Sinistral Transtension Research Articles

Devonian-Mississippian faulting controlled by WNW-ESE-striking structural grain in Proterozoic basement rocks in Billefjorden, central Spitsbergen

In Billefjorden, central Spitsbergen, Devonian collapse and Carboniferous rift-related sedimentary strata were deposited unconformably over Proterozoic basement rocks displaying well developed N-S-trending Caledonian grain. Caledonian structures and fabrics are thought to have controlled the location and trend of subsequent Devonian and Carboniferous basin-bounding faults like the Billefjorden fault zone and Lemströmfjellet–Løvehovden fault. However, fieldwork and interpretation of aerial photographs in Proterozoic basement rocks reveal the existence of steep, abundant, WNW-ESE-striking brittle faults that are sub-orthogonal to known major Caledonian and post-Caledonian structures in Billefjorden, but that do not extend into adjacent-overlying, rift-related, Pennsylvanian rocks of the Gipsdalen Group. Structural analysis of field data and aerial photographs suggest that WNW-ESE-striking faults in basement rocks in Billefjorden formed as (sinistral) strike-slip and normal faults during Devonian-Mississippian extension in agreement with previously inferred models of sinistral transtension. The abundance of these faults suggest that their formation was controlled by analogously trending, preexisting structural grain (planar anisotropies) at depth, and their pronounced WNW-ESE strike suggest that the strike of preexisting anisotropies were comparable to recently identified, crustal-scale, WNW-ESE-striking Timanian thrust systems in Svalbard and the northern Barents Sea.

Strike-slip influenced stratigraphic and structural development of the Foula Sandstone Group, Shetland: implications for offshore Devonian basin development on the northern UK continental shelf

The island of Foula, located 25 km SW of Shetland, preserves a gently folded, 1.6 km thick sequence of Middle Devonian sandstones spectacularly exposed in kilometre-long cliff sections >350 m high. These rocks unconformably overlie likely Precambrian-age amphibolite facies basement rocks that are intruded by sheeted granites. The onshore succession is similar in age to the nearby Lower Clair Group offshore to the west. New mapping, incorporating the use of drone imagery in the inaccessible cliff sections, uses down-plunge projections to show that growth folding and faulting on Foula were contemporaneous with sedimentation during basin filling. The large-scale structural geometry is consistent with the regional constrictional strain due to the sinistral transtension associated with movements along the Walls Boundary–Great Glen fault zone system during the Mid-Devonian. Detrital zircon provenance studies indicate that the Devonian sequences of Foula (and nearby Melby in western Shetland) show similarities with the Clair Group and Orkney successions. We suggest that NE–SW transtensional fold development contemporaneous with regional subsidence in the Devonian basins of Scotland may be more widespread than previously realized. Large, kilometre-scale folds previously interpreted to be related to Permo-Carboniferous inversion may therefore have initiated earlier in the basin evolution sequence than previously realized. Supplementary material: Appendices A, Methodologies; B, Regional stratigraphic correlation table; C, Heavy mineral data; D, Detrital zircon data; and E, 3D virtual outcrop models are available at https://doi.org/10.6084/m9.figshare.c.6442552

The emplacement mechanism of the Early Carboniferous Nepluyevka batholith according to petromagnetic data (the Southern Urals)

In this article we present the first data on the anisotropy of magnetic susceptibility (AMS) for the rocks of the Early Carboniferous Nepluyevka polyphase pluton, which is situated in the southern part of the East Urals megazone. We consider the properties of magnetic mineralogy of all phases of the batholith, and show that the leading role in determining the magnetic properties of the rocks belongs to an isotropic grain population of MD magnetite and multiple populations of SD magnetite, which are the chief contributors to AMS. We consider the characteristic parameters of AMS for each phase, and show that the AMS is magmatic (non-deformational) in nature. An emplacement mechanism of “magmatic duplex” developing in a kinematic setting of sinistral transtension is proposed for the batholith, based on the analysis of the characteristic magnetic fabrics.

