Lower friction of the Xianshuihe–Xiaojiang fault system and its effect on active deformation around the south‐eastern Tibetan margin

We use three‐dimensional mechanical modelling with fault as Coulomb‐type frictional surface to explore the active deformation of the Xianshuihe–Xiaojiang fault system in south‐eastern Tibet. Crustal rheology is simplified as an elastoplastic upper crust and a viscoelastic lower crust. Far‐field GPS velocities and Quaternary fault slip rates are used to constrain the model results. Numerical experiments show that effective fault friction lower than ∼0.1–0.08 leads to a high slip rate that fits well with geological estimates of the slip rate on the fault system. Associating with the modelled fault slip rate, strain in the surrounding crust distributes broadly, and is partitioned into strike–slip and thrust senses. This means that in the Indian‐Eurasia convergence, accommodation of the large fraction of sinistral motion on the fault system is achieved mainly due to its lower fault friction. This in turn affects crustal deformation around the south‐eastern Tibetan margin, resulting in negligible compression across the Longmen Shan.     

滇中小江走滑剪切带晚新生代挤压变形研究

对滇中小江走滑剪切带中的挤压变形作用进行研究发现,尽管这些变形多发生于中元古界昆阳群浅变质岩中,其形成时代却可能是第三纪末和第四纪初,和小江断裂带的走滑运动有成因关系,并导致了走滑位移量的衰减。昆阳群的隆起可能主要是第四纪变形的结果。     

3D modeling of earthquake cycles of the Xianshuihe fault,southwestern China

We perform 3D modeling of earthquake generation of the Xianshuihe fault, southwestern China, which is a highly active strike-slip fault with a length of about 350 km, in order to understand earthquake cycles and segmentations for a long-term forecasting and earthquake nucleation process for a short-term forecasting. Historical earthquake data over the last 300 years indicates repeated periods of seismic activity, and migration of large earthquake along the fault during active seismic periods. To develop the 3D model of earthquake cycles along the Xianshuihe fault, we use a rate- and state-dependent friction law. After analyzing the result, we find that the earthquakes occur in the reoccurrence intervals of 400–500 years. Simulation result of slip velocity distribution along the fault at the depth of 10 km during 2694 years along the Xianshuihe fault indicates that since the third earthquake cycle, the fault has been divided into 3 parts. Some earthquake ruptures terminate at the bending part of the fault line, which may means the shape of the fault line controls how earthquake ruptures. The change of slip velocity and displacement at 10 km depth is more tremendous than the change of the shallow and deep part of the fault and the largest slip velocity occurs at the depth of 10 km which is the exact depth of the seismic zone where fast rupture occurs.     

Variable behavior of the Dead Sea Fault along the southern Arava segment from GPS measurements

The Dead Sea Fault is a major strike-slip fault bounding the Arabia plate and the Sinai subplate. On the basis of three GPS campaign measurements, 12 years apart, at 19 sites distributed in Israel and Jordan, complemented by Israeli permanent stations, we compute the present-day deformation across the Wadi Arava fault, the southern segment of the Dead Sea Fault. Elastic locked-fault modelling of fault-parallel velocities provides a slip rate of 4.7 ± 0.7 mm/yr and a locking depth of 11.6 ± 5.3 km in its central part. Along its northern part, south of the Dead Sea, the simple model proposed for the central profile does not fit the velocity field well. To fit the data, two faults have to be taken into account, on both sides of the sedimentary basin of the Dead Sea, each fault accommodating ∼ 2 mm/yr. Locking depths are small (less than 2 km on the western branch, ∼ 6 km on the eastern branch). Along the southern profile, we are once again unable to fit the data using the simple model, similar to the central profile. It is very difficult to propose a velocity greater than 4 mm/yr, i.e. smaller than that along the central profile. This leads us to propose that a part of the relative movement from Sinai to Arabia is accommodated along faults located west of our profiles.     

