Deep Seismic Reflection Data Research Articles

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.

A method for avoiding artifacts in the migration of deep seismic reflection data

Conventional wave-equation-based migration of deep seismic reflection data can produce severe artifacts, which appear as broad circular arcs or “smiles”, due to the existence of apparent truncations of reflections on the stack section arising from poor signal penetration, changes in orientation of the acquisition profile, and the existence of strong overlying lateral velocity variations. These artifacts limit the interpretation of deep seismic profiles, because they obscure weak reflections and reflection truncations that may, e.g., indicate the presence of subsurface faults. Here I present a new migration algorithm, in which each sample of the stack is migrated to a short linear segment whose position and dip are determined by its original position on the stack, an estimate of the local apparent dip at that point, and a user-specified migration velocity. No subjective interpretation of reflections on the stack section is required, and the algorithm produces no arc-like migration artifacts. The degree of lateral smearing can be easily controlled, allowing reflection truncations to be revealed. In practice, the algorithm is most effectively applied to data that have been coherency-filtered to remove low amplitude noise, which would otherwise be preserved.

The crustal structure of the NW Moroccan continental margin from wide-angle and reflection seismic data

SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

High‐resolution seismic traveltime tomography incorporating static corrections applied to a till‐covered bedrock environment

A major obstacle in tomographic inversion is near‐surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first‐arrival picks from deep seismic reflection data acquired in the mid‐ late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time‐varying, near‐surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near‐surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magneticmap.

Crustal structures across the central Kaapvaal craton from deep-seismic reflection data

Eight deep-seismic reflection profiles (16 seconds TWT; generated by the vibroseis method) are presented and interpreted. The profiles cover a total length of 500km and are projected into a common section that strikes northwest to southeast across the central part of the Kaapvaal Craton, including the Colesburg Magnetic Lineament (CML). The profiles clearly image the crust-mantle transition (Moho) between 11 and 13 seconds TWT (~ 36 to 41km). The sections link some of the brittle Ventersdorp (~2.7 Ga) faults near surface to more subhorizontal ductile shear zones and related features that transect the upper crust of the Kaapvaal Craton. Sharp shallow-dipping reflectors that predate the Ventersdorp faults subdivide the entire crust into four elongate panels (or slivers; >200km by 10 to 20km). These panels extend from below Vryburg in the west, across the CML, to the crystalline basement below the Witwatersrand Basin in the east. The upper-most crustal reflectors dip towards the west and indicate extensive east-directed tectonic accretion and emplacement of a ~10km-thick allochthonous upper crustal panel comprising the Amalia-Kraaipan granite-greenstone terrane, across a Witwatersrand basement panel. The latter, in turn, is underlain by an elongate crustal panel that dips west from the surface near Welkom, reaching the lower crust below the centre of the section. The CML that subdivides the craton into an eastern block and a western block at surface is not a feature that can be easily linked to an underlying suture in the lower crust and mantle. Instead, this lineament has a shallow origin related either (1) to the 3.0 to 2.8 Ga Amalia-Kraaipan greenstone belt lithologies (that include extensive BIF, and/or (2) to possible thin back-thrust wedges of highly magnetic shales of the lower Witwatersrand sequences with (as previously suggested by Corner et al. , 1990) shallow east-dipping magnetite-rich basement granites. The entire crustal section resembles a tectonically stacked series of crustal slivers derived from the (north-) west that amalgamated in the Neoarchean (2.92 to 2.76 Ga) along east-propagating thrusts that cut up-section from middle to lower crust to reach the surface next to the upper Witwatersrand foreland basin. Distinct shallow dipping reflectors below the Moho suggest that the upper mantle was involved in this “shingling” tectonics ( c.f. Cook, 1986). Any related suture zone must have been positioned to the west of the present craton boundary. The lowermost crustal panel is probably composed of old TTG- basement (>3.2 Ga) that was extended and partially melted during the Ventersdorp event (~2.7 Ga), leaving a near-flat smooth and annealed Moho. A seismic profile across the southern margin of the Kaapvaal craton images the tectonic boundary with the Mesoproterozoic Natal Namaqua Mobile Belt (NNMB), and is consistent with existing geological and geochemical models in which the Kaapvaal craton is overthrust by the NNMB. The data suggest that cratonic crust and probably mantle lithosphere underlie a significant section of the NNMB beneath Natal and the Eastern Cape.