Development of the Whitehorse trough as a strike-slip basin during Early to Middle Jurassic arc-continent collision in the Canadian Cordillera

Abstract The Whitehorse trough is a synorogenic basin in the northern Cordillera that resulted from arc-collision processes along the northwestern margin of North America, but its filling history and tectonic significance remain uncertain. New detrital zircon U-Pb-Hf isotope analyses of 12 rock samples, including six basal sandstones that sit unconformably on Triassic rocks of Stikinia, were combined with published detrital zircon and fossil data to establish the depositional ages of synorogenic Laberge Group strata in Yukon and test proposed links between Intermontane terrane exhumation and basin-filling events. Laberge Group strata yielded 205–170 Ma and 390–252 Ma detrital zircon populations that indicate derivation from local Late Triassic to Middle Jurassic arc and syncollisional plutons and metamorphosed Paleozoic basement rocks of the Stikinia and Yukon-Tanana terranes. Basal sandstone units have Early Jurassic depositional ages that show the Whitehorse trough filled during early Sinemurian, late Sinemurian to Pliensbachian, and Toarcian subsidence events. Late Triassic to Early Jurassic detrital zircon grains confirm that syncollisional plutons near the northern trough were exhumed at 0.5–7.5 mm/yr and replicate their excursion to subchondritic Hf isotope compositions as a result of increasing crustal contributions from Rhaetian to Sinemurian time. The new detrital zircon data, combined with recent constraints for Triassic–Jurassic metamorphism and magmatism in Yukon, require modification of published forearc to syncollisional basin models for the Whitehorse trough. We reinterpret Jurassic subsidence patterns and architecture of the Whitehorse trough to reflect sinistral transtension within a transform fault system that resulted from the reorganization of subduction after end-on arc collision.

Kinematics and 40Ar/39Ar geochronology of the Lincang-Inthanon tectonic belt: Implication for Cenozoic tectonic extrusion of SE Asia

Abstract The Lincang-Inthanon tectonic belt is a major tectonic boundary within the southeastern Tibetan Plateau and Indo-China Peninsula, which are typical examples of tectonic extrusion in SE Asia. The Lincang strain zone, Lancang-Gengma fault, and Inthanon metamorphic complex make up this nearly N–S-striking tectonic belt, which separates the Baoshan–Shan Thai and Simao-Indochina terranes. New petrographic, structural, and mica 40Ar/39Ar geochronological studies were conducted to reveal their deformation styles and constrain the timing of their tectonic evolution. W–E-directed compression related to the subduction of the Paleotethyan Ocean with subsequent continental collision and sinistral ductile shearing in the early Oligocene are recorded along the Lincang strain zone. The Lancang-Gengma fault zone switched from sinistral shearing to dextral motion in the late Cenozoic and shows a deformation history similar to that of the parallel Red River fault. The Inthanon metamorphic complex may have experienced crustal shortening in the early Cenozoic, followed by sinistral transtension in the early Miocene. The Lincang-Inthanon tectonic belt shows many lithological, tectonic evolutionary, and metamorphic similarities with the Gaoligong, Chongshan, and Ailaoshan–Red River shear zones. Therefore, the sinistral shearing along the Lincang-Inthanon tectonic belt and the Chongshan shear zone in the north, which may have initiated since the early Oligocene, played an important role in adjusting differential extrusion and rotation of the Baoshan–Shan Thai and Simao-Indochina terranes. Our results delineate the regional tectonic framework and provide insights into the characteristics and geodynamics of intracontinental deformation in the eastern India-Eurasia oblique convergence zone.

Tectonic transition from thrusting to polyphase non-confining deformation within the Semail Ophiolite along the sinistral, transtensional Issmaiya Fault Zone (Sultanate of Oman)

