Deep Seismic Reflection Profiles Research Articles

Seismic images of structural variations along the deformation front of the Andaman–Sumatra subduction zone: Implications for rupture propagation and tsunamigenesis

Seven deep seismic reflection profiles cover the 3000 km-long subduction system from Andaman to Southern Sumatra, including zones that ruptured in 2004, 2007, and 2010. We find that (1) the frontal zone is characterized by a series of thrusts bounding folded blocks of sediments with preserved layering, showing a northward transition from dominantly seaward vergence of the frontal thrusts to dominantly landward vergence of the frontal thrusts, (2) the accretionary wedge is characterized by poor reflection of the seismic energy likely to be due to a high degree of faulting and compaction of the sediments, and (3) the oceanic crust is highly disturbed by faults and topographic reliefs along most of the margin. Landward vergence at the deformation front is associated with a thick incoming sediment section. The segment of the subduction zone where landward vergence is observed corresponds to an area with high near-trench slip during the December 2004 earthquake, the main tsunami source, and lies just west of the hypocenters of several intraplate events (Mw>7) in the years following the 2004 event.

Imaging the crustal structure beneath the eastern Tibetan Plateau and implications for the uplift of the Longmen Shan range

Competing models have been proposed to account for the unusual uplift of the Longmen Shan fault zone in the eastern Tibetan Plateau and resulting hazards. However, due to the terrain and thick sedimentary cover, the crustal structure of the eastern Tibetan Plateau remains uncertain. Therefore, testing these models has been difficult. In 2011, a 310 km long, SE-trending SinoProbe-02 deep seismic reflection profile was recorded in easternmost Tibet in order to study its regional crustal structure. The resulting image revealed the detailed crustal structure of eastern Tibet, which when combined with geological, GPS (Global Positioning System), and geochemical evidence, strongly suggests that Yangtze sub-continent crust extends beneath the region. The seismic profile also images the tremendously thick Triassic sedimentary cover in the Songpan-Ganzi terrane (SGT). These Triassic sediments vary considerably in thickness across several crustal blocks. In addition, both the Longriba fault zone of the northeastern SGT and the Longmen Shan fault zone show strong intracrustal reflections that terminate at a depth coinciding largely with the crust–mantle boundary (Moho). Accordingly, we propose a new tectonic model based on an integrated analysis of this seismic reflection profile and previous GPS measurements. In this model, crustal-scale deformation is suggested to have participated in the oblique extrusion and uplift of the easternmost edge of the Tibetan Plateau along the Longmen Shan. In a broader context, the lithospheric configuration imaged by the seismic reflection profile will advance our understanding of the tectonic response of the eastern Tibetan Plateau to the ongoing India–Eurasia collision and has new implications for the estimating seismic risk in the region.

New constraints on crustal structure and Moho topography in Central Tibet revealed by SinoProbe deep seismic reflection profiling

Abstract From October 2009 to May 2010, through the support of the SinoProbe project a series of deep seismic reflection profiles were conducted. In order to image deep crustal structure of central Tibet, these profiles extend from the northern Lhasa terrane to the Qiangtang terrane by crossing the Bangong–Nujiang suture (BNS). The Moho depth varies from 75.1 km beneath the northmost Lhasa terrane to 68.9 km beneath southmost Qiangtang terrane. There is a 6.2 km sharp Moho offset across the BNS. Within 25 km to the north of the BNS, the Moho rises smoothly to 62.6 km. Distinct Moho reflector lies at 62.6–67.3 km beneath the Qiangtang terrane. The boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from INDEPTH velocity model correspond to the strong reflection horizons at about18.8 km and 31.3 km depth beneath south Qiangtang terrane, respectively. A north-dipping series of reflection packages in the mid- to lower crust may mark subduction of the Lhasa terrane beneath the Qiantang terrane. The central portion of the reflection profile exhibits an antiformal structure at 14.1 km in the upper crust, which corresponds with the blueschist-bearing metamorphic belt.

Deep seismic reflection profile in Central Europe reveals complex pattern of Paleozoic and Alpine accretion at the East European Craton margin

AbstractThe first deep reflection seismic profile was acquired across the Teisseyre‐Tornquist Zone (TTZ), a major European suture zone, southeast of the Avalonia‐Baltica collision zone, portraying different tectonic deformation styles (thick‐ versus thin‐skinned) and Phanerozoic crustal accretion episodes over a relatively short distance (240 km). The overall crustal structure preserves image of the Neoproterozoic passive margin setting but with a significant overprint of the repetitive tectonic convergence (Cadomian to Alpine) acting at the Baltica margin. In contrast to the Avalonia‐Baltica collision zone, here the polygenetic crustal accretion occurring at the cratonic margin was ultimately shaped by significant strike‐slip faulting. TTZ is imaged as a narrow (10–15 km) transform zone separating the thinned East European cratonic crust from the Małopolska Block.

