Change In Tectonic Regime Research Articles

Transformation of an Archean craton margin during Proterozoic basin formation and magmatism: The Albany–Fraser Orogen, Western Australia

The Albany–Fraser Orogen is a well-preserved example of Proterozoic modification of an Archean craton margin. The formation of two successive basin systems accompanied by magmatism along the southern and southeastern Yilgarn Craton margin in Western Australia reflect distinct changes in tectonic regimes, resulting in significant transformations of the Archean craton crust. Provenance analysis from detrital zircons indicates that the first basin system – the c. 1815–1600Ma Barren Basin – was dominantly filled with Neoarchean detritus derived from the Yilgarn Craton, and Paleoproterozoic detritus derived from coeval voluminous, mostly felsic magmatism. The abundance of locally derived sediment deposited onto a reworked Archean Yilgarn Craton substrate, coupled with the progressively more juvenile isotopic signature of magmas, indicates a largely extensional tectonic setting, consistent with either a continental rift basin or a long-lived backarc basin system. This extensional regime led to the formation of a passive margin and oceanic basin along the craton edge – the c. 1600–1305Ma Arid Basin – and formation of an ocean-continent transition in the most heavily modified, outboard region of the craton margin (the eastern Nornalup Zone). A change in tectonic regime is denoted by the formation of an oceanic-arc – the c. 1410Ma Loongana Arc, preserved in the adjoining Madura Province. Oceanic subduction led to accretion of the Loongana Arc onto the craton edge, recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin (uppermost Arid Basin). This accretion event triggered crustal thickening and the earliest phase of magmatism of Stage I of the Albany–Fraser Orogeny at c. 1330Ma. A change to west-dipping subduction beneath the accreted portion of the Loongana Arc placed the orogen into a backarc setting, into which the remainder of the Recherche Supersuite mafic and felsic magmas were intruded. Thus, the Albany–Fraser Orogen is regarded as an Archean craton margin that preserves a long history of Proterozoic transformation dominated by extensional processes that resulted in the formation of orogen-wide, basin systems, accompanied by magmatism. The recognition that the Albany–Fraser Orogen has always been part of the West Australia Craton, and the tectonic regimes that have affected it, provide important constraints on reconstructions of the Nuna and Rodinia supercontinents.

Distinct deformational history of two contrasting tectonic domains in the Chinese Altai: Their significance in understanding accretionary orogenic process

The Chinese Altai, a key component of the western Central Asian Orogenic Belt, is considered to be formed through multiple accretions of different terranes. However, the deformational histories of each terrane (tectonic domain), i.e. structural records before and after the accretion, are rarely studied, which has hindered our understanding of the accretionary processes. To fill the gap, a systematic macro- and microscopic structural analysis was carried out on two contrasting litho-tectonic units, i.e. the early Paleozoic low-grade Alegedayi Ophiolitic Complex (AOC) juxtaposed to the high grade Tarlang Granitic Massif (TGM). Selected rock samples were analyzed using zircon U–Pb isotopic dating to constrain the timing of polyphase deformation. Our structural and geochronological data suggest that the two litho-tectonic units were initially detached and located in different crustal levels and experienced distinct phases of deformation under contrasting P-T conditions. They were mutually accreted with each other in the early Devonian and jointly underwent a WNW-ESE-directed shortening deformational event (D1) at ∼390 Ma. The change of tectonic regime was further enhanced by a subsequent NNE-SSW-directed shortening deformation (D2) after ∼ 380 Ma. The shortening process ended before the crustal-scale sinistral strike-slip shearing deformation along the Erqis fault zone at 290 - 240 Ma. Results of this study provide solid field-based evidence for a model that the Chinese Altai initially underwent a nearly E–W-oriented subduction-accretional event in the middle Paleozoic, before it was reoriented to a nearly N–S-oriented convergence.