Displacement and timing of left-lateral faulting in the Kunlun Fault Zone,northern Tibet,inferred from geologic and geomorphic features

The E-W to WNW-ESE striking Kunlun Fault Zone, extending about 1600 km, is one of the large strike-slip faults in the northern Tibet, China. As a major strike-slip fault, it plays an important role on the extrusion of Tibet Plateau in accommodating northeastward shortening caused by the India-Asia convergence. However, the time of initiation left-lateral faulting of the Kunlun Fault Zone is still largely debated, ranging from the Middle to Late Triassic (240–200 Ma) to early Quaternary (2 Ma). We document displaced basement rocks and geomorphic features along the Kunlun Fault Zone, based on tectono-geomorphic interpretation of satellite remote sensing images and field geologic and geomorphic observations. Our results show that the largest cumulative offset of basement rocks is likely to be 100 ± 20 km. Meanwhile, a series of pull-apart basins (Kusai, Xiugou and Tuosu lake basins) and pressure ridges (East Deshuiwai and Maji Snow Mountains), each 45–70 km long and ∼8–12 km wide, are developed along the Kunlun Fault Zone, which resulted from long-term tectono-geomorphic growth since the Late Miocene or Early Pliocene. Geologic evidence indicates that the Kunlun Fault Zone had a long-term slip rate of ca.10 mm/yr during the late Quaternary. This slip rate is similar to that shown by present-day GPS measurements. Thus, we estimate that the Kunlun Fault Zone probably began left-lateral faulting at 10 ± 2 Ma based on a total displacement of 100 ± 20 km, and assuming a constant long-term slip rate of ca.10 mm/yr for several millions of years. And this timing constraint on initiation of left-lateral faulting of the Kunlun Fault Zone is consistent with widespread tectonic deformation which occurred in the Tibetan Plateau.     

青藏高原大型走滑断裂带晚新生代构造地貌生长及水系响应

大型走滑断裂带对调节印度板块和亚洲板块碰撞后产生的陆内构造变形和地貌生长起着非常重要作用。本文分析了沿青藏高原北缘主要大型左旋走滑断裂带：东昆仑、康西瓦和鲜水河-小江断裂带发育的错断地质体、大型错断水系或水系拐弯等新构造地貌特征,表明这些大型走滑断裂带在晚新生代以来发生了大规模的左旋走滑运动：前新生代地质体错位距离为80～120 km,大型水系累积的位移量可达80～90 km。根据这些走滑断裂带的长期走滑速率为8～12 mm/a,估算上述大型走滑断裂带的左旋走滑运动开始于中新世晚期：东昆仑和康西瓦断裂带左旋走滑运动开始于10±2 Ma; 鲜水河-小江断裂带甘孜-玉树段的左旋走滑运动的开始时间约为8～115 Ma。同样,如果大型水系的沿断裂带出现的大型错位或拐弯能够代表断裂带累积错位的上限,表明发源于青藏高原的黄河、金沙江、喀拉喀什河和玉龙喀什河等一级水系上游大致开始形成于9～7 Ma±。西昆仑山前盆地中河流相沉积的最早响应时间为8～6 Ma,与喀拉喀什河和玉龙喀什河等西昆仑山地区一级水系的形成时间基本一致,表明这些大型水系初始形成时间与左旋走滑构造运动的开始时间准同时。这表明中新世中晚期青藏高原构造演化发生了重要转变。     

Source characteristics of the Yutian earthquake in 2008 from inversion of the co-seismic deformation field mapped by InSAR

On 21 March 2008, an Ms7.3 earthquake occurred at Yutian County, Xinjiang Uygur Autonomous Region, which is in the same year as 2008 Mw 7.9 Wenchuan earthquake. These two earthquakes both took place in the Bayar Har block, while Yutian earthquake is located in the west edge and Wenchuan earthquake is in the east. The research on source characteristics of Yutian earthquake can serve to better understand Wenchuan earthquake mechanism. We attempt to reveal the features of the causative fault of Yutian shock and its co-seismic deformation field by a sensitivity-based iterative fitting (SBIF) method. Our work is based on analysis and interpretation to high-resolution satellite (Quickbird) images as well as D-InSAR data from the satellite Envisat ASAR, in conjunction with the analysis of seismicity, focal mechanism solutions and active tectonics in this region. The result shows that the 22 km long, nearly NS trending surface rupture zone by this event lies on a range-front alluvial platform in the Qira County. It is characterized by distinct linear traces and a simple structure with 1–3 m-wide individual seams and maximum 6.5 m width of a collapse fracture. Along the rupture zone are seen many secondary fractures and fault-bounded blocks by collapse, exhibiting remarkable extension. The co-seismic deformation affected a big range 100 km × 40 km. D-InSAR analysis indicates that the interferometric deformation field is dominated by extensional faulting with a small strike-slip component. Along the causative fault, the western wall fell down and the eastern wall, that is the active unit, rose up, both with westerly vergence. The maximum subsidence displacement is ～2.6 m in the LOS, and the maximum uplift is 1.2 m. The maximum relative vertical dislocation reaches 4.1 m, which is 10 km distant from the starting rupture point to south. The 42 km-long seismogenic fault in the subsurface extends in NS direction as an arc, and it dipping angle changes from 70° near the surface to 52° at depth ～10 km. The slip on the fault plane is concentrated in the depth range 0–8 km, forming a belt of length 30 km along strike on the fault plane. There are three areas of concentrating slip, in which the largest slip is 10.5 m located at the area 10 km distant from the initial point of the rupture.     