Active blind thrusts beneath the Tokyo metropolitan area: Seismic hazards and inversion tectonics

AbstractThe Tokyo metropolitan area, underlain by Neogene and Quaternary sediments more than 5 km thick, is currently deformed by blind thrusts that could generate hazardous earthquakes. However, their little geomorphic expression and dense urbanization make understanding of folds produced above them and recent deformation highly elusive. Here we show subsurface geometries of several active blind thrusts beneath this highly urbanized area, based on tectonic landforms, high‐resolution seismic reflection data, and Quaternary stratigraphy. Deep seismic reflection profiles corroborate the notion that steeply dipping blind thrusts are reactivated normal faults originally formed by middle Miocene extensional tectonics. Despite very slow (less than 0.1 mm/yr) late Quaternary slip rates, our work suggests the presence of previously unrecognized faults that pose seismic hazards to Tokyo and outlying communities, highlighting the need for additional information to define recent slip rates, magnitude, and recurrence of past earthquakes on them.

Fine crustal structure beneath the junction of the southwest Tian Shan and Tarim Basin, NW China

The geometry of the entire crust from the northern part of the Tarim Basin to the southwestern Tian Shan east of Kashi is imaged on a N-S–directed explosive-source deep seismic-reflection profile. The profile reflects the sedimentary formations in the northern part of the Tarim Basin and the fold-and-thrust belt of the southern Tian Shan. N-dipping reflectors of the lower crust, as well as fluctuations in Moho depth, below which several mantle reflectors were observed, reveal the fine crustal structure beneath the junction of the southwest Tian Shan and the Tarim Basin. Mesozoic–Cenozoic shortening of the southwestern Tian Shan occurred at a crustal scale involving detachment-related folding in the basin directed northward toward the mountains and reverse faulting in the mountains directed toward the basin. In addition, a crocodile fabric developed within the lower crust beneath the basin area. The lithospheric structure revealed by the seismic-reflection section between the Tarim Basin and the Tian Shan Mountains reflects a process of intracontinental collision.

Crustal structure of the northern margin of the North China Craton and adjacent region from SinoProbe02 North China seismic WAR/R experiment

Abstract The SinoProbe02 North China seismic experiment consisted of three seismic recording efforts along a 450 km long profile. The profile extended from northwest of Beijing, across the northern margin of the North China Craton (NCC), and across the southeastern portion of the Central Asian Orogenic Belt (CAOB) to the Solonker suture zone. We processed, analyzed, and modeled the data collected during a wide-angle reflection and refraction effort. The recording of seismic waves from 8 explosions was conducted in 4 deployments of 300 Texan recorders with station spacings of 1–1.5 km. The P-wave field provided good quality data for most of the profile. Arrivals of refracted and reflected waves from sediments and basement (Pg), intracrustal phases (PcP, PlP), and the Moho (PmP, Pn) were typically observed. Travel time modeling was done layer-by-layer using the top to bottom approach and was constrained by a coincident deep reflection seismic profile. The initial velocity model was altered by trial and error, and the forward model was updated by damped least-squares inversion. In our modeling, calculated travel times fit observed arrivals for all 1439 traces with an average RMS error of 0.113 s. The final 2-D P-wave velocity model contains many features of tectonic significance: (1) the velocity structure varies significantly from the NCC to the CAOB; (2) thicker crust appears beneath the Yinshan–Yanshan belt and was probably generated by compression in the early Jurassic and modified during the craton's destabilization and extension; the flat and relatively shallow Moho in the CAOB may be attributed to the extension; (3) strong velocity variations beneath the Bainaimiao arc and Ondor Sum subduction accretion complex indicate that multiple pulses of magmatism occurred during the complex tectonic evolution of this area.

