Listric Faults Research Articles

Bladed Texture and Exploration Implications. A Case Study from the Kuklitsa Deposit, Krumovgrad Goldfield, SE Bulgaria

This study demonstrates that the bladed texture, which is common in epithermal, low-sulfidation (adularia-sericite) precious metal deposits, can serves as exploration vector towards precious metal mineralization. The paper presents two styles of bladed texture in the Kuklitsa gold deposit (Krumovgrad goldfield, SE Bulgaria) observed at both different altitude and lateral position in respect to regional low-angle detachment fault. The first style has formed as a crackle breccia just above the detachment fault where bladed texture consists of 10–20 vol % pseudorhombic adularia, 90-80 vol % quartz, scarce pyrite, and electrum, which is often observed under optical microscope. The second style is present in steep veins which fill listric faults of sharp tectonic contacts. It is developed at a higher level relative to the detachment fault. Bladed texture there consists of 1–2 vol % pseudorhombic adularia, 99-98 vol % quartz, and scarce both electrum and pyrite. Electrum of the two styles of bladed texture comprises only gold and silver but in different proportions with a higher gold content for the first style: fineness of 765, on average, for the first style vs. fineness of 692 for the second one. In this way, it is found that the adularia abundance correlates positively with the electrum one and negatively with the quartz abundance. The author uses the proportions of adularia, quartz and electrum, the fineness of electrum, and the relative distance to the detachment fault to conclude that the first style of bladed texture has been formed at higher temperature relative to the second style. The author infers that the first style is promising for mineralization of higher grade. Methods used comprise field observations and sampling, optical and electron microscopy, powder X-ray diffraction and electron microprobe analysis.

Significant Techno-stratigraphic development of the Caroni Basin, recorded in outcrops of the late Neogene Manzanilla and Springvale Formations, NE Trinidad

The Manzanilla and Springvale Formations outcrop along a 1.9 km stretch on the eastern coast of Trinidad, and provide vital information about the development of the Caroni Basin and north-central Trinidad. The Manzanilla Formation contains deposits that range from deepwater to delta plain facies. Sandstone turbidites are the oldest deposits and these transition upward into muddy slope deposits then shelf mudstones, siltstones and sandstones. The occurrence of sporadic conglomerate beds suggests debris flows or other coarse-grained sediment gravity flows additionally entering the basin at this time. The upper parts of the Manzanilla succession reflect marginal marine environments with delta front (including mouth-bar), crevasse splay, bay fill and marsh deposits. Most of these deposits originated as the northernmost part of the east to northeast flowing paleo-Orinoco Delta. The anomalous interfingering of coarse-grained lithic-rich debris flow deposits with the delta-derived deposits, strongly suggests a derivation directly from the steep-sloped alluvial-fan and fan-delta deposits eroding off of the Northern Range orogenic belt, some 10 km to the north. The facies associations of the Springvale Formation reflect channelized estuarine environments strongly influenced by tidal processes. Subtidal channels associate with and are succeeded by wave and tidal influenced intertidal facies and their overlying supratidal mudstones, paleo-sols and lignite beds.Examination of these two formations reveals that the Neogene Caroni Basin developed quite differently from the coeval Southern and Columbus basins to the south. The Southern and Columbus Basins are true foreland basins with growing accommodation space and expanded shelf section due to the movement of huge listric faults. When compared to similar aged formations in the Southern Basin, the Caroni Basin succession is relatively thin, starved of sediment, contains internal discontinuities and tectonically derived coarse-grained conglomerates. This contrast is due to the fact that the Caroni Basin is sited immediately adjacent to the northern Trinidad zone of Caribbean and South American plate-margin convergence. The compressional forces acting on the basin produced alternating periods of uplift/erosion and subsidence which inhibited continuous deposition. The outcrops examined herein present new detail of the tectonic impact on the evolving Caroni foreland basin within northern Trinidad, where compression-driven base level changes left their signals in the foreland succession.