Geometric evolution of a plate interface–branch fault system: Its effects on the tectonic development of the Himalayas

Crustal deformation due to fault slip depends strongly on fault geometry, and fault geometry is changed by the deformation of the crust. This feedback mechanism causes the geometrical evolution of the fault system. We have studied the progress of the geometrical evolution of a plate interface–branch fault system through numerical simulation, based on elastic–viscoelastic dislocation theory. If the plate interface is smooth, no significant change occurs in fault geometry. If the plate interface has a ramp, we observe the gradual horizontal motion of the ramp toward the hanging-wall side of the interface at half the plate convergence rate. The offset of the ramp decreases with time. The dip-angle of thrust faults branching from the plate interface increases more rapidly as the dip of the fault increases. We have applied these results to the plate interface–branch fault system at the India–Eurasia collision boundary and obtained a scenario for the tectonic development of the Himalayas for the last 30 Myr.     

Present-day crustal deformation along the Philippine Fault in Luzon,Philippines

The Philippine Fault results from the oblique convergence between the Philippine Sea Plate and the Sunda Block/Eurasian Plate. The fault exhibits left-lateral slip and transects the Philippine archipelago from the northwest corner of Luzon to the southeast end of Mindanao for about 1200 km. To better understand fault slip behavior along the Philippine Fault, eight GPS surveys were conducted from 1996 to 2008 in the Luzon region. We combine the 12-yr survey-mode GPS data in the Luzon region and continuous GPS data in Taiwan, along with additional 15 International GNSS Service sites in the Asia-Pacific region, and use the GAMIT/GLOBK software to calculate site coordinates. We then estimate the site velocity from position time series by linear regression. Our results show that the horizontal velocities with respect to the Sunda Block gradually decrease from north to south along the western Luzon at rates of 85–49 mm/yr in the west–northwest direction. This feature also implies a southward decrease of convergence rate along the Manila Trench. Significant internal deformation is observed near the Philippine Fault. Using a two dimensional elastic dislocation model and GPS velocities, we invert for fault geometries and back-slip rates of the Philippine Fault. The results indicate that the back-slip rates on the Philippine Fault increase from north to south, with the rates of 22, 37 and 40 mm/yr, respectively, on the northern, central, and southern segments. The inferred long-term fault slip rates of 24–40 mm/yr are very close to back-slip rates on locked fault segments, suggesting the Philippine Fault is fully locked. The stress tensor inversions from earthquake focal mechanisms indicate a transpressional regime in the Luzon area. Directions of σ₁ axes and maximum horizontal compressive axes are between 90° and 110°, consistent with major tectonic features in the Philippines. The high angle between σ₁ axes and the Philippine Fault in central Luzon suggests a weak fault zone possibly associated with fluid pressure.     

