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The Doruneh Fault System is one of the major transcurrent faults in central Asia, extending ~900 km from western Afghanistan into West-Central Iran. The left-lateral Doruneh Fault System is also a key structure in the Arabia–Eurasia collisional zone, bounding the northern margin of the independent Central Iranian Microplate. The Doruneh Fault System exhibits a curved geometry, and is divided here into three segments: Eastern, Central and Western. We present the results of geological, structural and geomorphic studies into the nature of recent activity along the Doruneh Fault System segments. A surprising observation is that small, relatively young drainage systems often show recent systematic left-lateral displacement across the fault, whereas large rivers indicate a former more complex right-lateral history. Furthermore, the existence of right-lateral offsets of pre-Pliocene rocks and S-C fabrics confirm this earlier phase of right-lateral movement on the fault. We suggest that the early right-lateral kinematics resulted from an earlier NW–SE-directed regional shortening, associated with the anticlockwise rotation of the Central Iranian Microplate. The shortening is characterized by the NE–SW-striking en échelon folds within the fault slivers, the right-lateral Taknar imbricate fan and the superimposed folding exposed north of Kashmar. Thus, assuming an initiation age of Eocene (55.8 Ma) for the fault, we estimate a former right-lateral slip rate of about 5.2–5.5 mm yr−1, which accompanied the 35° anticlockwise rotation of the Central Iranian Microplate. According to our study, the youngest units exhibiting right-lateral displacement are Middle Miocene in age, suggesting a post-Middle Miocene timing for the onset of slip-sense inversion.

Based on the latest re-processed and newly acquired seismic data and using the integrated approaches including seismic,well drilling,inverse gravity and magnetic processing and land sea correlation,we studied the distribution of the Mesozoic in the southern East China Sea Shelf Basin,and got some new understandings about the three uplifts,namely the Yandang,Taibei and Guanyin uplifts,defined by previous studies.It is found that the Yandang Uplift is in fact not a continuously uplift from north to south,but an old Proterozoic uplift made up of three en echelon uplifts,which disappear southward.Mesozoic strata occur steadily on both the Guanyin and Taibei uplifts.Our results show that the Mesozoic strata in the Southern East China Sea Shelf Basin are quite thick and widely distributed.According to our data,it is thicker in the south and east,and thinner in the north and west.In this regard,the concept of Mesozoic Great East China Sea is proposed in this paper to conclude the new discoveries.

[1] The transpressive Alpine Fault is the boundary between the Pacific and Australian plates across the South Island of New Zealand. Earthquakes on the Alpine Fault and related structures pose a serious risk to many urban centers, including the city of Christchurch. Although it is a major feature on satellite images, the Alpine Fault is a difficult target for surface studies along much of its length; it mostly traverses densely forested and mountainous terrain and where it occurs in the lowlands it is usually covered by recent sediments. To investigate the Alpine Fault at a rare accessible location (Inchbonnie), we have acquired high-resolution seismic reflection data along five 380–1200 m long lines. Images produced from these data reveal a glacially overdeepened valley containing a thick sequence of diverse glacigenic sediments that have been disrupted by three en echelon strands of the principal Alpine Fault and several secondary fault strands. Based on their seismic facies, the sedimentary sequence is interpreted to comprise basal lacustrine beds overlain successively by alluvial-colluvial deposits that possibly include the remnants of large landslides, deltaic-fan units, and braided river gravels. Whereas the principal Alpine Fault strands disrupt the entire post-glacial sedimentary section and likely offset basement at depths up to 400 m, most of the secondary faults either merge with the principal fault strands at shallow depths or are surficial features limited to the sedimentary section.

The Noda area in the Hokusatsu District, southwest Kyushu is underlain by the Imanokoba, the Ikenodan and the Ogakura Formations of the Cretaceous Shimanto Supergroup. The Imanokoba Formation comprises beds of sandstone and mudstone. The beds generally trend NE-SW and dip in a moderate to steep angles to the east or west. The Ikenodan Formation comprises beds of sandstone, mudstone, siliceous mudstone and chert and basaltic lavas. The beds generally strike between NE and NW, and dip toward the west in moderate to high angles with a westerly way-up. The chert and basaltic lava beds appear in a left-stepping en echelon arrangement. The contact between the Imanokoba and the Ikenodan Formations is in a N-S trending fault known as the Noda Fault. The Ogakura Formation is composed of alternating beds of sandstone and mudstone. The beds generally strike between NE and NNE and dip toward the west in moderate to high angles with westerly way-up. The contact relationship between the Ikenodan and the Ogakura Formations is not distinct. The westward disappearance of the basaltic lava marks the western limit of the Ikenodan Formation. The en echelon array in the Ikenodan Formation is interpreted as an imbricate fan with the Noda Fault on its bottom as a decollement and is called "the Ikenodan imbricate fan". The Ikenodan imbricate fan is considered to have been formed in an accretionary complex developed in a northbound subduction zone. The primary attitude of the Ikenodan imbricate fan was probably south-vergent then turned over with its west side down. It was previously reported that the strata of the Shimanto Supergroup in the Hokusatsu area form a regional bending structure called the "the Hokustasu Bend". The Noda area is situated in a hinge zone of the Hokusatsu Bend, although the formations in the Noda area have not been involved in the bending structure. The Ikenodan Formation and the Noda Fault presumably extend to the north without bending, otherwise the bending structure occurs anywhere outside the Noda area. In either case, the Hokusatsu Bend is not present as interpreted in the previous studies.

This report presents interpretations developed from a detailed study of new three-dimensional (3-D) high-resolution reflection seismic data in a portion of Tarim Basin. The tectonic history began with oceanic spreading during the Cambrian–Early Ordovician and continues beyond the Silurian–Devonian time of oceanic closure. Paleozoic faults of the Tazhong Uplift in the hinterland of Tarim Basin are capped by thick and undisturbed Meso-Cenozoic sedimentary sequences. Four Paleozoic fault systems have been recognized: (1) the Cambrian–Early Ordovician extensional faulting, (2) the Late Ordovician NWW trending thrust faulting, (3) the Silurian–Devonian NNE strike-slip faulting and (4) the Permian plutonism influenced by pre-existing fault planes. Zones of weakness created during Cambrian–Early Ordovician extensional faulting influenced subsequent tectonic movements. The Late Ordovician fault system divides the Tazhong Uplift into several deformation zones. Their mechanical characteristics vary across the study area, with stronger thrusting in the east. The Silurian–Devonian strike-slip fault system consists of three components: main faults, subordinate en echelon faults, and fault troughs. The main faults appear as steeply dipping, almost vertical, offsets on seismic map view, with associated flower structures on seismic profiles, together with other levels of faults. The Permian magmatic plugs have a spotty or bandy distribution, and are interpreted to have utilized former faults.

Petrology, 21 (6), ISSN:0869-5911, ISSN:1556-2085

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A3 (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.

The terminus of the Middle America Trench has been traditionally represented as an arc, concave towards the continent. Tres Marias Islands are located at the terminus of the Middle America Trench in western Mexico, and their location is not only intriguing but also a key to the re-construction of the position of Baja California peninsula before separation from the North America plate. Previous re-constructions suggested various places around the location of Tres Marias Islands for the position of the tip of the peninsula, and several converge to a position that invades the area occupied by the islands, offering no explanation for the overlap. Before peninsular separation from North America, the Guadalupe trench followed a smooth curve; recreating the position of this paleo-trench yields a baseline for fixing the position of the peninsula, as well as the original position of the Tres Marias Islands fragment. A new tectonic view of the structure of the Middle America Trench terminus is proposed, replacing the traditional arc representation with a series of en echelon blocks, the northernmost terminates at the Tres Marias Escarpment. The long sides of the blocks correspond to previously identified geological faults in the Bahia de Banderas region, while their offshore continuation is supported by topographic observations. As a test of this model I show the corresponding re-construction of the position of Baja California prior to separation from the North America plate and the positional evolution of the peninsula and the Tres Marias fragment from Chron 4n.2 (7.90 Ma) to Chron 3n.4 (5.12 Ma).

