Search

Results of long-term explorations in the Kongqueting region, located in an East China Sea depression, suggest that the strong fault activity during the sedimentary period of the Pinghu formation significantly influenced the development of the sedimentary system. However, the aggregation and enrichment of the sand body under a tectonic background has become a problem that requires an immediate solution. Considering research outcomes of previous studies, this study used three-dimensional seismic and logging data to identify Y- or H-type and the en echelon distribution patterns of the fault plane, as well as identify the locations wherein the fault growth index value is greater than 1 in the study region, indicating the tectonic background of the fault transfer zone in the region. Second, the development type of the fault transfer zone was studied, and the sand body enrichment site was identified based on seismic inversion data and the development position of the fault transfer point. This helps clarify the evolution of sedimentary facies of the Pinghu formation combined with the sedimentary environment of the braided-river delta. Finally, after summarizing the coupling relationship between the synsedimentary fault systems and the sand body enrichment patterns, three sand-control models were determined, namely, the flexural-parallel, the en echelon collinear, and the torsional-reformed superimposed transfer zones. The findings of this study lay a foundation for the subsequent search of tectonic lithologic oil and gas reservoirs.

Landforms and sediments on the palaeo–ice stream beds of central Alberta record glacitectonic raft production and subsequent progressive disaggregation and moulding, associated substrate ploughing, and grooving. We identify a subglacial temporal or developmental hierarchy that begins with incipient rafts, including en échelon hill-hole complexes, hill-hole pairs, and strike-slip raft complexes, all of which display patterns typical of transcurrent fault activation and pull apart. Many display jigsaw puzzle–style fragmentation, indicative of substrate displacement along shallow décollement zones and potentially related to patchy ice stream freeze-on. Their gradual fragmentation and smoothing produces ice flow-transverse ridges (ribbed moraine), hill-groove pairs, and paraxial ridge and groove associations. Initiator scarp and megafluting associations are indicative of raft dislodgement and groove ploughing, leading to the formation of murdlins, crag-and-tails, stoss-and-lee type flutings and drumlins, and Type 1 hogsback flutings. Downflow modification of rafts creates linear block trains (rubble stripes), stoss-and-lee type megaflutings, horned crag-and-tails, rubble drumlinoids, and murdlins, diagnostic of an immature palaeo–ice stream footprint. Lateral ice stream margin migration ingests disaggregated thrust masses to form ridged spindles, ladder-type morphologies, and narrow zones of ribbed terrain and Type 2 hogsback flutings, an assemblage diagnostic of ice stream shear margin moraine formation.

From November 2012 to May 2013, the coastal region of Bejaia (northeastern Algeria), experienced three distinct seismic sequences in about 6 months. The first one was on 28 November 2012 started with a mainshock of magnitude Mw = 5.1, and the second one occurred on 22 February 2013, a few kilometers southeast from the first one and culminated with a magnitude Mw = 4.3 earthquake. The last sequence comprised two shocks of moderate magnitude: a Mw 5.2 event on 19 May 2013, followed by a Mw 5.0 event on 26 May. Earthquakes were relocated using the double-difference method with differential times derived from phase-picked data and waveform cross-correlation delays. We compiled a dataset of 252 accurately relocated events that are associated with focal mechanism solutions consistent with a near-vertical, right-lateral strike-slip fault striking NW–SE. The data analysis is coherent with a new fault extending over 35 km from the Gulf of Bejaia to the northeast of Kherrata. This newly discovered active fault, we name it the Babors Transverse Fault, appears to be segmented in four segments (S1–S4) evidenced by separate earthquake clusters. Static Coulomb stress changes and the spatiotemporal evolution of seismicity suggest static stress triggering, with the rupture first activated in the S1 segment in the northwest, migrating to S2, and terminating in S3 and S4 in the southeast. Stress tensor inversion from focal mechanisms implies that the dominant local stress field is a pure strike-slip regime, with N–S maximum horizontal compression in good agreement with the velocity field displaying reorientation compared to the oblique convergence of Africa–Eurasia. In this study, the newly identified Babors Transverse and Tizi N’Berber–Darguinah faults, in association with the NNW–SSE offshore active fault of the Greater Kabylian block (in the north) and the NNW–SSE right-lateral strike-slip Lâalam fault (in the south), form an en echelon fault system that acts as a transfer zone between offshore thrust faulting and the Ghardimaou–North Constantine strike-slip fault.

Delimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F1, F2 and F3, are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F1 folds by F2 folds; and (ii) the parallelism between the GBF and F2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.

Rock shear fracturing occurs frequently in engineering sites, such as underground tunnels and rock slopes. One main factor is that the multiple presence of crack is complicated, especially in terms of crack status. This study aims to investigate the crack status from both crack existence and development. Rock fracturing is carried out by shear loading. Acoustic emission (AE) monitoring system and CCD camera are used to catch the characteristics of crack evolution. Results demonstrate that crack status is influenced by rock physical properties and its stress boundary, there exists a positive correlation between normal force and crack geometry (such as the angle of en echelon crack). Crack existence follows a distribution of “the whole size of en echelon crack and the local region of pinnate crack” in terms of spatial-temporal aspects. Crack development manifests the characteristics of “Inheritance and Variability”.

On Friday, January 24, 2020 at 20.55:11 local time (17:55 UTC), an earthquake with a magnitude of Mw = 6.8 has occurred in Sivrice district of Elazig (Eastern Turkey). Focal mechanism solution is consistent with pure left-lateral strike-slip faulting; the location of the epicenter and fault mechanism suggest deformation along the Puturge segment of the East Anatolian Fault Zone. A 10-day fieldwork was carried out along the Puturge segment to study surface deformation; the geometry of the surface rupture and other seismic geomorphological structures were mapped and studied in detail. The field data are also correlated with satellite images. This paper, therefore, presents classification of seismic geomorphological structures and discuss intimate relationship between fault geometry and stress field in the region. Seismic geomorphological deformation and related features of the Sivrice (Elazig) earthquake are observed in the area between Gezin (Elazig) and Ormanici (Puturge) villages; they are classified into two as seismotectonic and seismo-gravitational features. Field observations confirm that seismo-gravitational structures develop along both Gezin-Sivrice–Doganbagi and Doganbagi–Cevrimtas–Ilincak–Koldere–Ormanici sections of the Puturge segment, while surface rupture is mapped as seismotectonic structure only along the Doganbagi–Cevrimtas–Ilincak–Koldere–Ormanici section. Small-scale landslides, rock falls, feather cracks along asphaltic roads, and laterally discontinues ground failure-related features are common seismo-gravitational structures that developed along the fault zone. In addition, small-scale lateral spreading and liquefaction structures are common especially in areas where fault-perpendicular streams meet the Karakaya Dam reservoir. The surface rupture is mapped as stepping and overlapping en echelon fractures along elongated pressure ridges between Cevrimtas and Doganbag villages, to northwest of Ilincak village, along 1.5-km-long pressure ridge between Topalusagi and Doganyol, across the elongated hill that developed on an alluvial fan to the northwest of Doganyol and in the area between Koldere and Ormanici villages. Surface fractures deforming the pressure ridges are all aligned parallel to the long axes of the ridges and display reverse components that give rise to small-scale pop-up structures. Interferometric SAR (DInSAR) studies indicate a 10-cm uplift in the northwestern block of the fault and a 6-cm subsidence in the southeast block. The difference in vertical movements between two blocks of the fault is interpreted to suggest that at least 30-km-long section of the Puturge segment in the area between southwest of Sivrice and Puturge is broken during the main shock. Although the focal mechanism solution of the main shock gives pure left-lateral strike-slip faulting, there is no significant left-lateral displacement observed during the fieldwork. This can be explained by the following: (1) left-lateral strike-slip displacement was not able to reach the surface; (2) left-lateral torque movement of the fault around a vertical axis during the earthquake, (3) restraining bend nature of the Puturge segment, or (4) the presence of Puturge metamorphics along the fault strike. It is also important to note that along most part of the Puturge segment where surface rupture is observed, talus, colluvial or alluvial fan sediments are exposed; unconsolidated and/or poorly consolidated nature of these sediments may also be counted as one of the main reason for not observing horizontal displacement on the surface. When we compare these surface deformations with the surface ruptures that occurred in the last 100 years in Turkey, we suggest that the formation of the surface deformations is variable depending on: (1) the fault type and the state of regional stress, (2) the magnitude of the earthquake, (3) the duration time of the earthquake and (4) the geomorphologic feature of landscape in relation to the lithologic and structural features of the rock units along the active fault zone.

