled zones. Subhorizontal reflectivity zones are interpreted as horizontal fluid-filled fracture-type reservoir rocks. Results suggest the presence of fluids down to a depth of at least 12 km in the upper crust; the presence of these fluids lowering seismic velocity causes estimates of upper crustal composition to be too felsic.

Elsevier online abstract

Seismic lower crustal reflectivity and signal penetration in the Siljan Ring area, Central Sweden

Pages 17-30
Niklas Juhojuntti and Christopher Juhlin

During the period between 1984 and 1990, several reflection seismic profiles, with a total length of 100 km. were shot in the area of the Siljan impact structure, Central Sweden. The aim was to study the Proterozoic crystalline crust, prior to the drilling of a deep well. Parts of this data have been re-processed, addressing questions concerning signal penetration and lower crustal/upper mantle reflectivity, as strong lower crustal reflectivity is often absent on land data from the Baltic Shield. Earlier processing showed clear reflectors at upper crustal levels, some of which have been attributed to dolerites, through the use of borehole logging. At lower crustal levels, the picture was less clear with only diffuse bands of reflectors. We have tried to improve the image of the deeper parts, mainly by using standard processing techniques for correcting static shifts and for spectral balancing. The standard stacked sections show, as before, strong reflectors in the upper crust and some discontinuous events at lower crustal levels, primarily in the time window between 11 and 14 s TWT. From this we place the base of the crust at 13-14 s TWT (43-47 km at 6.6-6.7 km/s) in this area. There are strong lateral variations in this apparent reflectivity. These variations could, in part, be caused by differences in charge sizes. The depth of signal penetration, estimated through prestack amplitude decay, was not fully sufficient for the smallest charges (<10 kg). This is supported by coincident stacked sections, where data collected with smaller charges contain less coherent events at lower crustal levels. Some variations in apparent reflectivity are not related to the charge size. The effects of lateral variations in signal penetration have been studied, for 10-kg charges, by calculating amplitudes at fixed-offset channels for different shot positions. Although there are some variations in these amplitudes, we argue that this is not the only explanation for variations in apparent deep reflectivity and that the lower crustal reflectivity is weaker below the Transscandinavian Granite Porphyry Belt than below the Southern Svecofennian Province. From earlier studies, the meteor impact that has occurred in this area is interpreted not to have affected the lower crust.

Elsevier online abstract

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

Pages 31-33
B. Rajendra Prasad, H. C. Tewari, V. Vijaya Rao, M. M. Dixit and P. R. Reddy

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

Elsevier online abstract

Crustal-scale structures in the Proterozoic Mount Isa Inlier of north Australia: their seismic response and influence on mineralization

Pages 43-56
Barry J. Drummond, Bruce R. Goleby, Alexey G. Goncharov, L. A. I. Wyborn, C. D. N. Collins and Tyler MacCready

The Proterozoic Mount Isa Inlier in northern Australia is prospective for base metals and gold. It contains a Western Fold Belt (including the Leichhardt River Fault Trough) and an Eastern Fold Belt, separated by older basement rocks of the Kalkadoon Leichhardt Belt. Sediments and volcanics in both fold belts were deposited in rifts which were subsequently shortened by up to 50%. Mineralisation appears to be partitioned: large-tonnage lead and zinc deposits are more prevalent in the Leichhardt River Fault Trough, and most gold and copper occurrences are in the Eastern Fold Belt. Cross-sections of the inlier derived from coincident seismic reflection and refraction profiling are dominated by the youngest tectonic events. The refraction data imply a west-dipping lens of high-velocity intermediate-to-mafic rock in the middle to upper crust in the east of the inlier. It is collinear with another lens in the lower crust in the west of the inlier. The lenses form a belt of high-velocity rock cutting the crust from top to bottom and from east to west. The reflection data reveal different styles of compression-related structures in the east and west of the inlier. Thin-skinned tectonics dominate in the Eastern Fold Belt. The sediments and volcanics are thrust to the west along a number of shallowly east-dipping upper-crustal detachments. The detachments in turn are cut by steeply east-dipping reverse faults which link into the zone of high-velocity rocks defined by the refraction data. In contrast, faults in the Western Fold Belt are steep and penetrate to mid-crustal levels and probably also link into the belt of high-velocity material deeper in the crust. The partitioning into different tectonic styles occurs across the Kalkadoon Leichhardt Belt, which appears to have acted as a buttress during the crustal shortening. Published mineralisation models attribute the lead-zinc mineralisation to circulating fluids at shallow crustal levels within the Leichhardt River Fault Trough. High reflectivity of faults with a close spatial association with mineralisation is attributed to alteration along the fault caused by migrating fluids. Copper-gold mineralisation in the Eastern Fold Belt is scattered, but known major deposits lie along-trend from a thrust fault shown in the seismic data to be highly reflective. This fault links via the upper-crustal detachments and the high-velocity lenses into the middle to lower crust. and is seen as a likely control on fluid migration pathways from lower crustal levels into the supracrustal Eastern Fold Belt. The partitioning of the tectonic styles seen in the seismic data and the mode of linking of faults into the middle to lower crust are seen as primary factors in the partitioning of mineralisation in the region.