The 2016 Kumamoto earthquake sequence started with a M J (Japan Meteorological Agency magnitude) 6.5 event on April 14, and culminated in a M J 7.3 event on April 16. Associated with the sequence, approximately 34-km-long surface ruptures appeared along the eastern part of the Futagawa fault zone and the northernmost part of the Hinagu fault zone. We carried out an urgent field investigation soon after the earthquake to map the extent and displacement of surface ruptures with the following results. (1) The rupture zone generally consisted of a series of left-stepping en echelon arrays of discontinuous fault traces of various lengths. (2) Slip exceeding 100 cm occurred on previously unrecognized fault traces in the alluvial lowland of the Kiyama plain and on the western rim of the Aso volcano caldera. (3) Large slip with maximum dextral slip of 220 cm was measured throughout the central section of the rupture zone along the Futagawa segment, and the slip gradually decreased bilaterally on the adjoining northeastern and southwestern sections. (4) The surface rupture mostly occurred along fault traces mapped in previous active fault investigations. (5) Most of the surface ruptures were produced by the mainshock, and significant postseismic slip occurred after the mainshock.

International audience; Swarms of long-period (LP) events were recorded on Turrialba volcano, Costa Rica, during a seismic field experiment in 2009. Families of LP events were previously identified and located using a joint inversion for source location and mechanism; however, the spatial resolution of the obtained locations was not sufficient for imaging the structures on which they occur. Using a waveform similarity-based location method, we take advantage of the joint location-mechanism inversion by relocating events around the obtained familial location. The location method is successfully tested on a synthetic data set and is then applied to the Turrialba LP data set. The relocated events are jointly interpreted with their source mechanisms and reveal an en echelon structure within the upper edifice of the volcano. This can be interpreted as a response of a shearing band with high fluid pressure inducing tensile fractures at unconsolidated rock layer interfaces within the upper edifice of the volcano.

We used field data collected from the Bir Oum Ali structure (BOAS) and take into consideration recent published geophysical works to evaluate the deformation style and the structural evolution of the Southern Atlassic foreland in Northern Chotts Range in Tunisia. Various structural interpretations have been proposed for the genesis of the Northern Chotts Range. The BOAS was interpreted previously as (1) curved, right-stepping “en echelon” geometry folds as the result of NW-trending lateral strike-slip faulting (2), an anticline structure developed and the result of Ramp-related folding deformed the sedimentary cover (thin-skinned model) over the rigid basement during a single tectonic event (3), response of salt tectonic that began in Early Jurassic and allows the migration to nucleated folding during tertiary compressional events. New field data of the BOAS show a deformation style, in which shortening is differently accommodated in the eastern, southern, and northern areas. Data highlight a tight folding and steeply to overturned strata related to the N100–110° E-trending Bir Oum Ali–Hachichina fault systems (BOAHFS). This later exhibits fault kinematics, with striation showing a multiphase history. The second-order fault systems associated to the main trend of the BOAHFS are usually apparent strike-slip faults. Unfolding of the structure shows an inherited normal faulting. In addition, the normal faulting generates syntectonic conglomerates associated to the first normal faulting movement. The E-trending fold-related fault reactivation of the E-trending Northern Chotts Range might result from the reactivation of inherited Mesozoic faulting. A new structural data interpretation give information of tectonic inversion starting in Late Cretaceous (Campanian–Maastrichtian transition?). The Late Cretaceous to present-day history is dominated by two major events, i.e., Eocene Atlassic contractional event and Middle–Late Miocene to present-day Alpine event. The folding style, the partial reverse–reactivation of the pre-existing E-trending BOAHFS major faults, and the abundant thick tertiary siliciclastic growth strata sequences together with the recent published geophysical data provide a coherent model, in which the thick-skinned tectonic style (development with basement inversion at the depth) is synchronous of thin-skinned tectonic (shallow decollement in the sedimentary cover).

Meso-Cenozoic extensional structures are important for understanding the tectonics of the Chinese Central Asia. This paper presents a systematic investigation on the Meso-Cenozoic extensional structures in the Northern Tarim Basin. Close interpretations of seismic data reveal that the Meso-Cenozoic extensional structures were widely developed in the Northern Tarim Basin. These extensional structures are regionally composed of many small normal faults, which usually group into left- or right-step en echelon and form several transtensional fault zones. Combinations of normal faults in profile become small graben-horst or staircase-like cross-sections. Based on the areal distribution, structural style, combination relationship, formation and evolution time, and formation mechanism of the extensional structures, we found that the Meso-Cenozoic extensional structures in Northern Tarim Basin can be classified into two conjugate normal fault systems, which were formed separately in Jurassic-Early Cretaceous and Late Cretaceous-Neogene. The former is likely associated with the stress relaxation after a collisional orogeny accompanied with a certain degree of anticlockwise rotation of the Tarim block relative to the South Tianshan; the latter is possibly induced by the east by south tectonic escape of the Tarim block with a certain degree of clockwise rotation relative to the South Tianshan triggered by the far-field effect of the Himalayan orogeny.

According to the differences of structural deformation characteristics, the southern margin of the Junggar basin can be divided into two segments from east to west. Arcuate thrust-and-fold belts that protrude to the north are developed in the eastern segment. There are three rows of en echelon thrust-and-fold belts in the western segment. Thrust and fold structures of basement-involved styles are developed in the first row, and decollement fold structures are formed from the second row to the third row. In order to study the factors controlling the deformation of structures, sand-box experiments have been devised to simulate the evolution of plane and profile deformation. The planar simulation results indicate that the orthogonal compression coming from Bogeda Mountain and the oblique compression with an angle of 75° between the stress and the boundary originating from North Tianshan were responsible for the deformation differences between the eastern part and the western part. The Miquan-Urumqi fault in the basement is the pre-existing condition for generating fragments from east to west. The profile simulation results show that the main factors controlling the deformation in the eastern part are related to the decollement of Jurassic coal beds alone, while those controlling the deformation in the western segment are related to both the Jurassic coal beds and the Eogene clay beds. The total amount of shortening from the Yaomoshan anticline to the Gumudi anticline in the eastern part is ∼19.57 km as estimated from the simulation results, and the shortening rate is about 36.46%; that from the Qingshuihe anticline to the Anjihai anticline in the western part is ∼22.01 km as estimated by the simulation results, with a shortening rate of about 32.48%. These estimated values obtained from the model results are very close to the values calculated by means of the balanced cross section.

Foreland fold-and-thrust belts are challenging because their structural complexity highly impacts fluid flow and diagenetic processes. Paleofluid flow in the Rocky Mountain fold-and-thrust belt west of Calgary was investigated following a structural diagenetic approach. Paleofluid flow or mass transfer mechanisms have been inferred based on vein morphology (completely filled, en echelon, slickenfibre, bed-parallel, open or micro-breccia veins), orientation and geochemistry (stable oxygen and carbon isotope and strontium isotopes). Bed-parallel veins probably developed by flexural slip during Laramide folding, and most completely filled and slickenfibre veins formed or were reactivated during the Laramide Orogeny. Both types of veins associated with the Laramide Orogeny precipitated under low fluid flux conditions, dominated by diffusional processes as inferred from stable isotopic signatures similar to the host rock. The open veins with meteoric calcite cement, marked by cathodoluminescence zonations and strongly depleted oxygen isotopic signature, point to advectional fluid flow processes in a post-Laramide extensional stress field. Some slickenfibre and completely filled veins that originally formed during the compressional phase of the Laramide Orogeny were probably reactivated at time of the open vein formation. The latter inference is based on the occurrence of the same cement in the open veins as in the centre of the slickenfibre and completely filled veins. In conclusion, there is no evidence for large-scale fluid flow during the Laramide Orogeny, but evidence points to post-Laramide large-scale fluid advection being prevalent. Challenges in this structural diagenetic study consist of (i) the lack of abundant vein–vein and vein-stylolite cross-cutting relationships in the outcrops studied, (ii) the difference between structural versus diagenetic concepts, (iii) the uncertainty of chronology and attribution of fracture sets and diagenetic cements to either a single or discrete episodes, and (iv) reactivation of veins.