• The Oman Mountains record the transition between shortening during the obduction of the Semail ophiolite . • At least three extensional events: affected the autochthonous and allochthonous units after obduction: two between the latest Cretaceous and early Eocene and one after the mid-Eocene. • Allochthonous units underwent major extensional shearing immediately after the emplacement of the ophiolite. • The allochthonous units underwent two further extensional events: One between the latest Cretaceous and early Eocene, and the other one after mid-Eocene. Obduction of the Semail Ophiolite and deep-sea basin-derived rocks represent a major tectonic event during Late Cretaceous evolution of the Oman Mountains. These allochthonous units were thrust SW-ward onto the passive Arabian margin and platform, followed by the main uplift of the Saih Hatat Dome and minor uplift of the Jabal Akhdar Dome. We present new structural data that indicate major extensional and transtensional deformation affecting the ophiolite unit after its emplacement. The studied area is in the southeastern Oman Mountains and hosts the brittle “Issmaiya Fault Zone” (IFZ; ∼3 km in wide and ∼ 30 km long). Field work and satellite image analysis show that most faults affected the Semail Ophiolite and only a few faults cut through latest Cretaceous to Paleogene sedimentary rocks of the Ibra Basin. These constraints allow us to date most faulting to immediately after the Late Cretaceous emplacement of the Semail Ophiolite, lasting to the post-Paleogene. The IFZ strikes ∼NW-SE at an acute angle of ∼30° to the WNW-ESE oriented southern margin of the Saih Hatat Dome. Northwestward, the IFZ terminates at the southern boundary fault of the Saih Hatat Dome. Fault kinematics based on shear-fracture slip-indicators reveal vertical to steep NW/SE-striking fault planes with occurrence of mainly sinistral strike-slip and some extensional dip-slip faults. This highlights a latest phase in a constrictional strain field with N/S-extension, perpendicular to E/W-oriented and vertical shortening axes of a sinistral transtensional fault zone. We suggest that SW-ward thrusting of the Semail Ophiolite was followed by mainly sinistral transtension.

A new insight into the structural evolution of Rosetta Fault, eastern margin of Herodotus Basin, East Mediterranean

While exploration activities are flourishing in both the offshore Nile Delta Basin and the Levant Basin, Herodotus Basin remains unexplored. The study of the complex tectonic evolution of this basin is mainly hindered by large water depths, insufficient and poor seismic imaging, as well as the scarcity of well penetrations. The present study offers a new insight into the tectonic history of the eastern margin of Herodotus Basin through detailed structural study of the Rosetta Fault. Detailed structural mapping of Rosetta Fault using 3D seismic and borehole data reveals its complicated deformation history. Together with other NE-SW oriented faults in the Levant Basin and the northern Egypt onshore area, Rosetta Fault experienced different phases of slip at different times including normal slip due to extensional deformation at Triassic to early Cretaceous times and reverse slip at Late Cretaceous to Tortonian times due to the convergence of Eurasia and Afro-Arabia. Contrary to the findings of some plate tectonic models, which consider NE-SW drift of Afro-Arabia and Eurasia, the present study suggests that the direction of Mesozoic extension in Herodotus Basin (as well as the Levant Basin and the northern onshore areas of Egypt) was NW-SE based on the orientations of Mesozoic active faults. Rosetta Fault also shows deformation by sinistral transtension at Messinian and Holocene times intervened by Plio-Pleistocene normal slip. When the Rosetta Fault was affected by normal slip and transtensional deformation at Messinian to Holocene times, the Herodotus Basin lying to the NW of it was affected by NW-SE shortening associated with the Hellenic subduction. The obvious contrast in the deformation style of Rosetta Fault and the Herodotus Basin is attributed to the irregular shape of the convergent plate boundary extending from south Cyprus to SE Crete.

Critical Tectonic Limits for Geothermal Aquifer Use: Case Study from the East Slovakian Basin Rim

The Pannonian basin is a major geothermal heat system in Central Europe. Its peripheral basin, the East Slovakian basin, is an example of a geothermal structure with a linear, directed heat flow ranging from 90 to 100 mW/m2 from west to east. However, the use of the geothermal source is limited by several critical tectono-geologic factors: (a) Tectonics, and the associated disintegration of the aquifer block by multiple deformations during the pre-Paleogene, mainly Miocene, period. The main discontinuities of NW-SE and N-S direction negatively affect the permeability of the aquifer environment. For utilization, minor NE-SW dilatation open fractures are important, which have been developed by sinistral transtension on N–S faults and accelerated normal movements to the southeast. (b) Hydrogeologically, the geothermal structure is accommodated by three water types, namely, Na-HCO3 with 10.9 g·L−1 mineralization (in the north), the Ca-Mg-HCO3 with 0.5–4.5 g·L−1 mineralization (in the west), and Na-Cl water type containing 26.8–33.4 g·L−1 mineralization (in the southwest). The chemical composition of the water is influenced by the Middle Triassic dolomite aquifer, as well as by infiltration of saline solutions and meteoric waters along with open fractures/faults. (c) Geothermally anomalous heat flow of 123–129 °C with 170 L/s total flow near the Slanské vchy volcanic chain seems to be the perspective for heat production.