The eastern parts of the Sistan and Baluchistan province, southeast Iran is a tectono-magmatically active zone that continues to witness numerous small and large earthquakes. The volcanism in this region is related to the subduction of the Oman plate below the Eurasian plate. The study of the spatial distribution of the epicenter and focus of the earthquakes suggests close association with the numerous faults related to collisional tectonics. Evidence of ground fractures and faulting related to Saravan earthquake of 16th April 2013 which is the largest (7.8 Mw) recorded earthquake in the region is documented. This earthquake occurred along a segment of the NW-SE trending Saravan normal fault and its reverberations were felt as far away as New Delhi and Dubai. En echelon, 2-km long, northwest-southeast trending surface fractures and faults are recorded in the field near Hampani village southwest of Gosht city. The epicenter is located in the remote hilly terrain east of Gosht. Many of the fractures are listric. The fault records a downthrow of about half a meter and some segments record brecciation of phyllite, liquefaction of sediments and emergence of saline springs are associated features. Despite the high magnitude only a few casualties and minor damage to civil structures were recorded in Iran due to the sparse population and its remoteness.

Interpretation of a 3D seismic survey located on the western margin of the Northern North Sea Basin demonstrates how the propagation, overlap and linkage of two north–south-striking, en echelon normal fault segments exerted a powerful influence over prospective subtle stratigraphic traps. The relay ramp that formed between the segments appears to have focused sediment dispersal, controlled reservoir distribution and aided post-depositional petroleum migration. Integration of electrical well log data, root-mean-square (RMS)-amplitude analysis and biostratigraphy with seismic interpretation demonstrates that a series of elongate, linear, sand-prone (reservoir) channel complexes characterize the depositional slopes generated by fault growth. The combination of synsedimentary rotation of bedding due to fault propagation and associated footwall uplift led to erosion and truncation of a laterally extensive, older channelized system (Lower Sequence), the downdip parts of which extend beyond the relay ramp. Its subsequent drape by transgressive shales created the subtle stratigraphic trap that now hosts the Cladhan Field, with charge occurring because the sandstones belonging to the Lower Sequence extend as far as the active kitchen in the neighbouring (hanging-wall) depocentre situated downdip and to the east. In contrast, the exploration of a younger, Upper Sequence of sandstones has proven to be disappointing due to their more restricted distribution, lack of access to charge, and occurrence of faults that offset and breach the thin end of the stratigraphic wedge. The implication is that partially breached relay ramps not only provide a preferential site for synrift clastic reservoirs to develop but also form important migration pathways through which oil passed from a petroleum kitchen into a trap. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: http://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

En echelon fractures and veins are among the most common and distinctive geological structures, yet their three-dimensional forms and relationships to surrounding structures are commonly unclear. X-ray computed tomography (CT) offers an unrivaled ability to examine structures within rocks in three dimensions, and it is applied here to a sample of drill core from the Marcellus Shale of southwestern Pennsylvania (USA). CT images yield qualitative and quantitative data on the transition from a pyrite-rich planar vein to an en echelon veinlet array, and on the heterogeneity of veinlets within the array. Using a combination of three- and two-dimensional images, geometric data, and traditional petrography, we identify a range of veinlet shapes consistent with deformation during formation of an antitaxial graphite-calcite-pyrite vein system. Each of the veinlets is rooted in the underlying planar vein where it is narrowest. The transition from planar vein to en echelon array coincides with a change in bedding, suggesting that competency contrasts between adjacent beds controlled the fracture morphology. Veinlets initiated as short, lenticular fractures at ∼45° to the planar vein before lengthening, dilating, and rotating. None of the veinlets are strongly sigmoidal, nor is there measurable offset across the margins of the planar vein; therefore, finite non-shear strain was very limited, and fluid overpressure–induced fracturing during burial and diagenesis is probably the most likely process for fracturing and vein formation.

This work studies the characteristics of seismic coda waves attenuation in Eastern Anatolia. The short-period coda waves from about 400 earthquakes that occurred in Eastern Anatolia between 1989 and 2018 are analyzed. The three-component digital records of seismic events with M ≥ 3.5, recorded at epicentral distances of 10–600 kilometers, are used. S-wave attenuation was estimated from short-period coda envelopes (including those of Lg wave coda) at a frequency of ~1 Hz. The structural features of the S-wave attenuation field in the upper mantle of the Lake Van area are similar to those for other seismoactive zones. The attenuation field is represented by blocks, which have weak attenuation and are subisometric in plan view, and strong attenuation zones; in the case of blocks, attenuation decreases in the direction from the boundaries to the central of these blocks (Q-factor is up to 300–700). Among the zones of strong attenuation, which are usually linearly elongated, the most noticeable are the en echelon zone (north of 39° N), associated with the North Anatolian Fault; here, QS ~ 80–110 (up to QS ~ 60 in the north); and the Bitlis-Zagros fold and thrust belt (QS ~ 60–70). Sources of the strongest earthquakes with M > 6.0, including the 1976 Caldiran–Muradiye and 2011 Van earthquakes, are confined to the boundaries of the blocks. The attenuation field structure corresponds to the velocity field structure: low-velocity anomalies correspond to low-Q zones. Vertically elongated volumes, with which intensive localized seismicity (earthquake swarms) is associated and along which deep fluids can migrate, are revealed. Seismic activation in such volumes precedes large seismic events (for example, the 2011 Van earthquake). It is assumed that such volumes can be interpreted as local seismogenic sources.