Recent research on rates of crustal shortening within the Palmyra Fold Belt (PFB) in Syria has drawn attention to the possibility that reverse faults near the city of Damascus, which adjoins the SW PFB, have significant slip rates. We infer that the Damascus Fault, directly adjacent to the city, has developed a throw of ~ 2500 m and report the discovery of the en echelon Bassimeh Fault, with a throw of ~ 1000 m, this fault being revealed by warping of the local bedrock and of a terrace, of inferred Late Pleistocene age, of the River Barada. We estimate that this set of faults became active circa 0.9 Ma, synchronous with changes to the pattern of faulting previously reported farther southwest in the northern Jordan Valley. Vertical slip rates on the Bassimeh and Damascus faults of ~ 1.1 and ~ 2.8 mm a− 1, respectively, are thus estimated. We also infer that large historical earthquakes, previously attributed to left-lateral faulting farther west on the Dead Sea Fault Zone (DSFZ), probably occurred on this set of reverse faults; these faults thus represent a significant hazard to the city of Damascus. Our observations indicate that as much as half of the northward motion of the Arabian plate, relative to the African plate, may be ‘absorbed’ by crustal shortening within the PFB, potentially explaining the low slip rate recently measured geodetically on the northern DSFZ in western Syria.

Seismicity in the New Madrid seismic zone (NMSZ) of the central United States is generally attributed to a stepover structure in which the Reelfoot thrust fault transfers slip between parallel strike-slip faults. However, some arms of the seismic zone do not fit this simple model. Comparison of the NMSZ with an analog sandbox model of a restraining stepover structure explains all of the arms of seismicity as only part of the extensive pattern of faults that characterizes stepover structures. Computer models show that the stepover structure may form because differences in the trends of lower crustal shearing and inherited upper crustal faults make a step between en echelon fault segments the easiest path for slip in the upper crust. The models predict that the modern seismicity occurs only on a subset of the faults in the New Madrid stepover structure, that only the southern part of the stepover structure ruptured in the A.D. 1811–1812 earthquakes, and that the stepover formed because the trends of older faults are not the same as the current direction of shearing.

The 26 November 2005 Jiujiang-Ruichang, Jiangxi, Ms 5.7 earthquake occurred in a seismotectonic setting of moderate earthquake. The northwest-trending Xiangfan-Guangji fault (XFG) does not enter into the epicenter vicinity, but the northeast-trending Ruichang-Wuning fault (RWF) as a regional fault extends to the epicenter nearby, appearing as the Ruichang basin and its marginal faults. Tilting of the Ruichang Basin (RCB) in the Quaternary was controlled by the RCB southeast-marginal, buried fault (RSMBF). Shallow geophysical survey reveals that the RSMBF caused an offset of the reflection layers. Drill hole columnar section demonstrates that there are about 10–12 m displacement in the lower section of the middle-Pleistocene Series along the RSMBF, but no disruption is found in the upper section of the middle-Pleistocene Series. The RSMBF not only has activity in the Quaternary, but also coincides with the nodal plane I from the focal mechanism of the Jiujiang-Ruichang Ms 5.7 earthquake. This evidence, including aftershock distribution and isoseismic lines, strongly suggests that the RSMBF might be the seismogenic tectonics. The RWF is discontinuous at the surface, and consists of three en echelon Quaternary basins, which are the Ruichang, Fanzhen and Wuning basins. Three moderate earthquakes, the Fanzhen ML 4.9 earthquake, the Yejiapu ML 4.1 earthquake and the Jiujiang-Ruichang Ms 5.7 earthquake, have happened in the basins since 1995. The seismogenic tectonics of the Jiujiang-Ruichang Ms 5.7 earthquake is not isolated, but may be controlled by the RWF at depth, the slip of which causes the accumulation of energy for earthquake occurrence.

Perennial frost mounds are present near the edges of ponds located on the terrace between the mountain flank and a valley-bottom lake in the Akkol valley of the Russian Altai Mountains. These mounds are 10–50 m in diameter and 3–6 m in height. We describe one mound, identified as a lithalsa, which had been eroded so as to expose almost its entire vertical cross-section, revealing its internal structure. The frozen core consisted mainly of soil segments suspended in reticulate ice lenses with a mean thickness of 11–48 mm and a maximum thickness of about 160 mm. The shapes of the soil segments matched their neighbours. Other features included soil segments suspended in the ice veins shaped like ‘En echelon gash veins’, and the presence of a radial structure of ice-rich and sediment-rich frozen bands. These features all suggest the greater importance of a differential stress field during heaving of the mound and after ice segregation, compared to the thermal gradient and water supply. Copyright © 2012 John Wiley & Sons, Ltd.

Online material: Field-measured offsets of the surface rupture. The Ms 7.1 Yushu earthquake on 14 April 2010, 07:49:38 (epicenter, 36.2°N 96.6°E; focal depth, 14 km; http://www.csi.ac.cn/manage/html/4028861611c5c2ba0111c5c558b00001/qhyushu7.1/index.html (in Chinese) was located in a remote, mountainous, and sparsely populated region ∼30 km west of Yushu, Qinghai, China. The earthquake, which resulted in 2,698 dead, 12,135 injured, and 270 missing persons, caused widespread damage in the central Tibetan Plateau. (http://news.sina.com.cn/c/2010-05-31/162820381075.shtml; in Chinese). The quake occurred along the Yushu segment of the Ganzi (Garze)-Yushu fault (Chen et al. 2010; Lin, Rao et al. 2011), which constitutes the NW-SE–striking Xianshuihe fault system (Figure 1), a principal left-lateral strike-slip fault system accommodating eastward extrusion of crustal blocks of the eastern Tibetan Plateau due to the collision between the Indian and Eurasian plates. The Xianshuihe fault accommodates approximately 5–15 mm/yr of the lateral motion, which is around one-third of the overall eastward motion of Tibet (Allen et al. 1991; Zhou et al. 1997; Wen et al. 2003; Peng et al. 2006). The Yushu earthquake is the first rapidly investigated moderate strike-slip event in Tibet. In general, the mapping of surface ruptures is best made immediately after an earthquake, when fragile and ephemeral structural features associated with faulting are fresh. Our field observations were made from April 24 to April 30. An important finding is that the mapped fracture pattern is compatible with a self-similar hierarchical system of deformation that involves segment structures at three different scales, which range from kilometer-scale fault sections, to hundreds to tens of meter-scale fracture arrays, to meter-scale individual fractures. Surface ruptures of the Yushu earthquake are composed of three sections, each of which is separated by en echelon step-over structures (Figures 2 and 3). Each section is accompanied by mole tracks, en echelon shear fractures, and …

Snow avalanches often form levees and en-echelon shear planes in the run-out zone. We describe the formation of these depositional structures using a simple model that accounts for the role of granular fluctuations in avalanche motion. A mathematical feature of this model is the existence of a bifurcation saddle point, describing how granular fluctuations control the avalanche velocity in the runout zone. The saddle point discriminates between a flowing and stopping regime and defines the physical boundary between the flow and non-flow regions of the avalanche, i.e. the location of shear planes in the avalanche deposits. The formation of a shear plane depends on the interplay between terrain slope and avalanche mass flux, which varies from avalanche head to tail. Levees can form immediately at the avalanche front or, for steep slopes and low mass fluxes, at the avalanche tail. At ravine and gully shoulders the mass flux is restricted, thus initiating levee formation. We find that the levee lines are parallel to the flow direction when the mass flux is constant; en-echelon shear lines occur when the mass flux is decreasing. We test the model using several case studies where we have accurate laser scans of avalanche deposits. Our results suggest that avalanche flow parameters can be determined from simple levee measurements or, conversely, formation of levees and flow fingers can be predicted once the parameters governing the granular fluctuations are known.