Geodynamic significance of the boundary between the Thomson Orogen and the Lachlan Orogen, northwestern New South Wales and implications for Tasmanide tectonics

Interpretation of deep seismic reflection profiling, coupled with forward modelling of gravity and aeromagnetic data, new zircon U–Pb dating and the interpretation of the basement geology beneath the southern margin of the Eromanga Basin, has provided insights into the southern part of the underlying Thomson Orogen and its relationship with the Lachlan Orogen to the south. Our interpretations of these data suggest that the northern Lachlan and southern Thomson orogens had a shared history from the mid-Silurian to the Carboniferous. Major older differences, however, are suggested by the presence in the southern Thomson Orogen of: (i) a possible Neoproterozoic arc, (ii) latest Cambrian to earliest Ordovician turbidites, (iii) Late Ordovician turbidites, and (iv) geophysical evidence for thrusting of reflective ocean crust rocks high into the crust on a north-dipping detachment. The seismically imaged, north-dipping, crustal-scale Olepoloko Fault corresponds to the ‘surface expression’ of the Thomson–Lachlan boundary. We speculate that it reflects the partial reactivation and short-cutting of an older fault in the post-Devonian (?Carboniferous) and probably also in the latest Silurian and Early Devonian. Comparisons with the seismic architecture of the Lachlan Orogen immediately to the south, and with the central part of the Thomson Orogen ∼450 and 650 km to the north, suggest that the part of the Thomson Orogen west of the Quilpie Trough and the Nebine Ridge developed on inferred Neoproterozoic to Cambrian oceanic crust that floors the Barcoo Basin. This basin separated the continental margin at that time on the west from a sliver of continental crust preserved at Anakie on the east that was overlain by one or more, poorly dated, passive margin sedimentary ± volcanic sequences that predate a 500 Ma deformation. The southern margin of the Thomson Orogen also contains a sliver of old continental crust, sandwiched between the southern strike-slip margin of the Barcoo Basin to the north and the open proto-Pacific ocean to the south. It was locally the site of ca 580 Ma subduction, because seafloor spreading to the south lay oblique to the orogen margin. We suggest that the Thomson Orogen and Lachlan Orogen were amalgamated by the late Middle Ordovician, although the Thomson–Lachlan boundary remained a zone of weakness at least until the Triassic.

Seismic images of the continental Moho of the Indian shield

Abstract Deep seismic reflection profiling has imaged different patterns of the Moho across the Indian shield with a variety of tectonic environments from Archean to Recent. The character of the Moho varies from a discrete strong event, the base of strong coherent lower crustal sub-horizontal reflections, the base of dipping lower crustal reflections into mantle, to no clear reflection boundary. The seismic reflection data suggest a laminated lower crust in several places and offsets in the Moho at others. Kinematic and dynamic modeling of wide-angle reflection data across the Mesoproterozoic South Delhi Fold Belt and the Central Indian Tectonic zone suggests a laminated lower crust for these regions. In general, the lower crust of the Indian shield is heterogeneous. A clear Moho is identified in some of the Precambrian orogenic belts and sedimentary basins along with reflective lower-crust, whereas the cratonic areas exhibit a diffused Moho. Post-collisional extensional process, such as orogenic collapse, delamination, magmatic intrusions, low-viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger Moho. The termination of lower-crustal reflectivity at the Moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle.

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

Abstract The tectonosedimentary development of the South Atlantic is compared with the Central Atlantic margins, which are associated with major episodes of magmatism during the Mesozoic continental break-up. Subsequently, the Cenozoic break-up in the North Atlantic led to the formation of the volcanic Norwegian–Greenland conjugate margins. The DSDP boreholes in the magma-poor Iberian–Newfoundland margins have confirmed the occurrence of exhumed mantle at the ocean–continent transition. This possibility has been suggested for the South Atlantic margins, but still lacks confirmation from drilling. The Red Sea and the Gulf of Aden may be considered as natural laboratories to study the break-up processes and formation of divergent continental margins. Using key geological and geophysical data, we compare some of the structures observed in incipient stages of basin formation between the African and the Arabian plates with the structures observed in older sedimentary basins associated with the Gondwana break-up. We also analyse deep seismic reflection profiles and potential field data at the continent–ocean boundary of these conjugate margins, using palinspastic reconstructions to define the corresponding seismic pairs. We conclude that the Red Sea and the Gulf of Aden display remarkable differences to the Iberian–Newfoundland margins, and notable similarities with the South Atlantic margins.