Elsevier online abstract

Crustal architecture of central Australia based on deep seismic reflection profiling

Pages 57-69
R. J. Korsch, B. R. Goleby, J. H. Leven and B. J. Drummond

The crustal architecture of central Australia is interpreted from deep seismic reflection profiling conducted by the Australian Geological Survey Organisation in two surveys in 1985 and 1993. The seismic traverses, oriented normal to the main surface structures, ran north-south in central Australia, and crossed parts of the Arunta Block, Amadeus Basin, Musgrave Block and Officer Basin. The present crustal fabric was set in place by the end of the Mesoproterozoic (by about 1100 Ma). Reactivation of the structures took place in a continental intraplate setting mainly during the Middle-Late Palaeozoic Alice Springs Orogeny, but also during other orogenic events. In the Arunta Block. the crust is dominated by major north-dipping planar structures, interpreted as thick-skinned thrusts. Many of these thrusts cut deep into the crust, and at least one, the Redbank Thrust Zone, appears to cut and offset the Moho. In contrast, in the northern Musgrave Block, limited field mapping and teleseismic data suggest that the major crustal-scale planar structures are south-dipping. In the central to southern Amadeus Basin, deformation is essentially thin-skinned, with north-directed thrusting confined to the sedimentary succession. Thus, the deep seismic profiles in central Australia show a present day crustal architecture that is the response of the crust to Mesoproterozoic terrane amalgamation and to later reactivation by intraplate deformational events. Therefore. central Australia is a model for intraplate cratonic deformation that occurs in continental crust that is cold. thick and strong.

Elsevier online abstract

The HSI bright reflector: further evidence for extensive magmatism in the Precambrian of western Canada

Pages 71-81
H. A. F. Mandler and R. M. Clowes

Seismic reflection lines acquired in 1995 as part of LITHOPROBE, Canada's national geoscientific research program, revealed unusually bright reflectors of extensive lateral scale in the Archaean Medicine Hat block of southern Alberta. The herein-named Head-Smashed-In (HSI) reflector is imaged over an area of at least 6000 km2 in the upper half of the crust. The reflector appears as a mostly horizontal to subhorizontal feature. Although continuous, it has a complex and laterally variable internal structure. Comparison with a reflector of known impedance contrast in the overlying western Canadian sedimentary basin indicates a high-impedance structure to cause the reflections. Dense one-dimensional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres and lateral scales of discrete reflecting elements between tens of metres and hundreds of metres. Polarity constraints, overall geometry and reflection character strongly suggest that mafic/ultramafic horizontal sheets are the cause of HSI reflectivity. The HSI is the third major sequence of strong crustal reflectivity imaged by LITHOPROBE deep seismic data in the Precambrian crust of western Canada. All three features are very similar in overall geometry, reflector strength and character and have been interpreted as horizontal, sheetlike intrusions. Small-scale variations of the reflectivity signatures suggest a pronounced rheological heterogeneity in the upper crustal rocks within the host tectonic domains.

