Major Strike-slip Fault System Research Articles

Unveiling a major strike-slip fault system associated with the Somún Curá Large Igneous Province in central Patagonia, Argentina

This study presents new evidence of a major, regional strike-slip fault system linked to the emplacement of the Somún Curá Large Igneous Province in central Patagonia, Argentina. Employing a combination of remote sensing techniques and fieldwork, we provide additional insights into the structural complexities underlying this magmatic province, shedding light on its geological evolution. Our findings indicate a compelling correlation between the magmatic products of the Somún Curá Large Igneous Province and the left-lateral strike-slip fault system, resembling similar relationships observed in other magmatic large igneous provinces. Moreover, the strike-slip tectonics delineated in this study likely represent the culmination of the fault system's evolution, possibly originating from a longstanding basement fault and extending beyond the limits of the Somún Curá basaltic plateau.

Slab tearing and lithospheric structures in Luzon island, Philippines: constraints from P- and S-wave local earthquake tomography

Luzon Island is a complex setting of seismicity and magmatism caused by the subduction of the South China Sea lithosphere and the presence of a major strike-slip fault system, the Philippine Fault. Previous studies of the structure of this subduction zone have suggested that a ridge subduction system resulted in a slab tearing along the ridge. On the other hand, the Philippine Fault plays an important role in understanding how major strike-slip faults deform and displace at a continental scale. To constrain the lithospheric geological structure in the area and refine the slab tearing model, we performed a P- and S-wave seismic tomography travel time inversion using local earthquakes. The dataset has been combined from seismic phases reported by the International Seismological Centre and new pickings from six broadband seismic stations in northern Luzon. The three-dimensional P- and S-wave velocity models in Luzon Island were analyzed by applying the LOTOS package with a one-dimensional velocity model obtained from the VELEST program. Our tomographic images indicate contrasting velocity structures across the Philippine Fault to a depth of 60 km. Therefore, we suggest that the Philippine Fault might be a lithospheric structure that displaces both the crust and the upper mantle. The results also indicate regions of low-velocity slab windows from a depth of 40 km, which are interpreted as the sites of slab tearing. Compared with focal mechanisms and earthquake occurrence in this region, we propose that slab tearing extends from the fossil ridge and creates regional kinematic perturbations. The tearing produces shallow upwelling magma to stay in the chambers beneath the crust, which is in contrast to the magmatic system observed in other regions.

Rock-uplift history of the Central Pontides from river-profile inversions and implications for development of the North Anatolian Fault

Major strike-slip fault systems on Earth, like the North Anatolian Fault (NAF), play an important role in accommodating plate motion, but surprisingly little is known about how such structures evolve through space and time. Along the central sector of the NAF in the Central Pontides, transpression and crustal thickening along the northward restraining bend of the fault are thought to have generated rock-uplift rates of 0.2–0.3 km/Myr since at least 400 ka based on Quaternary marine and river terraces, while data from low-temperature thermochronology suggest that an enhanced exhumation phase occurred within the last 11 Myr. However, the precise onset of this faster uplift phase, which likely reflects deformation associated with the development of the central sector of the NAF, is poorly constrained.Here we define the spatiotemporal pattern of rock-uplift rates within the Central Pontides over the last ∼10 Myr by performing linear inversions of 19 river profiles that drain the northern margin of the Central Pontides, from the Sinop Range to the Black Sea. We use 21 new 10Be-derived basin-average denudation rates to calibrate an erodibility parameter, which we use to convert our χ-transformed river profiles into rock-uplift histories. Our results document an increase in rock-uplift rates after 10 Ma, with peak rates of ∼0.15–0.25 km/Myr occurring between 4 and 2 Ma. Moreover, the spatiotemporal pattern of uplift suggests that faster rock uplift started in the eastern part of the Sinop Range and migrated westward over a period of ca. 2 to 2.5 Myr, which we relate to the westward propagation of the NAF through this sector at a rate of 74 ± 13 km/Myr. In the context of previously published constraints on the westward propagation of the NAF starting in eastern Turkey at ∼12 Ma, our results suggest differences in fault-propagation rates that coincide with differences in the orientation of the NAF relative to plate-convergence velocity vectors. Fault segments with higher obliquity appear to have propagated at rates up to 2-fold slower than those oriented more parallel to the plate-convergence vector.

