Strike-slip Deformation Research Articles

Synrift basin inversion: Significant role of synchronous strike-slip motion in a rift basin

Abstract In rift basins with superposed strike-slip deformation, the structural style of wrench elements and the roles they play in synrift architecture and evolution are important, poorly understood issues for basin analysis and hydrocarbon exploration. The NE-SW–striking Tan-Lu fault zone, located in eastern China, runs through the Liaodong Bay subbasin within the Cenozoic Bohai Bay Basin and experienced dextral strike-slip motion during the later synrift stage of the basin (ca. 40 Ma to 23 Ma). Investigations of the Liaodong Bay subbasin indicate that rift-fault reactivation and wrench-fault development during strike-slip reactivation were strongly controlled by the distribution and geometry of preexisting rift faults, and local synrift basin inversion, induced by strike-slip reactivation of a preexisting graben during a later synrift stage, was a significant manifestation of synchronous strike-slip motion modifying synrift architecture and evolution. Moreover, synrift basin inversion within the Liaodong Bay subbasin manifested in two ways. First, stronger inversion occurred along the restraining bends of preexisting extensional faults. This induced uplift of the footwalls of graben-controlling faults, leading to deformation characterized by abundant shortcut thrusts and folds. The Liaodong uplift formed via this mechanism, triggered by strike-slip movement along the Tan-Lu fault zone at ca. 40 Ma. Second, weaker inversion induced by newly formed, subvertical, strike-slip faults occurred near the central part of the graben, with the characteristics of positive flower structures. Although inversion was limited to a very local area along a narrow fault zone, it substantially modified the basin’s physiography. In this rift system, coincident with local inversion-induced uplift, large-scale, rift-related subsidence occurred beyond the inversion belt within the flanking graben, leading to complexity and variety in intrabasinal structural deformation and filling, and exerting a complex influence on hydrocarbon prospects. This model of synrift basin inversion has profound implications for the interpretation of inversion structures and basin dynamics in any rift basin with superposed strike-slip deformation.

Carboniferous arc-related volcanism in SW Bogda Mountain, Northwest China, and its implications for regional tectonics

The Upper Carboniferous of Bogda Mountain is a critical area for understanding final oceanic closure of the Junggar Ocean in the Central Asian Orogenic Belt (CAOB). The 4 km thick Liushugou and Qijiaojing formations, comprising tuffs and lesser lavas that accumulated in a subsiding marine basin, were studied to understand the architecture and petrogenesis of the volcanic succession. Tuffs, lavas and hypabyssal intrusions are mostly of calc-alkaline composition, ranging from basalt through andesite and dacite to rhyolite, interbedded with rare high-K rocks of basaltic trachyandesite to trachyte composition. Clinopyroxene and FeTi oxides are the main fractionating minerals in the calc-alkaline rocks. Plagioclase has altered to albite by sea-floor metasomatism. The rocks are enriched in LILE and show relative depletion in Ta and Nb, suggesting an arc-related origin. The tuffs and lavas pass northward into dominantly trachytic rocks, suggesting a northward subducting, intra-oceanic arc to back-arc system, with polarity contrary to previous literature, that was active from the time of the Kalamaili arc collision (~350 Ma) to shortly after the North Tianshan–Junggar collision (~315 Ma). Following that collision, West Junggar experienced sinistral strike-slip deformation, and related crustal scale faults in Bogda Mountain allowed the rise of basaltic magma, forming pillow lavas and hyaloclastites in a deepening basin. Finally, in the latest Carboniferous, the subduction-related volcanism ended and the Bogda basin was then onlapped by turbidites. The recognition of northward subduction in the Bogda segment of the CAOB has regional implications for final closure of the Junggar Ocean. Comparison with published data from Kalamaili, the Junggar Basin, West Junggar and Arbassy demonstrates three tectonic phases with different volcanism, sedimentation and deformation in different parts of the amalgamating orogen.