Geochronology, geochemistry and geodynamic implications of the Late Mesozoic volcanic rocks in the southern Great Xing’an Mountains, NE China

Large-scale Mesozoic volcanisms in the Great Xing’an Mountains (GXM), northeastern (NE) China, are being extensively interested recently. However, the petrogenesis and geodynamic implications of these volcanic rocks are still on controversy. In this study, geochronology and geochemistry of the intermediate and felsic volcanic rocks of Late Mesozoic from the southern GXM was investigated. Zircon U–Pb geochronology yields 206Pb/238U ages ranging from 154.3±1.7Ma to 127.0±2.4Ma for the eight selected volcanic rocks, where the major period of the Mesozoic volcanisms is constrained during the Early Cretaceous. The presented volcanic rocks are andesite–trachydacite–rhyolite and show calc-alkaline to shoshonitic features. They have elevated incompatible trace element concentrations and significantly negative Nb–Ta–Ti and positive Pb anomalies, which display characteristics of typical subduction-related magmas rather than within-plate magmatic signature. Sr–Nd isotopic data indicate that their magma sources were mixed with proportions of old crustal components. Geochemical features, therefore, indicate that the intermediate and felsic shoshonitic magma was plausibly derived from partial melting of the subduction-modified lithospheric mantle and thickened lower crust, respectively, induced by delamination and/or asthenosphere upwelling in extensional setting. Combined with the tectonic evolution of the Great Xing’an area, we propose a hypothesis that the large-scale Mesozoic volcanisms in NE China were controlled by the transition of regional stress from southward to westward, which are induced by the changes of tectonic regimes from the previous Mongol–Okhotsk subduction to the succeeding paleo-Pacific subduction during Late Mesozoic.

Crustal thickening and clay: Controls on O isotope variation in global magmatism and siliciclastic sedimentary rocks

New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100–200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.

Marine connections of Amazonia: Evidence from foraminifera and dinoflagellate cysts (early to middle Miocene, Colombia/Peru)

Species composition in the present-day Amazonian heartland has an imprint of past marine influence. The exact nature, timing and extent of this marine influence, however, are largely unresolved. Here we use calcareous tests of foraminifera and marine palynomorphs from Miocene sediments in northwestern Amazonia to extend on current estimates for salinity ranges, paleoenvironments and paleogeography. Our samples mostly contain tests and/or organic linings of euryhaline (tolerant to a wide range of salinity) foraminifera of the genera Ammonia, Trochammina and Elphidium, with Ammonia being by far the dominant genus at all locations. Organic-walled dinoflagellate cysts (dinocysts), such as Spiniferites, Brigantedinium and Tuberculodinium vancampoae, are also common at a number of sites. This association of foraminifera and dinocyst taxa points at varying salinities, with aberrant forms of Ammonia indicating lower limits of 0–10psu (practical salinity units) whereas dinocyst associations suggest more marine conditions. Such regional heterogeneity is common at the interface of shallow marine to freshwater environments, like estuaries. We conclude that during the early and middle Miocene marginal marine conditions reached at least 2000km inland from the Caribbean portal. Global high sea level and fast subsidence in the sub-Andean zone are thought to be the controlling mechanisms of the marine incursions. Lowering of global sea level and a change in tectonic regime terminated the incursions in the course of the Plio-Pleistocene.

Evolutionary process of Beppu Bay in central Kyushu, Japan: a quantitative study of the basin-forming process controlled by plate convergence modes

This integrated tectonic study reveals the basin-forming and deforming processes on an active margin. Southwest Japan (SWJ) is an island arc under the influence of oblique subduction of the Philippine Sea plate, which has provoked dextral slips on the arc-bisecting Median Tectonic Line (MTL). Plio-/Pleistocene sediments in Beppu Bay, a tectonic depression at the westernmost portion of the MTL, are categorized into lower (5 ~ 0.7 Ma), upper (0.7 ~ 0.3 Ma), and an auxiliary uppermost (0.3 Ma ~ present) unit in ascending order. Detailed seismic interpretation demonstrates that major structures in the deep interior of the basin are an older half-graben under a strong N-S extensional regime and a younger pull-apart sag that developed in a right-stepping part of the MTL as a result of late Quaternary-enhanced strike-slip rates on the fault. Sediments within the pull-apart have been deformed by later inversion events as a contraction phase arose. Conspicuous deformation of the hanging wall of the low-angle detachment of the basin was successfully reproduced by numerical modeling. Based on a discrete element method, this suggests that structural differences in the deformed sedimentary layers are caused by differences in the dip angles of the faults. Remarkable temporal changes in tectonic regimes around Beppu Bay and other areas of SWJ are probably related to transient modes of convergence, including the migration of the Euler pole, of the Philippine Sea plate since ca. 6 Ma.

Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny

AbstractThe Rogart igneous complex is unique within the northern Scottish Caledonides because it comprises an apparent continuum of magma types that records a progressive change in emplacement mechanisms related to large-scale tectonic controls. Syn-D2 leucogranites and late-D2 quartz monzodiorites were emplaced during crustal thickening and focused within the broad zone of ductile deformation associated with the Naver Thrust. In contrast, emplacement of the post-D2 composite central pluton was controlled by development of a steeply dipping dextral shear zone along the Loch Shin Line, interpreted as an anti-Riedel shear within the Great Glen Fault system. The mantle-derived nature of the late-to-post-D2 melts implies that the Naver Thrust and the Loch Shin Line were both crustal-scale structures along which magmas were channelled during deformation. A U–Pb zircon age of 425±1.5 Ma for the outer component of the central pluton provides an upper limit on regional deformation and metamorphism within host Moine rocks. These findings are consistent with the view that a fundamental change in tectonic regime occurred in the Scottish Caledonides at c. 425 Ma, corresponding to the switch from regional thrusting that resulted from the collision of Baltica and Laurentia, to the development of the orogen-parallel Great Glen Fault system.

Double difference relocation of local earthquakes in the Nepal Himalaya

Interseismic strain across the Himalaya is associated with intense microseismic activity. In this study we analyze in detail this seismicity to explore in more details how it relates to Himalayan tectonics. We use the Double Difference Relocation Method to relocate local earthquakes recorded by National Seismological Centre in the period between 1995 and 2003. We also determined fault plane solutions for 10 earthquakes based on waveforms modeling and first P-motion data to complement the existing dataset of focal mechanisms. The results depict a narrow belt of intense seismic activity, at depth between 10 and 20km, which can be traced all along the topographic front of the Higher Himalaya in Nepal. This zone coincides with the zone of interseismic strain build up at the down dip end of the locked portion of the Main Himalayan Thrust fault. These earthquakes probably activate minor faults within the Himalayan wedge, in a volume which experiences stress build up in the interseismic period. The intense seismic activity generally cease abruptly as the elevation of topography gets higher than 3500 m. This elevation also coincides with a change of tectonic regime. Earthquakes within the seismicity belt at front of the Higher Himalaya are of thrust type indicating N-S shortening. Where the topographic elevation is higher than 3500 m focal mechanisms indicate E-W extension. The effect of topography on the regional stress field thus provides a simple explanation for the distribution of focal mechanisms and the seismicity cut-off at the 3500m elevation contour line.

Age and sources of late precambrian sedimentary sequences of the Southern Baikal Region: Results of the U-Pb LA-ICP-MS dating of detrital zircons

The first data on the age of detrital zircons are given for Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation of the southern flank of the Siberian Craton. The ages obtained for 348 zircons cover the Paleoarchean to Late Ediacaran period, demonstrate the dynamics of change of sources of the clastic material in the sedimentation basin, and mark the changes of the Late Precambrian tectonic regimes. The age of the youngest group of detrital zircons extracted from the rocks of the Kachergat Formation allows us to restrict the upper age limit of accumulation of the rocks of the Baikal Group to the Late Ediacaran (Late Vendian).

Mesozoic to recent evolution of intraplate stress fields under multiple remote stresses: The case of Signy Island (South Orkney Microcontinent, Antarctica)

Abstract Palaeostresses inferred from approximately 3200 brittle mesostructures measured at 43 sites in Signy Island show a stress field characterized by compressional, strike-slip and extensional stress states. There is a dominant NW–SE horizontal compression direction as well as three (NNW–SSE, WNW–ESE and ENE–WSW) secondary σ 1 horizontal stress directions. Orientation of σ 3 shows a main mode trending NW–SE together with secondary north–south and NE–SW extensional stress directions. The NNW–SSE horizontal compression is consistent with the stretching–shearing direction defined from the fold-axis trend. This is owing to a subduction-related tectonic regime during early to middle Jurassic times. In middle Jurassic/early Cretaceous times, the tectonic regime changes to extensional with north–south extensional stress directions. The early processes of subduction of the northern sector of the Weddell Sea oceanic crust below the South Orkney Microcontinent, from the Paleocene until the middle Miocene, is related to the σ 1 direction WNW–ESE to NW–SE. The spreading of Powell Basin took place from the late Eocene to the early Miocene and is the cause of east–west and NE–SW extensional processes in the southern South Orkney Microcontinent. Finally, the NW–SE σ 3 direction is related to the current left-lateral strike-slip movement between the Scotia and Antarctic plates.

