Dextral Strike-slip Motion Research Articles

Evidence of positive tectonic inversion in the north-central sector of the Sicily Channel (Central Mediterranean)

In order to unravel the tectonic evolution of the north-central sector of the Sicily Channel (Central Mediterranean), a seismo-stratigraphic analysis of single- and multi-channel seismic reflection profiles has been carried out. This allowed to identify, between 20 and 50 km offshore the central-southern coast of Sicily, a ~80-km-long deformation belt, characterized by a set of WNW–ESE to NW–SE fault segments showing a poly-phasic activity. Within this belt, we observed: i) Miocene normal faults reactivated during Zanclean–Piacenzian time by dextral strike-slip motion, as a consequence of the Africa–Europe convergence; ii) releasing and restraining bend geometries forming well-developed pull-apart basins and compressive structures. In the central and western sectors of the belt, we identified local transpressional reactivations of Piacenzian time, attested by well-defined compressive features like push-up structures and fault-bend anticlines. The reconstruction of timing and style of tectonic deformation suggest a strike-slip reactivation of inherited normal faults and the local subsequent positive tectonic inversion, often documented along oblique thrust ramps. This pattern represents a key for an improved knowledge of the structural style of foreland fold-and-thrust belts propagating in a preexisting extensional domain. With regard to active tectonics and seismic hazards, recent GPS data and local seismicity events suggest that this deformation process could be still active and accomplished through deep-buried structures; moreover, several normal faults showing moderate displacements have been identified on top of the Madrepore Bank and Malta High, offsetting the Late Quaternary deposits. Finally, inside the northern part of the Gela Basin, multiple slope failures, originated during Pleistocene by the further advancing of the Gela Nappe, reveal tectonically induced potential instability processes.

Hydrocarbon accumulation in strike-slip fault restraining bends: New insights into the tectonic controls on the Penglai 19-3 and Penglai 25-6 oil fields, Tan-Lu fault zone, east China

As the largest active strike-slip fault zone of east Asia, the Tan-Lu fault zone is the most significant tectonic feature controlling the hydrocarbon accumulation in Bohai Bay. The Penglai 19-3 and Penglai 25-6 fields are the most typical examples among the fields found in the Tan-Lu fault zone. The structures related to the two fields are fault restraining bends produced by dextral strike-slip movement on faults within the Tan-Lu fault zone. The structures initiated at the late depositional stage of the third member of the Eocene Shahejie Formation (ca. 40 Ma) after the deposition of the main source rocks of the basin. They then experienced a main development stage during deposition of the second and first members of the Eocene Shahejie Formation and the Oligocene Dongying Formation (40–25 Ma). During the Neogene, the structures continued to be enhanced slightly because of continued strike-slip until the early to middle Pleistocene. These structures were characterized by the absence of the preponderance of the reverse separations on faults and might represent the restraining bends in a divergent wrench deformation zone. This study shows that restraining bend structures along intrabasinal strike-slip systems formed after the deposition of the source rocks are very favorable for hydrocarbon accumulation.

Structural Evolution of the Rio Blanco-Los Bronces District, Andes of Central Chile: Controls on Stratigraphy, Magmatism, and Mineralization

The late Miocene to early Pliocene mineralized centers of the Rio Blanco-Los Bronces porphyry copper cluster constitute the largest known copper concentration in the Earth’s crust. Based on new district-scale geological mapping and cross-section interpretation, this study describes the structural evolution of the Main Cordillera of central Chile in the Rio Blanco-Los Bronces district and its relationship with the emplacement of magmatic bodies and mineral deposits. The tectonic evolution of this Andean segment was strongly controlled by regional-scale fault systems, oriented oblique to the north-south axis of the orogen. These NW- and NE-striking faults were active as normal faults during the late Eocene-Oligocene, and controlled the compartmentalization of the Abanico volcanotectonic basin into individual subbasins with characteristic volcanosedimentary facies and thicknesses. The faults were selectively reactivated during Mio-Pliocene east-west contraction, with the NW-striking faults showing a composite reverse-sinistral movement and the NE-striking faults showing mainly dextral strike-slip movements. This reactivation occurred coeval with the deposition of the Farellones Formation and the emplacement of the Rio Blanco-San Francisco batholith, the last pulses of which are genetically related with the formation of the Rio Blanco-Los Bronces cluster. Magmatic and hydrothermal fluid flow was channeled and focused by both sets of preexisting oblique structures and, in turn, fault rupture was driven by high fluid pressures. A proper understanding of similar long-lived, regional-scale arc-oblique fault systems might clarify the structural controls on magmatism and mineralization in the broader Mio-Pliocene metallogenic belt of central Chile and also in other Andean segments.