Detrital zircon U-Pb-Hf isotope signatures of Old Red Sandstone strata constrain the Silurian to Devonian paleogeography, tectonics, and crustal evolution of the Svalbard Caledonides

Abstract Detrital zircon provenance studies of Mesoproterozoic basement and overlying Old Red Sandstone strata in northwestern Svalbard, Arctic Norway, were conducted to test competing models for Caledonian paleogeography and tectonics and constrain the magnitude of orogen-parallel, Silurian to Devonian strike-slip faulting following the Laurentia-Baltica collision. Mesoproterozoic basement strata, cut by earliest Tonian orthogneiss units, mostly yielded 1640–1050 Ma detrital zircon populations that are consistent with pre-Caledonian locations near northeast Greenland. Basal Old Red Sandstone deposits that filled pull-apart basins showed basement-derived signatures but also contained 530–450 Ma and 670–570 Ma populations with slightly subchondritic (ε Hf[t] = –4 to –2) Hf isotope compositions. These results are consistent with late Silurian–Early Devonian proximity to the northeast Greenland Caledonides and Pearya, which indicates limited (<200 km) strike-slip displacement of Svalbard’s Caledonian allochthons after the Laurentia-Baltica collision. Previously interpreted connections between the Svalbard Old Red Sandstone and British Caledonides are incompatible with these detrital zircon results. Lochkovian Old Red Sandstone strata were deposited after a second episode of strike-slip faulting and show recycled basement signatures. The lack of 530–450 Ma and 670–570 Ma populations suggests that the second deformation episode reorganized local drainages. Pragian–Givetian strata have provenance from local Old Red Sandstone sources that were uplifted during a third and final episode of strike-slip deformation. The results indicate that northern Caledonian (Svalbard, Pearya) crustal evolution was characterized by the reworking of Mesoproterozoic–Paleoproterozoic sources and mostly <600 m.y. crustal residence times, whereas the southern Caledonides (UK, Ireland) show evidence for the reworking of older basement and mostly >600 m.y. crustal residence times.

Geomorphic indices and longitudinal profile of the Daba Shan, northeastern Sichuan Basin: Evidence for the late Cenozoic eastward growth of the Tibetan Plateau

The Daba Shan Orocline is located in the northeastern Sichuan Basin, and has undergone foreland propagation and distinct surface uplift owing to the eastward expansion of the Tibetan Plateau since the late Cenozoic. In order to describe and quantify the earth surface processes respond to deep geological mechanisms, and to detect the kind of tectonic deformation in the Daba Shan area during the late Cenozoic, various geomorphic indices were analyzed for study area. The obtained results are: (1) The northeast of the Hanzhong Basin, the Hannan-Micang Massifs, along the Chengkou-Fangxian fault, and the Shennongjia Massif exhibit intense tectonic activity. (2) The spatial patterns of the channel steepness indices indicate that the western segment of the northern Daba Shan (Fenghuang Shan and Paotaizi region) have higher uplift rates than the eastern. (3) The longitudinal profile analysis revealed that the landscape evolution of the Shennongjia Massif is in a transient state, with an uplift amplitude of 1700–1900 m since the end of the Paleogene (25 Ma), and an average uplift rate of 0.072 ± 0.004 mm/a. The paleochannel reconstruction indicate an incision of ~900–1000 m since the late Neogene (3.0–3.4 Ma), with the average uplift rate being 0.3 ± 0.03 mm/a. (4) Three tectonic deformation zones can be distinguished in the Daba Shan (from west to east), the shortening deformation, strike-slip deformation, and vertical uplift zones. These observations suggest that the eastward growth of the Tibetan Plateau has affected the Daba Shan area and shaped the modern landscape.