3-D structure of the Australian lithosphere from evolving seismic datasets

During the last 20 years, seismic tomography has frequently been used to provide information on the structure of the lithosphere beneath the Australian continent. New tomographic models are presented using two complementary seismological techniques in order to illustrate the current state-of-knowledge. Surface wave tomography is the ideal method to obtain information of velocity variations across the whole continent. The latest models use data from over 13 000 source–receiver paths, allowing a higher resolution than in previous studies using the same technique. In Western Australia the results at 100 km depth clearly reveal the contrast in structure between the Pilbara and Yilgarn Cratons and the Capricorn Orogen. At greater depths, the Kimberley Block has a distinct fast velocity anomaly in comparison with the surrounding mobile belts. In the east of the continent, strong horizontal gradients in velocity indicate transitions in lithospheric structure, although the new high resolution models reveal a complexity in the transitions through central Victoria and New South Wales. Complementing the surface wave tomography, we also present the results from the inversion of over 25 000 relative arrival times from body wave phases recorded in southeast Australia and Tasmania. The body wave tomography uses the surface wave model to provide information on long-wavelength structure and absolute velocities that would otherwise be lost. The new results indicate a distinct boundary between the Delamerian and Lachlan orogens within the upper mantle, the location of which is consistent with an east-dipping Moyston Fault, as observed by deep seismic reflection profiling. The new models also confirm a distinct region of fast velocities beneath the central sub province of the Lachlan Orogen. A significant new observation is that the inferred eastern edge of this central sub-province has a strong correlation with the location of copper/gold deposits; a similar relationship is observed at a larger scale in Western Australia where mineral deposits appear to flank the regions of fastest velocity within the West Australian Craton.

Crustal architecture and geodynamics of North Queensland, Australia: Insights from deep seismic reflection profiling

A deep crustal seismic reflection and magnetotelluric survey, conducted in 2007, established the architecture and geodynamic framework of north Queensland, Australia. Results based on the interpretation of the deep seismic data include the discovery of a major, west-dipping, Paleoproterozoic (or older) crustal boundary, considered to be an ancient suture zone, separating relatively nonreflective, thick crust of the Mount Isa Province from thinner, two layered crust to the east. Farther to the east, a second major crustal boundary also dips west or southwest, offsetting the Moho and extending below it, and is interpreted as a fossil subduction zone. Across the region, the lower crust is mostly highly reflective and is subdivided into three mappable seismic provinces, but they have not been tracked to the surface. In the east, the Greenvale and Charters Towers Provinces, part of the Thomson Orogen, have been mapped on the surface as two discrete provinces, but the seismic interpretation raises the possibility that these two provinces are continuous in the subsurface, and also extend northwards to beneath the Hodgkinson Province, originally forming part of an extensive Neoproterozoic–Cambrian passive margin. Continuation of the Thomson Orogen at depth beneath the Hodgkinson and Broken River Provinces suggests that these provinces (which formed in an oceanic environment, possibly as an accretionary wedge at a convergent margin) have been thrust westwards onto the older continental passive margin. The Tasman Line, originally defined to represent the eastern limit of Precambrian rocks in Australia, has a complicated geometry in three dimensions, which is related to regional deformational events during the Paleozoic. Overall, the seismic data show evidence for a continental margin with a long history (Paleoproterozoic to early Mesozoic) but showing only limited outward growth by crustal accretion, because of a repeated history of overthrust shortening during repeated phases of orogenesis.

P-receiver functions of anisotropic continental crust: a hierarchic catalogue of crustal models and azimuthal waveform patterns

Azimuthal P-receiver functions are influenced by crustal anisotropy and heterogeneity. To study these effects we created a catalogue of 30 crustal models and corresponding azimuthal P-receiver functions. The crustal models are grouped in five classes of anisotropic and isotropic horizontal and dipping plane layers. The model design was motivated by geological structure found by deep seismic reflection profiling and deep drilling. In the synthetic seismogram catalogue, we identified 14 azimuthal waveform features that could be used to narrow the range of possible crustal model for seismic inversion or interactive forward modelling. Based on a hierarchical classification of these waveform features 80 per cent of the input models could be uniquely identified. In case of field data the hierarchical classification can be used to find a starting model for forward modelling or non-linear inversion. To test the applicability of the method we applied it to records of four permanent stations of the German Regional Seismic Network. In two cases the procedure led to a satisfactory agreement of observed and modelled azimuthal data in both R and T components. The obtained subsurface models are plausible because they correspond to anisotropy values and structures found for similar settings by deep drilling and deep seismic profiling. In the other two cases only partial agreement of real and synthetic data could be obtained. These stations are located in a complicated tectonic setting that cannot be modelled satisfactorily by an approach based on plane dipping layers.