Late Miocene to Quaternary Development of the Jiujing Basin, Southern Beishan Block, China: Implications for the Kinematics and Timing of Crustal Reactivation North of Tibet

AbstractWe present results from a multidisciplinary investigation of the Jiujing fault (JJF) system and adjacent Jiujing Basin in the southern Beishan block, western China. Structural and geomorphological fieldwork involving fault and landform investigations, remote sensing analysis of satellite and drone imagery, analysis of drill-core data, paleoseismological trench studies, and Quaternary dating of alluvial sediments suggest the JJF is a late Pleistocene to Holocene oblique sinistral-slip normal fault. Satellite image analysis indicates that the JJF is a connecting structure between two regional E-W-trending Quaternary left-lateral fault systems. The Jiujing Basin is the largest and best developed of three parallel NE-striking transtensional basins within an evolving sinistral transtensional duplex. Sinistral transtension is compatible with the orientation of inherited basement strike belts, NE-directed SHmax, and the modern E-NE-directed geodetic velocity field. Cosmogenic 26Al/10Be burial dating of the deepest sediments in the Jiujing Basin indicates that the basin began to form at ~5.5 Ma. Our study reveals a previously unreported actively deforming domain of transtensional deformation 100 km north of Tibet in a sector of the Beishan previously considered tectonically quiescent. Recognition of latest Miocene-Recent crustal reactivation in the Jiujing region has important implications for earthquake hazards in the Beishan and western Hexi Corridor/North Tibetan foreland sectors of the Silk Road Economic Belt. Additionally, we compare the timing of latest Miocene-Recent crustal reactivation in the southern Beishan with the documented onset of reactivation in other deforming regions north of Tibet.

Kinematic variability of late Cenozoic fault systems and contrasting mountain building processes in the Alxa block, western China

Abstract In this study, new structural field data and geological observations are coupled with remote sensing analysis and compilation of previous published work to establish a new kinematic framework for the neotectonic reactivation of the Alxa block, China. The Alxa block occupies a key position in Central Asia north and east of the contractional and sinistrally transpressive deforming zones of the Qilianshan-Hexi Corridor-easternmost Altyn Tagh Fault and Beishan deforming belts, south of the sinistrally transpressive southernmost Gobi Altai, and west of – or overlapping with - Ordos block extensional and transtensional graben systems. In addition, the Alxa block is internally transected by important fault systems (Bayan Nuru, Bayanwula, Chahanbulage) that record previously unrecognized Late Cenozoic activity. Active mountain building processes in the Alxa block show remarkable variation from large extensional horst blocks (Helanshan, SW Langshan, SW Yabrai) to localized uplifts along sinistral wrench-belts (Bigesitai) to dominantly contractional (Longshoushan) and sinistral transpressional ranges (Helishan). The triangular wedge-shape of the Alxa block likely pre-disposed it to eastward extrusion in response to NE-directed India-Eurasia collision-derived compressional stresses from the south, and E-W tensional forces driven by Pacific margin rollback to the east. A combination of inherited regional lithospheric structure and the modern east-directed geodetically derived crustal velocity field led to: 1) crustal thinning and subsidence of the Ejina basin to the west in the wake of the eastward extruding Alxa block, and 2) distributed sinistral transtension and crustal extension within the Alxa block interior, and in the Hetao and Yinchuan grabens to the east.

Tectonic evolution of the Cretaceous Gyeongsang Back-arc Basin, SE Korea: Transition from sinistral transtension to strike-slip kinematics