Age of metamorphism and deformation in the Montagne Noire dome (French Massif Central): Tapping into the memory of fine-grained gneisses using monazite U-Th-Pb geochronology

Recent work has revealed that the Montagne Noire dome, located in the foreland of the Variscan belt (French Massif Central), contains a record of nearly coeval, late Variscan eclogitization and migmatization. Given these new results, it is important to understand the chronology of events that produced high-grade metamorphism and exhumation. Using U-Th-Pb dating of monazite, this study confirms ages of ~315–300 Ma for high-T metamorphism in the augen gneiss that makes up a large fraction of the Montagne Noire dome, and documents for the first time ~295 Ma monazite ages in compositionally varied fine-grained gneisses that form folded continuous layers within the core of the dome. The fine-grained gneiss layers are intensely sheared and are interpreted to have localized late, high-T deformation in the core of the dome. These sheared fine-grained gneisses form a network of shear zones that were kinematically linked to the extensional and strike-slip deformation zones that exhumed the Montagne Noire dome in a pull-apart (s.l.) domain. Continued deformation-recrystallization and fluid flow within these shear zones likely drove rejuvenation of monazite for ~5 million years after much of the melt had crystallized in the Montagne Noire dome.

On the Problem of Distinction between Recent Geodynamics of Central and East Asia

Abstract—The Central Asian and East Asian transitional zones are distinguished in Central and East Asia south of the Late Precambrian–Paleozoic fold belts of the Southern Russia and Kazakhstan and east of the Siberian Platform which are parts of the North Eurasian lithospheric plate. These transitional zones separate the plate from the Indian, Australian, and Pacific plates. The Central Asian zone is presently dominated by tectonic transpression accompanied by the thrusts and strike-slip deformations extensively developed under the influence of the Hindustan indenter pressure. The East Asian zone is mainly controlled by transtension with extensive shear strain processes associated with the deep submergence and, apparently, gradual disintegration and disappearance of the Pacific slab at a depth. This regime can also be connected with the ascending mantle plumes beneath Transbaikalia and North Mongolia. The Cenozoic rift structures and large depressions prevail in the East Asian transition zone. The difference in the geodynamic regimes in the transitional zones is demonstrated by seismic data and by the analysis of geophysical fields and anomalies in their deep structure. The boundary between the zones approximately coincides with the extended submeridional 102°–104° E geodivider which is identified based on numerous geological and tectonic manifestations in the North, Central, and South Asia. In the central part, the geodivider is supported by the data on recent crustal block structure, seismicity, active faults, and deep structure anomalies.

Processes and deformation rates generating seismicity in metropolitan France and conterminous Western Europe

Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extensionca.0.5 mm yr−1in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.

Magmatic-hydrothermal evolution of rare metal pegmatites from the Mesoproterozoic Orange River pegmatite belt (Namaqualand, South Africa)

