Deep Seismic Reflection Profiles Research Articles

Structural geometry of the Urumqi foreland thrust system, northern Tian Shan: Insights into the seismotectonics of the 2013 M 5.1 Urumqi earthquake and deformation pattern in the Urumqi area

In foreland thrust systems, the complex structural geometry creates greater difficulties in constraining the seismotectonics of moderate magnitude earthquakes without pronounced surface ruptures and in assessing future seismic risk. The 2013 M 5.1 Urumqi earthquake was a moderate event that occurred in the Urumqi foreland thrust system. Based on surface investigations, geological mapping, and interpretations of deep seismic reflection profiles, we suggest that the Urumqi foreland thrust belt is a typical active wedge thrust system. Two connected fault segments of the gently south-dipping blind fault ramp and the north-dipping Xishan back-thrust fault merge at a depth of ~11 km and bound the wedge-shaped fault block. Analyses of the focal mechanism and aftershock distribution indicate that the seismogenic fault responsible for the M 5.1 Urumqi event is the south-dipping Xishan fault ramp with a dip angle of ~10–30°, which highlights the potential seismic hazards of the unruptured north-dipping Xishan back-thrust ramp. The four fault branches on the hanging wall of the Xishan fault are interpreted as flexural-slip thrusts. Our results indicate that multiple surface ruptures over a 6 km-wide area may form if a large earthquake occurs along the Xishan fault system.

North-dipping relict subduction of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane: Study of two parallel deep seismic profiles

The final closure of the Paleo-Asian Ocean (PAO) and its tectonic characteristics have been debated for several decades owing to a lack of high-resolution information on the lithosphere structure. Scholars have been attempting to explain deep tectonic evolutionary processes while studying continental growth at the southern margin of the Mongolian Terrane. In a bid to provide a new interpretation of the deep structure with a higher resolution, we study two reprocessed deep seismic reflection profiles. We studied the northern part (210 km long) of the 630-km-long deep seismic reflection profile extending across the North China Craton (NCC) margin to the northern Sino-Mongolia border in the west; and a parallel profile (80 km long) in the east near the Sino-Mongolia border. Both profiles are characterized by consistently north-dipping layered reflections projecting from the lower crust to the upper mantle, with an estimated thickness of 3.6 to 6 km between adjacent reflections beneath the Uliastai and Hegenshan belts. Arched reflections are observed in the middle and lower crust; these may have been caused by later magmatic activities. In addition, the Moho reflection is observed to be fairly continuous and flat in most parts of these two profiles. The layered lower crust reflections and mantle reflections serve as important evidence that northward subduction occurred during the closure of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane. We propose a detailed model of the evolutionary processes from the early Paleozoic to early Mesozoic. The proposed model explains how these deep reflections were formed.

The India‐Eurasia convergence system: Late Oligocene to early Miocene passive roof thrusting driven by deep‐rooted duplex stacking

• The India-Eurasia convergence zone hosts a set of post-Eocene tectonic events. • Enhanced duplex stacking from continuous indian underthrusting. • Passive roof thrust system promoted the tectonic exchange. The Late Oligocene-Early Miocene epoch represents a critical stage in the collisional history of the India-Eurasia dominant collision zone. Within this timeframe, a set of tectonic events occurred simultaneously or progressively. However, the key responsible for triggering these nearly coeval events remains unclear. This study used an integrated analysis of both geological and geophysical data to document the tectonic interactions throughout the Himalayan Orogenic Belt from west to east. Deep seismic reflection profiles outline crustal geometry of a decoupled Indian subduction front, and the overlying sheets over the Main Himalayan Thrust (MHT) is evidenced with a sequence of detachment-associated ramp-anticlines. Meanwhile, regional magnetotelluric (MT) profiles document rheologic connections in the form of a high-conductivity anomaly running top-to-the south between the southernmost Tibet and the areas beyond the Yarlung-Zangbo Suture Zone to the south. The overall architecture provides a complete picture of the complex deformation pattern beneath the tectonic convergence system. Together with previous studies in surface geological investigations, we propose that the enhanced duplex stacking of the underthrusting Indian crust increased crustal shortening of the convergence system. The consequent sudden exposure of the northern Himalayan domes released the accumulated stress to trigger the onset of a south-dipping passive roof thrusting through the convergence system to the southernmost Tibetan Plateau. Recognition of this exchange pattern from crustal duplex stacking to passive roof thrusting replenished an understanding of the tectonic interactions of the ongoing India-Eurasia collision.