Geodynamic evolution of the offshore Indus Basin Pakistan: the western Indian Plate Passive Continental Margin

A complex Meso-Cenozoic history characterizes the offshore Indus Basin Pakistan, including rifting, drifting, collision, transpression and recent inverted extension that is poorly understood. We use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes. Synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons. Seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences. Additionally, the basement represented by chaotic and semitransparent reflections, and Moho discontinuity by moderate amplitude bright reflections are also identified on seismic records. Syn-depositional normal faulting in extensional regime (Cretaceous-Oligocene) are identified on seismic profiles that converted into strike slip faults in Miocene and listric faults in Recent sediments forming inverted extensional regime. We infer that the basin inversion from extension to inverted extension and transpression occurred in recent times. Number of depositional features including shelves, deltaic deposits, canyons and channel levee complexes have been identified that developed in different tectonic episodes. Five tectonic subsidence curves using back-stripping procedure have been calculated in order to constrain the geodynamic evolution of the Meso-Cenozoic sedimentary succession of the offshore Indus Pakistan. The analysis of tectonic subsidence curves, covering a time span of 140 Ma, allowed us to quantify the total subsidence and to document the activities of syn-rift and post-rift faulting systems. Different stages, in terms of duration and magnitude of subsidence-uplift trends, have been identified in the evolution of the basin. Two uplift events are prominent in Eocene (similar to 40 Ma) and Miocene time (similar to 12 Ma). A rapid resumption of subsidence with non-conformity in Early Palaeocene is seen along the margin.

Rift structures and its related unconformities on and adjacent the Dongsha Rise: insights into the nature of the high-velocity layer in the northern South China Sea

It is curious that the typical feature of high-velocity layer (HVL) underneath the crust for the magma-rich margin was found in the magma-poor margin of the South China Sea. In order to better understand the nature and age of the HVL in the South China Sea, here we provide seismic imaging on the Cenozoic structure and sedimentation using a regional seismic reflection profile combined with existing drilling data and seismic refraction lines in the northeastern South China Sea. An important finding is that the extent of erosion since Neogene is only on the Dongsha Rise and is much smaller than that of the HVL observed across the northern margin of the South China Sea, which suggests the occurrence of the HVL has no influence on the erosion on Dongsha Rise through Neogene. On the regional seismic profiles, the different structures between the deepwater rifted margins and the continental shelf were also observed. The faults in the Zhuyi Sub-basins on the continental shelf are imaged to be high-angle and bound a series of grabens and horsts. In contrast, the deepwater region is characterized by tilted fault blocks and an array of listric normal faults, which extremely thinned the crust to be ~ 5–10 km. After middle Miocene, active magmatism occurred in the deepwater region at the time of post-spreading of oceanic crust. These listric faults on the hyper-extended crust is interpreted to be conduits for magma migration to the shallower levels. This typical structure in the deepwater region changed the geophysical nature of the HVL underneath the extreme thinned crust, which should be formed before the event of hyper-extension and is likely to be associated with the subduction of the paleo-Pacific toward the Asia in the late Cretaceous. Our study will answer the question why the South China Sea with the HVL underneath the crust is a magma-poor passive margin.

Depositional architecture and structural evolution of a region immediately inboard of the locus of continental breakup (Liwan Sub-basin, South China Sea)

AbstractNew 3-D seismic data and regional 2-D seismic profiles from the northern South China Sea, the most extensive data set imaging a distal rifted margin in the world, are used to characterize a region located immediately inboard of the locus of Cenozoic continental breakup. The interpreted data set images a ∼6-km-thick continental crust in which the Moho and the base of syn-rift sediment are observed as clear, well-resolved seismic reflections. This extremely thinned continental crust was offset at its base by a complex detachment fault system from which oceanward-dipping listric faults propagated vertically to bound six separate tilted blocks, in a style akin to tectonic rafts. The seismic reflection data allowed us to investigate the thickness of syn- and post-rift strata above tilt blocks, revealing that the early-middle Eocene syn-rift topography was gradually blanketed in the late Eocene (ca. 38 Ma). After 33 Ma (earliest Oligocene), the main depocenter on the margin migrated to the south of the Liwan Sub-basin, i.e., oceanwards, as recorded by the thickening of strata within a breakup sequence. This work is important as it demonstrates how closely structures and sedimentation within the Liwan Sub-basin were controlled by a basal, rift-related detachment system, which is imaged in detail by 3-D seismic data for the first time on a rifted continental margin. Continental breakup was marked by a shift in the locus of subsidence (and crustal stretching) toward ocean crust, within a time period spanning ∼16 m.y. We extrapolate our findings from the South China Sea to the development of asymmetric passive margins across the world.

