Listric Normal Faults Research Articles

Interaction between gravity-driven listric normal fault linkage and their hanging-wall rollover development: a case study from the western Niger Delta, Nigeria

Abstract Rollover is the folding of the hanging-wall sedimentary record in response to slip on listric normal faults, and is a common feature of sediment-rich, gravity-driven tectonic provinces. Rollovers have been extensively studied by means of geometrical reconstruction, and numerical and analogue modelling. However, the detailed interaction between the kinematics of bounding listric normal faults and their hanging-wall deformation is not yet fully understood. In this study, we use 3D seismic-reflection data from the Forcados-Yokri area, western Niger Delta, Nigeria, to study the lateral linkage and landwards backstepping history of an array of listric normal faults, particularly focusing on their influence on the development and evolution of hanging-wall rollovers. Five individual, partly overlapping rollover structures have been studied with respect to their relative initiation and decay time, their spatial distribution, and their relationship to the tectonic history of their respective bounding faults. We demonstrate that the studied rollovers are highly dependent on the development of their bounding faults in terms of initiation time, lateral linkage, internal structural development and decay. Fault–rollover interaction is dynamic and changes through time depending on the temporal evolution of listric faults. Four genetic types of fault–rollover interaction were identified in this study: (1) the rotation of a rollover–crestal-collapse system, controlled by a changing lateral bounding-fault orientation during fault growth; (2) a stepwise shift of rollover–crestal-collapse systems associated with rollover abandonment, controlled by the initiation of a new fault in the footwall of an older structure; (3) a gradual shift of successive rollovers controlled by branching main faults; and (4) a general landwards and upwards migration of crestal-collapse faults within a rollover above stationary listric main faults.

High-resolution shear-wave seismic reflection as a tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany

Abstract. Subrosion is the subsurface leaching of soluble rocks that results in the formation of depression and collapse structures. This global phenomenon is a geohazard in urban areas. To study near-surface subrosion structures, four shear-wave seismic reflection profiles, with a total length of ca. 332 m, were carried out around the famous leaning church tower of Bad Frankenhausen in northern Thuringia, Germany, which shows an inclination of 4.93° from the vertical. Most of the geological underground of Thuringia is characterized by soluble Permian deposits, and the Kyffhäuser Southern Margin Fault is assumed to be a main pathway for water to leach the evaporite. The seismic profiles were acquired with the horizontal micro-vibrator ELVIS, developed at Leibniz Institute for Applied Geophysics (LIAG), and a 72 m long landstreamer equipped with 72 horizontal geophones. The high-resolution seismic sections show subrosion-induced structures to a depth of ca. 100 m and reveal five features associated with the leaching of Permian deposits: (1) lateral and vertical varying reflection patterns caused by strongly heterogeneous strata, (2) discontinuous reflectors, small offsets, and faults, which show the underground is heavily fractured, (3) formation of depression structures in the near-surface, (4) diffractions in the unmigrated seismic sections that indicate increased scattering of the seismic waves, and (5) varying seismic velocities and low-velocity zones that are presumably caused by fractures and upward-migrating cavities. A previously undiscovered southward-dipping listric normal fault was also found, to the north of the church. It probably serves as a pathway for water to leach the Permian formations below the church and causes the tilting of the church tower. This case study shows the potential of horizontal shear-wave seismic reflection to image near-surface subrosion structures in an urban environment with a horizontal resolution of less than 1 m in the uppermost 10–15 m.

Initial Opening of the Eurasian Basin, Arctic Ocean

Analysis of the transition from the NE Yermak Plateau into the oceanic Eurasian Basin sheds light on the Paleocene formation of this Arctic basin. Newly acquired multichannel seismic data with a 3600 m long streamer shot during ice-free conditions enables the interpretation of crustal structures. Evidence is provided that no major compressional deformation affected the NE Yermak Plateau. The seismic data reveal that the margin is around 80 km wide and consists of rotated fault blocks, major listric normal faults, and half-grabens filled with syn-rift sediments. Taking into account published magnetic and gravimetric data, this setting is interpreted as a rifted continental margin, implying that the NE Yermak Plateau is of continental origin. The transition from the Yermak Plateau to the oceanic Eurasian Basin might be located at a prominent basement high, probably formed by exhumed mantle. In contrast to the Yermak Plateau margin, the North Barents Sea continental margin shows a steep continental slope with a relatively abrupt transition to the oceanic domain. Based on one composite seismic line, it is speculated that the initial opening direction of the Eurasian Basin in the Arctic Ocean was highly oblique to the present day seafloor spreading direction.