SUMMARY Centroid moment tensor solutions are recomputed for 190 earthquakes from 1976 to 2010 along the Heezen, Tharp and Hollister transform faults of the Eltanin system using a 3-D seismic velocity model. The total length of the three en echelon faults is nearly 1000 km; each is characterized by fast long-term rates of displacement of about 80 mm yr–1. Strike-slip faulting with moment magnitudes Mw up to 6.4 characterizes most of these events. The few involving normal faulting are located up to 40 km on either side of the transforms and involve extension nearly normal to the transforms. This partitioning of slip likely results from changes during the last few million years in the Euler pole for relative motion between the Antarctic and Pacific plates. Some parts of the Heezen and Tharp transforms exhibit strong seismic coupling but others were aseismic at the resolution of our study, Mw > 5.0–5.5. Earthquakes were not found along nearby fast spreading ridges at that resolution. We calculate downdip widths of seismic coupling of about 5 km for four strongly coupled segments from observed moment rates and lengths along strike assuming earthquake activity accounts for the entire plate motion. Major differences in seismic coupling along strike are not in accord with common thermal models of plate cooling but instead are attributed to varying degrees of metamorphism, rock type and effective normal stress and possibly to the presence of short intratransform spreading centres. One 30–42-km-long segment of the Heezen transform that appears to be an isolated well-coupled asperity has ruptured in eight earthquakes of Mw 5.9–6.1 quasi-periodically with a coefficient of variation of 0.26 every 4.0 ± 1.0 yr. Other well-coupled fault segments, which were sites with earthquakes up to Mw 6.39 and fewer events since 1976, have average repeat times of about 7–24 yr. The fast rate of plate motion, maximum size of events and relatively short repeat times make these fault segments a good laboratory for research on quasi-periodic behaviour and earthquake prediction.

The Gaw-Khuni Basin is located in the central part of the Sanandaj-Sirjan zone, on the northeastern margin of the Zagros orogen. This basin is one of the continuous zones of Sanandaj-Sirjan zone basins that is filled by Pliocene–Holocene sediments. The Gaw-Khuni Basin is surrounded by an en echelon to parallel arrangement of active faults in dextral strike-slip fault zones that indicate a right-step dilational stepover structure. The boundary faults of this basin are the Eastern and Western Gaw-Khuni, Ramsheh, and Nain-Dehshir faults. Earthquake distribution determined along the faults, dissection and displacement of Holocene sediments, and also the development of pull-apart basins filled with Holocene deposits along the faults confirm that these faults are active. The Gaw-Khuni stepover was formed during transtensional movements produced between two dextral strike-slip faults (Nain-Dehshir and Ramsheh). Also, the Gaw-Khuni pull-apart basin and the Eastern and Western Gaw-Khuni faults developed in a releasing step between the faults and grew in two directions during the time of their formation.

A distinct element method analysis is carried out to examine the development of shear bands in overburden soil subjected to a strike-slip fault. About 2.3 million spherical particles are used in the analysis and the results are compared with the shears observed at the Nojima earthquake fault during the 1995 Hyogoken Nanbu earthquake. En echelon shears and secondary shears which strike at lower angles to the basement fault – typical in strike-slip faults – are observed in the numerical analysis. Simple shear in the horizontal plane and drag due to the dependence of velocity on depth are confirmed to control the helicoidal shape of Riedel shears. Rotation of the compressional direction toward the fault strike as a result of slip along Riedel shears is also verified. It is found that the compressional direction is more horizontal within the area enclosed by Riedel shears than in outside areas and that these compressional directions produce secondary lower-angle shears that are less helicoidal. It is shown that the formation of column-like structures of particles and their subsequent buckling play significant micromechanical roles in three-dimensionally wrenched shears. The results of the numerical analysis, such as shear intervals and striking angles, show a resemblance to observational results at sites where sediment contains coarse grains and is subjected to strike slip with a small dip component, although they are not exactly the same as those observed at locations with similar overburden thicknesses.

Some factors affecting the accuracy of magnitude calibration of palaeoearthquakes are discussed. The general problem is illustrated by two case studies. First, the 2003 strong earthquake in the Altai region (Russia–Mongolia border), which has generated co-seismic surface faulting is considered. An analysis is conducted to determine how accurately its magnitude could be defined after 2000 years based on surface faulting parameters, retrieved when future palaeoearthquake studies are launched. This example demonstrates that accurate evaluation of magnitude is possible if specific regional seismotectonic features are taken into account and surface faulting parameters are measured over a dense grid. Next, the Muya earthquake that occurred in the Baikal seismic region in 1957 is studied with respect to the problem of palaeoearthquake magnitude calibration in this region. The intensity of the Muya earthquake is assessed based on macroseismic and geological data. Macroseismic effect distribution confirms source depth at 20–22 km, agreeing with a seismic rupture length of 25 km: only a part of the source is exposed on the surface. Comparison with length of paleoseismodislocations shows that it is a regional feature. The source mechanism with three sub-sources is in agreement with the segmentation of surface ruptures. Sub-sources are of strike-slip type with small normal components. Essential normal slip at surface is probably not representative for the source and is due to accommodation of strike-slip movement along with a system of sub-parallel en echelon ruptures under tension.

The magnitude (MW) 6.9 (Ms 7.1) Yushu earthquake occurred on 14 April 2010 in the Yushu area, central Tibetan Plateau, causing widespread damage in high mountain regions. Field investigations reveal that the earthquake produced a 33-km-long surface rupture zone, with dominantly left-lateral strike-slip along the Yushu Fault of the pre-existing strike-slip Ganzi-Yushu Fault Zone. The co-seismic surface ruptures are characterized by discontinuous shear faults, right-stepping en echelon tensional cracks, and left-stepping mole track structures that indicate a left-lateral strike-slip shear sense for the seismic fault. Field measurements indicate co-seismic left-lateral strike-slip displacements of approximately 0.3–3.2 m (typically 1–2 m), accompanied by a minor vertical component of