Elsevier online abstract

Expanding spread profiles across the Trans-Hudson Orogen

Pages 83-91
Sándor Bezdán and Zoltan Hajnal

Four vibroseis expanding spread profiles (ESP) were acquired through the LITHOPROBE Trans-Hudson Orogen Transect program. These ESP's, with a maximum offset of 20 km, were centred on areas where prominent crustal reflectivity was detected by the regional survey. The small source stepout distance (100 m) generated high-fold (>30) data. The Common Midpoint (CMP) stacked sections of ESP and regional reflection data of the 1991 survey exhibit similar crustal images. The regional profiles provide better resolution of shallow and dipping features, whereas the continuity and strength of reflections below 6 s two-way traveltimes (TWT) are better defined on the ESP sections. Modelling results reveal that velocity estimation becomes more robust and accurate when crustal seismic surveys utilize longer offsets than commonly implemented by standard practice. These larger source-receiver separations, however, must be generally limited to offset/depth ratios not exceeding 1.5 when conventional velocity analysis techniques, based on the hyperbolic moveout assumptions, are implemented. The advantages provided by the long-offset data acquisition include increased S/N ratio and a greater number of traces with sufficiently large moveouts which improve velocity resolution, especially below mid-crustal depths. To achieve similar advantages in a regional crustal reflection survey would require the adoption of longer spread lengths than those presently implemented in standard data acquisition procedures.

Elsevier online abstract

Structure of the lithospheric mantle beneath the Trans-Hudson Orogen, Canada

Pages 93-104
Balázs Németh and Zoltan Hajnal

Refraction and wide-angle reflection seismic data collected in northern Saskatchewan and Manitoba in 1993 by LITHOPROBE provide new glimpses of the deep structure of the lithospheric mantle beneath the Trans-Hudson Orogen. Velocity models obtained through ray-tracing modelling techniques revealed the presence of complicated mantle structures down to a depth of 160 km beneath the orogen. Like the crust, the lithospheric mantle appears to show a seismically transparent upper and a highly reflective lower part, suggesting that the rheological environment responsible for development of reflections in the lithospheric mantle and of that in the crust are similar. This appears to be consistent with the observation that the strong and brittle upper 30 km of the mantle contain remnants of former tectonic processes in a form of velocity anisotropy, and the lower, more ductile zone seems highly reflective and isotropic. The suspected beginning of the lithosphere-asthenosphere transition zone at a depth of 160 km, imaged as gradual disappearance of mantle reflections, coincides well with models established using other geophysical methods.

Elsevier online abstract

Crustal structure of China from deep seismic sounding profiles

Pages 105-113
Songlin Li and Walter D. Mooney

More than 36,000 km of Deep Seismic Sounding (DSS) profiles have been collected in China since 1958. However, the results of these profiles are not well known in the West due to the language barrier. In this paper, we summarize the crustal structure of China with a new contour map of crustal thickness, nine representative crustal columns, and maps showing profile locations, average crustal velocity, and Pn velocity. The most remarkable aspect of the crustal structure of China is the well known 70+ km thickness of the crust of the Tibetan Plateau. The thick (45-70 km) crust of western China is separated from the thinner (30-45 km) crust of eastern China by the north-south trending seismic belt (105°E). The average crustal velocity of China ranges from 6.15 to 6.45 km/s, indicating a felsic-to-intermediate bulk crustal composition. Upper mantle (Pn) velocities are 8.0 ± 0.2 km/s, equal to the global continental average. We interpret these results in terms of the most recent thermo-tectonic events that have modified the crust. In much of eastern China, Cenozoic crustal extension has produced a thin crust with a low average crustal velocity, similar to western Europe and the Basin and Range Province, western USA. In western China, Mesozoic and Cenozoic arc-continent and continent-continent collisions have led to crustal growth and thickening. Inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by DSS profiles and other seismological studies. A high velocity (7.0-7.4 km/s) lower-crustal layer has been reported in western China only beneath the southernmost Tibetan Plateau. We identify this high-velocity layer as the cold lower crust of the subducting Indian plate. As the Indian crust is injected northward into the Tibetan lower crust, it heats and assimilates by partial melting, a process that results in a reduction in the seismic velocity of the lower crust in the central and northern Tibetan Plateau.