Three-dimensional magnetotelluric imaging of the Mérida Andes, Venezuela

The 100km wide Mérida Andes extend from the Colombian/Venezuelan border to the Coastal Cordillera. The mountain chain and its associated major strike-slip fault systems in western Venezuela formed due to oblique convergence of the Caribbean with the South American Plates and the north-eastwards expulsion of the North Andean Block. Due to the limited knowledge of lithospheric structures related to the formation of the Mérida Andes research projects have been developed to illuminate this zone with deep geophysical data. In this study, we present three-dimensional inversion of broadband magnetotelluric data, collected along a 240km long profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins. The distribution of the stations limits resolution of the model to off-profile features. Combining 3D inversion of synthetic data sets derived from 3D modelling with 3D inversion of measured data, we could derive a 10 to 15km wide corridor with good lateral resolution to develop hypotheses about the origin of deep-reaching anomalies of high electrical conductivity. The Mérida Andes appear generally as electrically resistive structures, separated by anomalies associated with the most important fault systems of the region, the Boconó and Valera faults. Sensitivity tests suggest that the Valera Fault reaches to depths of up to 12km and the Boconó Fault to more than 35km depth. Both structures are connected to a sizeable conductor located east of the profile at 12–15km depth. We propose that the high conductivity associated with this off-profile conductor may be related to the detachment of the Trujillo Block. We also identified a conductive zone that correlates spatially with the location of a gravity low, possibly representing a SE tilt of the Maracaibo Triangular Block under the mountain chain to great depths (>30km). The relevance of these tectonic blocks in our models at crustal depths seems to be consistent with proposed theories that describe the geodynamics of western Venezuela as dominated by floating blocks or orogens. Our results stress the importance of the Trujillo Block for the current tectonic evolution of western Venezuela and confirm the relevance of the Boconó Fault carrying deformation to the lower crust and upper mantle. The Barinas–Apure and the Maracaibo sedimentary basins are imaged as electrically conductive with depths of 4 to 5km and 5 to 10km, respectively. The Barinas–Apure basin is imaged as a simple 1D structure, in contrast to the Maracaibo Basin, where a series of conductive and resistive bodies could be related to active deformation causing the juxtaposition of older geological formations and younger basin sediments.

The sediments of Lake Singkarak and Lake Maninjau in West Sumatra reveal their earthquake, volcanic and rainfall history

Natural hazards such as earthquakes, volcanic activity, and heavy rainfall causing floods and debris avalanches are common phenomena in many tropical settings, including the island of Sumatra, located in the Indonesian archipelago. To enhance our understanding of the recurrence of these often destructive events, we studied the sedimentary infill of two lakes in the Padang highlands, West Sumatra. This includes Lake Singkarak, a tectonically-formed lake located on a step-over of a major strike-slip fault system (the Sumatran Fault), and Lake Maninjau, a caldera lake. Both lakes are located ~300 km from the Sunda subduction trench and surrounded by steep slopes and a chain of active volcanoes. Hence, considering their unique tectonic setting, these lakes may potentially record a wide range of natural hazards that affect the region. A combination of seismic-reflection profiles and short sediment cores revealed that both lakes indeed record various types of natural hazards, each with their own sedimentary response to a specific type of event. Lake Singkarak can be used to study past floods and major debris avalanches in addition to high-magnitude megathrust earthquakes, while traces of past intraplate earthquakes have been identified in both lakes. Furthermore, we argue that Lake Singkarak is an ideal recorder of volcanic activity in the region, while Lake Maninjau itself can pose a volcanic hazard to its surroundings as demonstrated by potential activity of the volcano below the lake.