Forward Modelling of Bouguer Anomalies along a transect of the Southern Apennines and the Southern Tyrrhenian Sea (Italy)

In the present study, we perform a gravity modelling at crustal scale along the trace of the CROP-04 (on-shore) and M-6B (off-shore) deep seismic reflection profiles, crossing the Southern Apennines and the Southern Tyrrhenian Sea (Italy). Along the 321 km-long modelled profile, we investigate the crustal-scale sources for the observed gravity anomalies through a simplified model of the crust and upper mantle across both onshore and offshore areas.After a compelling review of the published Moho geometries in the area, that were retrieved from either active or passive seismic methods, we test them in the observed gravity field through forward modelling of the Bouguer gravity anomalies. The comparison between the different Moho interpretations highlights the major contributors to the observed Bouguer gravity at the crustal scale, defining a set of starting values of these parameters for our final model.The proposed model locates the westward flexure of the Adriatic Moho, mimicking the subduction of the Adriatic lithosphere beneath the Peri-Tyrrhenian block and locates the step between the western (Tyrrhenian) and the eastern (Adriatic) Moho beneath the Apennines range providing a valuable geometrical and compositional model at the crustal scale. The model depicts a typical oceanic-to-continental crust transition in the Tyrrhenian domain and represents a solid starting base for further detailed modelling across the area.

Constraining Crustal Properties With Bayesian Joint Inversion of Vertical and Radial Teleseismic P‐Wave Coda Autocorrelations

AbstractThe sensitivity of seismic compressional and shear waves and their velocity ratios to rock lithology, pore fluids, and high‐temperature materials makes these parameters very useful for constraining the physical state of the crust. In this study, we develop a joint inversion approach utilizing both radial and vertical components’ autocorrelations of teleseismic P‐wave coda for imaging the crust by simultaneously characterizing the crustal , , and ratio. Autocorrelations of the radial and vertical components contain P and S waves that are reflected from the subsurface. Therefore, joint inversion of them can account for the variations of both and , and consequently, the ratio. Synthetic inversions show significant improvement in the estimation of these parameters compared to those from the inversion of either, P receiver functions or the autocorrelation of the vertical component. The velocity models inferred from the application of the approach to teleseismic data recorded along a north‐south passive seismic profile (BILBY experiment) in central Australia reveal a distinct pattern of the Moho and the variations across the crustal blocks/domains. The general trend of the Moho structure corresponds well with the change of the reflectivity that can normally be seen at the base of the crust and also with the Moho estimated from previous studies including the deep seismic reflection profiling method. The structure at depths greater than 10 km shows dominant high values beneath locations where the crustal domains interact (e.g., at transition from one domain to another).

Lithospheric structures of the central Solonker-Xar Moron-Changchun-Yanji Suture (Inner Mongolia) revealed by a deep seismic reflection profile

The Solonker-Xar Moron-Changchun-Yanji Suture (SXCYS) is regarded as a tectonic boundary between the Central Asian Orogenic Belt and the North China Craton. Formed during the closure of Paleo-Asian Ocean and modified by the subsequent magmatic activities, this suture might have recorded complex processes of accretionary orogeny and crustal recycling. Thus, seismic reflection profiling was conducted along a length of 160 km from Naiman to Ar Horqin Banner, Inner Mongolia, and the lithospheric structures of central SXCYS have been examined. A prominent feature is a group of strong reflections from the middle crust to the topmost mantle beneath the center of the profile, which consists of massive south dipping reflections and a few middle crustal reflections with a opposite vergence. This group of strong reflections is interpreted to be the subduction relict formed during the final closure of the Paleo-Asian Ocean. The estimated lateral extent of the suture zone is ca. 100 km across strike, and its subduction polarity is southward with an apparent dip angle of 15 to 20°. The upper crust along the whole profile is characterized by weak reflectivity, which also extends to the middle crust. This absence of reflective structures may be attributed to the magmatic activities during or after the continental convergence. Two strong reflection clusters extending from the bottom of upper crust to the lower crust are also identified, which may be caused by distinct magmatic intrusions. Overprinted on the dipping reflections, the Moho is identified at a depth of 33 to 36 km, which is inferred to be formed at an extensional background contemporary with the subsurface Mesozoic sediments and extensional faults. A detailed model of the evolution processes of the central SXCYS is proposed, which includes the oceanic subduction, the final collision, and the modification by subsequent magmatic activities.