Elsevier online abstract

Seismic reflection fabrics of continental collision and post-orogenic extension in the Middle Urals, central Russia

Pages 115-126
J. H. Knapp, C. C. Diaconescu, M. A. Bader, V. B. Sokolov, S. N. Kashubin and A. V. Rybalka

Deep seismic reflection surveys in the Middle Urals, central Russia, provide the basis for a revised structural and tectonic interpretation of late Paleozoic continental collision and early Mesozoic post-orogenic extension in this orogen. These profiles (ESRU '93 and ESRU '95) form a transect from the former East European platform margin in the west to accreted island-arc and oceanic terranes in the east, crossing the Main Uralian fault (MUF), the principal suture zone in the Urals. The foreland of the Middle Urals is characterized by a fold-and-thrust belt developed above a gently E-dipping detachment at 10 km depth, and a deeper, presumably older, and previously unrecognized detachment system of probable Late Proterozoic (Baikalian) age. A large ( 5 km) frontal ramp marks the axis of the regional Kvarkush anticlinorium which dominates the foreland structure. Reflectivity in the middle and lower crust of the East European platform is inferred to be primarily a Precambrian fabric. Based on correlation of the seismic data with surface geology, the MUF is redefined spatially, seismically and structurally as the top of an 40°E-dipping reflective package, placing it (1) at the boundary between upper greenschist to amphibolite facies rocks of continental affinity to the west and unmetamorphosed to lower greenschist facies rocks of the Tagil island arc to the east, (2) at the primary topographic break in the Middle Urals, and (3) 10 km further east at this latitude than recent interpretations, but consistent with previous Russian maps. At depth, the MUF appears to merge in a listric fashion with a zone of subhorizontal mid-crustal reflectivity ( 15-20 km depth) that truncates a series of large-scale antiforms and associated W-dipping (E-vergent?) shear zones of inferred Paleozoic age across the Uralian hinterland. These relationships suggest that the MUF was reactivated as a normal fault during late- to post-orogenic evolution of the orogen. Based on interpretation of the Moho as the base of lower-crustal reflectivity, crustal thickness varies from 48 to 51 km ( 14-15 s) across the ESRU '93 profile, in concert with existing Russian studies showing a thickened crust beneath the Urals. To the east, on the ESRU '95 profile (beneath the Tagil island arc), a strongly W-dipping reflective package in the lower crust suggests abrupt thinning of the crust to 45 km ( 13 s), consistent with nearby wide-angle reflection data. At the surface, this crustal thinning coincides with the Triassic Imenovskii graben. We interpret these collective observations as evidence for post-orogenic extension of the Middle Urals in early Mesozoic time, in association with reactivation of the MUF as a breakaway normal fault at the westernmost boundary of the West Siberian basin extensional province. In contrast, results from the URSEIS '95 experiment indicate that the Southern Urals are largely unextended, implying that post-orogenic evolution varied along strike within the orogen.

Elsevier online abstract

Some seismic signatures in the Romanian crust

Pages 127-136
V. Raileanu and C. C. Diaconescu

Some new seismic reflection data in the W of the Moesian platform, Focsani depression and Transylvania depression show seismic patterns which reflect the local structure and evolution of the each area. These local areas were affected by subsidence which over-printed the changes on the old pattern. The old and more or less reflective lower crust was fractured and some rigid blocks have preserved the reflectivity up to the present. The Bailesti section illustrates a transparent or diffractive upper crust and a well-marked lower crust by an alternation of reflective and non-reflective zones. The crystalline crust of the Râmnicu Srat section has two patterns: one relatively reflective, near the bottom of the depression (the W side), whilst the other, on the E flank of depression, is transparent. The E flank seems to be more fractured due to some tensile fractures originating in the bending stress during subsidence. The Târgu Mures section has a specific crustal pattern. A near-transparent crystalline crust overlies a very large crust-mantle transition zone which is fragmented in alternated reflective and transparent blocks.

Elsevier online abstract

A multidisciplinary geophysical study in the Betic chain (southern Iberia Peninsula)

Pages 137-152
R. Carbonell, V. Sallares, J. Pous, J. J. Dañobeitia, P. Queralt, J. J. Ledo and V. García Dueñas

The combined analysis of magnetotelluric measurements, tomographic velocity models and deep seismic reflection images confirms that the Betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties. At depth these data sets show evidence for a non-coincidence of the petrological and the seismic Moho beneath the Betics chain. The data sets reveal the geophysical properties of the Alboran domain (Internal Betics) and the Iberian Massif (External Betics). According to this, the Iberian crust features a relatively high seismic velocity, is seismically transparent in the seismic reflection images and is electrically resistive. The Alboran domain crust is characterized by a low average velocity, displays high reflectivity in the seismic reflection images and is electrically conductive. The outcrops of the metamorphic complexes (Alpujárride and Nevado Filábride), showing relatively high velocities coupled with low Vp/Vs values (1.67) derived from the Wadati slopes, suggest the existence of rocks rich in silica beneath the Alboran domain crust. An interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the Iberian upper crust. Partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels. These would account also for the reflectivity and the low Vp/Vs ratios mapped beneath the Alboran domain.