Deformation style in the damage zone of the Mondy fault: GPR evidence (Tunka basin,southern East Siberia)

The Mondy strike-slip fault connects the W-E Tunka and N-S Hovsgol basins on the southern flank of the Baikal rift system. Ground penetrating radar (GPR) surveys in its damage zone provide constraints on thicknesses, dips, and plunges of fault planes, as well as on the amount and sense of vertical slip. Strike-slip faulting in the southern segment of the Mondy fault within the territory of Russia bears a normal slip component of motion along the W-E and NW planes. These motions have produced negative flower structures in shallow crust appearing as grabens upon Pleistocene fluvioglacial terraces. The amount of normal slip estimated from the displacement of reflection events varies over the area and reaches its maximum of 3.4 m near Mondy Village. In the Kharadaban basin link, left-lateral strike slip displaces valleys of ephemeral streams to 22 m, while normal slip detected by GPR reaches 2.2 m; this normal-to-strike slip ratio corresponds to a direction of ~ 6° to the horizon. The angles of dips of faults are in the range 75°-79°; the thicknesses of fault planes marked by low- or high-frequency anomalies in GPR records vary from 2.5 to 17.0 m along strike and decrease with depth within a few meters below the surface, which is common to near-surface coseismic motions. Many ruptures fail to reach the surface but appear rather as sinkholes localized mainly in fault hanging walls. The deformation style in the damage zone of the Mondy fault bears impact of the NW Yaminshin fault lying between its two segments. According to photoelasticity, the stress field changes locally at the intersection of the two faults, under NE compression at 38°, till the inverse orientations of principal compression and extension stresses. This stress pattern leads to a combination of normal and left-lateral strike slip components.     

Detailed fault structure of the Tarutung Pull-Apart Basin in Sumatra,Indonesia, derived from local earthquake data

The Tarutung Basin is located at a right step-over in the northern central segment of the dextral strike-slip Sumatran Fault System (SFS). Details of the fault structure along the Tarutung Basin are derived from the relocations of seismicity as well as from focal mechanism and structural geology. The seismicity distribution derived by a 3D inversion for hypocenter relocation is clustered according to a fault-like seismicity distribution. The seismicity is relocated with a double-difference technique (HYPODD) involving the waveform cross-correlations. We used 46,904 and 3191 arrival differences obtained from catalogue data and cross-correlation analysis, respectively. Focal mechanisms of events were analyzed by applying a grid search method (HASH code). Although there is no significant shift of the hypocenters (10.8 m in average) and centroids (167 m in average), the application of the double difference relocation sharpens the earthquake distribution. The earthquake lineation reflects the fault system, the extensional duplex fault system, and the negative flower structure within the Tarutung Basin. The focal mechanisms of events at the edge of the basin are dominantly of strike-slip type representing the dextral strike-slip Sumatran Fault System. The almost north–south striking normal fault events along extensional zones beneath the basin correlate with the maximum principal stress direction which is the direction of the Indo-Australian plate motion. The extensional zones form an en-echelon pattern indicated by the presence of strike-slip faults striking NE–SW to NW–SE events. The detailed characteristics of the fault system derived from the seismological study are also corroborated by structural geology at the surface.     

Termination of the northwestern Basin and Range province into a clockwise rotating region of transtension and volcanism,southeast Oregon

New data indicate that northeast-directed extensional faulting characterizes slip across the Brothers fault zone (BFZ), which marks the northern limit of the northwestern Basin and Range (NWBR) extensional province in southeastern Oregon. Structural separation across individual north-northeast striking NWBR faults decreases to zero south of the BFZ. Field relationships and cross-sections demonstrate limited kinematic linkage and independent evolution of the two fault systems since ∼7 Ma. West-directed extension accumulated on NWBR faults at 0.01 mm/yr and lengthened northward after 7.05 Ma. BFZ faults accumulated northeast-directed extension at rates of 0.01 mm/yr since 5.68 Ma. Deformation coincides with periods of heightened basaltic magmatism in the High Lava Plains, implying that volcanism weakened the crust and promoted extension in the BFZ. In a new model, we reconcile the observed northward diminishing rate and clockwise motion of the modern NWBR deformation field with regional geology. The BFZ defines a small circle about the pole of rotation and separates a stable block to the NE from the extending region to the south. Faults to the south are growing northward, consistent with the northward decrease in rate and magnitude of extension in the NWBR.     