An investigation of how intracratonic rifting is “seeded”: Case study of the Late Devonian Dniepr-Donets Basin rift within the East European Craton

This paper analyses the role of pre-existing Precambrian structures for the localisation of the intracratonic rifts from a case study of the Dniepr-Donets Basin (DDB) in Ukraine. The DDB was formed as a result of Late Palaeozoic rifting in the Archaean-Paleoproterozoic Sarmatian segment of the East European craton (EEC). It separates the Ukrainian Shield (UkS) to its southwest from the Voronezh Massif (VM) to its northeast. The Donbas Foldbelt (DF) constitutes the tectonically inverted part of the DDB in its southeastern extent and has been imaged by coincident wide-angle reflection and refraction seismic as well as deep near-vertical reflection seismic profiles (project “DOBRE”). It is almost completely filled with a highly indurated succession of upper Palaeozoic sediments and metasediments, up to some 20 km in thickness, now exposed at the surface. Here, a crustal and upper mantle structural-compositional model that is tightly constrained by the seismic data and gravity anomalies along the profile is used to search for Precambrian pre-rift crustal features that could have played a role in localising Late Palaeozoic rifting. The results suggest that there may be a different tectonic history for Sarmatian crystalline crust on either side of the DF. Density isolines in the AM crust are shallower than corresponding ones of equal value in the VM crust and, accordingly, the mean density of the crust of the AM is higher. This effect, calibrated on the DOBRE profile, is expressed along the margins of the entire DDB with a higher background level of the gravity field seen generally for the UkS than for the VM. The associated gravity gradient coinciding with the location of Palaeozoic rifting means that there is a perturbation to the horizontal deviatoric stress in this position that likely predates rifting. Further, a well-constrained upper crustal low-density granitic body beneath the northeastern flank of the DF produces a significant negative gravity anomaly superimposed upon the background gravity gradient. This adds a significant additional extensional component to the ambient deviatoric stress field. It cannot be concluded with certainty that either of these crustal features was necessary or sufficient for “seeding” Late Palaeozoic rifting but modern passive seismology surveys across the DDB as well as new bedrock geological studies would help test such a hypothesis.

Estimation of lateral correlation length from deep seismic reflection profile based on stochastic model

Estimation of lateral correlation length from deep seismic reflection profile based on stochastic model

The Mabja Dome Structure in Southern Tibet Revealed by Deep Seismic Reflection Data and Its Tectonic Implications

AbstractThe ongoing India‐Asia collision has led to the formation of the northern Himalayan gneiss domes belt in southern Tibet. The domes are the result of the ongoing convergence and were formed by geological processes that may include crustal thickening, metamorphism, partial melting, and exhumation of middle crustal rocks to the surface. A combination of compressional, extensional, and diapiric processes has been invoked to explain the formation and evolution of these domes. Differentiating among these competing hypotheses requires well‐defined geophysical images of the internal structure of the domes. The Mabja dome is the largest dome within the northern Himalayan gneiss domes belt. A 70‐km long deep seismic reflection profile across Mabja dome was acquired in 2016. The seismic data could provide new information about the structural elements beneath the domes to the depth of 25 Km. We address the structure of the Mabja dome by conducting an integrated analysis of shallow crustal velocity structure and a normal‐incidence seismic reflection image of deeper crust. Our work suggests that the Mabja dome is underlain by shear zones at depths of 10–15 km and two high‐velocity bodies at depths of 3 km possibly representing the eclogitic‐facies rocks or mafic intrusions. We propose that the dome formation may have been controlled by collision‐induced north‐south shortening expressed by thrust stacking of middle crustal rocks, which led to the doming of the upper‐crustal rocks. The proposed mechanism inferred for Mabja dome can be applied to interpret the widespread domes throughout the southern Tibet and other related structures in orogenic mountain belts.