Impact of inversion tectonics on hydrocarbon entrapment in the Baggara Basin, western Sudan

The Baggara Basin of Sudan is an ENE-WSW trending basin located within the West and Central Africa Rift System (WCARS) and NW of the Muglad Basin. The trend of the Baggara Basin is similar to other basins in eastern Chad, where oil has been discovered. Since there are no published studies for the Baggara Basin, the present paper represents an addition to the geology of eastern extension of the WCARS in Sudan. The 2D seismic data interpretation sheds light on the development and evolution of key regional and local structures of the basin. The Baggara Basin is characterized by graben geometry with two boundary faults, characterized by dextral strike slip movement, and a depocenter at the central part. The interpretation reveals the presence of two types of structures within the Baggara Basin; inversion tectonics restricted to the larger basin-bounding faults in the western part and en-echelon faults in the central part. The timing of inversion tectonics is during the Eocene and it is related to a brief major compressional event due to the collision of the African and Eurasian plates. Although the inversion created a four-way dip closed anticline structure, this anticline postdated the initial migration from the source rock. Accordingly, the anticline structural is unsuccessful for hydrocarbon entrapment.

Late Quaternary slip-rate along the central Bangong-Chaxikang segment of the Karakorum fault, western Tibet

Insight into the spatial and temporal changes of slip-rate is essential to understand the kinematic role of large strike-slip faults in continental collision zones. Geodetic and geologic rates from present to several million years ago along the Karakorum fault range from 0 to 11 mm/yr. Here, we determine the first late Quaternary slip-rate at the southern end of the linear Bangong-Chaxikang segment of the Karakorum fault, using cumulative offsets (20–200 m) of fans and terraces at three sites, as well as 74 new 10 Be surface-exposure ages to constrain the age of these offset geomorphic markers. The rate is >3 mm/yr at sites Gun and Chaxikang, and it is >1.7–2.2 mm/yr at the Gar fan site. Together with rates obtained along the southernmost Menshi-Kailas segment, the Karakorum fault slip-rate seems to increase southeastward from south of Bangong Lake to Kailas (from >3 to >8 mm/yr). These Karakorum fault slip-rate data (>3–8 mm/yr), together with the total length of the fault (>1000 km) and its initiation age (>13–23 Ma), confirm that the Karakorum fault is the major fault accommodating dextral strike-slip motion NE of the western Himalayas. The dextral Karakorum fault in the south and the conjugate left-lateral Longmu Co–Altyn Tagh fault system in the north are thus the major strike-slip faults of western Tibet, which contribute to eastward extrusion of Tibet.