Flower structures in sandstones of the Paleozoic Inkisi Group (Brazzaville, Republic of Congo): evidence for two major strike-slip fault systems and geodynamic implications

AbstractFew studies have reported field descriptions of flower structures associated with strike-slip faults. This study describes and illustrates flower structures near Brazzaville (Republic of Congo) and explains their implication for the tectonic history of the Paleozoic Inkisi Group. Field observations show that the Inkisi Group is affected by two major strike-slip fault systems. The oldest system is dominated by north-northwest–south-southeast striking sinistral strike-slip faults and minor east–west striking dextral strike-slip faults. The youngest system consists of dominant northeast–southwest striking dextral strike-slip faults and minor northwest–southeast striking sinistral strike-slip faults. Flower structures within these major strike slip faults show four types of arrangements that likely depend on fault growth, propagation and damage zones: (i) flower structures associated with wall damage zones; (ii) flower structures associated with linking damage zones; (iii) flower structures associated with tip damage zones; and (iv) “hourglass” flower structures.Paleostress analysis reveals that both major fault systems originated from two differently oriented pure strike-slip regime stress stages. The first stage, which engendered the first major fault system, developed under northwest–southeast compression (i.e, σ1 = 322°). This phase probably coincided with north–south collision in the southern part of Gondwana in the Permo-Triassic and the Late Cretaceous compression times. The second stress stage, creating the second major fault system, developed under east–west (i.e, σ1 = 078°) compression. This phase is correlated with compression from the east–west opening of the Atlantic Ocean in the Miocene times.

Magnetotelluric imaging of the Mérida Andes and surrounding areas in Venezuela

SUMMARYThe Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100-km-wide Mérida Andes extend from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broad-band magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with 1-D or 2-D subsurface structures for the sedimentary basins but also indicate a strong 3-D setting for the Mérida Andes. Using a combination of 2-D and 3-D modelling we systematically examined the influence of 3-D structures on 2-D inversions. Synthetic data sets derived from 3-D modelling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2-D inversion models show electrically conductive basins with depths of 2–5 km for the Barinas-Apure and 2–7 km for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE–SW direction along its strike on a length 400 km and N–S direction at its centre on a length 60 km, respectively. Both faults are associated with subvertical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2-D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the ‘floating orogen hypothesis’ developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.

Near-surface structure of the North Anatolian Fault zone from Rayleigh and Love wave tomography using ambient seismic noise

Abstract. We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼1200 km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10 km of a 70 × 30 km region around Lake Sapanca. We observe low S-wave velocities (<2.5 km s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (>3.2 km s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4 s. At longer periods up to 10 s, the fast direction aligns with the direction of maximum extension for the region (∼45∘). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.

Seismically-triggered soft-sediment deformation structures close to a major strike-slip fault system in the Eastern Alps (Hirlatz cave, Austria)

We investigate episodic soft-sediment deformation structures cross-cut by normal faults preserved in unlithified finely laminated calcite rich sediments in the Hirlatz cave in the Northern Calcareous Alps (Austria). These sediments comprise varve-like alternations of brighter carbonate/quartz rich layers, and darker clay mineral rich layers. The deformed sediments contain abundant millimeter to centimeter-scale soft-sediment structures (load casts, ball-and-pillow structures), sheet slumps (thrust faults and folds), erosive channels filled with slides and chaotic slumps. After deposition and soft-sediment deformation normal faults developed within the entire sedimentary succession, an event that probably correlates with an offset of c. 10 cm of the passage wall above the outcrop. Our major conclusions are: (i) The sediments have a glacial origin and were deposited in the Hirlatz cave under phreatic fluvio-lacustrine conditions. The deposition and the soft-sediment deformation occurred most likely during the last glaciation (i.e. around 25 ka ago); (ii) The liquefaction and formation of the soft-sediment structures in water-saturated stratified layers was triggered by episodic seismic events; (iii) The internally deformed sediments were later displaced by normal faults; (iv) A possible source for the seismic events is the active sinistral Salzach-Ennstal-Mariazeller-Puchberger (SEMP) strike-slip fault which is located about 10 km south of the outcrop and plays a major role in accommodating the extrusion of the Eastern Alps towards the Pannonian Basin. To our knowledge, the described structures are the first report of liquefaction and seismically induced soft-sediment deformations in Quaternary sediments in the Eastern Alps.