Refraction waves full waveform inversion of deep reflection seismic profiles in the central part of Lhasa Terrane

The deep reflection seismic method is an effective technique for detecting the fine structures of the lithosphere and solving deep geological problems. At present, the methods for obtaining the underground velocity from land deep reflection seismic profile mainly rely on the travel-time information based velocity analysis and tomography, which theoretically limit the resolution of the imaging results. To achieve the goal of using deep reflection seismic profile for crustal-scale high-precision velocity imaging, we must finally overcome the problem of full waveform inversion (FWI) using full wavefield data. In this paper, we successfully conduct FWI using the early-arrival wavefields (mainly primary and multiple refraction wavefields) in the deep reflection seismic profile, which can be seen as the first important step towards the goal of crustal-scale high-precision velocity imaging. We propose a robust refraction waves FWI workflow with global normalized cross-correlation, multiple refraction matching, reverse time source estimation and wavenumber domain gradient filtering as its core algorithm. The application of the proposed FWI method to the refraction waves of the deep reflection seismic profiles in the central part of Lhasa Terrane shows that the refraction FWI images have a higher resolution to underground structures both in horizontal and vertical directions than the first-arrival tomography images. The lateral velocity change can be well-matched with the typical geological characteristics of the work area. The FWI result can better describe the lateral boundaries of the strata in different eras. At a depth of less than 3 km from the surface, the refraction FWI can correct the interface morphology of the tomography results and recover some small-scale velocity structures and high-velocity anomalies.

Vp/Vs ratios in the Parnaíba Basin from joint active-passive seismic analysis – Implications for continental amalgamation and basin formation

The Phanerozoic intracontinental Parnaíba Basin in northeast Brazil lies atop crust composed of Archaean to Mesoproterozoic cratonic blocks and Neoproterozoic mobile belts. Recently, active and passive source geophysical surveys characterised the crustal structure beneath the basin. We use information from published active-source seismic and new, coincident receiver function (RF) data to obtain Vp/Vs ratios for sedimentary and crustal structure and make inferences about crustal compositions and tectonic evolution. In our approach, sedimentary and crustal Vp/Vs ratios are adjusted to match common conversion point (CCP) images of RFs and known Moho and basement geometry. We use a P-wave model from published wide-angle reflection/refraction (WARR) seismics, and structural features from a deep seismic reflection (DSR) profile. CCP images of the primary RF conversions were used to model the crust, whilst conversions of multiples were used for the sediment-basement interface. The maximum uncertainties in Vp/Vs are estimated to be 0.15 for the basin and 0.03 for the crust. Vp/Vs ratios in the basin were estimated between 1.7 and 2.2. Lower values correlate with the exposure of older units primarily in the east of the basin, whilst higher values coincide with exposed younger units of the Parnaíba Basin. The obtained crustal Vp/Vs ratios between 1.73 and 1.81 support the previously published segmentation of the crust. In particular, we identified three regions of elevated Vp/Vs ratios, which can be related to proposed Neoproterozoic suture zones underlying the Parnaíba Basin, as well as high velocity lower crust beneath. The high Vp/Vs ratios can be explained by mafic compositions, for example metamorphosed or intruded crust, or fluids and sedimentary rocks entrained into highly deformed crust, typical for modifications related to suture zones. These new deep geophysical models provide important and complementary evidence for crustal amalgamation and the formation of the Parnaíba Basin.