Chapter 5 Frontier onshore petroleum basins of Myanmar

The Myanmar Ministry of Energy has identified as prospective the three basins discussed here, but they have seen little exploration activity. Arguably the Hukawng Basin is distinct from the others insofar as exploration drilling has been carried out, and a potentially prospective stratigraphic succession has been encountered. Moreover, this basin lies on trend with the petroliferous basins of the Central Myanmar Depression. But it would be hard to argue that it is anything other than a frontier basin, and for that reason it is included here. For various reasons we consider the other basins discussed here to carry higher risk. The Ministry's onshore map accompanying the details of the 2013 bidding round included two blocks on the western flank of the Shan Plateau in what MOGE calls the Kalaw Basin: Blocks PSC-U and PSC-V. These blocks, which lie NE and east of the capital Naypyidaw (see Chapter 4, Fig. 4.2), were not however included in their list of blocks on offer. It is an area of complex folding (e.g. Garson et al. 1976), much affected by NNW–SSE faults of which some (such as the Panlaung Fault) are considered by Ridd & Watkinson (2013) to have had a history of dextral strike-slip movement during the Late Cretaceous–Palaeogene. The western part of the two blocks (i.e. west of the Panlaung Fault) exposes Palaeozoic sediments of the Slate Belt intruded by granites (Mitchell et al. 2007), and is considered to have no petroleum potential. The eastern part of the blocks is more difficult to assess. Above the ubiquitous Permian–Triassic Plateau Limestone (=Shan Dolomite), Triassic flysch (Shweminbon Formation) passes up into non-marine younger Mesozoic rocks including red beds and coal-bearing intervals. We are not aware of any petroleum exploration having taken place, and any potential this area may have is considered to be high …

A new Triassic shortening-extrusion tectonic model for Central-Eastern Asia: Structural, geochronological and paleomagnetic investigations in the Xilamulun Fault (North China)

At the northern margin of the North China Block (NCB), the Xilamulun Fault (XMF) is a key belt to decipher the tectonic evolution of Central-Eastern Asia, as it records the Paleozoic final closure of the Paleo-Asian Ocean, and localizes a Late Triassic intracontinental deformation. In this study, structural analysis, 40Ar–39Ar dating, and paleomagnetic studies were performed to investigate the kinematics of the XMF and to further discuss its Triassic geodynamic significance in the Central-Eastern Asia framework after the Paleozoic Central Asian Orogenic evolution. The structural analyses reveal two phases of ductile deformation. The first one (D1), which displays N-verging and E–W trending folds, is related to the Early Paleozoic collisional event between the NCB and the Songliao–Hunshandake Block (SHB). The second phase (D2) displays a high-angle foliation and a pervasive sub-horizontal E–W stretching lineation with kinematic criteria indicative of dextral strike–slip shearing. The 40Ar–39Ar dating on mylonitic granite places the main shearing event around 227–209 Ma. This D2 shearing is coeval with that of the dextral strike–slip Bayan Obo–Chifeng Fault (BCF) and the Chicheng–Fengning–Longhua Fault to the south, which together constitute a dextral shearing fault system on the northern margin of the NCB during the Late Triassic. The paleomagnetic study performed on the Middle Permian Guangxingyuan pluton, located between the XMF and BCF, documents a local clockwise rotation of this pluton with respect to the NCB and SHB. Our multidisciplinary study suggests an NNW–SSE shortening and strike–slip shearing dominated tectonic setting on the northern margin of the NCB during the Late Triassic. Combining the contemporaneous dextral strike–slip movements of the XMF and BCF in northern China and the sinistral strike–slip movement of East Gobi Fault (EGF) in southeastern Mongolia with the large-scale tectonic framework, a Late Triassic NNW–SSE shortening-eastward extrusion tectonic model for Central-Eastern Asia is firstly proposed. The NNW–SSE shortening results in the eastward extrusion of the continental wedge bounded by the BCF and EGF, which is accommodated by the different kinematic patterns of the southern (XMF and BCF) and northwestern (EGF) bounding faults. This shortening-extrusion tectonic framework is tentatively interpreted as the result of the far field forces associated with three Late Triassic lithosphere-scale convergences in East Asia: i) northward intracontinental subduction between the NCB and South China Block, ii) collision of the Qiangtang Block with the Qaidam Block, and iii) southward subduction of the Mongol–Okhotsk Ocean beneath the Mongolia Block.