The 450 km long Orange River pegmatite belt intruded the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Province, Southern Africa, at ca. 1 Ga. The western part of the belt is characterized by the occurrence of LCT pegmatites locally mineralized in Li, Ta, Nb, Be and Bi. Most of these mineralized pegmatites display zonation with a quartz-feldspar-muscovite-garnet-beryl-bearing aplite border and wall zone, a quartz-feldspar-spodumene-lepidolite-bearing intermediate zone, and a quartz-K-feldspar core. The pegmatites and their granodioritic country rocks commonly show evidence of albitization and greisenization (i.e. secondary muscovitization). In this study, detailed petrographic observations and the compositions of mica, feldspar, spodumene and Nb-Ta oxide minerals are reported. This information is combined with bulk country rock compositions and new U-Pb zircon and monazite ages plus Sm-Nd isotope compositions of monazite to constrain the origin and magmatic-hydrothermal evolution of weakly and strongly mineralized LCT pegmatites.The Li-Ta-Be Kokerboomrand I pegmatite emplaced at ca. 985 Ma during the main stage of pegmatite emplacement in the Orange River belt and regional strike-slip deformation. It has εNd(t)monazite = −14 ± 3 and potentially formed by partial melting of Paleoproterozoic rocks from the Richtersveld magmatic arc. Alternatively, Mesoproterozoic sediments of the Bushmanland Subprovince may also provide a suitable magma source for the Kokerboomrand I pegmatite.The rare metal contents of muscovite and columbite group minerals (CGM) increase from the weakly mineralized to strongly mineralized pegmatites, suggesting early-stage fractional crystallization or variable partial melting conditions. Mica Nb/Ta values of <14 and K/Rb ratios of <40 can be used as geochemical tools for Li-Ta-(Nb)-Be exploration in the region, and possibly in other pegmatite belts worldwide.Rare metal-bearing minerals crystallized throughout the magmatic-hydrothermal evolution of mineralized pegmatites. During the magmatic-dominated stage I, constitutional zone refining likely induced a decrease of spodumene Fe contents and increase of CGM Ta contents toward the pegmatite core. Stage II marks the pervasive circulation of an immiscible hydrosaline melt. This stage is characterized by the replacement of the primary mineral assemblage by lamellar albite (cleavelandite), (fluor)calciomicrolite and Cs-Ta-rich Li-muscovite and zinnwaldite. Stage III represents the progressive transition from a magmatic-hydrothermal system dominated by a Li-Cs-Ta-rich hydrosaline melt toward a system mainly involving (Li)-Cs-(Ta)-Bi-rich aqueous fluids. This stage is characterized by the crystallization of lepidolite and zero-valence-dominant pyrochlore as well as strong localized greisenization and Bi mineralization.Magmatic-hydrothermal alteration of the immediate country rock of the mineralized pegmatites is marked by the crystallization of albite, Li-Ta-Cs-rich mica, various Nb-Ta oxide minerals, and led to a strong decrease of the whole-rock Nb/Ta ratio plus significant rare metal enrichment.Our findings from the Orange River belt demonstrate that an important role can be ascribed to melt-melt and melt-fluid immiscibility and metasomatism in the formation of pegmatite-related rare metal deposits. Greisens and albitites associated with pegmatites and their immediate country rocks can be considered as favorable targets for Li-Ta mineral exploration. Strong and pervasive metasomatism across the mineralized pegmatites and their country rocks suggests involvement of a parental medium that was highly enriched in water.

Modelling the influence of pre-existing brittle fabrics on the development and architecture pull-apart basins

We use new analogue modelling experiments to analyse the development of pull-apart basins in an upper crust characterised by the presence of pre-existing discrete fabrics. As in previous models, lateral movement of rigid basal plates induced strike-slip deformation of a sand-pack. Local extension allowing the formation of a pull-apart basin was produced by the step-over geometry of the master faults; in this area, a basal silicone layer was introduced to distribute deformation and reproduced a weaker crust in the basin. Conditions of neutral, overlapping and underlapping interacting master faults were reproduced. The model upper crust, modelled by a sand mixture, was characterised by the presence of pre-existing structures; the orientation of these inherited heterogeneities was systematically varied in different experiments. Model results indicate that, depending on their orientation with respect to the strike-slip displacement, pre-existing structures can be reactivated both within and at the margins of the pull-apart basins. Inside the basin, reactivation occurs when the pre-existing structures are orthogonal or sub-orthogonal to the strike-slip displacement; in this case, the pre-existing fabrics delay the development and linkage of cross-basin faults and increase the complexity of the deformation pattern giving rise to a new set of faults characterised by an atypical trend. Pre-existing fabrics oblique to the local extension direction may be partly reactivated in the central part of the basin as segments of cross-basin faults. At the margins of the pull-apart, reactivation occurs if the fabrics spatially coincide with the lateral boundaries of the silicone layer. In these conditions, reactivation allows a faster development of the border faults, which are less segmented than in the homogenous models; this also results in a more regular final geometry of the pull-apart.