Did a change in tectonic regime occur between the Phanerozoic and earlier Epochs?

A compilation of palaeogeographical maps of the Neo Proterozoic (Ptz) and Phanerozoic (Pz) 0.6 Ga BP to the present is analyzed in the terms of the (a) ratio between continental to oceanic crust areas (in short continent-to-ocean ratio), (b) length of spreading centres and (c) length of subduction zones. The variation of these tectonic processes is then compared to the variations occurred during the Archean (Arc 3.8–2.5 Ga BP) and Proterozoic (Ptz 2.5–0.54 Ga BP). The discussion takes the geomagnetic field and changes in length of the day (LOD) into account as well. From the constancy of the continent-to-ocean ratio through Pz and from the small size of the continental area above sea level in Neoproterozoic it follows that at the border between Ptz and Pz there has been a large change of the length of the shelf zones. This change can explain contemporary change of the despinning rate from about 0.35 to about 1.79 ms/century. In general, our findings suggest a change in tectonic regime at the border between Ptz and Pz.

Palaeoproterozoic of India: an introduction

A relatively rapid change in the Earth’s surface processes has been anticipated across the Archaean– Palaeoproterozoic boundary as a consequence of changes in the crust–mantle system and tectonic regime (Condie 1989, 1997; Eriksson et al. 2004; Reddy & Evans 2009). The Palaeoproterozoic era (2500–1600 Ma: Plumb 1991) represented perhaps the first supercontinent cycle, from the amalgamation and dispersal of a Neoarchaean supercontinent to the formation of the 1.9–1.8 Ga supercontinent Nuna (Reddy & Evans 2009), and encompasses one or more global tectonic event that coincides with fundamental changes in the integrated system of core, mantle, lithosphere, hydrosphere, atmosphere and biosphere (i.e. an integrated Earth System). An integration of seemingly disparate geoscience disciplines is therefore an essential prerequisite to understand these changes (Reddy & Evans 2009); and that was the aim of the UNESCOIGCP 509 project (2005–2009) on Palaeoproterozoic Supercontinents and Global Evolution. The fifth and final conference and postconference field workshop (in the Singhbhum Craton) related to the UNESCO-IGCP 509 project was organized by the Indian Statistical Institute (ISI), Kolkata with financial support from the UNESCO, the ISI, and the Council of Scientific and Industrial Research, Government of India during the period 26 October–3 November 2009. An entire session was devoted to the Palaeoproterozoic geology of India, as the Indian Shield represents a vast repository of the Palaeoproterozoic geological record. While most of the papers presented in this session were essentially on Palaeoproterozoic geology of the Indian Shield, a few were on the geological record during the Archaean– Palaeoproterozoic and Palaeo-Mesoproterozoic transitions. Some clues to the tectonic regime changes during these transitions also add to our understanding of the Palaeoproterozoic Earth processes, and how they connect with earlier or later periods of Earth’s evolutionary history. A thematic set of 13 papers on various regional as well as broader issues of the Indian Palaeoproterozoic geology and their global context constitute the present Special Publication. Built over the four major amalgamated Archaean nucleii, the major and minor Palaeoproterozoic sedimentary basins and supracrustal sequences in India contain potential information comparable only in scales to those of North America, Africa, Australia and Brazil. The deformation of these supracrustal sequences, attendant metamorphism and emplacement of plutonic bodies hold important clues to their connection with major orogenies, and facilitate investigations into global correlations. The latter has a direct bearing on refining models of Palaeoproterozoic supercontinent assembly and break-up. This volume begins with an overview of the Palaeoproterozoic geological records of the Indian Shield by Saha & Mazumder, wherein the key stratigraphic and tectonic issues are highlighted. Major Palaeoproterozoic sedimentary systems, according to these authors, developed in an intracratonic to marginal-marine setting. In significant contrast to the presence of banded iron formations (BIFs) in the Archaean of Singhbhum and Dharwar, the Indian Palaeoproterozoic supracrustals are devoid of BIFs. This article also provides a brief overview of the fold belts that straddle the suture between major Archaean nuclei, and the attendant tectonic processes. A comprehensive tectonic model for the amalgamation of the major Archaean nuclei of Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand, according to these authors, is yet to emerge and is of paramount importance to further our understanding of Palaeoproterozoic supercontinent development. An updated critical analysis of the Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum Craton and a synthesis thereof has been undertaken by Mazumder et al. According to these authors, the stratigraphic record of the Singhbhum Craton across the Archaean–Palaeoproterozoic boundary implies a changing tectonic scenario. They have pointed out problematic and controversial regional stratigraphic issues that need to be resolved through further geochronological studies in order to understand the relationship between Singhbhum and other cratonic provinces of India, Australia and South Africa. In a complementary paper, Mazumder et al. have presented a critical analysis of the Singhbhum Palaeoproterozoic supracrustals, and compared the northern and southern Singhbhum successions, and their possible relationship with global-scale events and those identified on Kaapvaal. Abbott et al. have reported native Fe, native Si and (Fe, Mn)S