The present-day architecture of the Saharan Atlas in Tunisia can be defined by two principal models: (1) The first model emphasizes a general SW–NE geological structure in the North forming successive and parallel bands (the Tellian zone, the diapir zone) and the central Atlas, which are cut by the southern Atlas ranges located within a NW–SE corridor. These zones are bordered to the East by the “North–South Axis”. (2) The second model defines the Tunisian Atlas in terms of an E–W strike-slip corridor, which initially controls the sedimentary facies distribution during the Meso-Cenozoic, and which then generates elongate en echelon folds in the sedimentary cover by dextral shearing. In this study, we aim to show that the Saharan Atlas in Tunisia appears today as a triangular megablock, that we call the Tunisian Block (TB), bounded by three structural trends (N–S, SW–NE and NW–SE) belonging to the African strike-slip fault network: (1) The eastern boundary appears as a complex faulted and folded corridor limiting the folded zone of the central Atlas in the West and the depressed zone of the Sahel in the East: it corresponds to the “North–South Axis” as defined classically in the literature. (2) The southern boundary also corresponds to a faulted belt (Gafsa–Negrine-Tozeur corridor), which cuts off the continuation of the North–South axis southward into the Gabes region; it corresponds to the Southern Saharan Atlas, delimited by the Gafsa fault in the North and the Negrine-Tozeur fault in the South. (3) The northern boundary, trending SW–NE, appears rather in the form of a reverse tectonic bundle, facing SE or S (oblique convergence), whose major feature corresponds to the El Alia-Teboursouk fault. This northern boundary cuts across and delimits the N–S corridor towards the North, in such a way that its extension is limited at both extremities. Finally, the inner part of the TB actually corresponds to a mosaic of second-order blocks, each of which contains an arrangement of widely spaced SW–NE trending anticlines forming the main relief separated by vast plains very often occupied by sebkhas. The paleogeographic and structural evolution of this region during the Mesozoic and Palaeogene shows that the TB, along with its limits as defined here, developed an increasingly distinct identity at a very early stage, being characterized by an extensive and/or transtensive tectonic regime. Finally, the Tunisian Atlas Chain defines a triangular domain that owes its origin and particular character precisely because of the paleogeographic and structural history of this paleoblock. The boundaries of this paleoblock remain mobile, thus tectonically controlling the geometry and morphology of a typical intracontinental basin. The extension directions and the frequent changes of stress regime (or rotations) are related to the existence of two active basins: the strike-slip margin of the western branch of Tethys and the Mesogea oceanic basin, with tectonic activity becoming alternately dominant in one or other of the basins at different times. In this context, the Tunisian basin is characterized by rhythmic sedimentation, composed of a succession of filling sequences linked to the continuing tectonic instability of the sedimentary floor associated with two major crises: one at the end of the Aptian and the other at the end of the Ypresian. The vertical movements related to the extension and/or transtension of the blocks is accentuated by Triassic salt tectonics, giving rise to linear (salt axes) or point (salt domes) structures that lead to the formation of shoal zones during development of the basin, thus enhancing the vertical tectonics. The diapirism developed slowly and gradually from late Triassic through to Langhian times, leading to numerous sedimentary wedges on the flanks of the structures. The uprise of the diapirs exhibits three pauses corresponding to the end-Aptian, end-Ypresian and pre-Burdigalian. The vertical tectonics is characterized by abundant drape folds giving rise to an extensional fault-related folding and strike-slip/dip-slip faults creating frequent unconformities that are nevertheless always localized. Finally, the folded chain results from the structural inversion of this paleoblock from Tortonian times onward. We can only account for the various folds-axis directions in the context of an intracontinental chain where the pre-existing major vertical faults are able to develop on the surface as draped-folds in a transpressive regime by the local reorientation of stresses in crustal-scale faults. In detail, the structures produced by this vertical tectonic activity, which are profoundly controlled by inheritance, display a highly original style at very shallow levels in the crust.

Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic strike-slip surface rupture zone produced by the 2010 Mw 6.9 Yushu earthquake in the central Tibetan Plateau, China. Field surveys and structural analysis of the surface ruptures reveal that the co-seismic Riedel shear structures are characterized by (i) tension cracks (T fractures); (ii) compression structures, mainly mole tracks (P shears); and (iii) discrete shear faults (R and Y shears) developed in unconsolidated alluvial deposits along the pre-existing left-lateral strike-slip Ganzi–Yushu Fault Zone. The T fractures generally show a right-stepping en echelon pattern. In contrast, the P and R shears indicate a left-stepping en echelon pattern. The Riedel shears are inclined to the general trend of the co-seismic surface rupture zone at mean counterclockwise angles of 41° (T fractures) and 21° (R shears), and a mean clockwise angle of 35° for the P shears. The present results show that (i) the co-seismic Riedel shear structures indicate a left-lateral strike-slip sense for the seismogenic fault, and (ii) the T fractures, and P and R shears are the primary Riedel shear structures formed during the early stages of the evolution of a strike-slip fault constrained by pre-existing geologic structures.

A new model of the tectonic evolution of the Losevo Suture Zone is advanced based on the composition of the rocks occurring therein, the U/Pb age of the Voronezh Formation, and the specified age of collision. The model comprises the destruction of the Sarmatia continent; the formation of active continental margins of the West Pacific and Andean types, replacing one another en echelon from north to south; and the collision of Sarmatia and Volgo-Uralia with a distinctly expressed late orogenic stage marked by deposition of the Voronezh Formation crowned by the Baigora volcanic-plutonic structure.

Experimental modeling is used to study the geometry and evolution of structures and related secondary faults along releasing bends and offsets and restraining bends on strike-slip faults. The controls of the relative positions of adjacent strike-slip faults on the geometry of the structures and the difference in geometries between bends and offsets are investigated. A new method of laser scanning is used to map the geometry and evolution of the structures and related faults. The models show that oblique releasing bends connecting approaching faults result in spindle-shaped basins, whereas transverse bends result in more S-shaped or rhomboidal basins. Offsets result in the distribution of strain over a wider area and a larger number of faults compared with preexisting bends, which result in fewer well-defined basin-bounding faults. Secondary faults include R, R, and Y Riedel shears near the main strike-slip faults and oblique normal faults in the center of the basin. Fault patterns exhibit en echelon geometries with a progressive step down into the deepest parts of the basin. Symmetric, asymmetric, and double basins may form in any of the structural settings, depending on the slip distribution among faults on the basin margins. For restraining bends, oblique (45) bends connecting approaching faults result in spindle-shaped uplifts, whereas transverse or oblique (135) bends connecting overlapping faults result in more rhomboidal or rectangular uplifts. The fold trends are at increasingly higher angles with the strike faults for transverse and oblique (135) bends. Secondary faults include en echelon reverse faults, which typically form along the steep limbs of asymmetric uplifts, normal faults, which are transverse or oblique to the axis of the structure, and R, R, and Y Riedel shears near the main strike-slip faults. The aspect ratios of the basins and uplifts increase with increasing displacement on the strike-slip faults. The results of these models can be used to interpret the structural and fault geometries in surface and subsurface structures formed along strike-slip faults.

Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from

The structure and stratigraphy of the southeast Nechako Basin, which are poorly understood primarily because of substantial volcanic cover, are investigated in an analysis of seismic reflection, well, and potential field data. Formation and development of the SE Nechako Basin resulted in sub-basins containing Cretaceous and Eocene rocks. Interpretation reveals that dextral transtension in the Early to Middle Eocene created NNW-trending, en echelon, strike-slip faults linked by pull-apart basins, which locally contain a thickness of Eocene volcaniclastic rocks of >3 km. This structural pattern is consistent with regional observations that suggest the transfer of slip from the Yalakom fault to the north via a series of en echelon strike-slip faults. In the Middle to Late Eocene, faults associated with a change in the direction of stress, echoed by the north-trending right-lateral Fraser fault, reactivated and cut earlier structures. A simple model agrees with local observations, that northeast-directed compression was subparallel to the relic Cretaceous grain. Cretaceous rocks are discontinuous throughout the basin and may be remnants of a broader basin, or a number of contemporaneous basins, formed in a regional transpressional tectonic setting that caused northeast-directed thrusting along the eastern side of the Coast Plutonic Complex. Results suggest that thrusting affected most of the SE Nechako Basin, as observed across the Intermontane Belt to the northwest and southeast. The pattern of deposition of Neogene volcanic rocks of the Chilcotin Group was in part controlled by the Eocene structural grain, but we find no evidence of Neogene deformation.

Recently, the researches on structure controls on sandbodies have provided a new method for predicting petroleum reservoirs. The Yitong (伊通) graben is situated in the northern section of the Tan-Lu (郯-庐) fault system in eastern China. It was characterized by dual properties of strike-slip and extension in Cenozoic. Two types of intrabasinal structures were identified as oblique fault and transverse uplift in the graben. The oblique faults arranged en echelon in plain and locally presented negative rosette structures on seismic profile, so they were closely derived from strike-slip movement of the northwestern boundary faults. Moreover, these oblique faults were divided to five zones. The three transverse uplifts, located corresponding to flattened southeast boundary faults, were mainly originated by displacement-gradient folding due to segmental extensional activities of southeast boundary faults. The large-scale sandbodies of subaqueous fan facies and fan delta facies had developed at the two types of intrabasinal structure zone. Based on analyzing the seismic facies, logging facies and seismic attribute extractions, and on discovering many incised valleys at the oblique fault zones, the two types of intrabasinal structures were revealed to have conducted drainage entering basin and further dispersing, and to have consequently controlled the development and distribution of sandbodies.