Geophysical evidence of a mantle derived intrusion in SW Iberia

Deep seismic reflection data acquired as part of the SW‐Iberia EUROPROBE project across the transpressional Variscan orogen sample three tectonic terranes: the South Portuguese Zone, the Ossa‐Morena Zone, and the Central Iberian Zone. The seismic data reveal the existence of a mid‐crustal reflective body 140 km long and of variable thickness (up to 5 km), the Iberian Reflective body. The conductivity image provided by coincident MT soundings, the amplitude characteristics of the seismics, mineralization studies related to magmatic ore deposits, and the surface geology suggest that the IRB is a mantle‐derived mafic intrusion. The geophysical, geological and petrological data suggest that the IRB is most probably an Early Carboniferous (approximately at 350–340 Ma) mantle‐derived intrusion possibly linked to plume activity that took place in Europe in the Carboniferous and Permian.

Recent tectonics in the Turkana Rift (North Kenya): an integrated approach from drainage network, satellite imagery and reflection seismic analyses

AbstractThe Turkana rifted zone in northern Kenya is a long‐lived and polyphased rift system where the lack of well‐marked rift morphology makes it difficult to identify the zone of active deformation. A high‐density river network is exceptionally well developed over the study area and shows evidence of drainage anomalies that suggest recent fault‐induced movements at various scales. Correlation of surface drainage anomalies with Landsat remote sensing and deep seismic reflection data permits to characterize the deep geometry of the inferred fault structures. Seismic stratigraphy further allows distinction between the inherited (Oligocene–Pliocene) and the newly formed (<3.7 Ma) origin of the recent deformation. Evidence for neotectonics are observed (1) along a large‐scale transverse (EW) fault rooted at depth along a steep basement discontinuity (Turkwell), (2) along a rift‐parallel (NS) fault zone probably emplaced during the Pliocene–Pleistocene and currently bounding the Napedet volcanic plateau to the west and (3) over a round‐shaped uplifted zone caused by positive inversion tectonics (Kalabata). The major contribution of this work is the recognition of a broad (80 km wide) zone of recent/active extensional deformation in the Turkana Rift in contrast with the narrow (20 km wide) N10°E‐trending axial trough forming the Suguta valley to the south, and the Chew Bahir faulted basin to the north. These along‐strike variations in structural style are partly controlled by the occurrence of rejuvenated Oligocene–Miocene rift faults and long‐lived transverse discontinuities in the Turkana Rift area. More generally, this study has implications for the use of river drainage network about recent/active extensional domains with subdued topography and slow deformation rate.

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.

Free water and seismic reflectivity in the lower continental crust

Many deep seismic reflection data sets show bright reflectivity filling much of the lower continental crust. Where these reflections are well developed, they have apparent reflection coefficients of at least 0.1. It has been proposed that this reflectivity could be explained by the existence of free water in the lower crust with stratified porosity or pore pressure, and that this free water might also explain observations of high electrical conductivity. If this explanation is correct then it follows that seismic observations of apparent heterogeneity in the lower crust reveal only heterogeneities in fluid distribution, and are not necessarily related to changes in rock type. That hypothesis is tested here. Water in the lower crust has a density and velocity comparable with liquid water at the Earth's surface. Numerical modelling of the seismic properties of rocks containing free water is used to delineate the range of porosity and pore geometry required to produce bright reflections. Porosities of several per cent or thin sheet-like pores with high crack density are required. Rocks in textural equilibrium do not contain thin pores and require porosities in excess of 5% to explain the reflection coefficients. The lower crust will compact, water will be expelled, and sharp boundaries in porosity and pore pressure will diffuse in geologically short times unless the water is distributed in isolated equi-dimensional pores. It is concluded that free water cannot explain the seismic reflections seen from the lower crust in stable areas, and that there are good geophysical and petrological reasons for supposing that the lower crust in the stable continental interior and on stable continental shelves typically contains little or no free water. The widespread observations of seismic reflectivity in the lower continental crust do indeed therefore imply significant heterogeneities in rock type.

