Dextral Strike-slip Fault System Research Articles

Mesozoic strike-slip fault system at the margin of the Junggar Basin, NW China

The Junggar Basin in Northwest China is covered by thick sediments, causing challenges in recognizing buried structures. Seismic reflection and drilling data from petroleum exploration provide essential data for the study of subsurface structures. In this study, we focus on the Mesozoic structural characteristics and tectonic evolution along the margins of the Junggar Basin by using 2D and 3D seismic reflection, drilling data, and surface geology. Our results demonstrate that a unified dextral strike-slip fault system developed along the marginal area around the Junggar Basin. The structural style is characterized by both transpressional structures, such as positive flower structures, en echelon folds, restraining bends, and fan-shaped and horsetail fractures at the fault tip, and transtensional structures, including releasing bends, pull-apart basins (fault basins), and positive inverted structures. We further interpret that except for the southern margin of the Junggar Basin, the current structural pattern of the basin margin was mainly formed in the pre-Cretaceous, which was covered by Cretaceous and Cenozoic strata. Based on the timing and style of deformation, we propose that the vertical-axis counterclockwise rotation of the Junggar Basin during the Mesozoic caused right-lateral strike-slip faulting along the basin margin. Cenozoic reactivation of these structures has been localized in the southern margin of the Junggar Basin under the far-field influence of the India-Asia collision.

Cenozoic extension to strike-slip transition in the Liaodong Bay Subbasin along the Tan-Lu Fault Zone, Bohai Bay Basin: New insights from stress field modelling

The NE-trending Liaodong Bay Subbasin (LDBS), a subunit of the Bohai Bay Basin (BBB), developed along the Tan-Lu Fault Zone (TLFZ) in eastern China. The LDBS is controlled by both extensional and dextral strike-slip fault systems and has an episodic evolutionary history, leading to a poor understanding of its tectonic origin. In this paper, through interpretations of seismic data and other geological and geophysical data, four best-fit finite element models were generated to reveal the tectonic development of the LDBS and the whole BBB. The deformation features predicted by the modelling results are consistent with the geological evidence. The modelling results revealed that the extension directions of the LDBS changed from WNW-ESE during the Paleocene-early Eocene, to NW-SE during the middle-late Eocene and then to NWN-SES or nearly N-S in the Oligocene and Miocene. The changes in the extension direction resulted in a strike-slip transition of the TLFZ in approximately the middle-late Eocene, from normal to dextral transtensional, and transformed the LDBS and the whole BBB from an extensional basin into a dextral transtensional basin. This transformation was likely triggered by the kinematic adjustment of the Pacific Plate from NNW to WNW in the middle-late Eocene, but was maintained and enhanced by the Indian-Asian collision during the rest of the Cenozoic.

Mapping the Lithological Features and Ore-Controlling Structures Related to Ni–Cu Mineralization in the Eastern Tian Shan, NW China from ASTER Data

The study of lithological features and tectonic evolution related to mineralization in the eastern Tian Shan is crucial for understanding the ore-controlling mechanism. In this paper, the lithological features and ore-controlling structure of the Huangshan Ni–Cu ore belt in the eastern Tian Shan are documented using advanced spaceborne thermal emission and reflection radiometer (ASTER) multispectral data based on spectral image processing algorithms, mineral indices and directional filter technology. Our results show that the algorithms of b2/b1, b6/b7 and b4/b8 from ASTER visible and near-infrared (VNIR)- shortwave infrared (SWIR) bands and of mafic index (MI), carbonate index (CI) and silica index (SI) from thermal infrared (TIR) bands are helpful to extract regional pyroxenite, external foliated gabbro bearing Ni–Cu ore bodies as well as the country rocks in the study area. The detailed interpretations and analyses of the geometrical feature of fault system and intrusive facies suggest that the Ni–Cu metallogenic belts are related to Carboniferous arc intrusive rocks and Permian wrench tectonics locating at the intersection of EW- and NEE-striking dextral strike-slip fault system, and the emplacement at the releasing bends in the southern margin of Kanggur Fault obviously controlled by secondary faults orthogonal or oblique to the Kanggur Fault in the post-collision extensional environment. Therefore, the ASTER data-based approach to map lithological features and ore-controlling structures related to the Ni–Cu mineralization are well performed. Moreover, a 3D geodynamic sketch map proposes that the strike-slip movement of Kanggur Fault in Huangshan-Kanggur Shear Zone (HKSZ) during early Permian controlled the migration and emplacement of three mafic/ultramafic intrusions bearing Ni–Cu derived from partial mantle melting and also favored CO2-rich fluids leaking to the participation of metallogenic processes.