The occurrence of shale diapirs in the Yinggehai–Song Hong (YGH–SH) Basin is well documented, as is their association with big petroleum fields. In order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high-resolution two- and three-dimensional seismic reflection data, as well as drilling data that cover the diapirs in YGH–SH Basin. As many as 18 diapirs were identified and are arranged in six N–S-striking vertical en echelon zones. On seismic reflection sections gas chimney structures, diapiric faults and palaeo-craters are genetically linked with the process of diapirism. Here we use geophysical and geological observations to propose a three-stage model for diapirism: initiation, emplacement, and collapse. During these three stages, different diapiric structure styles are formed, which we describe in detail. These include buried diapirs, piercing diapirs and collapsed diapirs. We link the diapirism to activity on the offshore continuation of the Red River Fault, as shown on our high-resolution seismic reflection data, which is also related to a high paleogeothermal gradient caused by crustal thinning. We also recognize the role of loading by the very large volume of sediment eroded from the edges of the Tibetan Plateau and delivered by the Red River to the basin.

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke moving upward from a sill at 18 km depth in the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth's crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing before it erupts of molten rock generated within the underlying mantle. Here we show seismicity accompanying melt intrusion from 17.5 to 13.5 km depth along a dyke dipping at 50° in the mid-crust of the Icelandic rift zone. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location, but breaking along fault planes with the same orientations. We suggest several possible reasons for the flipping fault mechanisms: the breakage of solidified plugs of basalt within the dyke itself as more melt intrudes; intrusion along sub-parallel fractures or dykelet fingers into the local stress field created near the tip of a propagating dyke; or movement on small jogs or offsets between adjacent en echelon dykes. Although the faulting is caused ultimately by melt movement, there is no resolvable volumetric component in the moment tensor solutions. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. Melt injection occurs in bursts propagating at 2–3 m/min along channels c. 0.2 m thick, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours.

The analysis and synthesis of the seismic data, the local gravity field, and the geological evidence reveal that the faults representing a framework for a system of the Cenozoic depressions in the Amur and Primorye regions define various structural parageneses: en echelon sets, extension duplexes, and conjugate knots. Being regularly arranged, they form a single branched-reticulate regional disjunctive zone named the Ussuri-Okhotsk rifting zone (UORZ). Owing to the relation with the faults of the Tan-Lu system and the graben-shaped basins of the marginal seas, this zone is an element of the pericontinental zigzag lineament zone, which controls the East Asian graben belt. The Ussuri-Okhotsk rifting zone represents a wide incipient right-lateral transtensional zone, which was formed under conditions of a pure shear deformation accompanied with additional extension. The compression and extension axes were oriented in the northeastern and northwestern directions, respectively. This deformation was responsible for passive dispersed epiorogenic and epiplatform rifting. After the episode of Late Miocene tectonic inversion, the rifting acquired taphrogenesis features. The tectonic movements were largely confined to extended deep-seated faults and led to the formation of spacious two-stage depressions. Simultaneously, the role of active rifting substantially increased, which is evident from the development of numerous Neogene-Quaternary basaltic plateaus. The rifting became more intense to form larger depressions reflecting deeper endogenic processes. This could probably be related to the mantle magmatism, the regional consolidation of the crust, changes in its structural anisotropy, and the insignificant reorganization of the tectonic stress field.

The genetic mechanism of the 5.12 Wenchuan Earthquake is still being debated and there is still no convincing general explanation for most of the phenomena. This is because researchers have ignored the important role of the Minshan block in the seismogenic process. The authors present a new opinion based on geological survey and comprehensive analyses. The Minshan block is a key tectonic element of the earthquake occurrence in the northwest triangle faulty block of Sichuan Province. The Minshan block is bordered by Longmen Mountain Range fractures in the south, the Huya fracture in the east, the Tazang fracture in the north and the Mounigou Valley fracture in the west. The rigidity of the block is relatively larger than those of the adjacent regions. The block's eastward movement pushed by regional maximum main geo-stress is limited when it suddenly tapers off near the east triangle end with a bottle-neck effect, and this causes geo-stress concentration around it. The shape of the block is coffin-like, wide in the upper part and narrow in the lower part. When a strong earthquake occurs along the block margins, the lock-up effect temporarily released, resulting in geo-stress transmitted to the Pingwu-Qingchuan (Motianling block) region. This transmission caused the Wenchuan earthquake's aftershocks to be concentrated in Qingchuan region. As the block moved eastward, the back of the block, i.e. the south segment of Mounigou Valley fracture, became active after the Wenchuan Earthquake. Therefore the aftershocks were concentrated along the south segment of Mounigou Valley fracture. Because the south margin is composed of the front range fracture, the geo-stress gradually released, causing many aftershocks along the Guanxian-Anxian fracture. The geological survey made after the Wenchuan Earthquake reveals that the surface ruptures in the south margin of Minshan block occur not along the Beichuan-Yingxiu fracture (central fracture) also along the front range fracture. The length of the surface rupture in the south margin ranges from several meters to several kilometers and it is distributed in en echelon (closely-spaced, parallel or subparallel, step-like surface ruptures). The vertical and horizontal displacements range from place to place and the thrusting component is dominant in the middle segment of Longmen Mountain Range structure belt. Nevertheless, the strike slip of the surface ruptures is dominant in the north segment of Longmen Mountain Range structure belt. Therefore the south margin is the original seismic structure. The sudden thrusting of the south margin of the Minshan block is the source event for the Wenchuan Earthquake.

Based on data presented in this study, the E-W–trending Alima anticline in the Metlaoui region of the southern Tunisian Atlas Mountains formed due to far-foreland, brittle deformation. The Alima anticline is one in a series of en echelon folds in the Atlas fold-and-thrust belt of North Africa. Geologic mapping indicates that the Alima anticline has a steep southern limb, a gently dipping northern limb, and pervasive normal fault sets. Fracture orientations suggest that fracturing occurred early in the fold history as a synfolding process, not as a pre- or postdeformational process. Gravity data show positive Bouguer anomalies near fold crests, not the negative anomalies that would be expected if the anticline were salt cored. Seismic data, collected along lines in basins surrounding the Alima anticline, suggest the presence of several high-angle reverse faults. Based on surface and subsurface studies, we attribute the development of the Alima anticline to far-foreland deformation associated with late Cenozoic contraction. N-S–directed elongation in the Triassic reoriented to NW-SE–directed shortening in the Miocene, causing Triassic normal faults to be reactivated as oblique-slip reverse faults. A comparison of the Alima anticline to other anticlines in the region suggests that several different styles of folding are present, each representing a different time of initiation.

The Mw 7.1 Darfield (Canterbury) earthquake of 4 September 2010 (NZST) was the first earthquake in New Zealand to produce ground-surface fault rupture since the 1987 Edgecumbe earthquake. Surface rupture of the previously unrecognised Greendale Fault during the Darfield earthquake extends for at least 29.5 km and comprises an en echelon series of east-west striking, left-stepping traces. Displacement is predominantly dextral strike-slip, averaging ~2.5 m, with maxima of ~5 m along the central part of the rupture. Maximum vertical displacement is ~1.5 m, but generally < 0.75 m. The south side of the fault has been uplifted relative to the north for ~80% of the rupture length, except at the eastern end where the north side is up. The zone of surface rupture deformation ranges in width from ~30 to 300 m, and comprises discrete shears, localised bulges and, primarily, horizontal dextral flexure. At least a dozen buildings were affected by surface rupture, but none collapsed, largely because most of the buildings were relatively flexible and robust timber-framed structures and because deformation was distributed over tens to hundreds of metres width. Many linear features, such as roads, fences, power lines, and irrigation ditches were offset or deformed by fault rupture, providing markers for accurate determinations of displacement.