Inheritance and style of rifting: incremental structural restoration of the Laptev Sea Rift System, north-eastern Russian Arctic

The Laptev Sea Rift System, on the north-eastern continental margin of the Russian Arctic, is a key area to understand the opening of the Eurasia Basin. The rifts developed since Cretaceous/Early Cenozoic times and consists of five, roughly north–south trending depocentres, controlled by major listric normal faults. Three cross-sections from the rift system were incrementally restored to quantify the amount of extension over time and to reconstruct the geological evolution. We show that since the beginning of rifting, fault activity in the Anisin Basin was unevenly distributed between two symmetrical graben systems. The central Ust’ Lena Rift has a completely different structure: regularly spaced west-dipping faults are interrupted by minor east-dipping faults in only three places. Fault dip decreases from west to east, from 60° to 30°, respectively.

A nonlinear inversion of InSAR-observed coseismic surface deformation for estimating variable fault dips in the 2008 Wenchuan earthquake

Satellite Interferometric Synthetic Aperture Radar (InSAR) is playing an increasingly important role in the observation of coseismic surface deformation caused by earthquakes, and has been used to invert for subsurface fault structure and reveal earthquake source mechanisms. However, the mapping of complex non-planar or curved (e.g., listric-shaped) faults still remains a challenging task due to variable dips along the underground depth and the impenetrability of the deep crust. Here, we develop a set of new inversion algorithms to determine the listric fault geometry with InSAR- and GPS-observed surface deformation as the significant constraints. The fault surface with variable dip angles is discretized into consecutive sub-fault layers along the down-dip direction. A nonlinear iteration algorithm is used to minimize the objective function to determine the dip angle for each sub-fault layer. The proposed method is first tested using synthetic data to show its effectiveness for retrieval of varying fault geometry dips, and then applied to the 2008 Mw 7.9 Wenchuan earthquake that ruptured the Yingxiu-Beichuan fault for over 320 km along the southwest-northeast strike. The inversion shows that the dip angle of the seismogenic fault is up to 76° near the surface layer, and gradually decreases along the down-dip direction. A significant decrease in dip occurs within the depths of 6–15 km with a dip of 32° at a depth of 15 km. The dip angle decreases to 2° at a depth of 20 km, and finally merges with the subparallel PengGuan fault, which is basically consistent with geological investigations and seismic waveform data inversion. Using the inferred fault geometry, the slip model associated with the event is estimated. Five high-slip concentrations along the strike of the Yingxiu-Beichuan fault are recognized. The inversion misfit of InSAR data is reduced to 7.1 cm with a significant improvement compared to previous studies.

Gravity-driven structures and deposits resulting from slope collapse in the margin of a carbonate platform (Pennsylvanian, Cantabrian Zone, NW Iberia)

Lower-slope to basin-floor deposits associated to isolated microbial carbonate platforms of the Valdeteja Formation (Carboniferous foreland basin, Cantabrian Zone, NW Spain), containing large slid blocks and debrites, exhibit a combination of contractional and extensional structures, which include thrusts, backthrusts, folds, extensional faults and joints, and boudinaged beds. Several lines of evidence indicate that deformation took place in poorly lithified conditions, such as thickened hinges, hydroplastic fractures, fold axes scatter or folding of microbial limestones.Once restored, all structures indicate a transport direction towards the NNE-ENE. This azimuth coincides with the direction towards which the thrust nappes of the southern branch of the Cantabrian Zone were emplaced during the Variscan Orogeny. However, the interpretation of the deposits and structures as gravity-related, associated to the collapse of the carbonate platform slope, is supported by features such as the rootless character of thrusts, the parallelism between transport direction and paleocurrent orientation, the concomitant development of contractional and extensional structures, the development of normal listric faults affecting the Valdeteja platform, the sedimentary environment (lower slope to basin floor) and the geological context of the Valdeteja carbonate platforms, which predate the Variscan deformation in the study area.