The formation conditions and time sequences for various types of wrench-related fractures are not clear. Based on a parabola-type failure criterion, this paper has gotten new insights on those questions. In a simple shear, the occurrence of either tensional fractures or Riedel shears is controlled by the ratio (Rtc) of tensile strength to cohesion. In a pure shear, the occurrence of either second order tensional fractures or second order Riedel shears is controlled by the ratio (Rtci) of tensile strength to cohesion, given a constant inner frictional coefficient. Where the Rtc or the Rtci is less than a certain value, the en echelon tensional fractures will occur first. Where the Rtc or the Rtci is bigger than the certain value, the Riedel shears will occur first. Where the Rtc or the Rtci is equal to the certain value, the en echelon tensional fractures and the Riedel shears will occur simultaneously. The understandings will enhance the research on wrench related fractures and will be of significance in petroleum exploration and development, because fractures are both important accumulation spaces and key migration paths for oil and gas.

High-quality three-dimensional (3D) seismic reflection and borehole data are used to characterize in detail the structural style and evolution of inversion structures in Horus oil field. This field lies in the central part of Alamein Basin, northern Western Desert of Egypt, in the hanging-wall of a major Jurassic-Early Cretaceous normal fault. This fault delineates the northwestern margin of the basin and extends in an ENE-WSW direction, bounding a half-graben trough in its hanging-wall. This trough had continued subsidence during Early Cretaceous time with depositional thickening of the Lower Cretaceous Alam el Bueib Formation. Fault displacement had ceased near the top of Barremian level. Aptian-Lower Senonian seismically traced levels show a major NE-plunging asymmetrical anticline overlying the Jurassic-Barremian half-graben. This anticline is sub-parallel to the inherited Jurassic rifting fault. At shallower Cretaceous levels, this fault is replaced by a set of WNW-ESE-trending en echelon faults of considerable displacements, which probably indicates a strike-slip component of deformation. These faults display normal separation on all displaced stratigraphic levels and tip upwards in the upper part of the Upper Cretaceous succession. This marks a considerable change in the tectonic mode of the area. NW-SE extensional faults were developed perpendicular to the fold axis, dissecting the anticline into several blocks entrapping hydrocarbons. Detailed seismic structural analysis of growth strata indicates that the fold initiated in Early Senonian time was associated with the inversion of the earlier Jurassic-Barremian half-graben. This basin inversion is attributed to the Syrian-Arc event that dominated North Africa during the Late Cretaceous time and continued through to the Early Miocene. Low fold amplitude coupled with normal displacements on the deep-seated ENE-WSW-trending Jurassic-Barremian fault indicate a mild phase of positive inversion. The Upper Senonian-Eocene stratigraphic sequences increased in thickness in the back-troughs that were associated with basin inversion.

Strike-slip deformation belts are interesting structures in the crust and are of significance in petroleum exploration. The Shunbei 5 fault belt (SB5), a long strike-slip deformation belt in the Tarim Basin, played an important role in the formation of a recently discovered major oilfield known as the Shunbei oilfield. In this study, models of plan view and vertical profile were established to interpret SB5 with multi-cycled tectonic activities. To this end, its structural framework, tectonic evolution, and associated plate tectonics were investigated using 2D and 3D seismic data. SB5 was formed as a dextral simple shear belt at the end of the Middle Ordovician. In the plan view, R-shears and P-shears with local transpressional and transtensional structures were observed. Along the vertical profiles, various structural styles occurred at various depths and strata in response to various stratigraphy mechanisms. Although these structures show clear boundaries between them, they correspond to the same formation time, indicating that they underwent deformation simultaneously. The second activity of SB5 occurred at the end of the late Ordovician, during which it was a dextral transtensional strike-slip deformation belt consisting of left-stepping en echelon R-shears. The R-shears were transtensional during the progressive deformation. Subsequently, SB5 underwent several strike slips of weak strength. Notably, SB5 cut through a deep Middle Cambrian gypsum salt layer and connected the deep Lower Cambrian source rock with deep Lower and Middle Ordovician carbonates to form the oil and gas reservoirs. The established models are of reference value in the interpretation of other subsurface strike-slip deformation belts.

The Apenninic chain, in central Italy, has been recently struck by the Norcia 2016 seismic sequence. Three mainshocks, in 2016, occurred on August 24 (MW6.0), October 26 (MW5.9) and October 30 (MW6.5) along well-known late Quaternary active WSW-dipping normal faults. Coseismic fractures and hypocentral seismicity distribution are mostly associated with failure along the Mt Vettore-Mt Bove (VBF) fault. Nevertheless, following the October 26 shock, the aftershock spatial distribution suggests the activation of a source not previously mapped beyond the northern tip of the VBF system. In this area, a remarkable seismicity rate was observed also during 2017 and 2018, the most energetic event being the April 10, 2018 (MW4.6) normal fault earthquake. In this paper, we advance the hypothesis that the Norcia seismic sequence activated a previously unknown seismogenic source. We constrain its geometry and seismogenic behavior by exploiting: 1) morphometric analysis of high-resolution topographic data; 2) field geologic- and morphotectonic evidence within the context of long-term deformation constraints; 3) 3D seismological validation of fault activity, and 4) Coulomb stress transfer modeling. Our results support the existence of distributed and subtle deformation along normal fault segments related to an immature structure, the Pievebovigliana fault (PBF). The fault strikes in NNW-SSE direction, dips to SW and is in right-lateral en echelon setting with the VBF system. Its activation has been highlighted by most of the seismicity observed in the sector. The geometry and location are compatible with volumes of enhanced stress identified by Coulomb stress-transfer computations. Its reconstructed length (at least 13 km) is compatible with the occurrence of MW≥6.0 earthquakes in a sector heretofore characterized by low seismic activity. The evidence for PBF is a new observation associated with the Norcia 2016 seismic sequence and is consistent with the overall tectonic setting of the area. Its existence implies a northward extent of the intra-Apennine extensional domain and should be considered to address seismic hazard assessments in central Italy.

The Pillara Zn-Pb deposit is located on the northeast margin of the Canning Basin, Western Australia where it is hosted by a series north-northeast-trending, en echelon, conjugate normal faults tha...

For studying rock avalanches, complex surficial and internal characteristics have been regarded as important indicators for understanding avalanche transport mechanism. In the present study, deposits of a massive Holocene rock avalanche, termed the Nyixoi Chongco rock avalanche, located on the Lhasa Terrane of Tibetan Plateau in China are investigated, based on a combination of remote sensing data and in-situ observations. It is revealed that abundant surface landforms, including Toreva blocks, longitudinal and transverse ridges, en echelon ridges, and hummocks, are commonly developed. Corresponding to these surface morphologies, a series of internal sedimentary structures, including jigsaw structures, inner shear zones, aligned clasts, diapiric structures, convoluted laminations, and basal decollements, are also well developed. Through an analysis of these surficial and internal characteristics, we propose that a rapid motion, similar to a laminar flow, most likely occurred after the avalanche mass detaching from the source area. In this process, the detached avalanche mass was mainly emplaced by a frictional, simple shear process characterized by a bulldozing effect that occurred between the sliding mass and the substrate. From the proximal to the distal sections of the avalanche, the transport of the rock mass can be classified into four major processes: an extension-dominated sliding process in the transition zone, a compression-dominated sliding process in subzones III-1 and III-2 of the accumulation zone (III), a sliding process co-dominated by compression and lateral spreading in subzone III-3, and a rapid radial spreading process in subzone III-4. The results of our study provided an improved understanding of the transport mechanisms of this rock avalanche.