A volcanotectonic survey of Ascraeus Mons, Mars

Ascraeus Mons is one of the largest volcanoes on Mars. It is replete with well‐preserved features that can be used to understand its volcanotectonic evolution. Previous studies of this volcano focused on specific features, and were limited by the quality and coverage of contemporary data. Our objective is to review and enhance the existing developmental model for Ascraeus by considering all endogenic surface features on the volcano. We surveyed the volcano's caldera complex, flank terraces, pit structures, sinuous rilles, arcuate grabens, and small vents. We report the spatial and temporal distributions of these features, appraise their proposed formation mechanisms in light of our mapping results, and propose a detailed geological history for Ascraeus Mons. An initial shield‐building phase was followed by the formation of a summit caldera complex and small parasitic cones, while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks. An eruptive hiatus followed, ending with the construction of expansive rift aprons to the northeast and southwest. Against later, extensive flank resurfacing in the late Amazonian, continued flexure formed arcuate grabens concentric to the edifice. Localized eruption and surface flow of a fluid agent (lava and/or water) from within the volcano then produced a population of rilles on the lower flanks. Finally, in a change of flank tectonic regime from compression to extension, pit crater chains and troughs developed on the main shield and rift aprons, eventually coalescing to form large embayments at the northeast and southwest base of the volcano.

Changes of Late Mesozoic Tectonic Regimes around the Ordos Basin (North China) and their Geodynamic Implications

Abstract:A synthesis is given in this paper on late Mesozoic deformation pattern in the zones around the Ordos Basin based on lithostratigraphic and structural analyses. A relative chronology of the late Mesozoic tectonic stress evolution was established from the field analyses of fault kinematics and constrained by stratigraphic contact relationships. The results show alternation of tectonic compressional and extensional regimes. The Ordos Basin and its surroundings were in weak N‐S to NNE‐SSW extension during the Early to Middle Jurassic, which reactivated E‐W‐trending basement fractures. The tectonic regime changed to a multi‐directional compressional one during the Late Jurassic, which resulted in crustal shortening deformation along the marginal zones of the Ordos Basin. Then it changed to an extensional one during the Early Cretaceous, which rifted the western, northwestern and southeastern margins of the Ordos Basin. A NW‐SE compression occurred during the Late Cretaceous and caused the termination of sedimentation and uplift of the Ordos Basin. This phased evolution of the late Mesozoic tectonic stress regimes and associated deformation pattern around the Ordos Basin best records the changes in regional geodynamic settings in East Asia, from the Early to Middle Jurassic post‐orogenic extension following the Triassic collision between the North and South China Blocks, to the Late Jurassic multi‐directional compressions produced by synchronous convergence of the three plates (the Siberian Plate to the north, Paleo‐Pacific Plate to the east and Lhasa Block to the west) towards the East Asian continent. Early Cretaceous extension might be the response to collapse and lithospheric thinning of the North China Craton.