Tectonic-stretching models have been previously proposed to explain the process of continental break-up through the example of the Asal Rift, Djibouti, one of the few places where the early stages of seafloor spreading can be observed. In these models, deformation is distributed starting at the base of a shallow seismogenic zone, in which sub-vertical normal faults are responsible for subsidence whereas cracks accommodate extension. Alternative models suggest that extension results from localised magma intrusion, with normal faults accommodating extension and subsidence only above the maximum reach of the magma column. In these magmatic rifting models, or so-called magmatic intrusion models, normal faults have dips of 45–55° and root into dikes. Vertical profiles of normal fault scarps from levelling campaign in the Asal Rift, where normal faults seem sub-vertical at surface level, have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks (basalt lava flows), using mechanical and kinematics concepts. We show that the studied normal fault planes actually have an average dip ranging between 45° and 65° and are characterised by an irregular stepped form. We suggest that these normal fault scarps correspond to sub-vertical en echelon structures, and that, at greater depth, these scarps combine and give birth to dipping normal faults. The results of our analysis are compatible with the magmatic intrusion models instead of tectonic-stretching models. The geometry of faulting between the Fieale volcano and Lake Asal in the Asal Rift can be simply related to the depth of diking, which in turn can be related to magma supply. This new view supports the magmatic intrusion model of early stages of continental breaking.

In the Man-Leo Shield, Paleoproterozoic (Birimian) belts crop out in nine countries of West Africa. Dominant domains include: (i) greenstone belts composed of plutono-volcanic, volcano-clastic and sedimentary rocks, deformed and weakly metamorphosed under regional greenschist facies conditions; (ii) widespread granitoid batholiths. The domains display a basin and dome-like architecture, and are overprinted by partitioned structures from successively shallower crustal depth. Analyses of key ductile and brittle structures has shown that the structural evolution of Man-Leo Shield is characterized by early vertical magmato-tectonics and subsequently, horizontal transcurrent tectonics with progression from ductile to brittle behavior. Basin and dome-like architectures, and the formation of an ubiquitous vertical foliation (MF) formed during emplacement of early amphibole-bearing (PAG) granite plutons at ca. 2.2 Ga by diapirism during NW–SE crustal shortening. Subsequent to a late stage of predominantly NW–SE shortening that created steeply-dipping mylonite zones (Mz1), transcurrent faults became predominant. The formation of transcurrent faults began transpressively, with development of N–S trending regional-scale mylonite zones (Mz1), and a steeply-plunging stretching lineation that probably formed during emplacement of PAG-type granitoids ca. 2.15 Ga. NNE–SSW transpressive sinistral horsetail faults and many NW–SE trending tension veins are interpreted to have formed at this stage. After cooling of the upper crust ca. 2.1 Ga, transcurrent faults became strike-slip in character with formation of dominantly NE–SW dextral faults (Mz2) and the passive emplacement of biotite (PBG) granitoids. Clockwise rotation of the extensional stress axis (σ3) from NNE–SSW trending to ENE–SSW trending assisted the propagation of dextral NE–SW and sinistral NW–SE extensional en echelon horsetail faults. WNW–ESE trending extension jogs (Egz) are interpreted to have been initiated under the same stress conditions. Displacements on strike-slip/transcurrent faults are interpreted as the product of rotation of rigid nuclei blocks producing faults’ re-activation. On the Man-Leo Shield Paleoproterozoic rocks are poorly exposed, but the tectonic model proposed in this study can help to shed light on the structural setting in areas of the shield which are poorly exposed, and in particular, why regional-scale structures do not display significant horizontal displacements. For practical use, key structural criteria can help to identify mylonite zones and transcurrent faults at different scales of investigation.

We produced regional geologic maps of the Hi’iaka and Shamshu regions of Io’s antijovian hemisphere using Galileo mission data to assess the geologic processes that are involved in the formation of Io’s mountains and volcanic centers. Observations reveal that these regions are characterized by several types of volcanic activity and features whose orientation and texture indicate tectonic activity. Among the volcanic features are multiple hotspots and volcanic vents detected by Galileo , one at each of the major paterae: Hi’iaka, Shamshu, and Tawhaki. We mapped four primary types of geologic units: flows, paterae floors, plains, and mountains. The flows and patera floors are similar, but are subdivided based upon emplacement environments and mechanisms. The floors of Hi’iaka and Shamshu Paterae have been partially resurfaced by dark lava flows, although portions of the paterae floors appear bright and unchanged during the Galileo mission; this suggests that the floors did not undergo complete resurfacing as flooding lava lakes. However, the paterae do contain compound lava flow fields and show the greatest activity near the paterae walls, a characteristic of Pele type lava lakes. Mountain materials are tilted crustal blocks that exhibit varied degrees of degradation. Lineated mountains have characteristic en echelon grooves that likely formed as a result of gravitational sliding. Undivided mountains are partially grooved but exhibit evidence of slumping and are generally lower elevation than the lineated units. Debris lobes and aprons are representative of mottled mountain materials. We have explored the possibility that north and south Hi’iaka Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units and created a depression which, by further extension during the rifting event, became Hi’iaka Patera. This type of rifting and depression formation is similar to the mechanism of formation of terrestrial pull-apart basins. With comparison to other regional maps of Io and global studies of paterae and mountains, this work provides insight into the general geologic evolution of Io.

Hundreds of narrow, linear ridge segments are found in the transition zone between the Elysium Rise and the Utopia basin, occurring as both single and multiple ridges. The ridges are distinctive because of their very linear, steep-sided nature, their often sharp ridge crest (which sometimes is fractured), their association with stubby flows, their continuity over long distances and their cross-cutting of different terrain. The linear ridges are interpreted to be single dikes and dike swarms, either emplaced as normal dikes or as dikes emplaced subglacially feeding an explosive or effusive eruption. Five dike swarms are identified, having lengths ranging from 10–45 km and being between 1–7 km wide, while single ridges are up to 20 km long and 100–500 m wide. In the areas of dike swarms, crustal dilatation is estimated to vary from 15–60%. Dikes emplaced en echelon suggest that variations in the local stress field caused rotation during dike emplacement and dikes crosscutting flow units imply that dike emplacement can account for some of the observed linear fractures in the area. The ridges both modify and constrain Early Amazonian flows and flood plain deposits suggesting intense dike emplacement in the Early Amazonian. The association with different stages of inverted craters, as well as some features of ice-related origin (possible ice-cauldron and tindar-like features), indicate that the dikes may have been exposed due to eolian erosion and loss of volatile rich units subsequent to their emplacement.

To determine whether the structural evolution of the Northern Cerberus Fossae (NCF) was dominated by cryospheric melting and collapse or fault-related subsidence, we used MOC, THEMIS and HiRISE images, and MOLA data to document spatial variations in vertical offset along strike. The Fossae are a series of fractures on the martian surface that cross-cut Noachian, Hesperian and, in places, very young Late Amazonian terrain. Serial cross sections across the fracture-related topography, from MOLA data, show that vertical offsets are not greater where fractures traverse older terrain, showing that offsets have accumulated since the formation of the Amazonian terrain. Vertical offsets are greater in the central portions of the fracture system with the profile resembling that for a single fault system. Topographic features that pre-date deformation are preserved on the graben floors suggesting little sediment infill, so the MOLA elevation measurements constrain total vertical offsets since the fractures formed. Deficits in vertical offset occur where fractures have not linked and remain en echelon across relay zones, or have linked, leaving palaeo-graben-tips. This indicates that the traces of the fractures propagate along strike at the surface and intersect over time periods that are likely to be in the range of 105–106 years rather than in a single collapse event. Deficits are also in places associated with collapse pits, suggesting such collapse is the early stage of graben subsidence at propagating lateral graben-tips. We use these observations to argue that the primary mechanism causing subsidence is not cryospheric melting and collapse, but faulting.