Trenching exposures of the surface rupture of 2008 Mw 7.9 Wenchuan earthquake,China: Implications for coseismic deformation and paleoseismology along the Central Longmen Shan thrust fault

The May 12, 2008, Mw 7.9 Wenchuan earthquake was induced by failure of two of the major faults of the Longmen Shan thrust fault zone along the eastern margin of Tibet Plateau. Our study focused on trenches across the Yingxiu–Bichuan fault, the central fault in the Longmen Shan belt that has a coseismic surface break of more than 200 km long. Trenching excavation across the 2008 earthquake rupture on three representative sites reveals the styles and amounts of the deformation and paleoseismicity along the Longmen Shan fault. Styles of coseismic deformation along the 2008 earthquake rupture at these three sites represent three models of deformation along a thrust fault. Two of the three trench exposures reveal one pre-2008 earthquake event, which is coincident with the pre-existing scarps. Based on the observation of exposed stratigraphy and structures in the trenches and the geomorphic expressions on ground surface, we interpret the 2008 earthquake as a characteristic earthquake along this fault. The interval of reoccurrence of large earthquake events on the Central Longmen Shan fault (the Yingxiu–Beichuan fault) can be inferred to be about 11,000 years according to ¹⁴C and OSL dating. The amounts of the vertical displacement and shortening across the surface rupture during the 2008 earthquake are determined to be 1.0–2.8 m and 0.15–1.32 m, respectively. The shortening rate and uplift rate are then estimated to be 0.09–0.12 mm/yr and 0.18–0.2 mm/yr, respectively. It is indicated that the deformation is absorbed mainly not by shortening, but by uplift along the rupture during the 2008 earthquake.     

Mountain building processes in intracontinental oblique deformation belts: Lessons from the Gobi Corridor,Central Asia

This paper presents a review of the Quaternary–Recent deformation field and mountain building processes within the Gobi Corridor region of Central Asia, which includes the North Tibetan foreland, Beishan, Gobi Altai and easternmost Tien Shan. The region can be considered the ‘soft core’ of Central Asia which has been reactivated due to the continuing Indo-Eurasia collision to the south. Favourable preconditions for reactivation of Gobi Corridor basement include a mechanically weak Palaeozoic terrane collage sandwiched between rigid Precambrian basement blocks to the north and south, thermally weakened crust due to Jurassic–Miocene volcanism and widespread Palaeozoic–Mesozoic granitic magmatism with associated high radiogenic heat production, and crustal thinning due to widespread Cretaceous rift basin development. The network of Quaternary–Recent faults within the entire region defines a diffuse sinistral transpressional deformation field that has generated a transpressional basin and range physiographic province. Typically, thrust and oblique-slip thrust faults are WNW-striking and reactivate basement faults and fabrics, whereas left-lateral strike-slip faults are ENE-striking and cut across basement trends. The angular relationship between SHmax and pre-existing basement structural trends is the fundamental control on the kinematics of Late Cenozoic deformation. Along-strike and across-strike growth and coalescence of restraining bends, other transpressional ranges and thrust ridges is an important mountain building process. Thrust faults throughout the region are both NNE and SSW directed and thus there is no common structural vergence, nor orogenic foreland or hinterland. Root structures appear to be vertical faults, not low-angle decollements and flower structure fault geometries within individual ranges are common. Published earthquake and geodetic data are consistent with a diffusely deforming continental interior region with tectonic loading shared amongst a complex network of faults. Therefore, earthquake prediction is likely to be more complex than in plate boundary settings and extrapolation of derived Late Quaternary fault slip rates is not straightforward. Modern mountain building within the Gobi Corridor demonstrates that reactivation of ancient accretionary and collisional orogens within continental interiors can play an important role in continental evolution and the life cycle of orogenic belts.     

Discussion of tectonic models for Cenozoic strike-slip fault-affected continental margins of mainland SE Asia