THE GEOLOGICAL FEATURES OF LIGURIAN SEA - NORTHERN APENNINES - ADRIATIC FORELAND FROM CROP SEISMIC DATA

The structure and the geodynamics of the Northern Apennines, and of the Tyrrhenian and Balearic basins is reconstructed and discussed using deep seismic reflection data from the Italian CROP project. Three profiles cross the Northern Tyrrhenian Sea (CROP M12A), the Northern Apennines (CROP03) and the Adriatic Sea (CROP M16). Lines CROPM10 and M11 cross the northern Balearic Basin, the Ligurian Sea and the Corsica-Sardinia block while line CROP M12B crosses longitudinally the Corsica Basin. Profile interpretation allows imaging E-verging lithospheric-scale contractional structures superposing the Corsica-Sardinia block onto the western margin of Adria Plate (lines CROP M10, M11, M12B) and the Corsica Basin characteristically located above this collision zone. Interpretation of seismic profiles (CROP M10, M11, M12A) has also allowed identifying a culmination of the asthenosphere top in the northern Tyrrhenian Basin. This culmination strikes in a roughly NNW direction and above it nucleate most of the Tyrrhenian extensional structures. The oceanic crust, the architecture of the sedimentary fill and the geometry of the margins of the northern Balearic Basin are imaged in the western part of profiles CROP M10 and M11. Profiles interpretation supports that the Northern Apennines are characterised by a dominant compressional style expressed by a well developed thrust system and shear planes. Crustal extension generally assumed to affect the whole Tuscan Apennines area is instead mainly restricted to the Tyrrhenian Basin where major extensional features mostly reactivate early compressive lithospheric structures. Such a tectonic picture derived from deep seismic profile interpretation is in good agreement with the surface geological-structural data carried out in the hinterland basins and the axial thrust belt along line CROP03. Migration toward the east of compression resulted in major thrusts displacing the Moho beneath the Northern Apennines watershed. The deformation of this lithospheric-scale thrust system propagated far in the foreland and controlled the deposition and sedimentation in the easternmost Pliocene-Quaternary foredeep basin (line CROP M16). During this stage, the Tyrrhenian Basin has been developing in the innermost sectors of the chain.

Structure and development of a microcontinent: Elan Bank in the southern Indian Ocean

Microcontinents appear to commonly form on young continental margins close to hot spots, but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance. Thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue. Elan Bank, a large western salient of the Kerguelen Plateau, is a microcontinent that originally lay between India and Antarctica in Gondwana. Recent regional plate tectonic reconstructions suggest that during Gondwana breakup, Elan Bank and India initially separated from Antarctica, and Elan Bank became isolated in the Southern Ocean via a ridge jump to the north between Elan Bank and India. In Albian time (∼108 Ma), voluminous magmatism attributed to the Kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings. Recent ODP investigations, deep seismic reflection data, and a wide‐angle seismic line on Elan Bank allow us to gain the first insight into the feature's integrated crustal structure and geological evolution and the adjacent continent‐ocean transition zone. Our analysis shows that Elan Bank's crust is at least 16 km thick. The upper igneous crust consists of a 2–3 km thick layer with seismic velocities ranging from 4.4 to 5.9 km/s that can be interpreted as the result of accumulation of lava flows originating from the Kerguelen hot spot. Seismic velocities at the base of the crust are as low as 6.6 km/s, which is consistent with a fragment of thinned continental crust ∼14 km thick. A high velocity body, located at depths of 5 to 10 km, could be interpreted as plutonic rocks emplaced during the major regional magmatic episode. On the basis of deep seismic reflection data, we interpret extensional structures beneath the volcanic flows. In Albian time, when the area was affected by the Kerguelen hot spot, volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent‐ocean transition zone to the south, creating the appearance of flat, unstructured basement. The seismic structure and inferred composition of Elan Bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts.