Late tectonic reorientation of lineaments and fabrics in the northern Eastern Ghats Province, India: Evaluating the role of the Mahanadi Shear Zone

Abstract South of the MSZ that cuts across the Eastern Ghats Province (EGP) of India, lineament trends and early granulite facies fabrics rotate from NNE-SSW in the south towards the WNW-ESE striking, northerly dipping Mahanadi Shear Zone (MSZ) in the north. Using field, microstructural and EBSD techniques, we demonstrate that D1 and D2 planar fabrics are reoriented by an E-W trending, northerly dipping D3 extensional shear zone that has a subordinate sinistral, strike-slip component. D3 microstructures are pervasive, with quartz and feldspar microstructures characteristic of amphibolite facies metamorphism. Monazite chemical ages from the D3 shear zone yield ages in the range 730–680 Ma. The MSZ, representing D4 deformation in the area, is a predominantly dextral strike-slip deformation. During D4, microstructures evolve exclusively through plastic deformation of quartz and culminates in S4 mylonitic fabric formation in high D4 strains zones under greenschist facies conditions. These microstructures are restricted to the MSZ and localized high D4 strain zones, unlike the D3 microstructures. We conclude that the earlier, post-granulite extensional deformation D3, and not the MSZ, was responsible for the swing in lineament and fabric trends in the northern EGP.

Kinematic and thermal evolution of the Haymana Basin, a fore-arc to foreland basin in Central Anatolia (Turkey)

Gondwana (Tauride/Kırşehir blocks) and Eurasia (Pontides) derived continental blocks delimit the Haymana basin, central Turkey, to the south and the north, respectively. The boundaries of these blocks define the Izmir-Ankara-Erzincan and Intra-Tauride Suture zones which are straddled by a number of Late Cretaceous to Oligocene marine to continental basins. The Haymana Basin is located at the junction of the IAESZ and ITSZ and comprises Upper Cretaceous to Middle Eocene basin infill deposited in response to the interaction of these blocks. The basin provides a unique opportunity to unravel spatio-temporal relationships related to the timing of late stage subduction history of Neo-Tethys Ocean and subsequent collision of the intervening continental blocks. We have conducted a multidisciplinary study in the region that includes mapping of major structures combined with fault kinematic analyses. E-W striking folds dominate the basin, cross-section balancing of these structures indicates around 25% roughly N-S shortening in the region. Paleostress studies indicate that the basin was initially subjected to N-S to NNE-SSW extension until the middle Paleocene (phase 1) and then N-S directed syn-depositional compression and coeval E-W directed extension until the middle Miocene (phase 2) implying strike-slip deformation and pure shear shortening in the basin. These different deformation phases are attributed to first fore-arc (subduction) basin development then foreland (collision) stages of the basin. Apatite (U-Th)/He dating of 5 samples indicate that exhumation of the SE segment of the basin started in early Oligocene, whereas the NW segment of the basin exhumed in the early Miocene. The differential uplift is possibly related to progressive north-westwards movement of Dereköy basin bounding fault at the north. We propose that the Haymana basin evolved from extensional forearc basin during the late Cretaceous to early Paleocene and foreland basin after the terminal subduction and subsequent collision of Tauride and Pontide blocks.

Structure and kinematic evolution of the Southern Sivas Fold-Thrust Belt, Sivas Basin, Central Anatolia, Turkey

The architecture, kinematics, and timing of fold-and-thrust belt development provide essential first-order constraints on the structural evolution of continental collision zones. The southern Sivas fold-thrust belt (SSFTB) is a ~300 × 50-km-long, ENEtrending contractional belt located in the Sivas Basin at the western end of the Arabia-Eurasia collision zone, and along the Izmir- Ankara-Erzincan and Inner Tauride suture zones. We present new geologic mapping and the first detailed structural characterization and seismotectonic evidence of active deformation along the western and central segments of the SSFTB that provide new insights into the timing and kinematics of deformation associated with collision and escape in Central Anatolia. The SSFTB is a dominantly north-vergent contractional belt that initiated during the late Eocene and experienced several punctuated episodes of shortening and exhumation, primarily during the late Oligocene and late Miocene. Structural mapping and balanced cross-sections reveal that minimum cumulative shortening increases from west (29%) to east (34%) along the belt, along with a gradual eastward increase in fold asymmetry and a progressive transition from fold-related to overturned fold-and-thrust-related shortening. The kinematics of the central and eastern SSFTB changed from north-vergent thrusting during the late Eocene and Oligocene to predominantly south-vergent during the Miocene. Contraction in the SSFTB ended by ~7-6 Ma and was followed by a switch to distributed and kinematically linked sinistral strike-slip and normal faulting along the northeastern segment of the Central Anatolian fault zone. Offset strain markers suggest a total of at least 1.7-2.4 km of left-lateral slip on the Deliler fault and ~940 m of normal slip on the Hinzir fault. The latest Miocene to Pliocene switch to intraplate strike-slip deformation marks a regional transition to westward tectonic escape of the Anatolian microplate.