백악기 경상분지 진주층 상부 셰일층 내에 발달하는 점완형 정단층군과 방해석 광맥계의 형성 기작

백악기 경상분지 진주층 상부 셰일층 내에 발달하는 점완형 정단층군과 방해석 광맥계의 형성 기작

Hyper-extended rift systems in the Xisha Trough, northwestern South China Sea: Implications for extreme crustal thinning ahead of a propagating ocean

From the apex of the NW Sub-basin of the South China Sea, we present new long-cable deep-penetration seismic profiles and deepwater drillings together with corrected seismic interpretations from previous studies. Structural mapping demonstrates that the Xisha Trough is characterized by a hyper-extended rift system that shows evidence of extremely thinned crust and rotated fault blocks. A series of NE, NEE and W-E striking listric normal faults along the Xisha Trough, which merge at depth into Moho reflection, record strongly extensional deformation. Based on the absence of an acoustic basement beneath a wedge-shaped rifted sequence, we document three diapiric magma structures exiting the Moho and penetrating into the extremely thinned continental crust, which is covered by a Cenozoic sequence. For the Xisha Trough we can demonstrate, based on seismic data and calculated fault slip rates, the faults bounding the Xisha Trough were initially formed by distributed rifting, and were reactivated and focused on faulting along the axis of the Xisha Trough. However, the peak faulting migrated along the margin strike from east to west and terminated at the apex of the Xisha Trough. One of the key processes, controlling the evolution of the Xisha Trough, is depth-dependent extension, which may arise from differential lithospheric stretching by regional dynamic lower crustal/mantle flow.

Seamount subduction at the North-Ecuadorian convergent margin: Effects on structures, inter-seismic coupling and seismogenesis

At the North-Ecuadorian convergent margin (1°S–1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake.This dataset highlights a subducted ∼30×40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits.The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion.In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8–1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths.

Geometry and kinematics of the Grant Range brittle detachment system, eastern Nevada, U.S.A.: An end‐member style of upper crustal extension

AbstractDocumenting the range of styles of normal faulting is fundamental to understanding crustal extension. Here geologic mapping, field relationships, and deformed and restored cross sections illustrate the geometry and kinematic development of a system of west‐vergent detachment faults in the Grant Range in eastern Nevada. Faults exhibit brecciation and stratigraphic cutoff angles of 5–15° at all structural levels and deform a 10 km thick section of Paleozoic and Paleogene rocks. The fault system is folded across an anticlinal culmination, which grew during extension, as indicated by progressively increasing interlimb angles and incision in the axial zone. The eastern limb consists of an imbricate stack of faults that were emplaced from bottom to top. In the western limb, several faults exhibit apparent thrust relationships. The oldest faults are cut by a ~29 Ma dike, and the highest preserved fault cuts ~32 Ma volcanic rocks that restore to paleodepths of ~1 km. Retrodeformation of folding and minimal structural relief and angularity across a Paleogene unconformity indicate the faults were active at 5–15° angles. Retrodeformation of offset indicates ≥49 km (98%) extension. We propose a model of stationary, sustained isostatic uplift and incision at the culmination axis (a “fixed hinge”), with updip excision producing bottom‐to‐top growth of the imbricate stack and downdip excision producing apparent thrust relationships. The fault system exhibits similarities to core complex detachment systems, though it is confined to upper crustal levels, and there are no preserved high‐angle or listric normal faults, indicating a unique extension style dominated by low‐angle excision.

Hanging-wall deformation and gas-migration associated to a major salt detaching fault in the Norwegian Danish Basin

The central and southern regions of the North Sea Basin are characterized by mobile Zechstein Salt and represent an excellent area to study listric normal fault development related to halokinesis. The North Sea Basin–in spite of being a mature explored hydrocarbon producing basin-contains a generally not fully understood hydrocarbon plumbing system. Faults and associated fluid migration routes represent critical elements of the plumbing system. Here we present a combined 3D seismic mapping of a listric fault and detailed seismic attribute analysis of shallow gas anomalies near the fault implying a close and complex relationship between a major detaching fault, hangingwall deformation and fluid migration and entrapment. The fault and associated antithetic and secondary synthetic faults is controlled by the growth of the underlying salt structure. Three separate segments of the fault are recognized showing distinct and varying characteristics of structural maturity and fluid migration. The studied fault and shallow gas anomalies thus represent an example of a complex faulted shallow fluid migration system developing in relation to an underlying salt structure which could be relevant for understanding similar salt, fault and fluid migration systems around the world.