The Gyeongsang Basin, the largest Cretaceous nonmarine sedimentary basin in Korea, formed as a continental back-arc basin related to NNW-directed subduction of the (proto-) Pacific Plate underneath the Eurasia Plate. The basin can be divided into an earlier-formed western back-arc depression and a later-formed eastern volcanic arc platform. We investigated the basin evolution and the associated tectonic settings, largely based on an analysis of structures individuated in the field in the back-arc region. From 127 Ma, the basin initiated as a NNE-trending narrow depression bordered by NNE-striking sinistral faults, and then progressively expanded under a transtensional kinematics induced by progressive trench roll-back. Sinistral shearing of inherited NNE-striking structures played an important role in basin subsidence, and secondary WNW- to NW-striking transverse faults acted as normal faults. The NNE-striking principal displacement zone in the west of the basin runs along the western marginal area of the Jinju and Daegu domains and passes through the Uiseong domain from south to north, but most of this zone is now preserved as deep structures. Volcanic activity starting at ca. 115 Ma was characterized mainly by episodic basaltic eruptions occurring contemporaneously with back-arc deposition of a sedimentary sequence. After ca. 90 Ma, a transtensional kinematics changed to a strike-slip one, and the basin expansion and sedimentation in the back-arc region terminated. During the strike-slip event, rhyolitic-dacitic volcanism increased in intensity as a large NE-trending volcanic arc developed close to subduction zone and its loading caused the stratal flexure in the back-arc region, and the orientations of the shortening and stretching axes remained NW–SE and NE–SW, respectively. Additionally, continuing sinistral shear generated local depressions along the faults located in the west of the back-arc region and within the volcanic arc.

The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile

The Paposo fault zone is a major brittle-ductile strand of the Atacama fault system (AFS), which records sinistral shear associated with Cretaceous oblique subduction beneath northern Chile. New detailed geologic mapping, macro- and microstructural data, and zircon geo/thermochronology reveal insight into the structural evolution of the southern portion of the Paposo fault. The core of the Paposo fault is defined by a ~50 m-thick zone of illite-rich gouge that dips steeply ESE and juxtaposes fractured but unfoliated Early Jurassic tonalite west of the fault against mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault. The mylonite zone east of the fault is ~0.7–1 km thick and includes a ~100–500 m-thick band of hydrothermally-altered ultramylonite derived from Latest Jurassic (146.5 ± 1.5 Ma) to Early Cretaceous (138.5 ± 1.6 Ma) granodiorite. West of this ultramylonite zone, a ~150–350 m-thick zone of younger tonalite (136.0 ± 1.8 Ma) parallels the Paposo fault and grades from protomylonite to mylonite <10 m from the gouge zone on its western margin. Mylonitic foliations dip steeply to moderately SE with lineations that typically plunge ~10–30° SW. Most SE-dipping mylonitic fabrics record oblique sinistral-reverse shear that is consistent with the overall pattern of small-scale brittle faults and S-C-C’ fabrics in the Paposo fault gouge zone. However, in several parts of the hydrothermally-altered mylonite zone, symmetric microstructures and S > L tectonite fabrics most likely record a significant component of zone-normal flattening. The upper age limit of deformation is constrained to be younger than the Late Jurassic/Early Cretaceous granodiorite. Hydrothermal alteration and development of high-strain zones in the Late Jurassic/Early Cretaceous granodiorite are locally associated with mafic dikes that do not cut the younger protomylonitic tonalite, indicating that most of the hydrothermal alteration and mylonitic strain occurred between ~139 Ma and 136 Ma. The Paposo fault gouge zone formed between 150 °C and 200 °C based on clay mineralogy and the illite Kübler index. The timing of gouge formation most likely overlaps with cooling below ~180–190 °C recorded by a zircon (U-Th)/He date of 116.6 ± 6.2 Ma from the Late Jurassic/Early Cretaceous granodiorite. Together these data constrain brittle and ductile deformation to the Early Cretaceous, similar to the age of deformation along other segments of the AFS and coeval with co-spatial arc magmatism. Regionally, the Paposo segment of the AFS is arcuate, trending NNW-SSE in the northern end and NNE-SSW in the southern end. Previous studies of fault strands along the northern portion of the Paposo segment document sinistral transtension, whereas oblique sinistral-reverse shear and local coaxial flattening record sinistral transpression along the southern portion of the Paposo fault. We propose that transtension and transpression along the AFS are controlled by the arcuate geometry, and both are compatible with sinistral simple shear along the N-S-trending magmatic arc.