Analogue model of gravity driven deformation in the salt tectonics zone of northeastern Mexico

In the deep seated gravity-driven deformation systems of the Gulf of Mexico contemporaneous extension and contraction of the overburden is favored by mechanical decoupling from the basement along thick salt sequences (up to 4 km). The updip extension is located inland, on the continental shelf of northeast Mexico, and is characterized by extensional listric faults and roll-overs; the downdip shortening zone is located at the deep waters and is characterized by a fold and thrust belt detached above the salt layer. Two physical experiments are used to discuss some aspects of these gravity-driven systems. The experimental setup includes a motor-driven experimental table, an inclined brittle basement (1°), a silicone layer simulating the salt sequences, and sand layers simulating the pre-kinematic Jurassic-Cretaceous strata before Laramide shortening. Deformation resulted in further tilting of the basement (3° to 4°). After the onset of deformation, thin sand layers were added at regular time intervals simulating the syntectonic sedimentation. The experiments reproduced the geometry of the deformation at the frontal ramp characterized by a seaward vergent thrust and its associated deformed region (the Perdido fold belt). The fold and thrust belt localization was favored by the change in basement inclination (a built-in slope change). Key elements interpreted in one available section of the area were reproduced in the model: a) the presence of an antithethic roll-over in the extensional zone and, b) the basinward vergence of folds and thrusts observed in the downdip shortening zone in the mexican Perdido fold belt.

New insights into the structure of the South Caspian Basin from seismic reflection data, Gorgan Plain, Iran

This research presents an integrated study of 2D seismic reflections and calibrated well data to outline the structural development of the Gorgan Plain on the border of the South Caspian hydrocarbon basin, in the Alborz and Kopeh-dagh ranges of northeast Iran. Eight seismic horizons, including two unconformities, have been identified, ranging in age from Early Jurassic to Quaternary. Analysis and interpretation of geophysical data provide an understanding of the structural geology of the Gorgan Plain, crucial for investigation of structural traps. Seismic interpretation indicates structural features such as deep reverse listric faults (inverted normal faults) for older formations; strike-slip and normal faults in younger sequences; dome and basin interference patterns of folding for all the top formations. Well data demonstrate the occurrence of a disconformity at the base of Upper Cretaceous beds and an angular unconformity at the base of Paleogene. Evidence identified in well and seismic-reflection data proves a complex pattern of tectonic phases since the Early Jurassic. It seems reasonable to suppose that these structural patterns are related to a change in direction of tectonic compression, which in turn has produced structural hydrocarbon traps in crests of domes in the Gorgan area.

Discrete-element modelling of hanging wall deformation along the D-1 fault system, Danish North Sea, with implications for gas chimney interpretations

This study uses 3D seismic mapping, NNCC attribute analysis and 2D SDEM modelling of soft-sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller. The computational model experiments used the overall fault and salt-structure evolution as boundary conditions to predict the distribution of faults within the hanging wall. The modelling successfully reproduced geometries similar to the syn- and antithetic faults that are visible on seismics, but it also showed the presence of well-defined vertical deformation zones, both in the hanging wall and the foot-wall of the listric fault. In the model, salt movement controls the formation of the deformation zones. The similarity of NNCC-inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3D seismic data might actually be related to vertical to sub-vertical deformation zones caused by the presence and movement of the salt structure.

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.

The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

A number of discrete slate belts of limited size occur in the Paleoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian shield of south central Sweden. The Grythyttan Slate Belt, studied for more than a century, forms a single basin with the nearby Saxan Slate Belt. We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back- or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centers. The conglomerates are pre-orogenic, not a post-orogenic molasse, as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at ca. 1895 Ma.