Elsevier online abstract

Seismic structure across the Caledonian Deformation Front along MONA LISA profile 1 in the southeastern North Sea

Pages 153-176
Tanni Abramovitz, Hans Thybo and MONA LISA Working Group

Seismic data from the MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) project in the southeastern North Sea image the Caledonian Deformation Front (CDF), which is the collisional suture between Baltica to the north and east and Avalonia to the south and west. The N---S-trending MONA LISA normal-incidence reflection profile 1 was recorded to 26 s twt. Coincident wide-angle data were acquired on nine ocean bottom hydrophones and several onshore mobile seismographs along and off the profile. The model of compressional seismic velocity shows three different crustal types: (a) a typical three-layered shield-type crust below the Ringkøbing Fyn High to the north: (b) a highly complex transitional crust in the central part; and (c) a two-layered crust of Caledonian origin to the south. Sharp and strong normal-incidence and wide-angle reflections from Moho were recorded south of the Caledonian Deformation Front in contrast to less distinct reflections further north. S-dipping crustal reflections from 4 to 11 s twt over 70 km horizontal distance terminate at Moho and coincide with a change in the seismic velocity structure. This indicates northward obduction of Avalonian crust. Non-migrated normal-incidence seismic sections show crossing weak N-dipping and stronger S-dipping reflections to 20 s twt from the uppermost mantle. We propose a tectonic model where the closure of the Tornquist Sea took place along a N-dipping subduction zone which was later overprinted by a late-or post-Caledonian S-dipping shear zone. Sub-Moho velocities are 7.8-7.9 km/s under 34-35-km-thick Baltica crust and 8.1-8.3 km/s under 25-26-km-thick Caledonian crust. The sub-horizontal Moho across the Caledonian collision zone implies late- or post-Caledonian re-equilibration of the seismological Moho. We interpret the low-velocity upper mantle (7.8-8.1 km/s) to the north as former Baltica lower crust in eclogite facies after pressure-induced metamorphism as a result of lithospheric flexure during the Caledonian orogeny. These rocks today appear as upper mantle that was uplifted to their present position during the Middle Devonian collapse of the North German-Polish Caledonides.

Elsevier online abstract

A transect across Australia's southern margin in the Otway Basin region: crustal architecture and the nature of rifting from wide-angle seismic profiling

Pages 177-189
D. M. Finlayson, C. D. N. Collins, I. Lukaszyk and E. C. Chudyk

The Otway Basin in southeastern Australia formed on a triangular-shaped area of extended continental lithosphere during two extensional episodes in Cretaceous-to-Miocene times which ultimately led to the separation of Australia and Antarctica. The velocity structure and crustal architecture of the Otway continental margin has been interpreted from offshore-onshore wide-angle seismic profiling data along a transect extending from near the northern Otway Basin margin with Palaeozoic outcrop to the deep ocean basin under the Southern Ocean. Along this transect, the Otway Continental Margin (OCM) Transect, the onshore half-graben geometry of Early Cretaceous deposition gives way to a 5-km-thick basin sequence (P-wave velocity 2.2-4.6 km/s) extending down the continental slope offshore to at least 60 km from the shoreline. At 120 km from the nearest shore, sonobuoy data indicate a 4-5 km sedimentary sequence overlying 7 km of crustal basement rocks above the Moho at 15 km depth (water depth 4220 m). Conspicuous strong Moho reflections are evident under the continental slope at about 10.2 s TWT. Basement is interpreted to be attenuated/faulted Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. These rocks comprise a 3 km section that has a velocity of 5.5-5.7 km/s overlying deeper basement with a velocity of 6.15-6.35 km/s. Over the same distance the Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the profile (water depth 5200 m). The continent-ocean boundary (COB) is interpreted to be at a prominent topographic inflection point at the bottom of the continental slope in 4800 m of water. P-wave velocities in the lower crust are 6.4-6.8 km/s above a transition to the Moho, with an upper mantle velocity of 8.05 km/s. There is no evidence of massive high-velocity (> 7 km/s) intrusives/underplate material in the lower crust nor any syn-rift or early post-rift subaerial volcanics, indicating that the Otway continental margin can be considered a non-volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins, e.g. the Nova Scotia-Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula.