Deep seismic reflection insights into syn-Rodinian crustal recycling

Owing to most tectonic events in Precambrian have been modified by intensive post-Precambrian overprinting, the question how exactly crustal recycling operated on earlier Earth remains an open one. Here, we report a SE-striking, 175 km-long deep seismic reflection profile that traverses the Sichuan basin, China. The image shows a well-preserved syn-Rodinian orogen resting beneath flat lying sedimentary cover of the basin. Relative to the counterparts of modern Earth, this orogen shows a structurally complex Moho geometry composed of multiple layers of basaltic sills, exhibiting extensive crustal extraction. The outlined crustal architecture further depicts that growth of the upper plate arose dominantly through tectonothermal processes, while coeval ductile underthrusting and overthrusting drove contemporaneous crustal shortening of the lower plate. Together with accretionary stacking above, these processes coevally generated a crustal thickness to ~ 30 km. This well-preserved syn-Rodinian orogen provides direct evidence for understanding contemporaneous continental recycling that assembled the Rodinia supercontinent. Plain language summaryNear-vertical, deep seismic reflection profiling is an effective technique for mapping complicated subsurface structures. In this study, the obtained high-resolution deep seismic reflection image captured a non-modified syn-Rodinian orogenic belt resting beneath flay lying sedimentary cover of the Sichuan basin, western Yangtze block, China. Structural interpretation and associated tectonic implications tell us: (1) the Yangtze block was actually a tectonic collage of microcontinents accreted during Neoproterozoic Rodinian assembly, (2) contemporaneous crustal recycling is featured with extensive crustal extraction, and (3) crustal growth was dominantly driven by tectonothermal events in the upper plate of the convergent zone and crustal shortening in a way of coeval underthrusting and overthrusting in the lower plate, a scenario that has shown no big difference with the counterparts of modern Earth. This well-preserved Neoproterozoic orogenic belt provides direct evidence for helping understand the continental recycling that assembled the Rodinia Supercontinent.

Mantle influx compensates crustal thinning beneath the Cathaysia Block, South China: Evidence from SINOPROBE reflection profiling

The crustal architecture of the Cathaysia Block, South China, is a key element in understanding the late Mesozoic tectono-magmatic history of East Asia, and includes a spectacular (∼260,000 km2) basin and igneous province that forms a natural laboratory for unravelling the interplay of extension, magmatism and crust-mantle interaction. Here we analyze the whole crustal architecture of East Cathaysia through a ∼300-km-long SINOPROBE deep seismic reflection profile. This profile reveals that East Cathaysia is underlain by two seismically distinct terranes (i.e., the Wuyishan and Coastal terranes), separated by a low-angle, northwest-dipping set of mantle-penetrating reflectors interpreted as the Zhenghe-Dapu fault zone, which documents Neoproterozoic terrane amalgamation and Cretaceous extensional reactivation. The seismic image of the Coastal terrane shows large-scale low reflectivity truncated by arrays of high-reflective subhorizontal bands of layered reflectors, corresponding to an extension-related crustal architecture dominated by extensive Cretaceous magmatism. The layered reflectors are interpreted to represent mantle-derived mafic sills, which document significant magmatic underplating associated with asthenospheric upwelling beneath the terrane. This interpretation accounts for the localized distribution of Cretaceous magmatic rocks in the Coastal terrane. We suggest that the magmatic underplating added large volumes of mantle-derived material to the crust, compensating for the crustal thinning. Such a magma-compensating process might have formed an integral part of crustal reworking and growth in creating the magma-dominated crust and the broadly flat Moho of the Coastal terrane during the Cretaceous.

Depth of Moho surface in the central Sichuan region revealed by deep earthquake exploration

Deep seismic reflection technology has been recognized by the international geosciences as an effective technical means to detect the fine structure of the lithosphere and solve the deep geological problems. Since the 1970s, a large number of deep seismic reflection profiles have been collected in the Sichuan Basin and the periphery, providing a basis for deep research and seismic acquisition techniques for crustal structure research in the region. Because the deep reflection seismic method has the characteristics of large detection depth, high resolution, accuracy and reliability, deep reflection seismic technology is still the main means of deep crust fine structure research in the future. In this paper, the ultra-deep seismic data is collected to reveal the geological characteristics of the Moho surface in the Sichuan Basin in the Sichuan Basin, which is stable and shallow and has a depth of about 45 km.