Horizontally deplacement geomasses in the continental rift geostruсtures (on the example of the Dnipro-Donets paleorift). Part 2. Structural parageneses of the reid deformation of the sedimentary cover

Formulation of the problem. The inversion stages of the structural-material evolution of the continental crust are characterized by regional scale of the deformation of volume tectonic flow of platform sedimentary rocks, which caused significant horizontally-healthy movements of tectonically activated geo-mass in the intra-plate graben-rift. The dynamic deformation of geomases manifests itself at the final stages of the geological development of such structures; therefore, it determines the main features of the systemic organization of the modern architecture of the basement and sedimentary cover complexes. Review of previous publications and studies. Secondary deformation linear slope, according to Patalakha E. (1979); Alekseev V. (1990) is considered as a set of local strike-slip zones, formed on spherically-located viscous faults, forming joint tectonic flows. The structural skeleton of tectonic flows, as the basis of the tectonic dislocation process, make offsets with the horizontally and rotational component movements. Horizontal landslides are typical disjunctive elements of the continental structures formed in the geodynamic conditions of the transtension (grabens, rifts), transpression (folded mobile belts, intra-plate activation zones), as well as the basic structural elements of the strike-slip tectonics of the sedimentary basin. The purpose of the article. The second part of the trilogy continues regional geotectonic studies of post-rift complications of the sedimentary cover structure of Dnipro-Donets Paleorift (DDP), covering the three main stages of the platform tectonic activation. Late Hercynian epoch was characterized by the formation of large linear anticlinal zones and salt-shafts against the background of the general syneclizal deflection of sedimentary basin, located within the paleorift. Cimmerian epoch of tectonic activity led to significant thrust deformations of the primary structural forms of the cover in the geodynamic setting of collision compression with intensification of the horizons of the sedimentary cover. The structural manifestation of strike-slip tectonics at Alpine epoch in sedimentary complexes are mainly horizontal displacements of geomases of blocks, lineaments, local tectonic elements and structures along dynamically interconnected coulisse of strike-slip domains. Methods. Structural kinematic analysis of transformations of the fracture systems of the Upper-Visean sedimentary complex under the influence of natural mechanism of reid tectonic flow of rocks. Results. The secondary structures of strike-slip deformation, structural kinematic parageneses, dislocations zones and subregional structural waves were identified in the structure of the platform cover, which are natural geological objects formed at the inversion stages of geological development of DDP. Scientific novelty and practical significance. The result of structural manifestation of volume tectonic flow in the platform complex was horizontal displacements of the original tectonic elements, blocks, segments, structures along dynamically-linked strike-slip domains. This is caused by the formation of secondary deformation structures of various scales, morphology and genesis, which comprise longitudinal structural waves of strips of post-rift deformations in the structure of the sedimentary cover of DDP.