En-echelon fractures have been commonly observed in nature. To understand the interaction of en-echelon flaws, numerical simulation with flat-jointed model and laboratory experiment has been conducted on specimens containing en-echelon flaws of various configurations. With reliable micro-parameters of the flat-joint contact through a comprehensive calibration, numerical results reveal a strong interaction in flaw configurations of extensional bridges but a mild interaction in flaw configurations of contractional bridges. The patterns of linking cracks are found to be dependent on the flaw configurations. Furthermore, two typical linking cracks, tensile and shear cracks, are identified in terms of contact force evolution, crack development, micro-components and damage zone width. Experimental results not only verify the above conclusions given by simulation, but also provide more distinctions between tensile and shear cracks in grain scale. Based on these findings, an empirical correlation is suggested between debonding behavior in PFC models and microcracking behavior in laboratory experiments. Finally, the tendency of crack initiation stress and peak stress in simulation and experiment is compared and explained with respect to the flaw-array angle. A good agreement between simulation and experiment indicates the capability of the flat-jointed model of PFC2D in handling complex crack coalescence problems in rocks.

Topographic divides along the northeastern marginal zone of the Tibetan Plateau separate tributary drainage systems that flow into the Yellow River. Field investigations and topographic analysis indicate that: i) Late Quaternary–Holocene sedimentary sequences are deformed into a series of left-stepping WNW–ESE-trending en echelon flexural folds; ii) the folds form topographic divides that separate drainage systems that flow into the Yellow River; and iii) the regional axis of maximum compressive stress, as inferred from flexural folds and active fault-related structures, trends ENE–WSW, consistent with geophysical and seismic data. These results demonstrate that the topographic divides formed through active flexural folding and faulting in the northeastern marginal zone of the Tibetan Plateau, corresponding to the ongoing northeastward shortening of the plateau accommodating the Eurasia-India continental collision.

The northwestern foreland of Tunisia (El Kef region and its surroundings) is a key area to study the tectonics of main E-W basement strike-slip faults and related structures. To achieve this goal, we used multidisciplinary approaches including field work , paleo-stress analyses and geophysical data. These approaches allowed us to propose a new structural model based on the evolution in space and time of Riedel shear type faults and to highlight their role in the structuring of the study region. During the Mesozoic , E-W normal oblique-slip basement faults (D1: Ghardimaou fault, D2: El Kef-Ouergha fault, D3: Jebel Harraba-Guern Halfaya fault and D4: Tajerouine fault) controlled the architecture and the subsidence of the El Kef Basin within a transtensive setting by block tilting , NE-SW negative flower structures, Triassic salt ascent and deposits thicknesses and facies variations. During the Eocene and Late Miocene , the NW-SE shortening (tectonic inversion) is expressed by the oblique reactivation of the pre-existing E-W faults as dextral strike-slip faults. This reactivation is accompanied by map-scale Riedel shear type faults, where D1, D2, D3 and D4 formed the principal displacement zones (PDZs). The successive movements of these faults cut the study area in several right-lateral shear bands in which subsidiary Riedel fractures (R, R', X, P, T and Y) and NE-SW folds are developed. Fault slip data and fractures analyses, across the study area, show several fracture systems which interact under a NW-SE SH max. During Quaternary, the NNW-SSE shortening reactivated the preexisting E-W fault networks (PDZs). It also, formed N S left-lateral strike-slip faults, NW-SE grabens , NE-SW reverse faults, and ENE en echelon folds arranged within Riedel shear bands.

Within the Maritimes Basin of eastern Canada, the onshore Moncton sub-basin is one of the NE-trending second-order depocenters that recorded a complex tectonic history from Late Devonian to Late Carboniferous. Analysis of a 3D seismic volume in the McCully gas field area exemplifies the complex interplay between mainly continental sedimentation and several tectonic episodes that contribute to a net extension or shortening in an overall strike-slip setting. The new interpretation demonstrates the role of normal faults oblique to the sub-basin axis in initial phases of sub-basin development and their influence on the deposition of the Upper Devonian-Lower Mississipian Horton Group sediments. A subsequent episode of transpression is recorded by syn-folding growth strata (base of the Mississipian Sussex Group) preserved on both flanks of an anticline that belongs to a fold system that shows an en echelon geometry relative to sub-basin bounding faults. Deposition of a marine interval (Middle Mississipian Windsor Group), including evaporitic strata is succeeded by an halokinetic growth succession (Upper Mississipian-Pennsylvanian Mabou Group) found in minibasins that formed through drape folding (downbuilding) in response to salt-walls development. This polyphased history illustrates the complexity of strike-slip basins that are often controlled by structures with different geometry and kinematics in time and/or space.

The Ban Pong Basin (BPB) is a small intermontane basin of Neogene age on the western margin of the Chiang Mai Basin in northern Thailand. The BPB was discovered in 2012 because of new quarrying activity. The basin complex formed in the hanging wall of a major Cenozoic low-angle normal fault (LANF) called the Inthanon Detachment, and provides a unique record of the Neogene deformation related to the detachment. The BPB’s preservation is probably due to its location in a synform within the rising Western Ranges metamorphic complex, which forms the Suthep-Inthanon mountain range. Initially, extensional faults in the BPB were oriented ∼N-S but then rotated to NNW-SSE, NW-SE, and ENE-WSW to NE-SW directions, which we interpret as arising due to local stress rotation related to the adjacent rising metamorphic complex. Folds, thrusts, and inverted normal faults both within Neogene sediments of the Mae Rim Formation and adjacent Palaeozoic metasedimentary rocks indicate a phase of compression that affected the basin late in its history (probably latest Miocene-Pliocene based on the regional timing of inversion). Complete Bouguer anomaly interpretations of 235 reduced gravity stations reveal two en echelon sub-basins, bounded by gravity highs related to shallow-lying Palaeozoic rocks and underlying high-grade metamorphic basement rocks. Two gravity-density cross-section models indicate the basin fill comprises up to 250 m of weakly lithified syn-rift sediments. The absence of a classic half graben morphology is probably related to inversion and erosion that significantly modified the extensional basin geometry and original thickness.