Structure of the Earth’s crust of the Eastern Rhodopes (Southern Bulgaria) from the regional deep reflection seismic profile Ivaylovgrad–Ardino

This paper presents a geological interpretation of the deep seismic profile Ivaylovgrad–Ardino, which was published in 1996. Four plates are distinguished according to their seismic features that build up the Eastern Rhodope Terrane. They have a total thickness of 22–24 km and layered structures with sub-oceanic character. The lower two plates (1, 2) are westward obducted on the Rila-Western Rhodope Terrane, forming one obduction complex. Plate 3 is probably also a part of the obduction complex. Plate 4 is thrust southward on all plates. A Kobilino Crypto-dome, comprising Plates 1 and 2 and covered discordantly by the third one, is found at the eastern part of the profile at 7–7.5 km depth. The two lower plates do not appear on the surface, but some ultrabasic to basic bodies are supposed to be parts of them, rising as tectonic slices. The Plate 3 is revealed on the surface at the eastern part of the seismic profile (east of Avren Fault), and is built up mainly by the Pre-Rhodopean Supergroup metamorphic rocks, respectively from the seismically indistinguishable Upper and Lower Allochthon. The Plate 4 appears mainly at the western end of the profile (between Mishevsko Village and Ardino Town), and is built up by the metamorphic rocks of the Rhodopean Supergroup from the Startsevo Lithotectonic Unit (or Middle Allochthon). Five fault and shear zone systems of different ages are distinguishable in all plates. Transparent areas in the plates are interpreted as magma chambers of the Paleogene volcanoes, others as Upper Cretaceous and Paleogene (?) plutons. Because these magmatic chambers are located in the sub-oceanic crust, both intermediate and acid Paleogene volcanics have isotopic characteristics similar to those of the metasomatized mantle.

Seismic reflection imaging of crustal deformation within the eastern Yarlung-Zangbo suture zone

Various models have been proposed to explain formational mechanisms for the Himalayan orogeny but uncertainties remain concerning uplift processes and deeper crustal structure of the orogen. Imaging of the orogen's deep structure can help resolve these uncertainties. This paper describes one migrated deep seismic reflection profile across the eastern Yarlung-zangbo suture (YZS) at 92° E, which provides a high resolution image of crustal structure that reveals (1) Yarlhashampo dome is divided into upper, middle and lower tectonic units by two detachments from rim, mantle to the core of dome, (2) one blind north-dipping reverse fault that offsets the South Tibet detachment (STD) developed along the northern flank of the Yarlhashampo dome, (3) a shift of the Main Himalayan thrust (MHT) dip angle from 18° into relatively gentle dips (6°) in the central part of the profile (18–20 s, t.w.t). Meantime, further inferring that (4) North-south compression event happened after ca.10 Ma in North Himalayas, (5) multi-stage duplexing as the primary mechanism for crustal thickening in the eastern YZS based on interpreted structures.

Shortening of lower crust beneath the NE Tibetan Plateau

We reprocessed a high-resolution deep seismic reflection profile to reveal the detailed crustal structure of the junction between the Qilian Shan-Nan Shan thrust belt and the Hexi Corridor basin in the NE Tibetan Plateau. The profile shows that the Moho discontinuity is characterized by complex dislocations and overlapping geometries. Restoration of the deformed Moho discontinuity reveals 50.2 km (35.5%) of Cenozoic lower-crustal shortening along the seismic profile. Compression of the lower crust produces crustal thickening of ~12.8 km. This demonstrates that lower-crustal shortening plays a significant role in topographic uplift and crustal thickening at the NE margin of the Tibetan Plateau. Thus, we propose a double-layered shortening mechanism, with decoupled deformation of upper crust and lower crust. The proposed deformation mechanism differs from previous crustal shortening models for the northern, eastern, and northeastern Tibetan Plateau. Our model eliminates the need for lower-crustal flow to explain topographic uplift along the NE margin of the Tibetan Plateau.