Understanding the roles of Cenozoic strike-slip faults in SE Asia observed in outcrop onshore, with their offshore continuation has produced a variety of structural models (particularly pull-apart vs. oblique extension, escape tectonics vs. slab-pull-driven extension) to explain their relationships to sedimentary basins. Key problems with interpreting the offshore significance of major strike-slip faults are: (1) reconciling conflicting palaeomagnetic data, (2) discriminating extensional, and oblique-extensional fault geometries from strike-slip geometries on 2D seismic reflection data, and (3) estimating strike-slip displacements from seismic reflection data.Focus on basic strike-slip fault geometries such as restraining vs. releasing bends, and strongly splaying geometries approach the gulfs of Thailand and Tonkin, suggest major strike-slip faults probably do not extend far offshore Splays covering areas 10,000’s km² in extent are characteristic of the southern portions of the Sagaing, Mae Ping, Three Pagodas and Ailao Shan-Red River faults, and are indicative of major faults dying out. The areas of the fault tips associated with faults of potentially 100 km+ displacement, scale appropriately with global examples of strike-slip faults on log–log displacement vs. tip area plots. The fault geometries in the Song Hong-Yinggehai Basin are inappropriate for a sinistral pull-apart geometry, and instead the southern fault strands of the Ailao Shan-Red River fault are interpreted to die out within the NW part of the Song Hong-Yinggehai Basin. Hence the fault zone does not transfer displacement onto the South China Seas spreading centre. The strike-slip faults are replaced by more extensional, oblique-extensional fault systems offshore to the south. The Sagaing Fault is also superimposed on an older Paleogene–Early Miocene oblique-extensional rift system. The Sagaing Fault geometry is complex, and one branch of the offshore fault zone transfers displacement onto the Pliocene-Recent Andaman spreading centre, and links with the West Andaman and related faults to form a very large pull-apart basin.     

40Ar–39Ar dating of volcanic rocks of the Shyok suture zone in north–west trans-Himalaya: Implications for the post-collision evolution of the Shyok suture zone

⁴⁰Ar–³⁹Ar geochronological studies carried out on the Khardung volcanics of Ladakh, India and our earlier Ar–Ar results from the volcanics of the Shyok suture along with the available geological and geochemical data provide good constraints for post-collision evolution of the Shyok suture zone. Whole-rock samples from the Shyok volcanics yielded disturbed age-spectra and we have demonstrated earlier that the youngest tectonic event in the Shyok suture zone responsible for the thermal disturbance of these samples is Karakoram fault activation at ～14 Ma. Contrastingly whole-rock samples from the Khardung volcanics, which are in tectonic contact with these Shyok volcanics, and are exposed in the form of thick rhyolitic and ignimbritic flows, yielded undisturbed age-spectra and good plateau-ages. The whole-rock plateau-ages of two rhyolite samples are 52.8 ± 0.9 and 56.4 ± 0.4 Ma. We interpret these ages to be the time and duration of emplacement of these volcanics over thickened margin of the continental crust, which appears to be coeval with the initiation of the collision between the Indian and Asian plate. The lesser extent of post-emplacement isotopic re-equilibration in these samples unlike the Shyok volcanics indicate that these samples were present in different tectonic settings, away from the Karakoram fault, at the time of deformation in the Shyok suture zone. We propose that the two volcanic belts of contrasting nature were brought together in juxtaposition by the Karakoram strike slip faulting at ～14 Ma.     

塔里木盆地柯坪断隆断裂构造分析

柯坪断隆内断裂发育，笔者根据野外及地震数据对各主要断裂和二级断裂进行了分析，认为柯坪塔格断裂形成于晚第三纪，沙井子断裂早期与柯坪塔格具有不同的发育历史，阿合奇断裂形成于挤压而非走滑的背景下，皮羌断裂和印干走滑断裂其实是协调作用的捩断层。萨尔干断裂是一条假走滑断层，实际上应该是一条撕裂断层。在挤压背景下形成了二类主要的断裂构造组合样式；叠瓦推覆体、构造窗。笔者认为柯坪断隆上的构造其实是印度板块和欧亚板块远程碰撞造山和板内变形的一种表现。     

Plio-Quaternary stress states in NE Iran: Kopeh Dagh and Allah Dagh-Binalud mountain ranges