Postglacial tectonic activity within the Skjálfandadjúp Basin, Tjörnes Fracture Zone, offshore Northern Iceland, based on high resolution seismic stratigraphy

Abstract Multibeam bathymetric and high resolution seismic reflection data (Chirp) have been used to illuminate the structural framework and tectonic evolution of the 300 km2 Skjalfandadjup Basin, one of three large rift basins within the Tjornes Fracture Zone, a complex transform zone offshore Northern Iceland. The Skjalfandadjup Basin together with the Nafir seamounts, form the nascent Nafir–Skjalfandadjup volcanic system of the oblique Grimsey Volcanic Zone. The Skjalfandadjup Basin is made up of normal faults with maximum displacement of 50–60 m and a dominant strike of N10–20°W. Correlation of Chirp data with tephrochronology from sediment core MD99-2275 provided constraints on tectonic movements along individual faults from Late-glacial time throughout Holocene. Maximum vertical displacement is observed on three listric normal faults marking the eastern boundary of the main basin. Postglacial tectonic activity commenced as early as 14–15 kyr BP, followed by three major rifting episodes prior to 10 kyr BP and three smaller rifting episodes during the last 4.2 kyr. The 10–12 kyr period of enhanced tectonic activity was accompanied by eruptions within the Tjornes Fracture Zone. Isostatic rebound following rapid deglaciation caused rifting with significant vertical displacement on normal faults offshore N-Iceland.

The structural and tectonic relationships of the major fault systems of the Tan-Lu fault zone, with a focus on the segments within the North China region

The Jiashan-Lujiang, Yishu and Bohai segments have been the most tectonically active parts of the Tan-Lu fault system since the Mesozoic. Any analysis of faulting processes in the Tan-Lu system is highly dependent on the relationships among the three segments. Here we divide the Tan-Lu system into three styles of faulting; strike-slip faulting, regional extensional faulting and listric normal faulting on the basis of EMAP data, seismic profiles and geological evidence. We suggest that: (1) the strike-slip faults, which formed in the Late Jurassic, include mainly the Jiashan-Lujiang fault, the deep parts of the Chihe-Taihu Fault and the Guhe-Sanbing fault; these three faults have had the most dramatic effect on the Jiashan-Lujiang segment. The strike-slip style of faulting produced a huge positive-flower structure that extends between the deep section of the Chihe-Taihu fault and the Guhe-Sanbing fault, and overlaps with the position of the Zhangbaling ductile shear zone; (2) the regional extensional style of faulting, whose characteristics are reflected best in the Yishu fault zone, is composed of the Changyi-Dadian, Anqiu-Quxian, Yishui-Yangtou and Tangwu-Gegou faults, all of which were active during the Early Cretaceous. These extension-dominated faults extend southwards to the Jiashan and Hefei Basins and die out in front of the Dabie Mountains; to the north, the faults penetrate the deeper parts of the Weibei depression where they controlled Cretaceous deposition in “graben-horst-graben” structures; (3) the listric normal faults, which were initiated in the Paleogene, generally occur around the Bohai Bay Basin, and include a series of NE-trending faults in the Weibei depression, such as the Tandong and Tanxi faults. These listric faults differ from the regional extensional faults in their localized occurrence and association with basin formation. Our study indicates that these three styles of faulting varied not only in timing and mechanical properties but developed at different levels at different locations, resulting in different tectonic relationships and the formation of different geologic features in the different segments of the Tan-Lu master system.

Mid-crustal detachment beneath western Tibet exhumed where conjugate Karakoram and Longmu–Gozha Co faults intersect

Models for how shortening is accommodated in the India–Asia collision vary between an end member in which largely rigid blocks are extruded eastwards between lithospheric-scale strike-slip faults and an end member in which a hot, weak mid-crustal layer aids distributed deformation. Here the mode of crustal deformation is evaluated by studying the intersection of two main conjugate strike-slip faults at the west end of the Tibetan plateau. Our field mapping suggests that these faults, the right-lateral Karakoram fault and the left-lateral Longmu–Gozha Co fault, an eastward continuation of the Altyn Tagh fault, may be connected by a large-scale, east-dipping listric normal fault, exposing a wedge of mid-crustal rocks in its footwall. Pseudosection modeling of matrix and porphyroclast rim compositions from footwall metamorphic rocks yield late-syntectonic pressures of 640±100 MPa and temperatures around 600±50°C. Extensive networks of narrow granitic dikes give U–Pb zircon ages as young as 13.7±0.2 Ma, suggesting that footwall rocks remained hot until the late Miocene and were not exhumed until after this time. We infer >40 km heave across the Angmong fault, and suggest that it absorbs effectively all of the slip across the Longmu–Gozha Co fault (which it appears to truncate), so the Longmu–Gozha Co fault is seemingly confined to the upper crust. Similar mechanical decoupling likely occurs throughout the plateau, with strike-slip faulting in western Tibet limited to the upper, brittle part of the crust.