Examination of the repeatability of two Ms6.4 Menyuan earthquakes in Qilian-Haiyuan fault zone (NE Tibetan Plateau) based on source parameters

Repeating earthquakes are key evidence for understanding earthquake recurrence and help improve long-term seismic risk assessment. In 1986 and 2016, two Ms6.4 earthquakes occurred in Menyuan region, near the middle segment of Qilian-Haiyuan fault zone, NE Tibetan Plateau. Long period waveforms of the two events at teleseismic stations show high degree of similarity, however, previous solutions on earthquake location and focal mechanism show remarkable discrepancies, especially for the 1986 event, as well the ruptured faults also remain ambiguous. In this study, we determined the source parameters of these two earthquakes, including the relative location, focal depth, focal mechanism, source duration and rupture directivity. The results show that two earthquakes are thrust events in shallow crust and are located within 15 km, and the 2016 earthquake ruptured down-dip for ~6 km along the southwest dipping plane. Combing the source parameters, geological data and deep seismic-reflection profiles, we infer that these two earthquakes occurred on the same secondary fault of Lenglong Ling fault (the middle segment of Qilian-Haiyuan fault), but they ruptured on different sections of the fault. Although the two events are probably not repeating earthquakes of Qilian-Haiyuan fault zone, the seismic risk of ~M6 thrust earthquake on these secondary faults and even larger strike-slip earthquake on Lenglong Ling fault are not negligible based on the source parameters and ten-year GPS observations.

High‐resolution seismic imaging of crooked two‐dimensional profiles in greenstone belts of the Canadian shield: results from the Swayze area, Ontario, Canada

ABSTRACTIn 2017, the Metal Earth multi‐disciplinary exploration project acquired a total of 921 km of regional deep seismic reflection profiles and 184 km of high‐resolution seismic reflection profiles in the Abitibi and Wabigoon greenstone belts of the Superior province of Canada. The Abitibi belt hosts several world‐class mineral deposits, whereas the Wabigoon has sparse economic mineral deposits. Two high‐resolution surveys in the Swayze area, a poorly endowed part of the western Abitibi greenstone belt, served as pioneer surveys with which to better understand subsurface geology and design a strategy to process other surveys in the near future. Swayze seismic data were acquired with crooked survey geometries along roads. Designing an effective seismic processing flow to address these geometries and complex geology required straight common midpoint lines along which both two‐dimensional prestack dip‐moveout correction and poststack migration processing were applied. The resulting seismic sections revealed steeply dipping and subhorizontal reflections; some correlate with folded surface rocks. An interpreted fault/deformation zone imaged in Swayze north would be a target for metal endowment if it extends the Porcupine–Destor structure. Because of the crooked line geometry of the surveys, two‐dimensional /three‐dimensional prestack time migration and swath three‐dimensional processing were tested. The prestack time migration algorithm confirmed reflections at the interpreted base of the Abitibi greenstone belt. The swath three‐dimensional images provided additional spatial details about the geometries of some reflections, but also had less resolution and did not detect many reflectors observed in two dimensions. Geological contacts between felsic, mafic and ultramafic greenstone rock layers are thought the main cause of reflectivity in the Swayze area.

A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling

The Atlas and Tell domains constitute a major segment of the Circum-Mediterranean Alpine fold and thrust belts in the Maghreban part of North Africa. South of the Tell, the Atlas results from a progressive Late Cretaceous to present foreland inversion of former Jurassic to Cretaceous extensional tectonic features inherited from the opening of the Tethys along the northern African margin. In contrast, the Tell is made up of allochthonous Cretaceous to Neogene basinal units detached along a tectonic sole made up of remobilized Triassic salt and tectonic mélanges.Since the last decade, the major tectonic steps accounting for the development of the eastern Maghreb structures are becoming more precisely constrained by seismic data and associated structural analyses. However their overall configuration at depth, as well as the deep architecture of the underlying basement remain poorly understood because only few wells yet penetrated deeper than the Triassic series in the Atlas and adjacent foreland basin. Therefore, this paper is intended to give an overview on the deep structural features affecting the Pan-African basement in the eastern Maghreb by means of tentative regional cross-sections. Our purpose is to stimulate exploration of deep hydrocarbon plays and give some insights to help both the industry, universities and other public research institutes to undertake jointly a major research program on the deep crustal configuration in the eastern Maghreb with particular focus on the pre-Triassic series beneath the Tellian allochthon in the north, the parautochthonous inverted Atlasic structures in the center, and in the adjacent autochthonous foreland domain extending southeastward in the offshore.