The Itabira Iron District, located at the northeastern end of the Iron Quadrangle, still presents some uncertainties regarding the tectonic origin of its macrostructure. The Itabira synclinorium is characterized by a large synformal structure in the NE-SW direction, with anticlines and synclines distributed "en echelon". The analysis of the relationship between the primary structures and the plane-axial schistosity, linear and planar structures, allowed the development of a three-dimensional model for the region. The increase in deformation from E to W, observed from the Cauê mine to the Conceição mine, and the rotation of the axes of the macrostructural folds, characterized by the Cauê, Dois Córregos, and Conceição synclines, are explained in this study by a model with six deformation phases. The increase in shortening from the Cauê mine towards the Conceição mine can be explained based on the flanking fold model. The application of this mechanism to explain kilometric-scale folding has been presented for the eastern flank of the Santa Rita Syncline in the Iron Quadrangle.

Following observations of mobilized boulder trail populations from Cerberus Fossae, Mars, that have been interpreted as possible evidence of large magnitude marsquakes rupturing for distances of ~207 km along exposed active faults, additional boulder trail populations were measured along shorter faults within the region of Grjota Valles (50-150 km length) to test the hypotheses that (1) these faults are also candidate locations for marsquakes, and (2) that marsquake magnitude might be smaller, limited by fault dimensions available for rupture. For a region containing two en echelon graben, boulder trail data define two anomalies with maxima in (a) boulder trails per kilometer, and (b) maximum width of boulder trails, one that is ~116 km in length along strike and the other ~70 km in length along strike. Values for the maxima are 45 trails per km and 5 m mean trail width for the 70 km long anomaly, and 115 trails per km with 5.3 m mean trail width for the 116 km long anomaly, above background values measured elsewhere along these faults of zero trails per kilometer with zero boulder trail widths. If combined with published data from Cerberus Fossae with a ~207 km long anomaly in boulder trails per km (125 trails per km maxima) and maximum mean boulder trail width (8.5 m maximum trail width), the 3 datasets suggest correlations between the (a) along-strike length of boulder trail anomalies, (b) boulder trails per km and (c) maximum boulder trail width. If interpreted as due to single marsquakes, and if the dimensions of these anomalies are a proxy for rupture length, when combined, one interpretation of this is that boulders have been mobilized by marsquakes and that the marsquake magnitude is proportional to the along strike length of the anomalies. In other words, the data suggest that marsquake magnitude, if that is the cause of the anomalies, is limited by fault length as expected for terrestrial seismically active faults. Such findings suggest that the Martian surface may have been shaken, in the very recent past, by large magnitude marsquakes. We discuss this in terms of the seismicity of Mars.

Strike-slip faulting often develops en echelon faulting resulting from Riedel shears. When faulted basement rocks are covered by thick unconsolidated deposits, this en echelon faulting leads to the subsequent evolution of a series of pop-ups along the fault during cumulative displacement. Although previous sandbox experiments and numerical simulations have examined the evolutionary processes for Riedel-shear pop-up structure formation, few field examples have been reported until now concerning (1) the time-series of evolutionary processes of pop-up structure generation and (2) time periods necessary to reach the stage of a “throughgoing fault zone”. The Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone is among the largest and fastest-slipping active fault zones in Japan, and a series of tectonic pop-ups related to left-lateral strike-slip displacement have developed in the Fujimi area in the central to southern part of the fault zone. This area is covered by thick (~820 m) unconsolidated deposits, which have resulted in Riedel-shear pop-up structures. We conducted a geomorphic analysis, an extensive drilling survey, and pit excavation survey on a pop-up in the Misayama-Godo area of Fujimi, and concluded that (1) the pop-up structures in the Misayama-Godo area have been built up by two marginal faults between ~100 ka and ~10–20 ka, and (2) the fault displacement at depth has been concentrated on the faults that run across the pop-up structures (corresponding to the “throughgoing fault zone”) since ~10–20 ka. One of the faults that run across the pop-up structures probably slipped during the most recent earthquake in the central to southern part of the ISTL fault zone. This study is presented as a robust field example of the evolution of Riedel-shear pop-up structures during cumulative strike-slip faulting.

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, decollement 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.

This article discusses the structural control of the Precambrian Al Ji'lani basic intrusion, which is one of numerous oval igneous intrusions scattered within different terranes of the Arabian-Nubian Shield. The area in the Ad Dawadimi vicinity exposes 75 km long northwest-trending sinistral en echelon fault systems. During East African Orogen, the master fault segments propagate as sinistral system creating a rhomboidal shape releasing stepover area, defined in this work as Al Ji'lani pull-apart structure. The fault kinematic analysis shows significant variation in stress tensors between data measured outside and inside Al Ji'lani pull-apart structure. The stress tensor, computed from fault population inside the pull-apart, typify a NW-SE to NNW-SSE pure tectonic extension. Outside the pull-apart structure, the stress tensor illustrates a pure strike slip stress regime with a minimum stress axis that varies from NW-SE to NNW-SSE, and maximum stress axis that varies from ENE-WSW to NE-SW. The pull-apart northwestern corner develops, at the end stage, a circular to elliptic basic intrusion known as the Al Ji'lani layered gabbro intrusion. The intrusion is located in the releasing stepover's corner and characterized by inward dipping layers, suggesting an evolution genetically related to the pull-apart growth. Field data and faults kinematic analysis reveal that the Al Ji'lani structure is an ancient Ediacaran–Cryogenian gabbro intrusion developed in pull-apart corner, widely recognized as preferential crustal weakness zone in recent systems of fault-related-volcanism. The results support a predominant relationship between brittle tectonic and volcanism rather than shearing as it was previously accepted.

Landslide hazards frequently occur in the Chinese Loess Plateau every year, causing devastating casualties, damage to engineering structures, and reduction in land resources. This work presents an experimental investigation of four samples retrieved from Jingyang in the southeast of the Chinese Loess Plateau, using ring shear tests and microscopic observation, in an attempt to explore the shear behavior of loess and paleosol under the larger deformations that are involved in landslide hazards. The results of ring shear tests reveal that the loess and paleosol samples exhibited different strength characteristics. The residual strength envelopes could be assumed to be a straight line, and the overconsolidated state had negligible influence on the residual shear strength. After the shear experiment, shear bands were studied from both macroscopic and microscopic perspectives. Macroscopic observation and mechanical analysis indicate that all samples developed a main horizontal failure plane related to the maximum shear stress, while loess samples also developed en echelon failure planes tilting from the horizontal at angles of 14–18°, which are interpreted as Coulomb slip planes. Scanning electron microscopy reveals that the particles inside shear bands tend to be re-oriented. Particle crushing occurred in the shear process, which was also verified by particle size distribution analysis.

The aim of this paper is to investigate the brittle failure mechanism of rock slopes with non-persistent en-echelon joints using the extended finite element method (XFEM). In the research, a novel geometrical method distinguished from the conventional level set method is proposed to describe the cracks in the XFEM. The numerical form of the J-integral considering the effect of the bode force and the frictional contact force is provided. Moreover, a refined coalescence method is proposed. Then, the state function employed to define the state of the crack and the method used to define the crack growth increment are introduced. Next, some numerical tests are carried out to validate the feasibility of the XFEM. Finally, two typical rock slope modes are provided to illustrate the mechanism of the step-path brittle failure of rock slopes.