Decoding fjord water contribution and geochemical processes in the Aysen thermal springs (Southern Patagonia, Chile)

The Aysen region, located in Southern Patagonia, Chile (43°38′S to 49°16′S) has optimum conditions for the formation of geothermal systems: Magmatic processes, abundant rainfall and active faults systems. In fact, several thermal springs emerge in coastal and inland areas of the Aysen region. Thermal springs in the coastal areas are spatially related to the Liquiñe-Ofqui Fault System (LOFS), a major strike-slip fault system that runs along the southern segment of the Southern Volcanic Zone, and could be controlling groundwater circulation. Despite the existence of literature regarding the composition of thermal springs, they do not elucidate the origin of thermal waters, the source of their chemical components nor have their hydrogeochemical processes been investigated. This knowledge will provide a useful tool for exploration of geothermal resources in the Aysen region.Sixteen thermal waters, three fjord waters, and three meteoric water samples were collected and analysed for major-minor ions, some trace elements (B, Li) and stable isotopes of δ2H and δ18O. Classic geochemical tools (i.e.: ratio of elements, ion vs chloride content), Hierarchical Cluster Analysis (HCA) and Factorial Analysis (FA) suggest there are three water groups (G1, G2 and G3). G1 and G3 are NaCl type coastal thermal springs with electrical conductivity (EC) above 1000μS/cm and mostly neutral pH (6.4 to 8.4). G1 has the highest average concentrations of Cl−, SO42−, Na+, Ca2+ while G3 has the highest values T°, SiO2, HCO3−, Li, B. The G2 samples are from Aysen inland areas, have EC below 1000μS/cm, slightly alkaline pH (7.9 to 9.6) and are Na-Cl-HCO3 type with higher average values of T°, SiO2, HCO3− and Li than G1.Factorial analysis indicates that two factors explain 82.1% of the total dataset variance. Factor 1 is formed by Cl−, SO42−, Na+, Ca2+, Li and B, which we associated with fjord water mixing processes. The factor 2 formed by T°, SiO2, HCO3−, Li and B might be interpreted as: i) silicate weathering from the North Patagonian Batholith and ii) volatile elements transported through high temperature vapours from a volcanic degassing source. These two components which could interact and overlap are address as “magmatic-hydrothermal” in this work. The factorial scores distribution is consistent with the HCA suggesting that fjord water mixing is the dominant geochemical process for G1 samples while G2 samples are influenced by magmatic-hydrothermal fluids. The G3 thermal waters show both processes with a greater influence of magmatic-hydrothermal fluids. The calculated saline factor (3.4% to 42.3%) supports fjord water mixing processes as one of the dominant processes affecting the coastal thermal spring's composition.The stable isotopic data (δ2H and δ18O) fall in the local meteoric water line (LMWL), suggesting current recharge and limited oxygen isotopic exchange during rock-dissolution processes. This could indicate short residence times and/or abundant contribution of actual meteoric water at shallow depths.