NE Iran, including the Kopeh Dagh and Allah Dagh-Binalud deformation domains, comprises the northeastern boundary of the Arabia–Eurasia collision zone. This study focuses on the evolution of the Plio-Quaternary tectonic regimes of northeast Iran. We present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors. The inversions of fault kinematics data reveal distinct temporal changes in states of stress during the Plio-Quaternary (since ～ 5 Ma). The paleostress state is characterized by a regional transpressional tectonic regime with a mean N140 ± 10°E trending horizontal maximum stress axis (σ₁). The youngest (modern) state of stress shows two distinct strike-slip and compressional tectonic regimes with a regional mean of N030 ± 15°E trending horizontal σ₁. The change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional N trending σ₁. The inversion analysis of earthquake focal mechanisms reveals a homogeneous, transpressional tectonic regime with a regional N023 ± 5°E trending σ₁. The modern stress state, deduced from the youngest fault kinematics data, is in close agreement with the present-day stress state given by the inversions of earthquake focal mechanisms. According to our data and the deduced results, in northeast Iran, the Arabia–Eurasia convergence is taken up by strike-slip faulting along NE trending left-lateral and NNW trending right-lateral faults, as well as reverse to oblique-slip reverse faulting along NW trending faults. Such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field. This implies that no strain and/or stress partitioning or systematic block rotations have occurred in the Kopeh Dagh and Allah Dagh-Binalud deformation domains. The Plio-Quaternary stress changes documented in this paper call into question the extrapolation of the present-day seismic and GPS-derived deformation rates over geological time intervals encompassing tens of millions of years.     

Recent tectonics and seismicity of the western Altai-Sayan mountainous region,Junggar basin,and Chinese Tien Shan

According to GPS monitoring, recent tectonic process between Tarim and West Siberia in the band within 80°–95° E is generated by the northward movement of the Tarim block. During the accompanying horizontal compression of the area, orogeny takes place within linear mobile zones when blocks are squeezed into the upper half-space. When the orientation of the mobile zones is transverse to the compression direction, the leading orogenic process is reverse faulting. When these directions intersect at an acute angle, the principal features of the mountain relief are formed by oblique-slip and strike-slip faults.The spatial distribution of seismic activity A10 over a 40-year period of instrumental observations within the mobile zones of the study area is extremely nonuniform. Seismic activity increases to the south, toward the source of deformations—the Indo-Eurasian collision. The maximum activity is observed at the reverse-fault boundaries of the eastern Tien Shan (~ 40). The seismic activity of the strike-slip fault boundaries of the Great Altai is considerably lower (0.11–0.16).     

Palaeoseismology in steep terrain: The Big Bend of the San Andreas fault, Transverse Ranges, California

Transpressional tectonics are typically associated with restraining bends on major active strike-slip faults, resulting in uplift and steep terrain. This produces dynamic erosional and depositional conditions and difficulties for established lines of palaeoseismological investigation. Consequently, in these areas data are lacking to determine tectonic behaviour and future hazard potential along these important fault segments. The Big Bend of the San Andreas fault in the Transverse Ranges of southern California exemplifies these problems. However, landslides, probably seismically triggered, are widespread in the rugged terrain of the Big Bend. Fluvial reworking of these deposits rapidly produces geomorphic planes and lines that are markers for subsequent fault slip. The most useful are offset and abandoned stream channels, for these are relatively high precision markers for identifying individual faulting events. Palaeoseismological studies from the central Big Bend, involving ¹⁴C ages of charcoal fragments from trench exposures, illustrate these points and indicate that the past three faulting events, including the great 1857 earthquake, were relatively similar in scale, each producing offsets of about 7–7.5 m. The mean recurrence interval is 140–220 years. The pre-1857 event here may be the 1812 event documented south of the Big Bend or an event which took place probably between 1630 and 1690. Ground breakage in both events extended south of the Big Bend, unlike the 1857 event where rupture was skewed to the north. The preceding faulting event ruptured both to the north and south of the Big Bend and probably occurred between 1465 and 1495. All these events centred on the Big Bend and may be typical for this fault segment, suggesting that models of uniform long-term slip rates may not be applicable to the south-central San Andreas. A slip-rate estimate of 34–51 mm a^{− 1} for the central Big Bend, although uncertain, may also imply higher slip in the Big Bend and highlights difficulties in correlating slip-rates between sites with different tectonic settings. Slip rates on the San Andreas may increase within the broad compressional tectonics zone of the Big Bend, compared to the north and south where the plate boundary is a relatively linear and sub-parallel series of dominantly strike-slip faults. Partitioning slip within the Big Bend is inherently uncertain and insufficient suitably comparable data are available to sustain a uniform slip model, although such models are a common working assumption.