Shear heat model for gouge free dip-slip listric normal faults

Abstract Shear heating due to brittle faulting is important in petroleum geosciences, tectonics and seismic studies. Temporal variation of shear heat in a listric normal fault, with a circular arc-shaped fault plane is simulated in this work, which was not done so far. The work is expected to have a far reaching implication in tectonics and petroleum geosciences since listric faults can be closely associated with hydrocarbon reserves (e.g., Valdiya and Sanwal, 2017 ). For such a fault plane devoid of gouge and any secondary faulting, shear heat is proportional to the mass of the hanging-wall block, the coefficient of friction acting between the hanging-wall and the footwall block, and radius of the circular arc. Shear heating intensifies temporally as the hanging wall block slides down and reaches progressively gentler fault dip.

Mass Transport Complexes on a Cenozoic paleo-shelf edge, Gippsland basin, southeastern Australia

Abstract The Southern Platform of the Gippsland Basin of southeastern Australia contains a mixed carbonate-clastic succession deposited on the outer continental shelf in water depths of 50–200 m. Seismic geometry indicates the presence of likely carbonate build-ups formed in deeper water near the north-eastern edge of the Southern Platform by bryozoan mounds which pass to the northeast into distal bottom set deposits in shelf edge clinoforms. Mass Transport Complexes (MTCs) developed along the paleo-shelf edge and basin bounding faults through repeated failure from the Late Oligocene to the Mid Miocene. Two distinct Mass Transport Complexes are present. The Sailfish MTC affects a zone 7 km wide along the paleo-shelf edge in the SE of the area and features slumped paleo-shelf masses with listric faulting and toe thrusts in places. Some inversion of slump-related structures indicates continued movement after an initial failure event. The Devilfish MTC is up to 32 km wide and consists of a zone of extension which passes downslope into a broad zone of compressional structures which die out downslope without the development of a clear emergent slump mass or a terminal structure. Stratigraphic relationships suggest that it formed progressively by creep from the Late Oligocene to the Late Miocene. The difference in deformation styles between the two MTCs can be explained by proximity to major basin forming faults. The Devilfish MTC formed where the paleo–shelf edge sat adjacent to an underlying fault which juxtaposes deeper basin fill sediments against basement. This geometry provided abundant pore water to weaken the overlying sediments which failed under differential loading imposed by shelf edge build ups to form the MTC, possibly triggered by repeated earthquakes. Devilfish MTC differs from mass transport deposit described in the literature in comprising an extensional headwall zone and a main body affected by compressional structures which then die out downslope, probably because of thickening of the section and a return to normal porewater pressures basin-ward. This type of MTC could be termed “Frontally Diffuse” in contrast to the recognised Frontally Emergent and Frontally Confined MTC terminations.

Fault-scarp degradation in the central Exmouth Plateau, North West Shelf, Australia

Abstract Latest Triassic–earliest Late Jurassic domino-style extensional faulting in the central Exmouth Plateau, North West Shelf of Australia, exhibits footwall degradation scarps with up to 1.8 km of scarp retreat of the Upper Triassic Mungaroo Formation. Extensional fault-propagation folding, rotation and uplift produced gravitationally driven scarp collapse of the incompetent and mudstone-dominated uppermost Mungaroo Formation. Scarp degradation occurred along the entire extent of the footwalls of three major faults within the research area. Individual segments display listric fault surfaces in cross-section and scoop-shaped scars in three dimensions. The listric collapse faults dip towards the erosional scarp and sole out at different levels within the upper Triassic Mungaroo Formation. Footwall crestal collapse formed coalesced, scoop-shaped degradation scarps with Mungaroo Formation debris deposited as wedges within the adjacent hanging-wall synclines. Maximum scarp degradation occurred at the fault centres and decreased towards the fault tips. This study proposes new three-dimensional evolutionary structural models for the fault-scarp degradation in the central Exmouth Plateau.