The sub-crustal stress estimation in central Eurasia from gravity, terrain and crustal structure models

We investigate the horizontal stress field beneath crustal structures of central Eurasia. The numerical procedure applied for a simultaneous determination of the sub-crustal stress and the crustal thickness from the global gravity, terrain and crustal structure models is based on solving Navier-Stokes’ problem which incorporates the inverse solution to the Vening Meinesz- Moritz’s problem of isostasy. The numerical results reveal that a spatial distribution of the sub-crustal stress in this study area closely resembles the regional tectonic configuration comprising parts of the Eurasian, Indian and Arabian lithospheric plates. The maximum shear stress intensity is generated by a subduction of the Indian plate beneath the Tibetan block. The intra-plate tectonic configuration is marked by the stress anomalies distributed along major active strike-slip fault systems and sections of subduction which separate the Tibetan and Iranian blocks from the rest of the Eurasian plate. The most pronounced intra-plate stress anomalies are related with a subduction of the Eurasian plate beneath the Tibetan block. We also demonstrate that a prevailing convergent orientation of stress vectors agree with the compressional tectonism of orogenic formations (Himalaya and Tibet Plateau, Than Shan, Zargos and Iranian Plateau), while the extensional tectonism of continental basins (Tarim, Ganges-Brahmaputra, Sichuan) is manifested by a divergence of stress vectors.

A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits

Research Article| March 01, 2015 A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits Caroline S. Perring Caroline S. Perring † BHP Billiton Nickel West, 125 St George’s Terrace, Perth, WA 6000, Australia †E-mail, Caroline.Perring@bhpbilliton.com Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 469–503. https://doi.org/10.2113/econgeo.110.2.469 Article history received: 06 Mar 2014 accepted: 04 Aug 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Caroline S. Perring; A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits. Economic Geology 2015;; 110 (2): 469–503. doi: https://doi.org/10.2113/econgeo.110.2.469 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Leinster area is situated toward the southern end of the Agnew-Wiluna Belt in the Yilgarn craton of Western Australia and hosts the giant Perseverance komatiite-associated nickel sulfide deposit. At in excess of 1.2 Mt contained Ni, this is the biggest type 1 (basal sulfide-rich accumulation) deposit in the world, and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the Perseverance fault on the eastern margin of the belt. In broad terms, the greenstone sequence comprises an eastern, rift-proximal komatiite-dacite sequence and a western and more marginal basalt-dominated sequence with minor komatiite.The komatiites display a wide range of compositions and textures, from highly magnesian, olivine meso- to adcumulate rocks composed of olivine with forsterite contents of up to 94.5% to sequences of thin, spinifex-textured komatiite flow units. This compositional range indicates considerable variation in volcanic facies, from rift-proximal to distal. Loci of focused flow (termed pathways), indicated by thickened cord-like bodies of more olivine rich rock, occur at all scales and are intimately associated with the Ni sulfide mineralization. The pathways are linked by thinner sequences of lower-MgO rocks, either layered olivine orthocumulate rocks or sequences of thin spinifex-textured flow units. Some of the lower-MgO komatiite sequences appear to emanate from the tops of pathways, in the manner of breakout flows in tube-fed basaltic terrains. Variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks.The overprinting effects of regional deformation and metamorphism, spanning at least 80 m.y., are considerable. At least eight local deformation events have been recognized by previous workers. These have been used to constrain the 3-D model and can be linked to craton-wide structural schemes. Some of the major faults are clearly accretionary and reflect reactivation of underlying basin-forming structures. Early north-over-south thrusting was followed by E-W–directed basin closure and the development of NNW-trending open regional folds. Extension accompanied batholithic granitoid intrusion and reactivated rift-parallel NNW-trending faults, producing the major strike-slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains. The ensuing orogenic collapse resulted in overturning of the stratigraphy to the east. Subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation.There is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the Ni sulfide deposits in this ancient terrane. Three-dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt, allowed the identification of komatiite pathway positions and their plunges, and provided quantitative data on the magnitude and direction of fault offsets of different generations. The modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of Ni sulfide deposits, even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism. This is illustrated with case studies of the Perseverance, Venus, Rocky’s Reward, Harmony, and Sir Lancelot deposits.Three-dimensional modeling has the capacity to integrate geologic, geochemical, and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration. The recent discovery of the Venus deposit is, in part, a tribute to the power of this approach. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic velocity modelling of the Carboneras Fault Zone, SE Spain