Along‐strike variations of crustal reflectivity related to the Andean subduction process

This paper presents the results of Kirchhoff prestack depth migration applied to two onshore deep seismic reflection data sets (ANCORP'96 and PRECORP'95). The prestack depth migration method was implemented in 3D (ANCORP) and in 2D (PRECORP), respectively, from topography. Selective stacking of the migrated depth slices yielded a 3D depth volume (ANCORP) and a 2D depth section (PRECORP), respectively. An additional “offline stacking” provided a final 370 km long 2D depth section (ANCORP). In this paper a 180 km long depth section will be presented. At the beginning of the ANCORP section a double reflection zone is observed, which is associated with the upper and lower boundary of the oceanic crust. Except for some areas a nearly complete image of the Nazca reflector at depths between 60–90 km has been obtained. Moreover, one Bright Spot in each section has been detected: the Quebrada Blanca Bright Spot (QBBS, ANCORP) and the Calama Bright Spot (CBS, PRECORP). The reprocessing yielded two new aspects regarding these structures: Besides the west dip of the QBBS, which was already known from standard poststack processing results, a 3D analysis of the ANCORP data set shows an additional north‐dipping trend of the QBBS. Furthermore, the CBS is discovered for the first time.

Tsunamis in the Sea of Marmara: Historical documents for the past, models for the future

More than 30 tsunami events have impacted the coasts of the Sea of Marmara in the past two millennium, clustering in İzmit Bay, the shores of İstanbul, Gemlik Bay, the shores of the Kapıdağ and Gelibolu Peninsulas. With respect to the last well-known tsunami, the İzmit tsunami of 17 August, 1999, available field survey run-up data and marine surveys provide an opportunity to evaluate how these events were triggered. The main purpose of this study is to determine the slope failure potential as a possible tsunamigenic source in the Sea of Marmara by utilising multi-beam bathymetry, shallow and deep seismic reflection data. On the basis of the landslide geomorphology, the generation, propagation and coastal amplifications of tsunamis related to earthquake and slope failure scenarios were tested by using tsunami simulation model TWO_LAYER. The maximum water surface elevations near the shores along the north and south coasts are obtained according to the selected scenarios of tsunami generation by using available data.

Palaeozoic tectonic evolution of the Middle Urals in the light of the ESRU seismic experiments

Abstract: A new compilation of deep seismic reflection data, in combination with the results of recent geological and isotopic investigations, provides the basis for significantly refined structural and evolutionary models of the Middle Urals. Many of the major structural boundaries observed at the surface (e.g. Main Uralian Thrust, Main Uralian Normal Fault, Serov–Mauk Fault, Prianitchnikova Shear Zone) can be followed on the seismic reflection data to depths of 8–15 km. Only a few features can be traced through the horizontal to shallow dipping zones of middle-crustal reflectivity that extend beneath much of the Middle Urals. Exceptions might be the Deevo Thrust and a newly discovered deep band of moderately dipping reflections beneath the West Siberian Basin that may represent the remnants of an ancient subduction zone. The reflectivity of the lower crust and Moho are similar beneath the hinterland of the Urals and below the western part of the West Siberian Basin, suggesting that in both places they have the same extensional origin. Accretion of various exotic terranes to the eastern margin of Baltica during Late Palaeozoic time involved considerable shortening of the continental margin. The accreted terranes (e.g. Tagil, Petrokamensk and Alapaevsk Arc Complexes) consist mostly of island-arc material formed during Mid- and Late Palaeozoic time. Closely associated with the island arcs are several high-grade metamorphic complexes (e.g. Salda and Murzinka–Adui Complexes) that were exhumed during and after Late Palaeozoic convergence. Towards the end of the orogeny, several north–south-trending strike-slip faults (e.g. Sisert Fault) were active. After the orogeny, major periods of tectonism and magmatism in the Middle Urals resulted in the extrusion of the Siberian trap basalts, crustal stretching and/or underplating, and the formation of the West Siberian Basin, the planet’s largest intracontinental sedimentary basin. Finally, a period of renewed thrust faulting, often reactivating older faults, led to the inversion of the Mesozoic basins.