High-resolution Galileo data of Ganymede's complex surface provide strong and ubiquitous evidence of strike-slip motion: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and strained craters. In a previous study, we performed a detailed mapping of these strike-slip morphologies within nine regions of Ganymede: Dardanus Sulcus, Tiamat Sulcus, Nun Sulci, Byblus Sulcus, Nippur and Philus Sulci, the Transitional Terrain of Northern Marius Regio, Anshar Sulcus, Arbela Sulcus, and Uruk Sulcus. Strike-slip indicators were inferred in various combinations at each site, in addition to extensional processes. However, the prominence of strike-slip indicators suggests shearing has been an important process to the structural development of Ganymede's surface. Moreover, tidal stresses on Ganymede, under particular circumstances, may have been sufficient to induce Coulomb failure and generate strike-slip faulting. Here we investigate the role of both diurnal and non-synchronous rotation (NSR) tidal stresses on Ganymede using the numerical code SatStress. We resolve normal and shear tractions onto discrete fault segments of specified orientation and assess Coulomb failure potential for the nine inferred fault zones. Testing a range of subsurface fault depths (z = 0–5 km) and ice friction (μf = 0.2–0.6), we find that tidal stress models of combined diurnal and NSR stress readily promote Coulomb failure within each studied fault zone. High friction (μf = 0.6) limits failure depths to ∼1 km, while low friction (μf = 0.2) extends failure depths to ∼2 km, consistent with elastic thickness estimates. We also compare each fault zone's predicted shear sense to the inferred shear sense from our structural mapping efforts and find compatible senses of shear among six of the nine regions that exhibit notable fault offset and/or prevalent inferences of en echelon, duplexes, and strained craters. In addition, principal stress orientations and failure feasibility computed on a global scale suggest that combined diurnal and NSR tidal stresses promote Coulomb failure within the shallow (

Veins in an array are characterised by measurement of their length (L), width (W) and orientation (θ), and by the number (0, 1 or 2) of adjacent veins to which they are linked. Data from a single array (129 veins), and two regions (comprising 146 and 64 arrays) have been measured and analysed. The system for measurement provides a sound basis for the quantitative assessment of both the array geometry and the effects of interaction of adjacent veins. Linkage is achieved by a breaching of the bridges of wall rock between veins, either by extension fractures, shear failure or the development of stylolites. It is primarily controlled by the strain, as measured by vein width. The size and shape of the veins is found to be controlled by two factors: the degree of linkage and the kinematics of the array (transtension/transpression). Interplay of these produces variability both within and between arrays, and this is examined for two large datasets from the Lower Jurassic of Somerset and the Lower Carboniferous of the Gower Peninsula, South Wales.

Pre-Cambrian to Paleocene age sedimentary rocks predominantly characterize the fold and thrust belt of eastern Hazara division. The Hazara Slate Formation is the oldest rock unit which represents the Precambrian sequence. The Permian and Triassic sequences are missing. The Jurassic sequence comprises Samana Suk Formation whereas the Cretaceous exposed is Chichali and Lumshiwal formations. The Eocene sequence consists of Nammal and Sakessar formations. The structural fabric of the area is mainly attributed to a series of northwest trending parallel to en echelon anticlines and synclines. Most of these folds are found to be asymmetric and are Northwest facing. Several thrust faults verging both to the north have been mapped that generally dissect the forelimbs of the anticlinal structures. But the following study going to be focus on surface structural features as well as subsurface projections of folds and faults. Study of such structural features has get prime importance in economic geology such as petroleum geology, mining geology and engineering geology. In our study area subsurface projection of folds and faults along the structural transects of the area suggests that these structures have formed as a result of shortening associated with ramping from a regional basal decollement. All the structures clearly demonstrate that the eastern Hazara area has been subjected to compressional deformation/stresses oriented northeast southwest. The repetition of rock units indicates, folding in the area and thrusting of Pre-Cambrian Hazara Formation over younger Paleocene Lockhart Formation, evidence of thrust fault. There are unconformable contacts between Hazara and Samana Suk, Chichali and Lumshiwal, Kawagarh and Hangu and Lockhart formations indicate fluctuation in the environment of deposition. The Bagnotar Fault, Dhamtaur syncline and Thai anticline are the major structural features identified and reported in the study area.

The strike-slip fault systems in the central Tarim Basin, China, afford an exceptional opportunity to document the structural characteristics and evolution process of small displacement intracratonic strike-slip faults using three-dimensional seismic reflection data. These strike-slip faults display subvertical segments at depth and en echelon normal fault zones where relatively shallow. Fault segmentation and flower structures can be commonly observed in plan view and cross-section view, respectively. Consistent with the notion that segment coalescence is the fundamental process for fault evolution, the mean segment length of representative strike-slip faults examined in this study is positively correlated to the measured fault offset. The width of the en echelon normal fault zone is positively correlated with the estimated maximum overburden thickness. The integrated data sets suggest that the evolution of the conjugate fault array followed a sequential evolution process instead of forming simultaneously. The switch in slip direction of the master fault of the conjugate fault array is attributed to the change of stress orientation. Regarding individual strike-slip faults, increase in displacement induces the formation of faults with lower fault-array angles linking initially formed en echelon normal faults. In cross sections, throughgoing fault surfaces can also form, connecting the lower subvertical fault segment and the upper en echelon normal faults. The presented data sets and evolution models established in this study can be used as tools to better predict the structural attributes of subsurface strike-slip fault systems with important consequences for reservoir formation and hydrocarbon accumulation in the Tarim Basin in particular, and in ancient marine basins in general.

The Little Minch Sill Complex consists of a series of stacked, multi-leaved Paleocene dolerite sills, which have primarily intruded into Mesozoic sedimentary rocks and Paleocene tuffs/?hyaloclastites within the Sea of Hebrides Basin. The Sea of Hebrides Basin is situated to the west of the Scottish mainland on the NE Atlantic margin. Two previously proposed models for the emplacement of the sill complex have opposing ideas for the location of magma input and the emplacement mechanisms. Both models have been constructed using data primarily from onshore outcrops on the Isle of Skye, Raasay and the Shiant Isles. However, these onshore outcrops only represent a quarter (1040 km2) of the entire extent of the sill complex, which is largely located offshore. To understand how the sill complex as a whole was emplaced within the basin, both onshore and offshore magma transport needs to be considered. Using high-resolution multibeam bathymetry data (up to 2 m resolution) obtained between 2008 and 2011, along with supporting seismic reflection, sparker and pinger data, a new assessment of the offshore extent and character of the sill complex has been constructed. Mapping of the large-scale relationships between intrusions and the host rock, along with morphological features such as magma lobes, magma fingers, transgressive wings, en echelon feeder dykes and the axis of saucer-/half-saucer-shaped intrusions, has indicated the magma flow directions within the intrusive network. Assessing the flow kinematics of the sills has provided insights into magma transport and emplacement processes offshore. Combining data from previously mapped onshore sills with data from our newly constructed model for magma emplacement offshore has allowed us to construct a new model for the emplacement of the Little Minch Sill Complex. This model demonstrates that major basin-bounding faults may have a lesser role in channelling magma through sedimentary basins than previously thought. Applying the knowledge obtained from this study could further progress our understanding of the effect of sill emplacement on fluid flow within volcanic rift basins worldwide, with direct impacts on the exploitation of petroleum and geothermal systems.