Intracontinental deformation within the India-Eurasia oblique convergence zone: Case studies on the Nantinghe and Dayingjiang faults

AbstractThe most striking structural features in the interior of the Shan Plateau, southeast of the eastern Himalayan syntaxis, are a series of NE-trending faults that exhibit sinistral movement and an arcuate geometry. Their origin and tectonic evolution remain poorly understood. Furthermore, a switch in slip sense is recorded along many of these faults, but the timing of kinematic reversal is still unclear, hindering an understanding of the causal geodynamic mechanisms. We conducted an integrative study of apatite and zircon (U-Th)/He thermochronology, 40Ar/39Ar geochronology, and structural and geomorphic analysis to decipher the evolution of two major NE-trending faults: the Nantinghe and Dayingjiang faults. At least three deformation stages are identified within the Nantinghe fault zone, including top-to-the-SE/ESE thrusting, dextral ductile strike-slip shearing, and sinistral movement. Zircon and apatite (U-Th)/He data, collected from the northeastern terminus of the Nantinghe fault, reveal rapid cooling in the early Miocene. Combined with the 40Ar/39Ar data from sinistrally sheared mylonite, left-lateral movement on the Nantinghe fault is inferred to have initiated as early as ca. 20 Ma. The Dayingjiang fault reactivated as a sinistral brittle fault along the dextral Yingjiang shear zone. A two-stage thermal history is identified along the shear zone, with prominent cooling during dextral ductile shearing in the early- to mid-Miocene followed by a lower-magnitude cooling episode at ca. 11 Ma caused by sinistral transtension along the Dayingjiang fault. The evolution of the Nantinghe and Dayingjiang faults suggests that the NE-trending fault system in the Shan Plateau may have developed along preexisting structures and underwent diachronous slip-sense inversion in the late Cenozoic. The northward advance of the eastern Himalayan syntaxis caused a major change in both the regional stress field and fault geometries in the eastern India-Eurasia oblique convergence zone, contributing to the inversion of fault kinematics.

The 3D seismic characteristics and significance of the strike-slip faults in the Tazhong area (Tarim Basin, China)

The eastern part of Tazhong area in the Tarim Basin consists of three sets of vertical strike-slip faults oriented in north–northeast (36°azimuth), east–northeast (68° azimuth), and west–northwest (126°azimuth) directions that cut the strata from Cambrian to Carboniferous. The fault belts indicate significant horizon upwarp and downwarp deformations and variations in their stratigraphic thickness on seismic profiles. Through detailed interpretation of the 3D seismic data, we consider that these phenomena reflect the different stress properties and active stages of the faults. The horizon upwarp and downwarp within the fault belts correlated respectively to the decrease and increase in stratigraphic thickness within the fault belts in comparison to the coeval counterpart of the bilateral fault blocks. For the same fault, different stratigraphic intervals express different types of horizon deformation and thickness changes. The horizon downwarp and the contemporaneous stratigraphic thickening inside the fault belts suggest the transtensional actions of the fault. The horizon upwarp and the contemporaneous thinning within the fault belts suggest transpressional actions of the fault. Based on this, we inferred the active periods of the three sets of strike-slip faults. The north–northeast-striking faults were formed in the late Ordovician Sangtamu Formation. This set of faults experienced four stages, i.e., sinistral transpression, sinistral transtension, static, and transtension. The east–northeast and west–northwest-striking faults initiated in the mid-Cambrian period as coupled transtension. Activity ceased in the west–northwest faults after the mid-Cambrian and in the east–northeast faults during the late Ordovician. The three sets of strike-slip faults all affect the formation of the hydrothermal dissolution reservoirs that are distributed in the Ordovician carbonate rocks.

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

Although many intrusions are now known to have been incrementally emplaced, the mechanisms through which this takes place are generally poorly understood. The Newry igneous complex was incrementally emplaced within the Southern Uplands-Down-Longford terrane of Northern Ireland during late Caledonian sinistral transtension. This study uses a variety of new and existing data and techniques to provide a fuller and firmer understanding of incremental emplacement than has previously been available, addressing both deep-crustal processes and those operating within the emplacement site. Host-rock orientations suggest that some of the accommodation space for the Newry igneous complex was generated due to pull-apart tectonics operating within the Southern Uplands-Down-Longford terrane. Local host-rock deflections, concentric igneous foliations, and concentric linear anisotropy of magnetic susceptibility (AMS) fabrics show that inflation due to magma overpressure also generated significant space. Strong AMS fabrics close to the boundaries of some magma pulses in turn suggest that inflation was accomplished by injection of individual magma pulses and was thus incremental. The dome-like orientations of mineral foliations within plutons and the truncation of steep local host-rock tracts by the Newry igneous complex imply that the complex consists of four laccolithic bodies. On a larger scale, it is suggested that the deep-seated Argyll and Newry lineaments represent faults that allowed magma generated at depth to ascend to the crustal level of the Southern Uplands-Down-Longford tract boundaries. It is also inferred that sinistral movement along the Argyll and Newry lineaments may have produced the releasing bend within the Southern Uplands-Down-Longford terrane. Higher in the crust, reduced confining pressure resulted in tectonic opening along this releasing bend. This local stress field induced horizontal magma flow and emplacement of the Newry igneous complex as laccolithic bodies. This study suggests that simplistic emplacement models should largely be abandoned in favor of holistic models incorporating the multiple interdependent processes operating during magma ascent and emplacement.