Magnetic fabric of Late Jurassic arc plutons and kinematics of terrane accretion in the Blue Mountains, northeastern Oregon

The Blue Mountains Province of northeastern Oregon, western Idaho, and southeastern Washington (USA) consists of the amalgamated Wallowa distal island arc, Baker mélange-bearing accretionary wedge–forearc, and Olds Ferry fringing island arc terranes. Anisotropy of magnetic susceptibility (AMS) and particularly the orientation of magnetic lineations in Middle to Late Jurassic diorite–tonalite–granodiorite plutons intruding the Baker terrane indicate a change in tectonic regime from flattening with principal stretching at a high angle to the local orientation of the terrane boundaries at around 160Ma to constriction with boundary-subparallel subhorizontal stretching at ca. 146Ma. The former magnetic fabric is compatible with magnetic lineations recording strain related to top-to-the-southwest back-thrusting onto the Olds Ferry arc. In contrast, the ~146Ma boundary-parallel stretching may record strain partitioning whereby a part of the Baker terrane was extruded laterally parallel to terrane boundaries after the terrane collision. After restoration for the clockwise post-mid-Cretaceous terrane rotation, the ~160Ma restored principal stretching directions are compatible with the overall left-oblique subduction and convergence of the Wallowa distal and Olds Ferry fringing arc terranes. Magnetic fabric data from the plutons thus support the interpretation that the early displacement of the Blue Mountains Superterrane, and perhaps also its possible correlative Intermontane Superterrane of the Canadian Cordillera, was southward along the truncated North American continental margin during the late Middle Jurassic.

Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan

AbstractU–Pb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LA–MC–ICP–MS U–Pb lower intercept cooling ages of 10.1 ± 0.4, 10.8 ± 0.1 and 10.0 ± 0.3 Ma, respectively. Numerical finite‐difference diffusion models constrained by previously published temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ∼800 °C at 14 Ma in the granulite‐bearing unit and ∼650 °C at 12 Ma in the amphibolite‐bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation‐related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile‐bearing samples. Instead, the titanite age suggests cooling from ∼650 °C at an earlier time of 17–15 Ma, implying that the high‐grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high‐grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 °C Ma−1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along‐strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.

Plio–Quaternary tectonic regime changes in the transition zone between Alborz and Kopeh Dagh mountain ranges (NE Iran)

This paper is concerned with the changes in the Plio–Quaternary stress states in NE Iran and provides evidence for regional active deformation across a wide transition zone between the Alborz and Kopeh Dagh mountains. The Arabia–Eurasia convergence is involved in the strike–slip faulting along NE-trending left-lateral faults in the eastern Alborz as well as NW-trending right-lateral faults in the western Kopeh Dagh. The inversion result of fault kinematic data (slip-vector measurement of fault plane and focal mechanisms) strongly indicates an active transpressional tectonic regime in the study area. We provide evidence for drastic temporal changes in the stress state during the Plio–Quaternary based on the inversions of fault kinematic data. A regional transpressional tectonic regime with a mean N036 ± 20°E trending horizontal σ 1 is representative for the modern stress state. The older state reveals compressional to transpressional tectonic regimes comprising a N135 ± 20°E trending horizontal σ 1. The modern transpressional tectonic regime characterized by a regional NE-trending σ 1 is consistent with the inversion of earthquake focal mechanisms that yield a N47°E trending σ 1 for the present day stress state. The kinematic results are significantly homogeneous and mechanically compatible with the active geological structures, in the way that reverse and strike–slip faulting are coherent with a single σ 1 direction.

A chronostratigraphy for the Dinaride Lake System deposits of the Livno-Tomislavgrad Basin: the rise and fall of a long-lived lacustrine environment