Documenting neotectonic instabilities and determining the style and time of deformation in the vast and difficult to access central Amazonia region is challenging. We focus on these issues by investigating a large area of the Negro River drainage basin, applying morphostructural analysis based on synthetic aperture radar data. The digital elevation models of the C-band Shuttle Radar Topography Mission and the L-band Protection System of Amazonia were used as the database. We also used subsurface magnetic information from the Earth Magnetic Anomaly Grid global model to validate the morphostructures. The results revealed NW-oriented morphostructural lineaments bounding multiple depositional valley fills. These were extensively fragmented to form regularly distributed en échelon rectangular blocks commonly offset horizontally by several kilometers. Strike-slip faults and oblique, either normal or reverse, faults are present. These structures were reactivated along the main NE- and SW-oriented regional structural trends due to N-S-oriented horizontal compression and E-W-oriented horizontal extension in the late Pleistocene and Holocene. The extensive neotectonic faulting results from the interplay of plate motion and Andean uplifting since the late Pleistocene, combined with local stresses.

Summary We apply diffraction imaging to a 3D data set in the southern Malay Basin, where it has a wide range of applicability to both structural and stratigraphic targets. In the Tertiary section, diffraction imaging helps to resolve small scale faulting, including en echelon faulting. It assists the overall structural interpretation in both high and low reflectivity formations. Isolated channels as well as channel complexes can be identified using diffraction imaging. In addition, it provides superior illumination and definition of basement fractures.

Producing a sufficient volume of multiscale crack networks is key to enhancing recovery of shale gas. The formation of crack network largely depends on initiation and propagation of microcracks. To reveal the influence of different loading methods on the propagation of mineral-scale microcracks, this study used the Voronoi tessellation technique to establish a cohesive zone model of shale mineral distribution and applied six different boundary conditions to represent different loading methods. Crack path characteristics, rupture characteristics, continuous crack propagation and turning, and en echelon intermittent crack propagation under different loading methods were compared and analyzed. The essence of different loading methods affecting the length and complexity of cracks was the spreading range of tensile microcracks. The mechanical properties of minerals led to dissimilarities in continuous crack propagation and turning. The formation and propagation of en echelon intermittent fractures of different scales were mainly impacted by the heterogeneity of minerals and mineral aggregates. The spreading direction and connection form of en echelon intermittent fractures were mainly affected by the loading method. Conclusions arising from mineral-scale simulations contribute to understanding the mechanism of microcrack propagation resulting from different loading methods, and these conclusions have a guiding significance to enhanced shale gas recovery.

To understand crack coalescence patterns and local strain behaviors near flaw tip for flawed rock subjected to biaxial compression, a series of biaxial compression tests with a lateral stress of 2.0 MPa were carried out on rock-like specimens containing two flaws. The paper classified crack coalescence patterns, measured the local strain concentration near the flaw tip, and analyzed the stress–strain behavior related to crack initiation, propagation, and coalescence. Some interesting cracking behaviors of flawed specimens subjected to biaxial compression were observed. Seven basic crack types (wing crack, anti-wing crack, oblique and coplanar shear cracks, lateral crack, remote crack, and en echelon cracks) are identified and ten patterns of crack coalescences are observed subjected to biaxial compression. In general, the crack coalescence varies from the shear crack coalescence (S-mode) to the tensile wing crack coalescence (T-mode), and then to mixed shear/tensile crack coalescence (TS-mode), with the increase of the rock bridge ligament angle β, ranging from 25 to 120° subjected to biaxial compression. Crack initiation and coalescence usually can be reflected by a load stress rapid drop, a measured strain jump, or turning near flaw tip subjected to biaxial compression. The measured tensile or compression strains near flaw tip tend to increase almost simultaneously with higher axial stress before crack initiation; however, become very complex after crack initiation, due to the stress relaxation and adjustment near flaw tip induced by the combination of crack development and constant lateral stress. The existence of lateral stress results in many fragments in a shear failure mode with local surface spallings in flawed specimens containing two flaws.

We used satellite images, earthquake catalogues and field observations to study several active fault systems and their interactions in Sabzevaran Area in SE Iran. The focus of this study is to verify the link between the active faults, their kinematics and seismic activity. Field observations and geomorphological analysis highlight the interaction of the active faults. Moreover, most of the tectonic activity is observed in the area, related to the Chahmazrae- North Faryab shear zone. Most of the earthquakes in this shear zone are reverse and occur in the deeper crust while aftershocks dominantly occur in the shallower crust. The Main Zagros Reverse Fault (MZRF) is the source of reverse events and the Chahmazrae-North Faryab shear zone is source of left-lateral, oblique reverse faulting events, and strike-slip events. These types of the earthquakes in the study area confirm the idea of tectonic proximity of the root faults and shear zone. In the interaction area, minor fractures begin to develop and are progressively linked to the main faults. In the en echelon arrangement of the faults, the minor faults have grown and linked the en echelon segments of the faults. It seems that the earthquake ruptures can spontaneously propagate across both extensional and compressional fault steps. This propagation occurs along strike-slip faults such as Sabzevaran fault and its branches.

Heterogeneous sequences of exhumed fault rocks preserve a record of the long-term evolution of fault strength and deformation behaviour during prolonged tectonic activity. Along the Outer Hebrides Fault Zone (OHFZ) in NW Scotland, numerous pseudotachylytes record palaeoseismic slip events within sequences of mylonites, cataclasites and phyllonites. To date, the kinematics and controls on seismicity within the long active history of the OHFZ have been poorly constrained. Additional uncertainties over the relative location of a meteorite impact and possible pre-OHFZ brittle faulting also complicate interpretation of the diffuse seismic record. We present kinematic analyses of seismicity in the OHFZ, combining observations of offset markers, en echelon injection veins and injection vein geometry to reconstruct slip directions and stress fields. This new dataset indicates that a range of fault orientations, slip directions and slip senses hosted seismicity in the OHFZ. Such complexity requires several stress field orientations, in contrast with the NW–SE Caledonian compression traditionally attributed to frictional melting along the OHFZ, indicating that seismicity had a long-term presence across the fault zone. Persistence of strong frictional failure alongside the simultaneous development of weak fault rocks and phyllonitic shear zones in parts of the OHFZ has significant implications for understanding seismic hazard along mature continental faults. Supplementary material: Tables listing analysed orientation measurements plus further information and sensitivity testing of palaeostress analysis parameters are available at https://doi.org/10.6084/m9.figshare.c.5134797