Structure and sediment budget of Yinggehai–Song Hong basin, South China Sea: Implications for Cenozoic tectonics and river basin reorganization in Southeast Asia

The temporal link between offshore stratigraphy and onshore topography is of key importance for understanding the long-term surface evolution of continental margins. Here we present a grid of regional, high-quality reflection seismic and well data to characterize the basin structure. We identify fast subsidence of the basin basement and a lack of brittle faulting of the offshore Red River fault in the Yinggehai–Song Hong basin since 5.5Ma, despite dextral strike-slip movement on the onshore Red River fault. We calculate the upper-crustal, whole-crustal, and whole-lithospheric stretching factors for the Yinggehai–Song Hong basin, which show that the overall extension observed in the upper crust is substantially less than that observed for the whole crust or whole lithosphere. We suggest that fast basement subsidence after 5.5Ma may arise from crustal to lithospheric stretching by the regional dynamic lower crustal/mantle flow originated by collision between India–Eurasia and Indian oceanic subduction below the Eurasian margin. In addition, we present a basin wide sediment budget in the Yinggehai–Song Hong basin to reconstruct the sedimentary flux from the Red River drainage constrained by high-resolution age and seismic stratigraphic data. The sediment accumulation rates show a sharp increase at 5.5Ma, which suggests enhanced onshore erosion rates despite a slowing of tectonic processes. This high sediment supply filled the accommodation space produced by the fast subsidence since 5.5Ma. Our data further highlight two prominent sharp decreases of the sediment accumulation at 23.3Ma and 12.5Ma, which could reflect a loss of drainage area following headwater capture from the Paleo-Red River. However, the low accumulation rate at 12.5Ma also correlates with drier and therefore less erosive climatic conditions.

Active transverse faulting within underthrust Indian crust beneath the Sikkim Himalaya

SUMMARY Deep focus earthquakes within the underthrust Indian lower crust beneath the Himalaya occur in very specific regions and have distinct source characteristics. The study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust Indian Plate, and its influence on the active deformation of the overlying Himalayan wedge. One of the most significant regions of these deep focus earthquakes is beneath the Sikkim and Bhutan Himalaya. We study the source characteristics of the 2011 September 18 (Mw 6.9) deep focus Sikkim main shock and its major aftershocks using global, regional and local waveform data. We determined the focal mechanism of the main shock using moment tensor inversion of global P and SH waveforms, and ascertained the earthquake fault plane using rupture directivity from regional P-wave spectra. The main shock originated at 53 ± 4 km depth and ruptured at least 20 km thickness of the underthrust Indian lower crust. Faulting occurred on a near vertical dextral strike-slip fault oriented NW-SE (strike 127 ◦ ,d ip 81 ◦ and rake 167 ◦ ), oblique to the local strike of the Himalayan arc. The rupture initiated from the SE end of the fault and propagated to the northwest. The main shock was followed by 20 small-to-moderate aftershocks (mb > 3.0), which we relocated using phase arrival times. We computed the focal mechanisms of the larger ones (mb ≥ 3.5) using local waveform inversion. We find that all aftershocks originated SE of the main shock, between depths of 12 and 50 km, and have dominantly strike-slip mechanisms. Our results, combined with the source mechanisms of earthquakes from previous studies, reveals that the entire underthrust Indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the Sikkim and Bhutan Himalaya. These active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating ‘microplates’ in northeast India observed from GPS geodesy.

The Influence of the Karatau Fault on Hydrocarbon Migration in the Aryskum Depression, Kazakhstan

In this paper, the history of Karatau strike-slip fault and its control on inversion structures and hydrocarbon migration of the Aryskum depression were discussed. Karatau fault was characterized by sinistral strike-slip in the early stage and dextral strike-slip in the late stage. Lots of positive inversion structures in the Aryskum depression formed by the dextral strike-slip movement during Eocene-Oligocene, which synchronized with the main generation and expulsion stages of major source rocks, becoming favorable traps for hydrocarbon accumulation. Karatau fault also dominated the direction of hydrocarbon expulsion and migration in the J1 and J2 source rocks by altering their layer shapes, resulting in higher hydrocarbon enrichment in the depression center than that in the flanks. Meanwhile, as the thickness of regional cap rock in the middle and northern parts of the Aryskum depression is obviously higher than that in the relevant vertical fault throws which has caused little hydrocarbon dissipation from the fault, the hydrocarbon enrichment in the middle and northern parts of the depression is higher than that in the southern part.