Crustal structure of the Ordovician Macquarie Arc, Eastern Lachlan Orogen, based on seismic‐reflection profiling

In the Eastern Lachlan Orogen, the mineralised Molong and Junee‐Narromine Volcanic Belts are two structural belts that once formed part of the Ordovician Macquarie Arc, but are now separated by younger Silurian‐Devonian strata as well as by Ordovician quartz‐rich turbidites. Interpretation of deep seismic reflection and refraction data across and along these belts provides answers to some of the key questions in understanding the evolution of the Eastern Lachlan Orogen—the relationship between coeval Ordovician volcanics and quartz‐rich turbidites, and the relationship between separate belts of Ordovician volcanics and the intervening strata. In particular, the data provide evidence for major thrust juxtaposition of the arc rocks and Ordovician quartz‐rich turbidites, with Wagga Belt rocks thrust eastward over the arc rocks of the Junee‐Narromine Volcanic Belt, and the Adaminaby Group thrust north over arc rocks in the southern part of the Molong Volcanic Belt. The seismic data also provide evidence for regional contraction, especially for crustal‐scale deformation in the western part of the Junee‐Narromine Volcanic Belt. The data further suggest that this belt and the Ordovician quartz‐rich turbidites to the east (Kirribilli Formation) were together thrust over ?Cambrian‐Ordovician rocks of the Jindalee Group and associated rocks along west‐dipping inferred faults that belong to a set that characterises the middle crust of the Eastern Lachlan Orogen. The Macquarie Arc was subsequently rifted apart in the Silurian‐Devonian, with Ordovician volcanics preserved under the younger troughs and shelves (e.g. Hill End Trough). The Molong Volcanic Belt, in particular, was reworked by major down‐to‐the‐east normal faults that were thrust‐reactivated with younger‐on‐older geometries in the late Early ‐ Middle Devonian and again in the Carboniferous.

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

Crustal architecture of the southern Uralides from true amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis profile

True amplitude processing of the Urals Seismic Experiment and Integrated Studies (URSEIS) vibroseis deep reflection seismic data acquired by the URSEIS consortium shows the southern Uralide crust to be composed of four major blocks with distinctive reflection characteristics. These blocks are juxtaposed along crustal‐scale boundaries. The foreland thrust and fold belt, developed from the East European craton crust, is imaged as subhorizontal to east dipping reflectivity that can be related to its Paleozoic and older tectonic history. The Moho beneath the foreland thrust and fold belt is not imaged in the vibroseis data set. The Main Uralian fault (the major arc‐continent suture) is unreflective, but its subsurface location can be inferred by the truncation of the reflection pattern of the East European craton and its contrast with that of the Magnitogorsk arc. The Magnitogorsk arc reflectivity is characterized by patchy, noncoherent to coherent reflections in the upper ∼10 – 15 km that are interpreted to be related to the arc volcanic rocks. Below this, reflectivity is diffuse, or the arc crust is transparent, and the Moho is not imaged. The East Magnitogorsk fault zone, which juxtaposes the arc against the East Uralian zone, is not imaged by the data. The upper 5 to 6 km of the East Uralian zone, corresponding to the Dzhabyk granite, is transparent. Below the granite the crust is characterised by east dipping patches of moderately coherent, high‐amplitude reflections that in the east become shallowly west‐dipping. A ∼10 km thick, west dipping band of coherent, high amplitude reflections between 12 and 35 km depth, corresponding to the Kartaly Reflection Sequence, extends beneath almost the entire East Uralian zone. The crust beneath the easternmost East Uralian zone reaches 53 km in thickness. The upper and middle crust of the Trans‐Uralian zone is characterized by a series of east and west dipping, concave upward, moderately coherent, high‐amplitude reflections. The lowermost middle and lower crust displays thin bands of west dipping, coherent, low‐amplitude reflectivity. The Moho is imaged as a sharp transition from reflective lower crust to transparent upper mantle at ∼49 km depth, and the lower crustal reflections appear to merge with it. The URSEIS vibroseis data are integrated with results from the explosion source reflection data, the wide‐angle reflection data, and the surface geology to place constraints on the crustal architecture of the orogen and on the timing of its assemblage.