The geodynamic evolution of the Famatinian orogen from the paleomagnetic record of El Hongo trondhjemite (Early Paleozoic, Sierras Pampeanas de Córdoba, Argentina)

The Pampia cratonic block is juxtaposed to the Paleoproterozoic Río de la Plata craton through the Córdoba fault, an extension of the Transbrasiliano lineament. As a fragment of hyperextended crust, it was the locus of several magmatic belts during the Paleozoic. The Ordovician Famatinian orogen, for example, developed on its western margin as a consequence of subduction, which would lead to the accretion of Cuyania at ~460 Ma. The El Hongo trondhjemite is a small Famatinian pluton emplaced at ~ 500 Ma, close to the eastern margin of Pampia, controlled by shear belts in the foreland setting. New Ar–Ar ages verified that the cooling lasted at least until 430 Ma. Protracted cooling of this intrusion recorded magnetic remanence comprising several magnetic components, all of them carried mainly by hematite, and subordinately by magnetite. Hematite is present as a product either of exsolution from ilmeno-hematitess, or of low-temperature oxidation of magnetite (martite). The remanence is interpreted to have been acquired thermochemically during repeated pulses of fluid migration and deuteric oxidation. The A and B directions are predominant and have an arcuate distribution identifying clockwise rotation during acquisition. Other two directional groups, C and D are less represented, restricted to narrow patches; they seem to be acquired later, and coincide roughly with the ill-defined Silurian-Devonian Gondwana apparent polar wander path for the Early Paleozoic, implying that rotations had finished by then. An emplacement along dextral strike-slip fault systems is envisaged for El Hongo trondhjemite, where faulting occurred through shear belts in the weakened back-arc and foreland of the Famatinian belt, in response to the oblique subduction that would have accompanied the clockwise rotation of Laurentia around Gondwana during the Early Paleozoic. Strike-slip and rotations would have finished at ~460 Ma, coinciding with the final docking of Cuyania.

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

Abstract The timing of slip on brittle faults in Earth’s upper crust is difficult to constrain, and direct radiometric dating of fault-generated materials is the most explicit approach. Here we make a direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibers and veins from the same fault. We have dated fault generated materials from the Big Creek fault, a northwest-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ca. 73 Ma and ca. 60–57 Ma, representing at least two periods of fault slip that form part of a complex fault and fluid-flow history. The Cretaceous result lies within previous indirect estimates for major slip on the fault. The Paleocene–Eocene result coincides with the estimated timing of slip of the nearby Tintina and Denali faults, which are crustal-scale, northwest-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model, where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for distinct fault-generated materials using different decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.

Imaging active faulting in the western Taiwan Strait

Large, destructive historical earthquakes off the coast of China’s Fujian Province point to important tectonic activity in the western Taiwan Strait that, until recently, has received little attention. We present newly acquired reflection seismic data that is used to study the shallow crustal structure of the western Taiwan Strait. With these data we map the location of the Benhai fault for the first time, describe its upper crustal geometry and, in combination with seismicity and earthquake focal mechanisms, interpret its kinematics. These new data demonstrate that there is wide spread evidence of faulting that reaches the sea floor in the western Taiwan Strait, clearly indicating that the Benhai fault is active. Faults that cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones, together with the fault and basin map pattern, are consistent with this fault zone being in the early developmental stages of a dextral strike-slip system. Earthquake focal mechanisms, although not definitive, support the model of an active dextral strike-slip fault system in the western Taiwan Strait.

Burma Terrane part of the Trans-Tethyan Arc during collision with India according to palaeomagnetic data.