The Dinarides are an integral part of the Alpine orogenic belt and stretch over large parts of Slovenia, Croatia, Bosnia-Herzegovina, Monte Negro and Serbia. A great number of intra-montane basins formed in the interior of this Late Eocene to EarlyOligocene orogen during the Miocene. These basins harbored a suite of long-lived lakes, collectively called the Dinaride Lake System (DLS). Lake Livno, with its 600km2 of preserved surface, was the second largest of these Dinaride Lakes. At present, its deposits are divided between the Livno and Tomislavgrad basins, which were part of a single basin when Lake Livno first formed. High resolution age constraints for the over 2km basin infill have been lacking up to now, partly due to the endemic nature of its lacustrine fauna. This severely hampered geodynamic as well as paleoenvironmental reconstructions. Here, we present a chronostratigraphy based on radioisotopic and magnetostratigraphic data. 40Ar/39Ar measurements reveal that the Tušnica volcanic ash, found in between the Gomphoteriumbearing coal seams at the base of the basin infill, is 17.00±0.17 Ma old. 40Ar/39Ar dating of the Mandek ash, correlative to the uppermost sedimentary unit, provides an age of 14.68±0.16 Ma. Correlation of the composite magnetostratigraphy for the main lacustrine depositional phase to the Astronomically Tuned Neogene Time Scale is straightforward and reveals that the majority of the deposits of Lake Livno accumulated between 17 Ma and approximately 13 Ma. The disappearance of Lake Livno is most likely attributable to a change in tectonic regime. Calcarenites and breccias, derived from the basin margins, first entered the lake around 14.8 Ma and subsequently coarsened and thickened upwards. The basinmargins were apparently gradually uplifted before subsidence stalled. Comparison with chronostratigraphic data for other constituents of the DLS leads to the conclusion that their lifetimes largely coincide. Finally, we calibrate the most important marker fossils of the various Dinaride basins to the geological time scale and we present a new biochronological scheme for the DLS.

Neoproterozoic SHRIMP U–Pb zircon ages of silica-rich Dokhan Volcanics in the North Eastern Desert, Egypt

Chronology of Neoproterozoic volcanosedimentary successions remains controversial for many regions of the Arabian–Nubian Shield, including the Dokhan Volcanics of NE Egypt. New U–Pb zircon SHRIMP ages have been obtained for 10 silica-rich ignimbrites and two subvolcanic dacitic bodies, mapped as Dokhan Volcanics, from the North Eastern Desert of Egypt. Crystallization ages range between 592 ± 5 and 630 ± 6 Ma (Early Ediacaran). Apparently, the late consolidation of the Arabian–Nubian Shield was accompanied by the evolution of isolated volcanic centres and basin systems which developed during a period of approx. 40 Ma, independently in space and time and probably under changing tectonic regimes. The obtained age data together with other previously published reliable ages for Dokhan Volcanics suggest two main pulses of volcanic activity: 630–623 Ma and 618–592 Ma. Five samples contain inherited zircons, with ages of 669, 715–746, 847 and 1530 Ma, supporting models that North Eastern Desert crust is mainly juvenile Neoproterozoic crust.

The Early Jurassic tectono-magmatic events in southern Jiangxi and northern Guangdong provinces, SE China: Constraints from the SHRIMP zircon U–Pb dating

The Jurassic to Cretaceous magmatism in the South China Block (SCB) has important geological significance for its tectonic evolution, particularly, the tectonic regime change. We report here new SHRIMP zircon U–Pb ages, geochemical and Sr–Nd isotopic results from the Early Jurassic magmatic rocks in eastern Nanling region of southern Jiangxi and northern Guangdong provinces, SE China. The SHRIMP zircon U–Pb analyses yield consistent Early Jurassic ages of 193 ± 2 Ma for granodiorite and 196 ± 1 Ma for gabbro-diabase from the Hanhu complex of southern Jiangxi Province. Field observation indicates that the gabbro-diabase coexists with syenite in granodiorite, and that the mafic components infiltrate into the syenite. Combined with the SHRIMP zircon U–Pb ages of 196 ± 2 Ma for granites and 195 ± 1 Ma for gabbros of the Xialan complex of northern Guangdong province, these contemporary ages represent an earliest tectono-magmatic events during the Yanshanian orogeny. These results reveal that there are tectonic extension and the post-collisional and intraplate magmatism events at or before 196 Ma, in contrast to the wide believed quiescent stage (205–180 Ma) of magmatic activity in SE China. The confirmation of these tectono-magmatic events in the eastern Nanling area thus sheds new light on the quiescent stage of magmatic activity in SE China during 205–180 Ma and provides new constraints to understand the Early Jurassic tectonic evolution of SE China.