The Carboneras fault zone forms part of a major strike-slip fault system in SE Spain, striking NE–SW, and accommodating up to 40km displacement. It affects basement metamorphic rocks and unconformably overlying upper Miocene sediments and volcanic rocks. High-resolution shallow seismic tomographic sections were made across the fault zone in two localities. From the same areas, fault rocks and their wallrocks were collected for laboratory seismic velocity measurements. The laboratory data were corrected for the substantial effects of near-surface crack damage. By combining these results with geological cross sections, forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ‘ground-truth’ the inferences made from them. These velocity/depth relationships matched moderately well with those extracted from the in-situ tomography results. Aspects of the in-situ seismic sections matched features on the forward-modelled sections, but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool.

Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator?

For the first time, a deep seismic data set acquired in the frame of the Algerian–French SPIRAL program provides new insights regarding the origin of the westernmost Algerian margin and basin. We performed a tomographic inversion of traveltimes along a 100-km-long wide-angle seismic profile shot over 40 ocean bottom seismometers offshore Mostaganem (Northwestern Algeria). The resulting velocity model and multichannel seismic reflection profiles show a thin (3–4 km thick) oceanic crust. The narrow ocean–continent transition (less than 10 km wide) is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with Miocene to Quaternary sediments. This fault system, as well as the faults organized in a negative-flower structure on the continent side, marks a major strike-slip fault system. The extremely sharp variation of the Moho depth (up to 45 ± 3°) beneath the continental border underscores the absence of continental extension in this area. All these features support the hypothesis that this part of the margin from Oran to Tenes, trending N65–N70°E, is a fossil subduction-transform edge propagator fault, vestige of the propagation of the edge of the Gibraltar subduction zone during the westward migration of the Alboran domain.

Distributed deformation around the eastern tip of the Kunlun fault

Whether active strain within the Indo-Asian collision zone is primarily localized along major strike-slip fault systems or is distributed throughout the intervening crust between faults remains uncertain. Despite refined estimates of slip rates along many of the major fault zones, relatively little is known about how displacement along these structures is accommodated at fault terminations. Here, we show that a systematic decrease in left-lateral slip rates along the eastern ~200 km of the Kunlun fault, from >10 mm/year to <1 mm/year, is coincident with high topography in the Anyemaqen Shan and with a broad zone of distributed shear and clockwise vorticity within the Tibetan Plateau. Geomorphic analysis of river longitudinal profiles, coupled with inventories of cosmogenic radionuclides in fluvial sediment, reveal correlated variations in fluvial relief and erosion rate across the Anyemaqen Shan that reflect ongoing differential rock uplift across the range. Our results imply that the termination of the Kunlun fault system is accommodated by a combination of distributed crustal thickening and by clockwise rotation of the eastern fault segments.

Late Palaeoproterozoic mafic dyking in the Ukrainian Shield of Volgo-Sarmatia caused by rotation during the assembly of supercontinent Columbia (Nuna)