SREDNEKEDROVAYA PALEOSEISMODISLOCATION IN THE BAIKAL RIDGE: ITS STRUCTURE AND THROWS ESTIMATED FROM GROUND‐PENETRATING RADAR DATA

Our study aimed to clarify the seismic potential of the Severobaikalsk fault and to discover the structural features of active faults on the NW shores of Lake Baikal. Seismogenic faults and large seismogravitational structures were mapped in the area of the Srednekedrovaya paleoseismodislocation, one of the most remarkable seismotectonic structures in the Baikal region. During the field trip, we tested the capacities of an OKO‐2 georadar and an ABDL‐ Triton antenna used to study cross‐sections of the Baikal ridge. Its slopes are steep, covered with Pinus pumila and abundant screes, many of which developed into boulder streams (‘kurumnik’). The first studies of the Sredneked‐ rovaya paleoseismodislocation were conducted by V.P. Solonenko and his team in 1964–1965. To some extent, this zone can be viewed as a reference object that can provide much information and thus deserves an in‐depth investiga‐ tion using new technologies. Our study combined the field observation and the interpretation of high‐resolution satel‐ lite images provided by DigitalGlobe (US) and downloaded by SAS.Planet. The consolidated database was sufficient for constructing a new schematic map showing the seismogenic faults associated with the Srednekedrovaya paleoseis‐ modislocation. The cumulative length of the ruptures observed on the surface amounted to almost 29.5 km. Some ruptures are separate from each other, and the rupture spacing ranges from the first tens of meters to the first kilome‐ ters. The width of the widest rupture zone is 1.9 km. The length of individual ruptures varies from 5.0 m to 2.7 km. Morphologically, the Srednekedrovaya paleoseismodislocation is represented by ledges and ditches that often comprise complex grabens disturbing the bedrock and slope deposits. The fault structure of this zone is a typical set‐ ting of orthogonal and slightly oblique crustal stretching, but its manifestation differs in the zone segments. In general, it is a combination of steeply dipping and listric faults traced to the depth of 13 m. In plan, the faults are observed to form the systems of subparallel ruptures that mainly strike at 30°. A linear relationship is established between the heights of the seismogenic ledges and the throws estimated from the ground‐penetrating radar data. The former are larger by 0.5–2.0 m than the throw measured from the radargrams. Apparently, this reflects the magnitude of expan‐ sion of the ledge upward along the sloping slope. In the zone of the main fault plane coinciding with the main ledge, the maximum and mean arithmetic throws are 8.3 and 4.93 m, respectively. On other fault planes, the throws range from 0.4 to 4.6 m. The paleoearthquake magnitude ranges from 6.8 to 7.6, according to the estimations from the seis‐ mic rock collapse volume, fault length, and the displacements. Our study of the Srednekedrovaya paleoseismodisloca‐ tion confirms that listric normal faulting is widespread along the western side of the North Baikal basin and gives in‐ direct evidence that conditions for accumulation and release of seismic energy are different on the western and east‐ ern shores of Lake Baikal. It should be noted, however, that in the studied near‐surface layer of the crust, the blocks of loose material may move along the flat planes due to gravitational sliding that increases under the impact of cryogenic processes on the steep slopes of the Baikal ridge.

Holocene eruptions of Mt. Popa, Myanmar: Volcanological evidence of the ongoing subduction of Indian Plate along Arakan Trench

In the western part of Myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of Indian Plate under Eurasian Plate. Volcanoes of Burma arc, however, did not produce eruptions in the recorded history and are considered extinct. Such perception questions the ongoing subduction, as well as keeping local officials unaware of possibility of future volcanic eruptions in the country. We have investigated the youngest lava flows and pyroclasts of Mt. Popa, which is the best-preserved polygenetic volcanic edifice in Myanmar. The Ar/Ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately. The radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that Mt. Popa produced several eruptions in the beginning of Holocene. The youngest eruption occurred ~8000 BP and included the 1.3 km3 gravitational collapse of the volcanic cone with deposition of the 11-km-long debris avalanche, immediately followed by emplacement of 0.1 km3 of pyroclastic flow of calc-alkaline basaltic andesite composition. The collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault, which was formed during the pre-collapse asymmetric gravitational spreading of the volcanic cone. The recently erupted products are geochemically similar to products of young monogenetic volcanoes of Monywa area located 150 km north Mt. Popa, and both display patterns consistent with magma generation at an active subduction system. Low magma production rate of Mt. Popa (3 × 10−5 km3/year averaged for the 1 Ma of the volcano history) is in agreement with the highly oblique angle and slow rate of the subduction. The fact of the Holocene eruptions of calc-alkaline composition in Burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between India and Asia.