Convergence between the Indian and Asian plates has reshaped large parts of Asia, changing regional climate and biodiversity. Yet geodynamic models fundamentally diverge on how convergence was accommodated since the India-Asia collision. Here we report paleomagnetic data from the Burma Terrane, at the eastern edge of the collision zone and famous for its Cretaceous amber biota, to better determine the evolution of the India-Asia collision. The Burma Terrane was part of a Trans-Tethyan island arc and stood at a near-equatorial southern latitude at ~95 Ma, suggesting island endemism for the Burmese amber biota. The Burma Terrane underwent significant clockwise rotation between ~80-50 Ma, causing its subduction margin to become hyper-oblique. Subsequently, it was translated northward on the Indian Plate, by an exceptional distance of at least 2000 km, along a dextral strike-slip fault system in the east. Our reconstructions are only compatible with geodynamic models involving a first collision of India with a near-equatorial Trans-Tethyan subduction system at ~60 Ma, followed by a later collision with the Asian margin.

Transfer of deformation during indentation: Inferences from the post- middle Miocene evolution of the Dinarides

Understanding the structural and kinematic effects of indentation is still debated due to the large number of competing mechanisms associated with the complex orogenic build-up. Among the many examples available worldwide, the evolution of the Adriatic continental microplate in the Mediterranean domain provides one of the best places to understand the mechanics of indentation. This understanding is hampered by the lack of structural and kinematic data in the Dinarides, an orogen situated at the critical transition between the Alps, Albanides and Hellenides, and across the Adriatic margin of the Apennines. We have studied the less known area of the central and south-eastern Dinarides by focussing on collecting a new kinematic dataset for structures formed during the Adriatic indentation, which postdates the main Late Jurassic – Paleogene orogenic structuration. Our results are in agreement with previous interpretations of an early-middle Miocene period of extension that affected the entire orogen across its strike and is incompatible with indentation effects in the studied parts of the Dinarides. More importantly, we demonstrate for the first time that the post- middle Miocene Dinarides deformation was characterized by a coherent regional system of large offset dextral strike-slip faults, which transfer gradually their offsets to thrusts and high-angle reverse faults. The overall deformation transfer mechanism can be described as a special class of continental restraining bends or stepovers, whose geometry is controlled by rheological distribution. The integration of our results in the larger geodynamic context shows that the post-middle Miocene Dinarides fault system accommodates the differential motion between the N- to NE-wards Adriatic indentation and the rapid S- to SW- ward movement of a Hellenides area situated SE of the Kefalonia Fault, driven by the Aegean slab-roll back.

Insights into the tectonic history of the Western Alps through dating of fissure monazite in the Mont Blanc and Aiguilles Rouges Massifs

Hydrothermal monazite-(Ce) crystals from sub-horizontal and younger sub-vertical Alpine fissures located in the Mont Blanc and Aiguilles Rouges Massifs were dated by in-situ SIMS Th-U-Pb methods to identify major growth phases during deformation. Sub-horizontal clefts formed in a compressive regime that led to steep foliation and down-dip lineation in the Mont Blanc shear zones. Sub-vertical clefts formed during younger, strike-slip movements along boundaries on both sides of the Mont Blanc Massif and along Permocarboniferous deposits in the Aiguilles Rouges. All analyzed monazite-(Ce) crystals show signs of recrystallized, porous and complexly-zoned grains, indicative of dissolution-reprecipitation processes. Consequently, the crystals show three main growth phases at 11.5–10.5 Ma, 9 Ma and 8–7 Ma, interpreted as representing (re)crystallization during the onset of dextral strike-slip faulting and subsequent steps of activity along the dextral strike-slip fault system. Whereas monazite-(Ce) from the older horizontal clefts may record all three crystallization steps, monazite-(Ce) crystallization in vertical clefts from the Aiguilles Rouges Massif starts only at around 9 Ma. Comparison of monazite-(Ce) crystallization ages with thermochronometers indicates that the anastomizing network of vertical shear zones with down-dip lineation in the Mont Blanc Massif became overprinted at 11.5 Ma and only traces of an older monazite-(Ce) generation survived. Heat advection seems to have been locally sufficient to reset zircon-fission track ages. The vertical clefts of ca. 9 Ma age in the Aiguilles Rouges Massif indicate that strike-slip deformation occurred not only on the south-eastern but also the north-western boundary of the Mont Blanc Massif and may be linked to the increase in exhumation rate around that time. The identified deformation phases show that the external Massifs of the Western Alps (Aar, Mont Blanc, Aiguilles Rouges, Pelvoux) not only share a comparable tectonic position within the Alpine arc but also underwent a very similar tectonic evolution.