Two new microscopic ductile kinematic indicators from the Oman Mountains

Two novel shear-sense indicators were observed in different tectonic settings. The first one (“PDT”) occurs in a calcmylonite, associated with extension. The second one (“Dragged Intergrowths”) was discovered in mylonitized plagiogranite (trondhjemite) dikes with vertical to sub-vertical foliation and strike-slip deformation within harzburgite.The calcite mylonite displays shear planes, characterized by parallel laminae which differ in composition. They contain light-colored, calcite, while darker impurities consist of hematite, goethite, and rutile. PDTs are associated with microthrusts. The thrusts form an acute angle with the mylonitic foliation. Thrusting between two microthrusts created pull-aparts, filled with drusy calcite. The shear sense of the PDTs is revealed by (1) shortening and related imbrication of the thrust laminae, (2) pull-apart structures and (3) drag folds at either end of the microthrusts. The indicated shear is confirmed by independent evidences. One PDT is up to ∼0.5 mm wide and 1 mm long.The mylonitized plagiogranite contains a K-feldspar δ-porphyroclast (2 mm long), featuring dragged K-feldspar/quartz intergrowth due to rotation during high-temperature conditions. Bending occurs on either end of the object. The original orientation of the intergrowth was subparallel to the foliation. As a result an “S”-shaped object formed. The deduced shear sense is independently confirmed.

Cenozoic cooling history of the North Qilian Shan, northern Tibetan Plateau, and the initiation of the Haiyuan fault: Constraints from apatite- and zircon-fission track thermochronology

The growth of the Tibetan Plateau resulted primarily from India-Asia convergence since initial collision at 65–55 Ma. The Cenozoic Qilian Shan thrust belt and the left-slip strike Haiyuan fault system together define the northeastern margin of the plateau. Although these thrust and strike-slip fault systems play an important role in accommodating continental convergence, our knowledge of the temporal and spatial distribution of upward and outward growth of the northeastern Tibetan Plateau is still lacking. In this study, we integrate new geologic mapping and low-temperature themochronometry (apatite- and zircon-fission-track ages) to provide constraints on the uplift and cooling history of the North Qilian Shan and the initiation of the Haiyuan fault. Cooling ages and thermal history modeling from a traverse across a prominent restraining bend on the Haiyuan fault suggest the North Qilian Shan experienced a three-phase cooling history, including: (1) broad cooling during the Late Triassic to early Cenozoic, (2) long-term tectonic quiescence from late Cretaceous to middle Miocene, and (3) rapid cooling and exhumation from ~15–10 Ma to present. We relate this most recent local cooling (~15–10 Ma) to the initiation of strike-slip faulting along the central Haiyuan fault, which corroborates other recent studies, suggesting a middle Miocene activation of strike-slip deformation along the Haiyuan fault in northern Tibet.

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.

Paleogene evolution of the Burmese forearc basin and implications for the history of India-Asia convergence

The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene – Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin - the northern part of the Burmese forearc - and integrate these results with previous data from other basins in central Myanmar. Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late Middle Eocene, with a forearc basin open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar was formed 39-37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern precollisional paleogeography for the Indochina Peninsula, and indicate that any postcollisional rotation of Indochina, if it occurred at all, must have been achieved by the late Middle Eocene.

An Ediacaran back-arc basin preserved in the Transversal Zone of the Borborema Province: Evidence from geochemistry, geochronology and isotope systematics of the Ipueirinha Group, NE Brazil