Structural and Tectonic Study of Brifka Anticline, Northern Iraq

A structural and tectonic study was carried out in the area between Baikher and Shaikhan anticlines. It is revealed that the main distinct structural feature is an anticline extending 16 Km northwest – southeasterly obeying the Zagros Range trend. This anticline with 6 km width and 16 km length called Brifka Anticline. Several geologic formations are exposed in the studied area, ranging in time span from the Paleocene to Miocene periods. These formations are Kolosh, Khurmala, Gercus, Avanah and Pila Spi where Kolosh represented the core and Pila Spi makes the carapace of the anticline. The geometrical analysis of this fold along four selected traverses revealed that the anticline is asymmetrical, mostly overturned and verged towards southwest, except in the fourth traverse it is appeared verging towards south. The anticline axis is oriented northwest -southeasterly in the first three traverses whereas it trended towards east- west in the fourth one. The study included the vertical investigation of fold style of folded formations. It appeared that the anticline axis in Pila Spi Fn. is slightly changed from that of Kolosh Fn. in the first and forth traverses, whereas it changed anti clockwise 11-16° from Kolosh to Pila Spi formations in the second and third traverses. The present study showed that the systems okl>c and hol>c are the most dominant joints in the first three traverses whereas hko>a system and ac set are abundant in the fourth traverse. It also revealed that the system hko>a are more abundance than hko>b system which indicate that the compression phase was tensive than the alternative tension. Faults were investigated by field work and satellite image interpretation. It revealed three strike-slip faults inclined to the fold axis. They are BF1, BF2 and Besary faults. Another fault was concluded from the geometric properties and vergency of Brifka anticline. It is a foreland listric fault. This was followed by the determination of the formation thicknesses in order to conclude the sense of displacement of this fault. The determination appeared that the thicker sediments are in the area of the southwestern limb reflecting that the fault is reactivated as reverse foreland listric fault.(.

The 2016 Mw 6.7 Aketao earthquake in Muji range, northern Pamir: Rupture on a strike-slip fault constrained by Sentinel-1 radar interferometry and GPS

On 25 November 2016, the Aketao, Xinjiang earthquake occurred on the Muji fault, which is located at the northernmost end of the right-lateral Karakorum Fault (KF). This event provides a rare chance to gain insights into how the stress accumulates in Pamir margin as the Indian plate collides with the Eurasian plate. Space geodetic measurements including InSAR and GPS were used to obtain coseismic surface displacements associated with this earthquake. Based on a finite fault model, the coseismic slip distribution inverted by the combined datasets indicates that the 2016 Aketao event is caused by a primary shallow strike-slip with minor normal-slip at a steep-dipping angle. To explore the real structure of Muji fault, listric fault model inferred by relocated aftershocks as well as the planar fault model, were used in our slip distribution inversion. The results suggest that the optimal fault model should be a highly-dipping planar fault with two separated asperities. The large slip zone is beneath the surface near the epicenter with a maximum slip of 1.1 m, while the small one in the east breaks the surface, in a good agreement with the field seismic geological survey. The total geodetic moment is 1.35 × 1019N∙m, equivalent to Mw 6.7. The nearly pure dextral strike-slip Aketao earthquake, and the recent 2015 Mw 7.2 sinistral strike-slip Tajikstan earthquake in this region, to some extent, manifest the extension motion is dominated in northern Pamir Plateau, in response to the northward convergence between Indian and Eurasian collision.