Regional strike-slip and initial subsidence of Korea Plateau, East Sea: tectonic implications for the opening of back-arc basins

This study focuses on the initial subsidence mechanism of Korea Plateau in the East Sea. New interpretation of digitized multichannel seismic profiles (vertically unexaggerated) in the southern part of the South Korea Plateau delineates that acoustic basement comprises a number of normal-fault blocks, forming intra-plateau basins and troughs. In the western part of the plateau, the normal faults are oblique to the continental margin, whereas in the eastern part, they are bounded by an orthogonal transfer fault. The entire faults are suggestive of a large-scale dextral strike-slip fault system. The dextral fault system in the plateau was accompanied by a regional strike-slip fault along the North Korean continental margin, which offsets the prominent NE-SW granite belts on land and under the sea. The regional dextral strike-slip fault system activated during the late Oligocene/early Miocene to middle Miocene. The regional dextral strike-slip along the western continental margin of the sea was coupled with the complex dextral fault on the eastern margin from Sakhalin to Honshu. The regional strike-slip deformation was due to the changes in plate motion, i.e., southward drift (late Oligocene/early Miocene) and clockwise rotation (middle Miocene) of the Japanese Arc away from the Eurasian Plate.

Antecedent geologic control on nearshore morphological development: The wave dominated, high sediment supply shoreface of southern Namibia

The inner continental shelf, nearshore zone and adjacent onland coastal strip of southern Namibia has been mapped using airborne electromagnetic and magnetic techniques, providing, for the first time in the area, a continuous onshore-offshore geophysical dataset. In addition, the inner continental shelf and nearshore zones of this vigorous, wave-dominated coastline were mapped, inshore of a pervasive acoustic blanketing layer, using multibeam bathymetry, singlebeam bathymetry and boomer sub-bottom profiling systems in water depths between ‐3 and ‐55 m. We use an integrated, iterative approach to the interpretation of these brand new datasets to map the structural framework of the Late Proterozoic -age bedrock of the area beneath thick Holocene sand cover and show how, despite the vigorous wave regime and high sediment supply from the Orange River, this framework acts as a recurring primary control to the area's geomorphic evolution.The bedrock morphology is divided into three main zones that are constrained by NNW-striking and coast-parallel faults. A southern zone, characterised by both relatively shallow bedrock depth and shallow bedrock slope is bounded in the north by a dextral strike-slip fault system. This basement promontory is the primary control on the shallow, low gradient seafloor in the southern zone. Strike-slip movement on the fault shifted the bedrock to the north both down and inshore relative to the shallow basement promontory to the south. However, in this central zone, both the seafloor and seafloor gradient remain shallow. Sediment accumulation in this area is explained by two processes. Firstly, the relative downward movement of the bedrock has resulted in the creation of relatively more accommodation space and sediment delivery begins to overprint the underlying bedrock framework, smoothing the general seafloor gradient in the central zone. Secondly, this central zone, directly north of the basement promontory has existed in a swell shadow as long-lived SW directed wave energy has continually been refracted around the palaeo-promontory, promoting natural accretion of sediment within the lee of this feature. A fault-related inflection point on the modern coastline marks a change in coastline orientation from NW to slightly more north striking, and the start of the northernmost bedrock zone. Here the shallow, wide, low-relief seafloor zone transitions to a wider nearshore high-relief zone that mimics the underlying uniformly steepening nearshore bedrock profile. The combination of the uniform, coast-normal basement gradient with the fault-related inflection changes the nearshore response to incoming wave energy. In the northern zone, waves approach at a slightly more oblique angle due to the fault-controlled coastline orientation, and reinforce an already vigorous longshore drift regime that is responsible for the steeper and deeper nearshore profile, underpinned by the underlying steep basement topography. The steep antecedent topography also limits the amount by which the seafloor can aggrade and subsequently adjust its slope; an imbalance between the sediment supply and the large volume of accommodation space thus exists that cannot be overcome given the context of the vigorous longshore drift. The continual inheritance of the underlying bedrock slope in the nearshore is further revealed with the unchanging slope of the reflectors that match the present-day nearshore seafloor gradient. We suggest that the geologic framework control remains a critical factor in nearshore-beach system development, even in an area of high sediment supply and high wave energy conditions.