Elsevier online abstract

Crustal structure at the SE Greenland margin from wide-angle and normal incidence seismic data

Pages 191-198
Trine Dahl-Jensen, Hans Thybo, John Hopper and Minik Rosing

Results from a seismic refraction and reflection line along the southeast coast of Greenland give information on both the Precambrian structures on the Greenland continent and on the effects of the Tertiary breakup of the North Atlantic. Three seismic stations on the Greenland coast recorded the airgun shots from a 279-km reflection seismic line approximately 20 km offshore. The maximum offset recorded was 313 km. The wide-angle data show crustal thickness variation from 39 km in the south to 49 km in the north, with an 8- to 17-km-thick, high-velocity (7.5 km/s) layer at the base of the crust, interpreted as underplating related to the opening of the North Atlantic in the Tertiary. The boundary between the early Proterozoic Ketilidian orogen in the south and the Archaean block to the north show little variation in seismic velocities, and the reflection pattern suggests that the Archaean underlies the Ketilidian at depth. We see no evidence that the Julianehåb Batholith at the boundary between rocks of the Ketilidian orogen and the North Atlantic block is a deep structure.

Elsevier online abstract

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

Pages 199-220
Webster Ueipass Mohriak, Marcelo Bassetto and Ines Santos Vieira

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.

Elsevier online abstract

Preliminary results from a geophysical study across a modern, continent-continent collisional plate boundary - the Southern Alps, New Zealand

Pages 221-235
F. J. Davey, T. Henyey, W. S. Holbrook, D. Okaya, T. A. Stern, A. Melhuish, S. Henrys, H. Anderson, D. Eberhart-Phillips, T. McEvilly, R. Uhrhammer, F. Wu, G. R. Jiracek, P. E. Wannamaker, G. Caldwell and N. Christensen

The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Elsevier online abstract

Structural insight into the south Ryukyu margin: effects of the subducting Gagua Ridge

Pages 237-250
Philippe Schnürle, Char-Shine Liu, Serge E. Lallemand and Donald L. Reed

This study presents three multi-channel deep seismic reflection profiles located in the south Ryukyu margin between 122°30′E and 123°30′E, where a N-S-trending oceanic ridge, the Gagua Ridge, is entering the subduction zone, for the purpose of examining the effects of ridge subduction on structures of the forearc region. Structural features which correspond to different stages of the oblique ridge subduction are observed. East of 123°E, a short-lived sequence of indentation, tunneling, then resumption of frontal accretion occurred in the accretionary wedge (the Yaeyama Ridge) as the subducted portion of the Gagua Ridge swept the overriding Ryukyu margin from below along the northwesterly convergent direction. Under the forearc basin, the subducted portion of the Gagua Ridge is uplifting the arc basement to form the Nanao Basement Rise which separates the sedimentary strata of the Nanao and East Nanao forearc basins. Results from this study suggest that the oblique subduction of the Gagua Ridge has not only affected accretionary wedge structures but also the arc basement of the south Ryukyu margin.

Elsevier online abstract

Seismic investigation of the continental margin off- and onshore Valparaiso, Chile

Pages 251-263
E. R. Flueh, N. Vidal, C. R. Ranero, A. Hojka, R. von Huene, J. Bialas, K. Hinz, D. Cordoba, J. J. Dañobeitia and C. Zelt