Evolution and Architecture of Blue Nile Basin Using Integrated Gravity,Reflection Seismic and Wire Line Logs

Parts of the Blue Nile basin showed good oil indications a decade ago, the puzzling question is why no oil draining. The common geological sense will go towards the lateral lithologic variations and its impacts on the porosity and permeability and thus towards the dynamics of the processes that governs and generate this architecture and the modes of its development. This study aims to reveal the basin architecture and its evolution, and it strongly suggests the application of constrained 2D backstripping technique using process oriented gravity modeling (POGM) technique and use the discrimination between the rift and sediment anomalies. By decompaction stratigraphic seismic sections, isostasy, recover the geometry of the crust at time of rift, calculating and adding the contribution of sediments and water to rift gravity anomaly, compare between the observed free air and the calculated gravity. POGM technique results a good discrimination between sediment and rift gravity contributions, and a value of 45 km thickness of elastic lithosphere, which is used to constrain the backstripping technique. Results from the backstripping shows the basin is a half graben type, the extension factor varies from 1.04 up to 1.1, which is accommodated by listric normal fault and the rotated blocks which compromise the hanging wall and developed to be the basin. The relative movement between these blocks and the foot wall goes different behaviors from acting independently to act as one unit this interplay change the normal faults into reverse faults, made temporal segmentation of the basin, relief, internal drainage patterns and migration of basin depocentre. From a tectonostratigraphic point of view, the Blue Nile basin evolution is given by different paleoenviroments and their positions in terms of time and space are controlled by interplay of sediment supply and basin capacity which are climatic and structural/thermal subsidence dependents respectively.

Relationship between syn-depositional faulting and carbonate growth in Central Luconia Province, Malaysia

Using 3D seismic and well data, detailed seismic interpretation has been conducted on two carbonate Platforms EX and FY located in the Central Luconia Province, Malaysia. The results provide an insight to understand the relationship of faulting with syn-depositional carbonate growth. Five geo-seismic units were interpreted from the Late Oligocene to present day sedimentation in the basin. Structural interpretation of both platforms shows that almost all the faults in the deeper part of the platforms are normal listric faults that resulted from the final rifting stage of the South China Sea. Small-scale, steep normal faults within the carbonate units behave relatively as syn-depositional faults that became the base and template for the reefs to grow as these faults created conjugate and branching faults systems. Apart from becoming templates for the reefs to grow, these syn-depositional faulting events had also interrupted the growth of reefs especially those that were located at the platform margin. Slight movement of the branching faults induced sub-marine landslide to create reefs collapse. The establishment of relationship between faulting and carbonate growth in Central Luconia Province might be useful for revisiting mature hydrocarbon reservoirs.

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Chaotic breccias and megabreccias – locally called gash breccias – hosted within the Pembroke Limestone Group (Visean, Mississippian, lower Carboniferous) of southwest Wales are re-mapped along with spatially-related crackle and mosaic breccias. Of thirteen studied megabreccia bodies, seven lie along steep, NNW- or NNE-striking strike-slip faults originating during north–south Variscan (late Carboniferous) shortening, though reactivated during later extension. Four bodies are conformable with E–W striking, steeply-dipping bedding, and two have irregular or indeterminate margins. The bedding-parallel zones are interpreted as the dilational tips of listric normal faults, and the cross-strike faults as transtensional transfer zones. Sub-horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults. Most fissures have geometrically matched margins produced by this dilational faulting, and only locally have the indented margins indicating solutional processes. The most likely age for the main fissure extension and fill is late Triassic, based on analogous dated fills at the eastern end of the Bristol Channel Basin. The Pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones.

Tectonic analysis of the Honaz Fault (western Anatolia) using geomorphic indices and the regional implications