Fault and fracture patterns around a strike-slip influenced salt wall

The trends of faults and fractures in overburden next to a salt diapir are generally considered to be either parallel to the salt margin to form concentric patterns, or at right angles to the salt contact to create an overall radial distribution around the diapir. However, these simple diapir-related patterns may become more complex if regional tectonics influences the siting and growth of a diapir. Using the Sedom salt wall in the Dead Sea Fault system as our case study, we examine the influence of regional strike-slip faulting on fracture patterns around a salt diapir. This type of influence is important in general as the distribution and orientation of fractures on all scales may influence permeability and hence control fluid and hydrocarbon flow. Fractures adjacent to the N-S trending salt wall contain fibrous gypsum veins and injected clastic dykes, attesting to high fluid pressures adjacent to the diapir. Next to the western flank of the salt wall, broad (∼1000 m) zones of upturn or ‘drape folds’ are associated with NW-SE striking conjugate extensional fractures within the overburden. Within 300 m of the salt contact, fracture patterns in map view display a progressive ∼30°–35° clockwise rotation with more NNW-SSE strikes immediately adjacent to the salt wall. While some extensional faults display growth geometries, indicating that they were syn-depositional and initiated prior to tilting of beds associated with drape folding, other fractures display increasing dips towards the salt, suggesting that they have formed during upturn of bedding near the diapir. These observations collectively suggest that many fractures developed to accommodate rotation of beds during drape folding. Extensional fractures in the overburden define a mean strike that is ∼45° anticlockwise (counter-clockwise) of the N-S trending salt wall, and are therefore consistent with sinistral transtension along the N-S trending Sedom Fault that underlies the salt wall. Our outcrop analysis reveals fracture geometries that are related to both tilting of beds during drape folding, and regional strike-slip tectonics. The presence of faults and fractures that interact with drape folds suggests that deformation in overburden next to salt cannot be simply pigeon-holed into ‘end-member’ scenarios of purely brittle faulting or viscous flow.

Oblique wedge extrusion of UHP/HP complexes in the Late Triassic: structural analysis and zircon ages of the Atbashi Complex, South Tianshan, Kyrgyzstan

ABSTRACTThe exhumation and tectonic emplacement of eclogites and blueschists takes place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan. Our field mapping and structural analysis demonstrate that the Atbashi Eclogite–Blueschist Complex is situated in a complicated duplex formed by a northerly dextral transpression system and a southerly sinistral transtension system. The two major shear systems suggest that the Atbashi Complex was extruded obliquely southwestwards during eastward penetration of the southern tip of the Yili–Central Tianshan Arc of the Kazakhstan Orocline during the Late Triassic. Also, we report new zircon U–Pb metamorphic ages of four eclogites and one garnet-bearing quartz-schist from the Atbashi complex of 217–221 Ma and 223.9 Ma, respectively, suggesting that the main extrusion was later than previously proposed and that the final orogenesis was not completed until the Late Triassic. The HP/UHP rocks have an oblique plunge to the NE and extrusion took place southwestwards during escape tectonics along the South Tianshan accretionary wedge in the Late Triassic. Our work shows that the movement of HP/UHP rocks had a 3D style with an arc-parallel structure, and sheds light on earlier 2D models with either forearc- or backarc-directed extrusions, which indicates that more systematic structural and geochronological work is needed to characterize the accretionary tectonics of many orogens around the world. Our data on the timing of extrusion and emplacement of the Atbashi Eclogite–Blueschist Complex also help to resolve the long-standing controversy about the time of terminal orogeny of the Central Asian Orogenic Belt.

Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc

We report fault slip data from exhumed fault surfaces along the NNE trending Alpenrhein valley and its intersection with the WNW striking Bodensee Graben near the Alpine thrust front of the Central Alps in the Swiss-Austrian-German border region. This conjugated graben system straddles the boundary between the Alps and its foreland and allows comparing the kinematics of graben formation between the two different tectonic domains. Our data show sinistral transtension along the Alpenrhein Graben and dextral transtension along the Bodensee Graben. Both transtensional graben systems resulted from the same kinematic regime of NW directed shortening and NE oriented extension. The graben faults are not older than NW striking tear faults associated with NW directed Oligocene nappe emplacement in the Helvetics starting at 35–30 Ma and ending by 25–20 Ma. Compatible with this are six U-Pb ages of calcite fibers from four samples yielding consistent ages ranging from 25.3 ± 5.6 Ma to 21.8 ± 3.4 Ma (2σ errors). Earthquake data since 1996 show that kinematic directions persisted until the Recent. Our data broadly fit with the kinematic evolution of the Oberrhein Graben, which shows significant E-W extension in the Oligocene. We suggest that Oligocene extension in the Alps and its foreland resulted from the increased curvature of the Western Alps arc and associated moderate tangential stretching in the internal parts of the Central Alps. We discuss a tectonic model of eastward rollback of the west Mediterranean subduction zone associated with counterclockwise rotation of Adria, the latter of which aided the formation of the Western Alps arc.

Fault kinematics of the Magallanes-Fagnano fault system, southern Chile; an example of diffuse strain and sinistral transtension along a continental transform margin

A system of left-lateral faults that separates the South American and Scotia plates, known as the Magallanes-Fagnano fault system, defines the modern tectonic setting of the southernmost Andes and is superimposed on the Late Cretaceous – Paleogene Patagonian fold-thrust belt. Fault kinematic data and crosscutting relationships from populations of thrust, strike-slip and normal faults from Peninsula Brunswick adjacent to the Magallanes-Fagnano fault system, presented herein, show kinematic and temporal relationships between thrust faults and sets of younger strike-slip and normal faults. Thrust fault kinematics are homogeneous in the study area and record subhorizontal northeast-directed shortening. Strike-slip faults record east—northeast-directed horizontal shortening, west—northwest-directed horizontal extension and form Riedel and P-shear geometries compatible with left-lateral slip on the main splay of the Magallanes-Fagnano fault system. Normal faults record north-south trending extension that is compatible with the strike-slip faults. The study area occurs in a releasing step-over between overlapping segments of the Magallanes-Fagnano fault system, which localized on antecedent sutures between basement terranes with differing geological origin. Results are consistent with regional tectonic models that suggest sinistral shearing and transtension in the southernmost Andes was contemporaneous with the onset of seafloor spreading in the Western Scotia Sea during the Early Miocene.

Torque exerted on the side of crustal blocks controls the kinematics of Ethiopian Rift

Plate tectonic stress at active plate boundary can arises from 1) a torque applied on the side of lithospheric blocks and 2) a torque at the base of the lithosphere due to the flow of the underlying mantle. In this paper we use a simple force balance analysis to compare side and basal shear stresses and their contribution in driving kinematics and deformation in the Ethiopian Rift (ER), in the northern part of the East African Rift System (EARS). Assuming the constraints of the ER given by the dimension of the lithospheric blocks, the strain rate, the viscosity of the low velocity zone (LVZ) and the depth of the brittle–ductile transition zone, the lateral torque is several orders of magnitude higher than the basal torque. The minor contribution of basal torque might be due to low viscosity in the LVZ. Both Africa and Somalia plates are moving to the ”west” relative to the mantle and there are not slabs that can justify this pull and consequent motion. Therefore, we invoke that westerly oriented tidal torque on Africa and Somalia plates in providing the necessary side torque in the region. This plate motion predicts significant sinistral transtension along the ER and rift parallel strike-slip faulting similar to the estimated angular velocity vector for tectonic blocks and GPS observations. Vertical axis block rotations are observed in areas where the lithospheric mantle is removed and strain is widely distributed.