Crustal and mantle structure beneath the Terre Adelie Craton, East Antarctica: insights from receiver function and seismic anisotropy measurements

The Terre Adelie and George V Land (East Antarctica) represent key areas for understanding tectonic relationships between terranes forming the Neoarchean-Palaeoproterozoic Terre Adelie Craton (TAC) and the neighbouring lithospheric blocks, together with the nature of its boundary. This region that represents the eastern border of the TAC is limited on its eastern side by the Mertz shear zone (MSZ) separating more recent Palaeozoic units from the craton. The MSZ, that recorded dextral strike-slip movement at 1.7 and 1.5 Ga, is likely correlated with the Kalinjala or Coorong shear zone in South Australia, east of the Gawler Craton and may therefore represent a frozen lithospheric-scale structure. In order to investigate the litho-spheric structure of the TAC and the MSZ, we deployed from 2009 October to 2011 October four temporary seismic stations, which sampled the various lithospheric units of the TAC and of the neighbouring Palaeozoic block, together with the MSZ. We used receiver function method to deduce Moho depths and seismic anisotropy technique to infer the upper mantle deformation. Results from receiver functions analysis reveal Moho at 40–44 km depth beneath the TAC, at 36 km under the MSZ and at 28 km beneath the eastern Palaeozoic domain. The MSZ therefore delimits two crustal blocks of different thicknesses with a vertical offset of the Moho of 12 km. Seismic anisotropy deduced from SKS splitting at stations on the TAC shows fast polarisation directions () trending E–W, that is, parallel to the continental margin, and delay times (δt) ranging from 0.8 to 1.6 s. These results are similar to the splitting parameters observed at the permanent GEOSCOPE Dumont D'Urville station (DRV: 95 • N, δt 1.1 s) located in the Palaeoproterozoic domain of TAC. On the MSZ, the small number of good quality measurements limits the investigation of the deep signature of the shear zone. However, the station in the Palaeozoic domain shows trending N60 • E, which is significantly different to the trending measurements from stations on the TAC, suggesting that the MSZ may also represent a major frontier between the Neoarchean-Palaeoproterozoic and Palaeozoic terranes.

Evolution of the stress field in the southern Scotia Arc from the late Mesozoic to the present-day

The geological evolution of the Scotia Arc, which developed between Antarctica and South America, has facilitated the connection between the Pacific and Atlantic oceans and, has important global implications. To improve the knowledge of the late Mesozoic evolution of the southern Scotia Arc, over 6000 brittle mesostructures were measured over the last 20years at different outcrops from the northern Antarctic Peninsula and the South Shetland Islands as well as the James Ross and South Orkney archipelagos. This dataset covers a length of more than 1000km of the arc. Fault data were analysed using the Etchecopar, y-R, Right Dihedra, Stress Inversion and Search Grid Inversion Palaeostress Determination methods. A total of 275 stress tensors were obtained. The results showed that the maximum horizontal stress was in the ENE-WSW and the NW-SE orientations and that the horizontal extension tensors were oriented NE-SW and NW-SE. In addition, seismic activity and focal mechanism solutions were analysed using the Gephart method to establish the present-day stress field and characterise the active tectonics. The results obtained suggest that there is a regional NE-SW compression and a NW-SE extension regime at the present day. The Southern Scotia Arc has a complex geological history due to the different tectonic settings (transform, convergent and divergent) that have affected this sector during its geological evolution from the late Mesozoic until the present day. Six stress fields were obtained from the brittle mesostructure population analysis in the region. The NW-SE and N-S maximum horizontal stresses were related to a combination of Mesozoic oceanic subduction of the former Phoenix Plate under the Pacific margin of the Antarctic Plate, Mesozoic–Cenozoic subduction of the northern Weddell Sea and the Oligocene to the Middle Miocene dextral strike-slip movement between the Scotia and Antarctic plates along the South Scotia Ridge. The NE-SW compression was related to late Miocene to present-day sinistral transcurrent movement along the South Scotia Ridge. Finally, the NW-SE extensional stress field may be related to the development of the following back-arc basins: the Late Cretaceous–Eocene Larsen Basin, the Lower to Middle Miocene Jane Basin and the Pliocene to present-day Bransfield Basin. In addition, the NE-SW and the E-W tensional stress fields were related to the Oligocene opening of the Powell Basin.