The Ipueirinha Group is a ca. 75 km-long by 13 km-wide succession of metavolcano-sedimentary rocks situated in the westernmost Transversal Zone, Borborema Province, NE Brazil. It is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites, local metaultramafites, rhyolitic metavolcaniclastic rocks and rare lenses of metamarl. The metavolcaniclastic rocks show highly fractionated chondrite-normalized REE patterns (LaN/YbN = 3.68–8.94) with slight to moderately negative Eu anomalies (Eu/Eu* = 0.72–0.94), incompatible trace element signatures similar to those of volcanic arc-related rocks, including a Nb-Ta trough, and negative ƐNd(t) ranging from −3.4 to −12.2. Metarhythmites show LREE enrichment (LaN/YbN = 4.83–15.32), moderate Eu anomalies (Eu/Eu* = 0.68–0.81), and incompatible element signatures consistent with provenance from a continental arc. Nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites, as reflected by the ƐNd(t) values between −3.6 and 0.3. Detrital zircon LA-ICP-MS U-Pb data reveal that the main sources of detritus for the metarhythmites are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and younger zircons at ca. 636 Ma). Hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (ƐHf(t) = 2.0–5.7), and suggest that the Neoproterozoic crustal evolution of this province involved mixing between older sources with negative ƐHf(t) and younger juvenile sources. Metaultramafites associated with the Ipueirinha Group show ƐNd(t) ranging from −1.3 to −8.5 and TDM = 1.33–1.64 Ga. The whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data obtained for a granitic sill (578 ± 13 Ma; zircon U-Pb) along with monazite U-Pb data from a chloritite sample (595 ± 29 Ma) indicate that strike-slip deformation, metamorphism and syn-orogenic melt generation coexisted in the Ipueirinha Group during the Late Ediacaran. We suggest that the Ipueirinha Group comprises a Cryogenian/Ediacaran back-arc basin to the Tamboril-Santa Quitéria magmatic arc, which is one of the few records of initial convergence tectonics in the West Gondwana margin of the Borborema Province.

Polyphase rifting and break-up of the central Mozambique margin

Abstract The break-up of the Gondwana supercontinent resulted in the formation of the Central Mozambique passive margin as Africa and Antarctica were separated during the mid-Jurassic period. Although plate kinematics during the oceanic spreading phase are well constrained, the initial fit of Africa and Antarctica, their earliest relative movements and margin architectures remain active areas of interest. This study uses high quality multi-channel seismic reflection profiles to identify the major crustal domains in the Angoche and Beira regions of the Central Mozambique margin. Our results show that the Central Mozambique passive margin is characterised by intense but localised magmatic activity, evidenced by the existence of seaward dipping reflectors (SDRs) in the Angoche region, and magmatic sills and volcanoclastic material marking the Beira High. The Angoche and Beira regions possess faulted upper-continental crusts, with possible exhumation of lower crustal material forming an extended ocean-continent transition (OCT). The Beira High segment reveals an offshore continental fragment, which is overlain by a faulted pre-rift sedimentary unit likely to belong to the Karoo Group. The combination of our seismic interpretation with existing geophysical and geological data has allowed us to propose a break-up model which supports the idea that the Central Mozambique margin was affected by polyphase rifting. The Beira High basement is formed by a strike-slip deformation along a proposed lithospheric weakness - the Lurio-Pebane shear zone. Northwestern-southeastern oriented extension follows and results in continental break-up and oceanic spreading. Our results suggest a segmentation of the Central Mozambique margin with oceanisation first occurring in the Angoche segment. The formation of the first oceanic crust in the Beira segment followed, likely delayed by the formation and failure of the northern Beira High rift.

Neotectonics in the South American passive margin: Evidence of Late Quaternary uplifting in the northern Paraiba Basin (NE Brazil)

The relief evolution of several areas of northeastern Brazil has been increasingly related to tectonic events. This is the case for the Paraíba Basin, where an abundance of faults and folds were recorded in Neogene and even Late Pleistocene-Holocene deposits. We tested whether dome-like morphologies in the northern sector of this basin, where geological exposures are scarce, were also formed by tectonic deformation. The work combined limited geological data with qualitative and quantitative analyses of the relief applying the 1 arc-second digital elevation model provided by the Shuttle Radar Topography Mission. The geomorphic indices of basin asymmetry factor (AF), swath topographic profiles, enhanced transverse hypsometry index (THi*) and normalized river long-profiles were used for validating a neotectonic origin for the dome-like morphologies. This interpretation was also supported by other morphological evidence of the dome relief, including: recurving channels featured in radial to recurved trellis patterns; tectonically-controlled drainage sub-basins; high relative relief values related to increased fluvial incision; and anomalous river long-profiles indicative of ungraded rivers. The domic structures were attributed to fault- and fold-related strike-slip deformation during the late Quaternary. This record supports the proposal that the South American passive margin remained under tectonic stress long after the main rifting in the late Jurassic-early Cretaceous.