The eastern margin of the Tibetan Plateau is bounded by a N–S-trending active fault zone, including the Zemuhe Fault, which is an important tectonic boundary in studies of continental dynamics and deformation modes. Here, we present the kinematic mechanism and deformation features of co-seismic surface ruptures caused by a large historic earthquake that occurred upon the Zemuhe Fault. Field investigations and interpretations of aerial photographs reveal a NNW–SSE-trending surface rupture zone, less than 100 m wide, defined by en echelon fractures, mole tracks, and fault scarps over a length of 60 km. Trench excavations and radiocarbon age data confirm that the surface rupture zone was produced by the 1850 M 7.5 Xichang earthquake. Gullies preserved on the youngest alluvial fans and the lowest terrace risers in the area record systematic left-lateral offsets of 1.4–6.0 m and vertical offsets of 0.2–0.65 m resulting from the 1850 earthquake. Topographic and geologic evidence reveals that the spatial distribution of the 1850 co-seismic surface rupture was controlled by pre-existing geological structures of the Zemuhe Fault. The present results confirm that the Zemuhe Fault plays an important role as a major strike-slip fault in the southeastward motion of the southeastern Tibetan Plateau, and that the southern segment of the Zemuhe Fault has high seismic potential.

Based on a detailed analysis of satellite imagery combined with field geologic and geomorphic observations, we have mapped late Cenozoic folds and faults in the northeastern Pamir–Tian Shan convergence zone. It is a unique example to understand intracontinental ongoing mountain building within India–Eurasia collision system. In the front of northeastern Pamir, our investigations reveal that the NW-WNW-trending folds display a right-stepping en echelon pattern and NW-WNW-striking faults are mainly characterized by south-dipping thrusts with an extensive dextral strike-slip component. Drainage systems across the active faults show a systematic right-lateral offset. In contrast, structural style of the ENE trending fold-and-thrust belts are predominated by south–north directed shortening southwest of the Tian Shan. Our results also infer that oblique thrusting accommodates as long-term dextral slip rate of ca. 4.0 mm/yr during the late Cenozoic time north of the Pamir topographic front. Tectono-stratigraphic evidence suggests that the tectonic deformation was initiated at ca. 3–5 Ma in the study area. We suggest that intracontinental mountain building in the Pamir–Tian Shan convergence zone should be attributed to the crustal shortening caused by folding and thrusting as well as block rotation related to strike-slip faulting within the India–Eurasia collision system.

We present a detailed structural and lithologic map of Robben Island, offshore Cape Town, South Africa. Robben Island is underlain by the Tygerberg Formation, part of the Neoproterozoic to early Cambrian Malmesbury Group of the Saldania Belt. The depositional setting and structural history of the Tygerberg Formation are poorly constrained due to limited outcrop and lack of previous structural studies. Sedimentary structures are indicative of deposition at relatively high rates in a high energy environment and we concur with previous workers that deposition occurred on turbidite fan systems in a tectonically deepening basin. By comparison with active and ancient examples, we suggest that a forearc or trench slope, supra-subduction zone basin is a possible match to the setting of the Tygerberg Formation. However, limits on preservation and insufficient age data prevent comparisons in basin geometry and deposition rates which could be used to test depositional setting with more certainty. Northwest-southeast striking subvertical pressure solution cleavage is pervasive throughout the exposures. Upright folds, with axial planes parallel to the cleavage, plunge 10 to 15° to the northwest or southeast with approximately 20° variation in trend azimuth. The folds are limited in along-axis extent and often occur in asymmetric pairs. Subtle bedding-parallel shear zones divide folds of different plunge directions. This pattern of folds is consistent with experiments and observations of en echelon folding during distributed strain associated with oblique transpression. This finding is consistent with previous studies of parallel, slightly earlier orogenic belts to the north (Gariep and Kaoko Belts) although our observations do not allow us to distinguish whether transpressional strain was sinistral or dextral. Sinistral transpression is considered more likely given the dominantly sinistral strike-slip history on the nearby Colenso Fault and the southward migration of collision along the western margin of Africa during the late Neoproterozoic to early Cambrian.

Intensity of the Muya, 1957 earthquake is assessed in localities based on macroseismic data and in epicentral area based on effects in natural environment; it is analyzed how these assessments correspond to each other and to instrumental location of epicenter, hypocentral depth, and magnitude; it is evaluated, how seismodislocations of the Muya earthquake could serve as control of palaeoseismostructure parameters in this region. Spatial distribution of macroseismic effect confirms relatively deep source (20–22 km). Deep source agrees with anomalously short surface rupture length (not more than 25 km); only a part of the source exposed on the surface. Comparison with length of palaeoseismostructures shows that it is a regional feature. Epicentral intensity based on surface ruptures is assed X degrees in ESI2007 scale. Ignoring geological effects will underestimate epicentral intensity up to two degrees. Source mechanism with three sub-sources is in agreement with segmentation of surface ruptures. Sub-sources are of strike-slip type with small normal component; essential normal slip at surface is probably not representative for the source and is due to accommodation of strike-slip movement along with a system of sub-parallel en echelon ruptures under tension.

Crustal deformation models of the Tibetan plateau are assessed by investigating the nature of Neogene deformation along the India–Asia suture zone through geologic mapping in south-central Tibet (84°30′E). Our mapping shows that the suture zone is dominated by a system of 3 to 4 ENE-striking, south-dipping thrust faults, rather than strike-slip faults as predicted by models calling upon eastward extrusion of the Tibetan plateau. Faults along the suture zone are not active, as they are cut by a system of NNW-striking oblique slip normal faults, referred to herein as the Lopukangri fault system. Fault-slip data from the Lopukangri fault system shows that the mean slip direction of its hanging wall is N36W. We estimate the net slip on the Lopukangri fault by restoring components of the thrust system. We estimate that the fault has accommodated ∼ 7 km of right-slip and ∼ 8 km of normal dip-slip, yielding a net slip of ∼ 10.5 km, and 6 km of horizontal east–west extension. The Lopukangri fault system is active and geomorphic offsets indicate right separations and westside-down dip-separation. The mapview curviplanar geometry and geomorphic expression of the Lopukangri fault system is similar to faults and rift basins to its east and west. These extensional faults are en echelon in map view and encompass a region that is 200 km long (east–west) and 95 km wide (north–south). Assuming our results for the Lopukangri fault are applicable to the entire system, we estimate a maximum of 18% extension across the zone. All active faults in the system terminate southward adjacent to the India–Asia suture zone. Because the individual rift geometries are similar and suggest a common kinematic relationship, we propose that the extensional system formed as a trailing extensional imbricate fan at the southern termination of the central Tibet conjugate fault zone. Alternatively, the extensional system may terminate to the north and represent a group of isolated crustal tears. Both kinematic interpretations imply a semi-smooth north–south variation in the magnitude of east–west extension in southern Tibet, with higher magnitudes in the north along the Bangong–Nujiang suture zone than in the south along the India–Asia suture zone. Our results from southern Tibet show that deformation between southern Tibet and the Himalayas is broad (95 km wide) and best described as a continuum possibly since the Late Miocene. Conversely, the structural boundary between western Tibet and the Himalayas, which is defined by the Karakoram fault, is presently a discrete boundary, and probably has been since the Middle Miocene. We think this variation in the displacement gradient and age of these structural boundaries within the interior of Tibet is best explained by the fault patterns and strain history describing wholesale E–W stretching and N–S shortening of the Tibetan crust.

Alteration of rocks and localization of uranium mineralization in the northwestern Strel’tsovka caldera are exemplified in the Dal’nee deposit. In the main parameters of hydrothermal mineralization (temperature, pH, pressure, and composition of solution), the Dal’nee deposit differs from the deposits of the Strel’tsovka ore field located in the central part of the caldera. The localization of high-grade stratiform orebodies are interpreted in light of kinematic relations between steeply and gently dipping faults that formed in the tectonic setting of the NE-SW-trending, long-living, right-lateral, strike-slip faulting. The wide halos of argillic alteration and the structural control of uranium mineralization are caused by the fact that the deposit is located at the margin of the geological block, which has developed since the Late Triassic in a regime of extension (pull-apart) to form a depression, which is arranged en echelon relative to the main caldera and comparable to it in area. Currently, this depression is overlapped by sediments of the Sukhoi Urulyungui Basin. Such a structure markedly increases the probability of finding hidden uranium ores associated with low-temperature argillic alteration in the volcanosedimantary rocks and granitoid basement of the northwestern Strel’tsovka caldera.