Structure and tectonic geomorphology of the Qujiang fault at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system, China

The northwest-striking Qujiang fault is located at the southeastern tip of the Sichuan–Yunnan block, which is bounded by the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system. The Qujiang fault is one of the most seismically active faults in western Yunnan, China and is considered to be the seismogenic fault of the 1970 Ms 7.7 Tonghai earthquake. In this study, we examine the geometry, kinematics, and geomorphology of this fault through field observations, satellite images, and DEMs. It is a Holocene active fault characterized by dextral strike–slip movements with dip–slip components. It can be divided into northwest and southeast segments depending on different kinematics. The northwest segment from Baiyizhai to Wujie shows right-lateral strike slip with normal components, whereas it is characterized by dextral movements with the northeast wall thrusting over the opposite in the southeast segment from Wujie to Miaobeishan. The geophysical data combined with our investigations indicate that the fault dips steeply to the NE at depth, while secondary faults with different dip directions developed at shallow surface owing to fault bifurcation. During the Tonghai earthquake, a blind fault may have developed on the southwest wall, resulting in aftershocks concentrated to the southwest of the surface raptures. Multiple tectonic landforms are developed along the fault. The right-lateral Holocene slip rate was determined to be 2.84–3.27mm/year based on radiometrically dated offset gullies. Our analyses show that the present deformation at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system is partitioned into a shortening distributed over a region ~150km wide and a right-lateral strike slip localized on the Qujiang and Jianshui faults. Since the late Pleistocene, the Qujiang fault has been playing an important role in accommodating the ongoing SE-directed movements of the Sichuan–Yunnan block, creating strong earthquakes to release strain energy.

Assessment of the geodynamical settings around the main active faults at Aswan area, Egypt

The proper evaluation of the level of crustal deformations in the Aswan region, especially around the main active faults, is crucial due to the existence of one major artificial structure: the Aswan High Dam. This construction created one of the major artificial lakes in the world: Lake Nasser. The Aswan area is considered as an active seismic area in Egypt due to the frequent recurrence of felted earthquakes. Two earthquakes occurred in the region (Kalabsha fault) after the construction of the Aswan High Dam: November 14, 1981 (ML = 5.6) and December 26, 2011 (ML = 4.1). The main goal of this study is to evaluate the active geological structures that can potentially affect the Aswan High Dam and that are being monitored in detail. For implementing such objective, several techniques (namely magnetic, seismic and GPS) have been utilized. A detailed land magnetic survey was carried out for the total component of the geomagnetic field using two proton magnetometers. The obtained magnetic results reveal that there are three major parallel faults affecting the area. The most dominant magnetic trend strikes those faults in the WNW-ESE direction. The seismicity and the fault plane solution associated with the December 26, 2011 earthquake have been also analyzed. The source mechanism of this event delineates two nodal planes. The trending ENE-WSW to E-W is consistent with the direction of the Kalabsha fault and its extension towards east. The trending NNW-SSE to N-S is consistent with the N-S fault trending. The movement along the ENE-WSW plane is right lateral, while left lateral along the NNW-SSE plane. Based on the estimated relative motions using GPS, dextral strike-slip motions at the Kalabsha and Seiyal fault systems are identified by the velocity gradient changes between the southern and northern stations. However, at the area between Kalabsha and Seiyal faults, other relative movements are also observed, which is consistent with other set of faults (N-S).