Major Strike-Slip Faults Identified Using Satellite Data in Central Borneo, SE Asia

We use freely available Google satellite data, instrumental seismicity, fault plane solutions, and previously mapped structural and geological maps to identify new fault zones in central Borneo. We have mapped a number of ~NW-SE trending dextral strike-slip faults and ~NE-SW to ~N-S trending sinistral strike-slip fault zones. The geomorphic expression of faulting is shown by the well-developed triangular facets, fault rupture scarps, truncated sedimentary beds, topographic breaks, displaced ridges, deflected streams, faulted Plio-Pleistocene volcanic deposits, and back-tilted Holocene to Recent sedimentary deposits. Some of the mapped faults are actively growing, and show text-book examples of dextral and sinistral offset, which ranges from ~450 m to tens of km. The dextral strike-slip fault systems are clearly developed in the central and eastern portions of Borneo where they cut through the folded sedimentary sequences for >220 km. The ~NE-SW to ~N-S trending sinistral strike-slip faults are dominantly developed in the eastern portion of central Borneo for >230 km. The geomorphic expression of faulting is clear and the fault scarps are ~SE facing for the sinistral fault system, and ~NE facing for the dextral fault system. The age of the faulting is constrained by the cross-cutting relationship where the fault cuts through Plio-Pleistocene volcanic deposits for >30 km, which suggests a neotectonic nature of faulting. The strike-slip fault systems that we have mapped here provide the first geomorphic evidence of large-scale strike-slip faulting in Borneo and suggest the presence of a major sinistral strike-slip fault that runs for >900 km through the center of Borneo, and forms a backbone onto which most of the mapped structures root. The mapped structures clearly suggest that plate tectonic forces dominantly control the geological structures that we have mapped and support the regional oblique convergence that is oblique with respect to the major trend of the Crocker Range, which forms the spine of the Borneo Island.

The Guanacaste Volcanic Arc Sliver of Northwestern Costa Rica

Recent studies have shown that the Nicoya Peninsula of northwestern Costa Rica is moving northwestward ~11 mm a−1 as part of a tectonic sliver. Toward the northwest in El Salvador the northern sliver boundary is marked by a dextral strike-slip fault system active since Late Pleistocene time. To the southeast there is no consensus on what constitutes the northern boundary of the sliver, although a system of active crustal faults has been described in central Costa Rica. Here we propose that the Haciendas-Chiripa fault system serves as the northeastern boundary for the sliver and that the sliver includes most of the Guanacaste volcanic arc, herein the Guanacaste Volcanic Arc Sliver. In this paper we provide constraints on the geometry and kinematics of the boundary of the Guanacaste Volcanic Arc Sliver that are timely and essential to any models aimed at resolving the driving mechanism for sliver motion. Our results are also critical for assessing geological hazards in northwestern Costa Rica.

Role Played by Strike-Slip Structures in the Development of Highly Curved Orogens: The Transcarpathian Fault System, South Carpathians

AbstractWe describe a previously unrecognized major dextral strike-slip fault system in the South Carpathians, hereafter referred to as the Transcarpathian fault system. The master fault has been active since the mid-Cretaceous and has a total offset of ∼150 km, of which only <35 km are post-Oligocene. The fault acted as a subduction-transform edge propagator (STEP) fault during the mid-Cretaceous subduction of the Ceahlau-Severin ocean system and separated an area to the north where the subduction system was accretionary (the East Carpathians) from an area to the south (the western half of the South Carpathians) where the subduction system was erosive. In the South Carpathians, the oceanic basin closed during the mid-Cretaceous after commencement of higher convergence rates and subduction erosion of the trench, leading to tectonic underplating and continental collision between the Dacia and Moesian microplates. The results bolster the idea that STEP-type strike-slip faults are critical in the development...