The Ukrainian Shield comprises the exposed crust of the large Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton (“Baltica”). Geological and geophysical data indicate that 1.80 to 1.75Ga mafic dykes related to anorthosite–mangerite–charnockite–granite (AMCG) plutons are widespread within the Ukrainian Shield. We examined their ages, distribution patterns, orientations and compositions in three different crustal blocks (Volyn, Ingul and Azov), and found close spatial relationships with major strike-slip fault systems developed during two phases of extension. The early, 1.80–1.77Ga, generation of mafic dykes mostly follows NW (330±20°) and more rarely N–S- or E–W-trending faults corresponding to major NE–SW extension (the Submoshorino phase). These dykes contain olivine dolerites, picrites, camptonites, lamprophyres, kimberlites and other rocks belonging to tholeiitic and subalkaline jotunitic series. The compositions of these dykes differ between the host blocks, but all feature upper mantle geochemical signatures such as high contents of Ni and Cr, and positive values of εNd(1800) up to +2.8. High degrees of REE fractionation indicate deep levels of mantle melting, which is particularly characteristic of the Ingul block as marked by the most extensive and dense mafic dyke swarms. The later, 1.76–1.75Ga, dyke swarms occur close to the most voluminous AMCG suites of similar age and were emplaced during the second (Korsun) phase of faulting when all the older strike-slip fault zones were reactivated and partly transformed to tensional faults by E–W extension. These dyke swarms mainly trend 030±20°. They are jotunitic and their isotopic signatures indicate a greater participation of crustal sources in the parent melts. The overall transtensional tectonic setting of the mafic dyking associated with the AMCG magmatism in Volgo-Sarmatia was created by convergent tectonics and postcollisional collapse of the thickened lithosphere, as well as by mantle delamination coupled with the rotation of Volgo-Sarmatia between 1.80 and 1.75Ga. This agrees with palaeomagnetic reconstruction suggesting rotation(s) of Volgo-Sarmatia during its protracted oblique docking with Fennoscandian terranes and Laurentia as supercontinent Columbia (Nuna) was assembled.

The 2008 Yutian normal faulting earthquake (Mw 7.1), NW Tibet: Non-planar fault modeling and implications for the Karakax Fault

The ENE striking Altyn-Tagh Fault and the WNW striking Karakax Fault are two major strike–slip fault systems in northern Tibet, and form a prominent ~ 2000 km long fault system. The 2008 Yutian normal faulting earthquake (Mw 7.1) struck near the southern edge of the Tarim Basin, where the two fault systems converge. While there are numerous NS-trending normal faults particularly in southern Tibet, their tectonic origins have remained contentious. Based on crustal deformation data sets obtained from synthetic aperture radar (SAR) as well as aftershock distribution, we developed a non-planar fault source model for the 2008 Yutian earthquake that exhibits a large normal-fault slip on a west-dipping surface with a nearly NS strike, thus suggesting a localized EW trending extensional stress field. The extensional stress was presumably generated at a step-over region of two NE-trending left-lateral strike–slip faults, which would probably belong to the Altyn-Tagh and Longmu–Gozha Co Fault Systems. In the epicentral area, there exists a fault scarp that coincides with the top edge of our fault model, and thus similar earthquakes must have occurred over geological time. Such normal faulting earthquakes must have been repeatedly suppressed the left-lateral slip behavior of the Karakax Fault. In addition, if the slip along the Altyn-Tagh Fault is transferred to the Longmu–Gozha Co Fault, which is SE across the normal fault system, the slip rate of the Karakax Fault would be less than that of the adjoining Altyn-Tagh Fault.

Mechanics of fore-arc slivers: Insights from simple analog models

[1] Convergent margins with oblique subduction commonly include a fore-arc sliver, a portion of the overlying plate bounded by the trench and a major strike-slip fault system. It has long been noted that relative plate motion near the deformation front, as indicated by earthquake focal mechanisms, is generally closer to margin-normal than would be expected from overall relative plate motion. This results from sliver motion as well as from shear within the fore arc. We carry out some simple calculations to determine how the rate of sliver motion should vary with plate convergence obliquity. We find that the results are related to rheology but that the measurement of sliver rates at natural fore arcs is unlikely to yield real insight into physical properties; even for very simple models; the system is too complicated and its relation to rheology is not unique. The proportion of oblique convergence taken up by sliver motion in simple tabletop experiments depends on the rate of slip and the smoothing of asperities. Similarities in taper and style of strain between frontal wedges forming with and without slivers suggest that structural observations of exhumed accretionary wedges are unlikely to allow geologists to draw definitive conclusions about the degree of obliquity of relative plate motion at the time when the wedges were formed and in some cases not even whether or not a sliver plate was present at the time of deformation.