At the latitude of Valparaiso, Chile, a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Three legs of the German R/V Sonne (cruises SO101, SO103 and SO104) surveyed the continental margin and oceanic plate offshore Valparaiso, aiming at studying the crustal structure and investigating possible causes for the change in slab configuration. Sonne cruise SO101 investigated the tectonic setting with swath-mapping bathymetry, magnetics and high-resolution seismics. Following these investigations cruise SO103 collected land-sea wide-angle seismic data, and coincident deep seismic reflection data were acquired during cruise SO104. Coincident near-vertical and wide-angle seismic measurements were made along two profiles. Profile 1, located at the south of the study area, away from the influence of the subducting ridge, crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed. Profile 2, located in the north, runs from the Juan Fernandez Ridge to the Chilean coast. The crustal velocity models obtained for the two profiles show that the continental crust extends to the middle-lower slope boundary, which is also reflected in morphology. In addition, they show that the crustal structure of the oceanic plate is rather similar, but the plate seems to be slightly more inclined along the northern profile (13° versus 10° in the south). The two profiles are only about 70 km apart but their structures differ significantly. No straightforward correlation exists between the two profiles that can be attributed to ridge collision. The data support that the 1985 central Chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate.

Elsevier online abstract

West margin of North America - a synthesis of recent seismic transects

Pages 265-269
Gary S. Fuis

A comparison of the deep structure along nine recent transects of the west margin of North America shows many important similarities and differences. Common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust, accreted oceanic/arc (or oceanic-like) lithosphere of Mesozoic through Cenozoic ages, Cenozoic accretionary prisms, Mesozoic accretionary prisms, backstops to the Mesozoic prisms, and undivided lower crust. Not all of these elements are present along all transects. In this study, nine transects, including four crossing subduction zones and five crossing transform faults, are plotted at the same scale and vertical exaggeration (V.E. 1 : 1), using the above scheme for identifying tectonic elements. The four subduction-zone transects contain actively subducting oceanic crust, Cenozoic accretionary prisms, and bodies of basaltic rocks accreted in the Cenozoic, including remnants of a large, oceanic plateau in the Oregon and Vancouver Island transects. Rocks of age and composition (Eocene basalt) similar to the oceanic plateau are currently subducting in southern Alaska, where they are doubled up on top of Pacific oceanic crust and have apparently created a giant asperity, or impediment to subduction. Most of the subduction-zone transects also contain Mesozoic accretionary prisms, and two of them, Vancouver Island and Alaska, also contain thick, tectonically underplated bodies of late Mesozoic/early Cenozoic oceanic lithosphere, interpreted as fragments of the extinct Kula plate. In the upper crust, most of the five transform-fault transects (all in California) reflect: (1) tectonic wedging of a Mesozoic accretionary prism into a backstop, which includes Mesozoic/early Cenozoic forearc rocks and Mesozoic ophiolitic/arc basement rocks; and (2) shuffling of the subduction margin of California by strike-slip faulting. In the lower crust, they may reflect migration of the Mendocino triple junction northward (seen in rocks east of the San Andreas fault) and cessation of Farallon-plate subduction (seen in rocks west of the San Andreas fault). In northern California, lower-crustal rocks east of the San Andreas fault have oceanic-crustal velocity and thickness and contain patches of high reflectivity. They may represent basaltic rocks magmatically underplated in the wake of the migration of the Mendocino triple junction, or they may represent stalled, subducted fragments of the Farallon/Gorda plate. The latter alternative does not fit the accepted 'slabless window' model for the migration of the triple junction. This lower-crustal layer and the Moho are offset at the San Andreas and Maacama faults. In central California, a similar lower-crustal layer is observed west of the San Andreas fault. West of the continental slope, it is Pacific oceanic crust, but beneath the continent it may represent either Pacific oceanic crust, stalled, subducted fragments (microplates) of the Farallon plate, or basaltic rocks magmatically underplated during subduction of the Pacific/Farallon ridge or during breakup of the subducted Farallon plate. The transect in southern California is only partly representative of regional structure, as the structure here is 3-dimensional. In the upper crust, a Mesozoic prism has been thrust beneath crystalline basement rocks of the San Gabriel Mountains and Mojave Desert. In the mid-crust, a bright reflective zone is interpreted as a possible 'master' decollement that can be traced from the fold-and-thrust belt of the Los Angeles basin northward to at least the San Andreas fault. A Moho depression beneath the San Gabriel Mountains is consistent with downwelling of lithospheric mantle beneath the Transverse Ranges that appears to be driving the compression across the Transverse Ranges and Los Angeles basin.

Elsevier online abstract