The near-surface of London is faulted, although the locations, architecture and tectonic origins of the faults are broadly unknown. This presents serious issues for geotechnical engineering in London and has implications for our structural understanding of the London Basin. The region is a product of Alpine compression, yet it is unclear whether these major faults are new Alpine shears or reactivated basement faults. We assess the plausibility of Alpine reactivation and inheritance of basement faults in London through three investigations: analysing structures in the near-surface; mechanically assessing the feasibility of basement fault reactivation under Alpine stress conditions; and comparing inheritance mechanisms with observations in London and the Thames Estuary. Three major en echelon fault sets are identified. These appear to have compartmentalized London9s near-surface geology and are all interpreted to be products of Alpine reactivation of the underlying basement faults. Fault interaction and linkage is evidenced by complex zones of intense faulting identified by tunnelling projects. The role of new structural development in accommodating Alpine compression is considered to have been comparatively minor. The lack of major faulting in the basin9s interior may reflect the competence of the underlying Anglo–Brabant Massif in restricting Alpine strains to its margins. Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

During extension of the continental lithosphere, rift basins develop. These are often initially offset, and must interact and connect in order to create a continuous rift system that may ultimately achieve break-up. When simulating extensional tectonics and rift interaction structures, analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system. Yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation, resulting in rotational extension, and associated rift propagation and structural gradients. Here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings. Our modelling efforts show that rotational extension and orthogonal extension produce significantly different large-scale structures. Rotational extension can cause important variations in rift maturity between rift segments, delay rift interaction zone development, and make rift segments propagate in opposite directions. Still, local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting. Furthermore, we find that various degrees of rift underlap produce three basic modes of rift linkage structures. Low underlap distance (high angle φ) experiments develop rift pass structures. With increasing underlap distance (φ = ca. 40°), transfer zone basins develop. High degrees of underlap (φ ≤ 30°) tend to result in en echelon sub-basins. Our results match with data from previous modelling efforts and natural examples. We furthermore propose a large-scale tectonic scenario for the East African Rift System based on rotational extension and associated rift propagation. These insights may also be applicable when studying other large-scale rift systems.

The borehole method is quite commonly utilized in underground excavation engineering, such as for mining and tunneling, to release high pressure in brittle rock materials for safety control. In this paper, with the purpose of better understanding the failure behavior of brittle rock material with en echelon boreholes, a series of uniaxial compression tests were carried out on sandstone samples in the laboratory. The geometric layout of boreholes was the focus in this study and varied in terms of the number of borehole rows and orientation. In accordance with the experimental results, three failure modes were observed to occur in the sandstone samples, which are tensile failure across the borehole row, tensile failure along the borehole row, and a mixed failure mode. The strength and deformability of sandstone samples are found to significantly depend on the layout of boreholes. The variation in the brittleness of the rock materials was also analyzed with respect to the ratio of the initial stress over peak stress. The findings of this study would enhance the understanding of the borehole method and provide guidance for underground excavation engineering.

An apophysis of the Sybella Granite intrudes strata of the Eastern Creek Volcanics in the vicinity of Slaughteryard Creek, 8 miles west-north-west of Mt. Isa. This has resulted in the impression of higher temperature metamorphic conditions on rocks of a tectonic block which is separated from the regional Amphibolite facies schists by a large shear zone.This tectonic block originated as a drag fold in the eastern overturned limb of the regional anticline structure. Detachment of this fold from the adjacent strata was accompanied by the emplacement of an en echelon series of granitic sills within the shear. Diapiric upwelling of the main Sybella Granite in the core of the regional structure caused truncation of its limbs by the Mt. Isa and May Downs faults respectively. This movement also induced the emplacement of the main sill-like apophysis. Introduction of a large gneiss mass along the decollement pushed the intruded tectonic block northwards, reversing the folded attitude of some strata by frictional drag. This movement also disrupted strata in the adjacent regional schists producing permeable zones which gave access to later metasomatic fluids. A regional greenschist facies metamorphism was impressed during unloading and uplift.

The Nankalayuergun fold zone in the North Tarim Basin, NW China, provides an exceptional opportunity for documenting the structural characteristics and evolution of en echelon folds along transpressional fault zone. However, the genetic mechanism of these en echelon detachment folds remains debatable due to poor understanding of the deep structure. Combined with seismic and borehole data, we characterized the geometries and kinematics of Nankalayuergun fold zone, revealed its Cenozoic evolution, and discussed the formation mechanism. The stratified fold zone was geometrically decoupled by salt structures, and the structural style of three salt-influenced folds had individual characteristics due to differences in salt thickness. The timing and strength of Cenozoic deformation of three en echelon detachment folds has a sequential evolution tendency from northwest to southeast. The structural relief of supra-salt fold is the sum of Cenozoic detachment and sub-salt Paleozoic-Mesozoic transpressional folds, indicating that sub- and supra-salt structures are kinematically coupled. Segmentation of Deep Nankayuergun Transpressional Fault (DNTF) can be observed by gravity and seismic data. The supra-salt detachment folds differ from classic echelon structures in that it is only located on the active side of the DNTF. Furthermore, the hinges of the supra-salt folds located right above the sub-salt transpressional fold scarps, corresponding to the reactivation of three DNTF segments. The transpressinoal regimes, sub-salt structures, and the heterogeneity of salt rock are major factors forming the polygenetic echelon detachment folds. The case presented in this study displayed a specific pattern of salt-influenced en echelon structures along transpressional faults and highlighted the influence of pre-exiting structures on the geometry and kinematics of shallow folds, even though salt can decouple sub- and supra-salt deformation.

In this field study we reinterpret the narrow eastern North Pyrenean Zone, France, as an inverted salt-rich transtensional rift system based on identification of halokinetic depositional sequences across rift platform to distal rift margin domains with a cumulative throw of >2.8 km on steep Cretaceous faults. The rift platform records extension on detached rotational faults above Triassic evaporites from Jurassic to Aptian with uplift and erosion during the Albian. Transtensional Aptian–Albian minibasins align along the salt-rich rift margin fault zone. In the Aptian–Albian main rift large en echelon synclinal minibasins developed between salt walls, although Jurassic diapiric evolution is likely. Upper Cretaceous units locally record continuing diapirism. The Boucheville and Bas Agly depocentres, altered by synrift HT metamorphism, form the distal rift domain terminating south against the North Pyrenean Fault. The narrowness of the Pyrenean rift, shape of minibasins, en echelon oblique synclinal depocentres and folds coupled with a discontinuous distribution and intensity of HT metamorphism support a transtensional regime along the Iberia–Europe plate margin during late Early and early Late Cretaceous. In this model, the distal European margin comprises deep faults limiting laterally discontinuous crustal domains and ‘hot’ pull-apart basins with mantle rocks directly beneath sedimentary cover., This work was funded by the French ANR project PYRAMID; the OROGEN project financed by Total, BRGM and the CNRS; the Spanish Ministry of Economy and Competitiveness, Grant/Award Number SUBTETIS (PIE-CSIC-201830E039); and Generalitat de Catalunya, Grant/Award Number AGAUR 2017 SGR 847.