Recognition of Yanshanian magmatic-hydrothermal gold and polymetallic gold mineralization in the Laowan gold metallogenic belt, Tongbai Mountains: New evidence from structural controls, geochronology and geochemistry

The Laowan metallogenic belt in China is an important metallogenic belt within the Tongbai orogenic belt, and contains the medium-sized Laowan and Shangshanghe gold deposits, the small Huangzhuyuan lead–zinc–silver–gold deposit and some gold and Cu–Pb occurrences. These deposits are hosted in Mesoproterozoic plagioclase amphibolite (or schist) and mica-quartz schist. The gold ores are mainly quartz veins and veinlets and disseminated altered ores. Subordinate ore types include massive sulfides and breccias. The Laowan gold deposit is characterized by three right-stepping en-echelon fracture-controlled alteration zones that dip gently to the south and includes disseminated, sheeted and stockwork ores. These lodes were formed by the interaction of ore-forming fluid with foliated-to laminated cataclasite within the transpressional faults. The Shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north, and includes quartz veins and breccias in addition to ores in altered wallrocks. These lodes were formed by focusing of fluids into transtensional faults. These ore controlling faults displaced early barren quartz veins 10m horizontally with a dextral sense of motion. The ore-hosting structures at the Laowan and Shangshanghe deposits correspond to the P and R-type shears of a brittle dextral strike-slip fault system, respectively, which make angles of about 15° and −15° to the Laowan and Songpa boundary faults. The ore-controlling fault system post-dated formation of a ductile shear zone, and peak regional metamorphism. This precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation. These gold deposits are not typical orogenic gold deposits. The metallogenic belt displays district-scale-zoning of Mo→Cu–Pb–Zn–Ag→Au relative to Songpa granite porphyry dike zone, suggesting the mineralization may be closely related to the granite porphyry. Measured δ34S of sulfides and δ18O and δD of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids. The similarity of Pb isotope ratios between the ores and Yanshanian granitoids suggests a similar source. As the age (139±3Ma) of granite porphyry obtained by zircon U–Pb isotope overlaps the mineralization age (138±1Ma: Zhang et al., 2008a), the gold and polymetallic metallogenesis of the Laowan gold belt has close spatial, temporal and possibly genetic relationships with Yanshanian high level magmatism.

Extracting ore-deposit-controlling structures from aeromagnetic, gravimetric, topographic, and regional geologic data in western Yukon and eastern Alaska

Aeromagnetic lineaments interpreted from reduced-to-pole (RTP) magnetic grids were compared with gravity, topography, and field-based geologic maps to infer regional structural controls on hydrothermal mineral occurrences in a poorly exposed portion of the North American Cordillera in western Yukon and eastern Alaska. High-frequency and variable-intensity aeromagnetic lineaments corresponding to discontinuities with an aeromagnetic domain change were interpreted as steep-dipping and either magnetite-destructive or magnetite-additive faults. These structures were interpreted to be predominantly Cretaceous in age and to have formed after the collision of the Intermontane terranes with the ancient Pacific margin of North America. To demonstrate the reliability of the aeromagnetic interpretation, we developed a multidata set stacking methodology that assigns numeric values to individual lineaments depending on whether they can be traced in residuals and first vertical derivative of RTP aeromagnetic grids, isostatic residual gravity grids, digital topography, and regional geologic maps. The sum of all numeric values was used to estimate the likelihood of the aeromagnetic lineament as a true geologic fault. Fault systems were interpreted from zones of lineaments with high spatial density. Using this procedure, 10 major northwest-trending fault systems were recognized. These were oriented subparallel to the regional Cordilleran deformation fabric, the mid-Cretaceous Dawson Range magmatic arc, and well-established crustal-scale dextral strike-slip fault systems in the area. These orogen-parallel fault systems were interpreted to play a structural role in the emplacement of known porphyry Cu-Au and epithermal Au systems of mid-Cretaceous (115–98 Ma) and Late Cretaceous (79–72 Ma) age. The procedure also identified seven northeast-trending, orogen-perpendicular fault-fracture systems that are prominent in eastern Alaska and exhibit sinistral-to-oblique extensional kinematics. These structures were interpreted to govern the emplacement of Late Cretaceous (72–67 Ma) porphyry Mo- and Ag-rich polymetallic vein and carbonate replacement systems in the region.