Amplitude effects due to multiscale impedance contrasts and multiple scattering: implications for Ivrea-type continental lower crust

SUMMARY Studying seismic wave propagation in non-deterministic media is interesting as some parts of the Earth (e.g. in the continental lower crust) may be amenable to a stochastic description. In this paper we study the acoustic response of a 1-D binary stochastic fractal medium and separate the single- from the multiple-scattered wavefield. We show that, for our models with scales that (partly) overlap the seismic bandwidth, amplitude changes occur due to both the presence of multiple scattering and the multiscale nature of the impedance contrasts. We study these changes as functions of sc (a measure of the nominal bimodal reflection coefficient), ka (a measure of the relative length scales of the medium compared to the wavelength of the probing waves) and c, the scatterer density. As our models are binary (two lithologies), the results should be interpreted as an upper bound for the general class of acoustic 1-D stochastic fractal model responses. We present a measure to quantify the interference of single- and multiple-scattered wavefields, and use this measure to study the importance of the three aforementioned dependences. Since stochastic fractal media have been proposed for Ivrea-type lower crust, we relate our results to such a model and find that amplitudes may increase by up to 45t15 per cent. The contribution of multiple scattering to this increase ranges from 1t 1t o 20t15 per cent, the main contribution being due to the multiscale nature of the impedance contrasts. We claim that the parameters used to model Ivrea-type lower crust (sc=5 per cent, c#5r10 x2 ,1 jkaj24 and m=9.6r10 x1 ) are robust, although we recognize that the scatterer density c is determined by the vertical sampling Dz in our models. The heuristic argument that extra geometrical spreading in two/three dimensions will further diminish the importance of multiple-scattering effects, combined with our results, makes it tempting to claim that multiple scattering is not important for 2/3-D generalizations of our 1-D models. However, in the light of indirect evidence for the importance of multiple scattering in Ivrea-type lower crust reported by other authors, we argue that elasticity (P‐S conversions) may be important. Therefore, we believe that it is necessary to study the 2-D case with a similar direct approach to that presented in this paper before a satisfactory conclusion can be drawn about the validity of the standard processing of deep seismic reflection data and the importance of multiple scattering.

Adaptive filtering of random noise in 2-D geophysical data

Random noise is often a problem in geophysical data visualization because it obscures fine details and complicates identification of image features. Adaptive filters have recently been used to suppress speckle (random) noise in synthetic aperture radar (SAR) images. SAR data are similar to seismic reflection data, both in their data acquisition approach and in their final data processed format. The nature of the random noise associated is also very similar, and adaptive filters can be applied to reduce random noise in both types of data sets. In this paper several popular adaptive filters—the Lee filter, the Frost filter, and the Kuan filter, which have been used frequently for speckle reduction in SAR data—are tested on Lithoprobe (AGT) deep seismic reflection data and on one set of oil industry shallow seismic reflection data. In addition, a standard band‐pass filter, which is common in many seismic data processing packages, is tested with the oil industry test data. Performance of the adaptive filters is also tested on Radarsat SAR data. The random (speckle) noise in both the Lithoprobe and the Radarsat (SAR) data sets is statistically very similar, and the adaptive filters tested successfully suppressed random noise while minimizing blurring. Among the tested filters the enhanced Lee filter performed best, closely followed by the enhanced Frost filter and the Kuan filter. The background noise in the oil industry seismic data is statistically quite different from the above two data sets; the results obtained were less than satifactory, although they were still encouraging. With the oil industry data, the enhanced Frost and Kuan filters performed better than the enhanced Lee filter. The commonly used band‐pass filter successfully removed the background (random) noise, but it also suppressed the reflection events, making the final result less desirable.

Europrobe seismic reflection profiling across the eastern Middle Urals and West Siberian Basin

New deep seismic reflection data provide images of the crust and uppermost mantle underlying the eastern Middle Urals and adjacent West Siberian Basin. Distinct truncations of reflections delineate the late‐orogenic strike‐slip Sisert Fault extending vertically to ∼28 km depth, and two gently E‐dipping reflection zones, traceable to 15–18 km depth, probably represent normal faults associated with the opening of the West Siberian Basin. A possible remnant Palaeozoic subduction zone in the lower crust under the West Siberian Basin is visible as a gently SW‐dipping zone of pronounced reflectivity truncated by the Moho. Continuity of shallow to intermediate‐depth reflections suggest that Palaeozoic accreted island‐arc terranes and overlying molasse sequences exposed in the hinterland of the Urals form the basement for Triassic and younger deposits in the West Siberian Basin. A highly reflective lower crust overlies a transparent mantle at about 43 km depth along the entire 100 km long seismic reflection section, suggesting that the lower crust and Moho below the eastern Middle Urals and West Siberian Basin have the same origin.