A mid-crustal strain-transfer model for continental deformation: A new perspective from high-resolution deep seismic-reflection profiling across NE Tibet

Understanding why continental deformation departs from the theory of plate tectonics requires a detailed knowledge of three-dimensional structures at a lithospheric scale. In Tibet, the end-member models of continental deformation make distinctively different predictions on strain distribution and contrasting structural geometry as a function of depth. Specifically, the thin-viscous-sheet model predicts vertically coherent deformation while channel-flow and continental-subduction models predict the presence of sub-horizontal detachment zones within or at the base of the Tibetan crust during the Cenozoic deformation. To differentiate the above models, we conducted a high-resolution seismic-reflection survey across the active left-slip Kunlun fault and its nearby contractional structures. The results of this work show that the actively deforming middle Tibetan crust is dominated by discrete sub-horizontal simple-shear zones that terminate the sub-vertical, left-slip Kunlun fault above and mantle-cutting thrusts below. The flat shear zones appear to act as roof and floor thrusts of large duplex structures that transfer shortening strain from locally deformed and coupled lower crust and mantle lithosphere below to the high-strain domains of the upper crust above. The middle-crustal strain-transfer model proposed here implies that the weak Tibetan middle crust may not be active everywhere during the Indo-Asian collision. It also predicts that the kinematics of the activated portions of the middle crust, whether being deformed by simple shear or channel-flow deformation, may vary from place to place, depending strongly on the lateral variation of mechanical strength at different depths of the lithosphere. Our approach of establishing the kinematics of middle-crust deformation departs significantly from the early work that emphasizes exclusively the role of vertically varying rheology in controlling the mode of continental deformation.

Imaging the Deep Structure of the Central DeathValley Basin Using Receiver Function, Gravity,and Magnetic Data

We use receiver function, gravity, and magnetic data to image the deep structures of central Death Valley. Receiver function analysis suggests the Moho is 24 km deep in the central part of the basin and deepens to 33 km to the east and 31 km to the west. The estimated lower crustal density is 2900 kg/m3, which suggests a gabbroic composition, whereas the upper crustal density, excluding basin sediments, is estimated to average 2690 kg/m3 or approximately a quartzofeldspathic composition. We modeled the magnetic sources as upper crustal to suggest a relatively shallow Curie depth in this region of high heat flow. We developed models to test the hypothesis that a low-density, non-magnetic body (magma or fluid-rich material?) within the lower crust at a depth of 15 km could coincide with the location of the Death Valley bright spot imaged on a deep seismic reflection profile. Those models suggest that if there is a low density region in the mid to lower crust in the area of the bright spot, then the region is also likely to be underplated by mafic or ultramafic materials which may have contributed to heating, uplift, and thinning of the crust during extension.

Reanalysis of the COCORP Utah Line 1 deep seismic reflection profile: Toward an improved understanding of the Sevier Desert detachment question

The Sevier Desert detachment (or reflection, SDR), which underlies the Sevier Desert basin along the eastern margin of the USA Basin and Range, is commonly cited as a type example of a low-angle normal fault (LANF) in continental crust. We present the results of reanalyzing the SDR on the COCORP (Consortium for Continental Reflection Profiling) deep seismic profile crossing the Sevier Desert basin (Utah Line 1). We employ a strategy of showing how shallow crustal velocity models, derived from first-break analysis of the shot records, may be used to reduce the effect of lateral velocity variations on imaging the SDR. Our results imply that the irregularities and discontinuities along the SDR are likely caused by overlying lateral velocity variations. The reprocessed versions of the section reveal a smoother, simpler, and more continuous SDR, lacking most of the apparent large offsets and structural variations on the currently available version of the profile. Seismic attribute and structural analyses of the reflection indicate significant variations along the profile, which are likely related in part to the field acquisition, but may also suggest lateral variations in the physical origin of the SDR. Such lateral variations may be consistent with previous studies that challenge the LANF interpretation of the SDR on the basis of attributing different origins to different parts of the reflector; however, a smoother and more continuous SDR points to a tectonically uniform origin and is thus interpreted to be more consistent with a LANF explanation.

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event.

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program)

We present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning.

Mapping of crustal scale tectonic boundaries in the Ossa-Morena Zone using reprocessed IBERSEIS reflection seismic data

The IBERSEIS deep seismic reflection profile imaged crustal scale structures in the SW Iberian Variscan belt, crossing the South Portuguese Zone, the Ossa-Morena Zone and the Central Iberian Zone in Spain. Two subsets of the profile, corresponding to the South Portuguese Zone–Ossa-Morena Zone and the Ossa-Morena Zone–Central Iberian Zone tectonic contacts, have been reprocessed with the aim of investigating the influence of cross-dip and to better image steeply dipping features. Alternative strategies for binning midpoints into common depth point (CDP) bins using different azimuths were examined for synthetic data. We show that the choice of the CDP-processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip-moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile. Multi-azimuth binning, normal moveout/dip-moveout, and migration velocity analysis on synthetic and real data show the presence of clear sub-vertical upper crustal structures near the South Portuguese Zone–Ossa-Morena Zone suture, the Aroche fault. This sub-vertical reflectivity that was not imaged earlier, projects into a location in the lower crust with low reflectivity.