Age, geochemical characteristics and petrogenesis of Late Cenozoic intraplate alkali basalts in the Lut–Sistan region, eastern Iran

Miocene to Quaternary alkali basalts in the Lut–Sistan region, eastern Iran are spatially associated with two active, N–S-trending dextral strike–slip fault systems in the region, i.e., the Neh faults in the Sistan suture zone and the Nayband fault ~200km further to the west in the Lut block. Here, we present new 40Ar/39Ar ages, geochemical and Sr–Nd isotopic data for these rocks to decipher the petrogenetic processes responsible for their formation and regional tectonic implications. Our new ages indicate that the volcanism commenced at ~14Ma in the western Lut region and ~11Ma in the northern Sistan suture zone. The rocks are composed dominantly of hawaiites and mugearites with minor basanites and basaltic andesites. Petrographic observations and major and trace elemental variations suggest that the alkali basalts underwent variable fractionation of olivine, clinopyroxene and Fe–Ti oxides. Chondrite-normalized rare earth element and mantle-normalized trace element patterns of these rocks largely resemble those of ocean island basalts. High εNd(t) (+1.4 to +3.6), low to moderate initial Sr isotopic ratios (0.7047–0.7073), and trace element ratios indicate that crustal contamination was insignificant in the petrogenesis. The rocks have neither geochemical features pointing to residual hornblende or phlogopite, nor arc-related signatures characteristic of the Iranian sub-continental lithospheric mantle. Thus, the alkali basalts most likely have asthenospheric origin. Modeling of REE suggests that they could have formed by low degrees of partial melting (~3–10%) of an enriched mantle source at garnet-stable depths. We propose that the east Iranian alkali basaltic volcanism was triggered by asthenospheric upwelling in an extensional setting, presumably caused by delamination of thickened lithospheric root following the Late Cretaceous collision between the Lut and Afghan continental blocks. Our results imply that two contrasting tectonic regimes coexist in Iran since the Middle Miocene, i.e., extensional in eastern and compressional in southwestern Iran.

Surface Versus Underground Measurements of Active Tectonic Displacements with TM 71 Exstensometers in Slovenia

The tectonic setting of W Slovenia is characterised by NW-SE trending dextral strike-slip fault systems and moderate seismicity. Monitoring of tectonic movements along five presumably active faults or in their vicinity using TM 71 extensometers was set up in 2004. In five years of monitoring some clear trends of displacement were established. The morphologically most prominent fault in W Slovenia is Idrija Fault having a total length of more than 120 km. The average lateral displacement measured along a crack in its inner fault zone in Učja valley was 0.26 mm/year. Short-term rates were even greater and reached 0.54 mm/year. Raša Fault monitoring site at the foot of Vremščica Mt. established first an average uplift of SW block for 0.16 mm/year and left-lateral displacement of 0.16 mm/ year. It was followed by down-slip of the same block at the rate of 0.37 mm/year. In Postojnska Jama two instruments, 260 m apart, were installed at the fault zone, which extends about 1 km northeast from Predjama Fault. We detect small tectonic deformations, dextral horizontal movement of 0.05 mm in 5 years for Postojna 1 and extension of 0.03 mm in 5 years for Postojna 2. Both devices recorded similar reactions to some earthquakes with magnitude range 3.1-5.2 and epicentral distance of 12-95 km. The amplitude of individual peaks is in the order of 0.08 mm. The monitoring at Kneža Fault started at the end of 2006. In two years clear oblique displacement was established with left-lateral rate of 0.06 mm/year and uplift of SW block for 0.06 mm/year. Monitoring in Pološka Jama situated in vicinity of the Ravne Fault started in 2008. Preliminary results show 0.08 mm of horizontal displacement between two limestone beds. Established displacements proved the active tectonic movement of all monitored faults. Observed deformation rates can be compared with the regional deformation rate in W Slovenia established from GPS measurement, which is in the order of 2 mm/year.