Determination of three-dimensional in situ stresses by anelastic strain recovery in Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Abstract The Wenchuan Earthquake Fault Scientific Drilling Project was implemented rapidly after the great 12 May 2008 earthquake (Mw 7.9) to better understand rupture mechanisms of the seismic faults. The first borehole of the project, WFSD-1 was located in Hongkou Township, Dujiangyan City, Sichuan Province, China on the hanging wall of the Yingxiu–Beichuan fault, which underwent large dextral and vertical displacement during the earthquake. The near-vertical borehole was 1201 m long. In-situ stresses were measured in rock samples from depths between 424 and 1173 m drilling depth by the anelastic strain recovery (ASR) method. The average trend of the maximum principal stress σ1 was N309° (ranging from N291° to N325°), rotated with increasing borehole depth from NW–SE to WNW–ESE. The magnitude of the in-situ maximum principal stress was estimated to be 35.3 MPa at the depth of 1173 m. The relations between horizontal and vertical stresses are vertical stress σv > maximum horizontal stress σH > minimum horizontal stress σh at depths above 424 m, σH > σh > σv at depths from 424 m to 800 m, and σH > σv > σh below the depth of 800 m. These indicate that the stress states are in a normal faulting stress regime above 424 m; a reverse faulting regime from 424 m to 800 m and a dextral strike-slip regime below 800 m. The stress measurements from WFSD-1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike-slip motion. The orientations of σ1 are roughly consistent with the tectonic displacement direction of the Longmenshan area, which provides further evidence of NW–SE movements representing compression of the Songpan–Ganzi block toward the Sichuan basin.

Sub-crustal earthquakes within the Australia–Pacific plate boundary zone beneath the Southern Alps, New Zealand

Sub-crustal earthquakes have been observed sporadically for ∼40 years in the central South Island of New Zealand. We report on 20 events recorded between December 2008 and February 2012 near the Alpine Fault in the continental collision zone between the Australian and Pacific plates. A subset of 18 events at depths of 47–74 km occurs south of Mt. Cook and together with recently reported tremor locations indicates along-strike variations in deformation behaviour along the plate boundary. The sub-crustal earthquakes south of Mt. Cook increase in depth, frequency and size southwards towards the Puysegur subduction zone. Focal mechanisms could be determined for 14 earthquakes and exhibit predominantly strike-slip and reverse faulting solutions. Stress inversion analysis of the focal mechanisms yields a stress field favouring oblique-reverse faulting. We interpret the geographic and vertical distributions of these sub-crustal events in relation to a previously proposed tectonic model of a remnant passive margin that formed south of New Zealand in the Eocene and was overridden when dextral strike-slip motion initiated on the Alpine Fault. We infer that sub-crustal earthquakes occur along the leading edge of this structure, which is attached to the continental Australian crust.

Relative location and source mechanism of inland earthquakes in Northern Egypt

We have relocated 259 inland earthquakes in northern Egypt using the double-difference hypocenter technique. Among this dataset we are able to determine source mechanisms of 200 events using P-wave polarities and amplitude ratios as well. The studied earthquakes have been recorded by the Egyptian National Seismological Network from October 1997 to December 2006 with local magnitude (ML) varies between 1.5 and 5.0. Three earthquake dislocations have been defined namely: Dahshour, southeast Beni-Suef, and Cairo-Suez district. Earthquake activities tend to occur in clusters along the first dislocation (Dahshour) however, relatively scattered along the second (southeast Beni-Suef) and the third (Cairo-Suez district) dislocations. At Dahshour dislocation three distinct clusters have been distinguished. Source mechanism solutions of Dahshour earthquakes displayed normal faulting with a strike-slip component to strike-slip faulting with a minor normal dip-slip component. Most of earthquake focal mechanism orientations are varying from NE-SW to NW-SE. The fault plane solutions of Beni-Suef earthquakes represented normal faulting with a strike-slip component. If the NNW-SSE striking plane has been chosen to be the actual fault plane, some solutions would indicated normal faulting with a sinistral strike-slip motion and other reflect normal faulting with a dextral strike-slip component. The fault plane solutions of Cairo-Suez district earthquakes are compatible with E-W to ENE-WSW striking normal fault with a dextral strike-slip motion.