Morphology, structure and evolution of California Continental Borderland restraining bends

Abstract Exceptional examples of restraining and releasing bend structures along major strike-slip fault zones are found in the California continental Borderland. Erosion in the deep sea is diminished, thereby preserving the morphology of active oblique fault deformation. Long-lived deposition of turbidites and other marine sediments preserve a high-resolution geological record of fault zone deformation and regional tectonic evolution. Two large restraining bends with varied structural styles are compared to derive a typical morphology of Borderland restraining bends. A 60-km-long, 15° left bend in the dextral San Clemente Fault creates two primary deformation zones. The southeastern uplift involves ‘soft’ turbidite sediments and is expressed as a broad asymmetrical ridge with right-stepping en echelon anticlines and local pull-apart basins at minor releasing stepovers along the fault. The northwest uplift involves more rigid sedimentary and possibly igneous or metamorphic basement rocks creating a steep-sided, narrow and more symmetrical pop-up. The restraining bend terminates in a releasing stepover basin at the NW end, but curves gently into a transtensional releasing bend to the SE. Seismic stratigraphy indicates that the uplift and transpression along this bend occurred within Quaternary times. The 80-km-long, 30–40° left bend in the San Diego Trough–Catalina fault zone creates a large pop-up structure that emerges to form Santa Catalina Island. This ridge of igneous and metamorphic basement rocks has steep flanks and a classic ‘rhomboid’ shape. For both major restraining bends, and most others in the Borderland, the uplift is asymmetrical, with the principal displacement zone lying along one flank of the pop-up. Faults within the pop-up structure are very steep dipping and subvertical for the principal displacement zone. In most cases, a Miocene basin has been structurally inverted by the transpression. Development of major restraining bends offshore of southern California appears to result from reactivation of major transform faults associated with Mid-Miocene oblique rifting during the evolution of the Pacific–North America plate boundary. Seismicity offshore of southern California demonstrates that deformation along these major strike-slip fault systems continues today.

Cretaceous epithermal gold–silver mineralization and geodynamic environment, Korea

Epithermal precious-metal mineralization in the Korean Peninsula mainly occurred along NNE-trending major strike-slip fault systems that are associated commonly with formation of pull-apart basins and major volcanic activity during the Cretaceous. Sedimentation in the basins was initiated in the Hauterivian and continued into the Albian, whereas much of the volcanism occurred sporadically from ca. 110 to 50 Ma, with a major episode between ca. 90 and 70 Ma. Epithermal Au–Ag mineralization in Korea took place between approximately 100 and 70 Ma, overlapping with the shallow magmatic activity. Styles of epithermal Au–Ag deposits in Korea include those of the Mugeug-type found in sediment-dominant basins in the central portion, and the Haenam-type in volcanic-dominant basins in the southwest. Epithermal Au–Ag deposits associated with the volcanic-dominant basins in the southern Korea generally formed at very shallow crustal levels (<0.5 kbar) and relatively low temperatures (<300 °C) from fluids containing large components of less-evolved meteoric waters than those associated with sediment-dominant basins. Orthogonal subduction following oblique subduction of the Izanagi Plate along the Pacific continental margin during the Cretaceous probably represents a major control of magmatism and associated Au–Ag mineralization in the Korean Peninsula. In the Early Cretaceous, the left-lateral strike-slip movements due to the northward (oblique) subduction of the plate resulted in the Gongju–Eumseong and Yeongdong–Gwangju fault systems. Late Cretaceous calc-alkaline volcanic activity and associated caldera-related fractures related to an orthogonal convergence that postdates the NNE-trending strike-slip movements may play an important role in the formation of epithermal Au–Ag deposits. Simultaneously with, or soon after heating related to magmatism, continued movement of strike-slip faults may also have been critical to the ore-forming process, leading to relaxation of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.