Structural Evolution of the Rock Creek Gold Deposit, Seward Peninsula, Alaska

Sheeted quartz veins at Rock Creek contain an indicated gold resource of 404,000 ounces (oz) at a grade of 1.31 grams per ton (g/t). The deposit, located on the Seward Peninsula, 12 km north of Nome, Alaska, occurs in variably graphitic greenschist-facies schist, fine-grained quartzite, and dirty marble of the lower Paleozoic to upper Proterozoic Nome Group. The veins at Rock Creek, as at many prospects in the southwestern Seward Peninsula, are hosted by the most carbonaceous part of the Nome Group. These strata underwent complex polyphase deformations that were progressively less penetrative through time. They include four distinct styles of folding, followed by an episode of thrust faulting. The Mid-Cretaceous dilational vein system formed during the onset of D4 deformation along the northeast-trending Albion fault. The overall shape of the vein system in relationship to controlling faults shows that a preexisting structural fabric may have influenced its formation; tension veins lie along but slightly oblique to the northeast-striking Albion fault in a left-stepping array. Younger “Albion” veins parallel and lie within the Albion fault zone. The highest ore tenor and greatest concentration of veins occur near the intersections of northeast and northwest structures. The veins diminish in frequency along strike across the northwest-trending structures. The vein system occurs primarily in the hanging wall of a curviplanar dextral thrust fault that splays from the high-angle Albion fault and becomes gently dipping in its upper reaches. Vein concentration diminishes abruptly below this curviplanar transpressional structure. Small stratigraphic offsets show that the vein system was not displaced by the structure but rather formed preferentially above it. The orientations and sequencing of structural elements show that the tension vein system formed during a period of highly focused dilation. The Albion fault zone subsequently propagated through the maximum zone of dilation as it evolved to a throughgoing dextral strike-slip fault system. Vein chemistry changed from primarily Au-As (tension veins) to Sb-Au-Ag-As-Pb (Albion-style veins) contemporaneously with a 10° counterclockwise migration of vein orientation. These changes mark a period of highly focused extension immediately prior to the onset of throughgoing transpression along the Albion fault, which resulted in a collapse of the hydrothermal system. The left-stepping geometry of tension veins and later evolution of the overall structural system to compression suggest that it formed within a restraining bend along the dextral strike-slip fault system. The general shape of the vein system, the positions and orientations of controlling structures, and their vectors and styles of displacement define a sidewall ripout that flanks the Albion fault zone.

The mechanics of a magma chamber-fault system in trans-tension with application to Coso

The mechanical interaction between an elliptically shaped magma chamber and a fault subject to transtension is investigated with particular reference to the Coso geothermal field. The geologic setting of the Coso field is interpreted as a releasing bend step-over structure formed by the Airport Lake and Owens Valley dextral strike-slip fault system. The role of the Coso volcano-magmatic center in the development of the “over-step” structure is examined by treating the magma chamber as a liquid inclusion in a viscoelastic crust containing a fault (Airport Lake). The problem is numerically solved using a 2D viscoelastic finite element model with thermally activated viscosity to account for thermal weakening of the rock. The temperature distribution around the magma body is calculated based on a 3D steady-state approach and using the mesh-less numerical method. The fault is modeled as a frictionless contact. The simulated distributions of stress and strain around the inclusion display a rotation caused by the shearing component of the applied transtension. The results indicate that the fault tends to overstep the chamber in a geometric pattern similar to a step-over. There is good correspondence between the computed distributions of the maximum shear stress in the vicinity of the magma chamber and the map of earthquake epicenters at a depth of 7–10 km in Coso.