The Phuket-Slate Belt terrane: tectonic evolution and strike-slip emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar

Abstract The Phuket-Slate Belt terrane can be traced for 1700 km from Phuket to Mandalay, and has a distinct stratigraphy and tectonic history. It is characterized by a very thick Carboniferous-Lower Permian succession which includes diamictites interpreted as glacio-marine rift-infill deposited when the Sibumasu block separated from Gondwana. It was emplaced in the Late Cretaceous-Palaeogene by dextral strike-slip movement on a fault system which includes the Khlong Marui and Panlaung Faults. Southwards the Khlong Marui bounding-fault and its close associate, the Ranong Fault, are postulated to extend to Sumatra where they align with the restored proto-Indian Ocean location of the India–Australia transform at the time that both were undergoing dextral displacement and Greater India was moving toward its collision with Eurasia. It is suggested that emplacement of the Phuket-Slate Belt terrane was the result of its coupling with the north-going India plate, resulting in up to about 450 km of dextral shift on the terrane's bounding fault system. Post-emplacement sinistral movement on the cross-cutting Mae Ping and Three Pagodas Faults offset the terrane boundary resulting in its present outline.

GEOLOGICAL STRUCTURE AND SEDIMENTARY SYSTEMS OF THE WAN’AN BASIN, NANSHA WATERS

The Wan'an Basin is a strike-slip and pull-apart basin formed by the tensile stress derived from the dextral strike-slip movement of the Wan'an fault on its east side.Main faults extend in NE、NEE and SN directions that formed a tectonic pattern of "three swells-four depressions"in the basin.The basin experienced a successive tectonic evolution of basement formation,initial rifting,rifting development(first phase of rifting),early postrifting(second phase of rifting)、tectonic inversion and postrifting(regional subsidence).Based on the relationship of two-way travel time vs.depth between stacking velocities from the seismic data and drilling data from wells,four megasequence boundaries were identified: MB1,the top of the acoustic Pre-Tertiary basement;MB2,the top of the Oligocene(24Ma);MB3,the Late Miocene(8Ma);MB4,the Early Pliocene(4Ma).Above each megasequence boundary(MB),there is a megasequence(MS).They are referred to as MS1 to MS4 from the oldest to youngest,corresponding to a lacustrine-deltaic depositional system of Late Eocene-Oligocene;a transitional system of non-marine-littoral-shelf in Late Miocene;a system of carbonate-shallow water shelf and deltaic system of Miocene-Early Pliocene and a shelf-slope-deep marine system of Early Pliocene-Quaternary,respectively.

Structural controls in the western offshore Midland Valley of Scotland: implications for Late Palaeozoic regional tectonics

AbstractNew detailed mapping and related field studies, together with re-assessment of prior investigations, have revealed that three groups of faults, orientated broadly NE, N and NW, have been the primary controls on stratigraphical, structural and geomorphological evolution in the upper Firth of Clyde since their initiation by proto-Variscan stresses in Late Devonian time. Extended control has been achieved through repeated episodic reactivation, during which existing lines of fracture were rejuvenated and others of similar orientation initiated. Movements on two (if not all three) groups of faults persisted until middle Palaeogene time at least. The faults have been augmented by two sets of irregularly distributed, open, plunging folds, broadly N–S and E–W in their axial orientations. Some N–S folds may be attributed to oblique or strike-slip movement on reactivated caledonoid faults, others to intermittent transpression, probably in Namurian–Westphalian times, affecting mainly the northeastern Midland Valley but stretching beyond the massif of the Clyde Lava Plateau to register a weakened presence as far W as the upper Firth. The N–S folds and dextral strike-slip movements on some faults may be far-field expressions of the Uralian Orogeny, whereas earlier, sinistral displacements on NE faults and the development of small, later and less-significant E–W folds may be related to different phases of long-lived Variscan compression from the S.