Honaz Dağı (Honaz Mountain, Denizli) is the highest peak in western Anatolia (2517 m) and represents the southern margin of the Denizli Graben Horst System in the western Anatolia Extensional Province. Honaz Fault is a 2-km-wide, 15-km-long northward arc-shaped dip-slip listric normal fault that bounds the northern margin of Honaz Dağı, and has been formed under an N–S-trending extensional tectonic regime since the early Quaternary. This study uses fieldwork and digital elevation model-based geomorphic analysis to identify the kinematic behaviour of the Honaz Fault. Morphometric indices, such as axial river profiles, drainage basin geometries, triangular facets, mountain‐front lineament patterns and sinuosities, document the impact of active tectonics on the evolution of the Honaz Fault and its surroundings, as do valley floor width-to-height ratios and mountain front facet characteristics. These neotectonic indices suggest a relatively high degree of tectonic activity along the Honaz Fault, and suggest that the Quaternary landscape evolution of Honaz Dağı was governed by structural and climate-related erosional processes. Combined geomorphic indices and field data suggest that the analysed dip-slip normal fault segments of the Honaz Fault are linear and highly active, and could generate earthquakes with magnitudes of 6.7. Morphometric analyses of the fault trace along the mountain front yield minimum slip rates of .15–.38 mm/y for the Honaz Fault, which are similar to slip rates calculated for active normal faults in the Aegean region.

Apparent paleomagnetic rotations reveal Pliocene–Holocene internal deformation of the Tengchong Block, southeastern Tibetan Plateau

We present new paleomagnetic data from early Pliocene to Holocene volcanic rocks of the Tengchong volcanic field and Mangbang basin of the Tengchong (Lhasa) Block, Yunnan, along the eastern margin of the India–Asia collision zone. Two remanent magnetization components occur: a higher coercive one, carried by Ti-rich titanomagnetite and magnetite and likely of primary origin, and a lower coercive one, residing in magnetite and representing a more recent overprint. The Pleistocene–Holocene rocks of the Tengchong volcanic field yield an in situ overall mean direction of D/I=352.5°/25.4°, indicating a southeastward tilt; we attribute the tilting to block rotation above listric normal faults that cut the Tengchong volcanic field. The in situ overall mean direction derived from Pliocene volcanic rocks of the Mangbang basin is D/I=330.5°/54.2°. We explain this apparent counterclockwise rotation by northeastward tilting due to half-graben formation during normal faulting along the Nuangdeng fault zone along the eastern basin margin.

Subsurface evidence for late Mesozoic extension in western Mongolia: tectonic and petroleum systems implications

AbstractNew seismic reflection profiles from the Tugrug basin in the Gobi‐Altai region of western Mongolia demonstrate the existence of preserved Mesozoic extensional basins by imaging listric normal faults, extensional growth strata, and partially inverted grabens. A core hole from this region recovered ca. 1600 continuous meters of Upper Jurassic – Lower Cretaceous (Kimmeridgian–Berriasian) strata overlying Late Triassic volcanic basement. The cored succession is dominated by lacustrine and marginal lacustrine deposits ranging from stratified lacustrine, to subaqueous fan and delta, to subaerial alluvial‐fluvial environments. Multiple unconformities are encountered, and these represent distinct phases in basin evolution including syn‐extensional deposition and basin inversion. Prospective petroleum source and reservoir intervals occur, and both fluid inclusions and oil staining in the core provide evidence of hydrocarbon migration. Ties to correlative outcrop sections underscore that, in general, this basin appears to share a similar tectono‐stratigraphic evolution with petroliferous rift basins in eastern Mongolia and China. Nevertheless, some interesting contrasts to these other basins are noted, including distinct sandstone provenance, less overburden, and younger (Neogene) inversion structures. The Tugrug basin occupies an important but perplexing paleogeographic position between late Mesozoic contractile and extensional provinces. Its formation may record a rapid temporal shift from orogenic crustal thickening to extensional collapse in the Late Jurassic, and/or an accommodation zone with a Mesozoic strike‐slip component.

Cataclastic bands in immature and poorly lithified sandstone, examples from Corsica, France

Cataclastic bands with intense cataclasis formed in phyllosilicate- and clay-rich poorly lithified sandstone have been identified and characterized for the first time. This study indicates that intense cataclasis could occur at shallow burial depth regardless of mineralogy of the rock at the time of deformation. The study was performed on a series of listric normal faults (displacement up to 10m) in the Aghione Formation at Aleria Basin, Corsica, France. In-situ measurements of permeability show a reduction in permeability up to two orders of magnitude in the damage zone (cataclastic bands), while permeability increases along the slip surfaces. The slip surfaces make conduits to fluid flow, which is also confirmed by the field observations. The permeability and porosity decrease within cataclastic bands is in agreement with an increase in P-wave velocity measured on the band possibly due to compaction, introducing anisotropy to the rock.

The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure?

Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2-D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks.

Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

AbstractWe study the erosive convergent margin of north‐central Chile (at ~31°S) by using high‐resolution bathymetric, wide‐angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2‐D velocity‐depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic‐continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large‐scale crustal thinning and long‐term subsidence of the outermost fore arc.