Southward Thrusting Research Articles

Testing alternative tectonic models for the Permian-Pleistocene tectonic development of the Kyrenia Range, N Cyprus: Implications for E Mediterranean Tethyan palaeogeography

Three published alternative tectonic models of the Permian-Pleistocene development of the Kyrenia Range, N Cyprus are tested, supported by new field, geochemical and micropalaeontological evidence: 1. The Kyrenia Range represents the northern continental margin of the S Neotethys, close to its present relative position. The range initiated as a Permian-Cretaceous rift/passive margin, switching to a N-facing active margin during Late Cretaceous-Neogene; 2. The Range was located along the N African continental margin until the Neogene when northward subduction transferred it to the southern margin of the Eurasian plate; 3. The Range is a far-travelled allochthon that was emplaced to near its present position, probably during the Eocene.In the light of regional comparisons, especially with southern Turkey, the combined evidence mainly supports tectonic model 1. Sedimentary and palaeontological data show that the restored stratigraphy of the Kyrenia Range indicates Late Permian initial rifting and Early-Middle Triassic advanced rifting, followed by Jurassic-Early Cretaceous passive margin subsidence. Small exposures of ophiolite-related melange located between the Mesozoic carbonate platform and the overlying latest Cretaceous-Palaeogene deep-water volcanic-sedimentary succession include evidence of HP/LT metamorphism, pointing to Late Cretaceous subduction. MORB/boninites, diabase-gabbro and extensive harzburgitic serpentinite originated as a SSZ ophiolite, together with a possible high-grade metamorphic sole (garnet amphibolite) and an accretionary prism (E-MORB/OIB; metachert). Microfossil evidence indicates exhumation of the melange and the underlying platform prior to Late Maastrichtian. A mass-transport complex formed within a compression-related foredeep during the Middle Eocene. Associated southward thrusting and folding culminated in emergence and subaerial erosion, generating a major unconformity, that was followed by subaerial and then marine deltaic deposits (Late Eocene-Oligocene). Following major Late Miocene southward thrusting, uplift of the Kyrenia Range took place during the Pleistocene, related to collision of the leading edge of the North African plate (Eratosthenes Seamount) with the Eurasian (Anatolian) plate.

The boundary between the North China Craton and the Central Asian Orogenic Belt in NE China: Seismic evidence from receiver function imaging

• 38 broadband seismic stations (profile NCISP-10, 400-km-long) were deployed across the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) in Northeast China. • Receiver function CCP images reveal a southward thrust nappe structure at the boundary of the NCC and the CAOB. • Yilan-Yitong fault (YYF) cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents an ultra-crust normal fault. This study imaged the crustal structure and tectonic state of the interacting North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using receiver function imaging. The seismic profile derived from the NCISP-10 broadband seismic array spanning a northeasterly area of the NCC and a southeasterly area of the CAOB. The profile shows a flat Moho with localized anomalies but generally resting at an average depth of 32 km. The average Vp/Vs ratio was about 1.75 but the crust around the Songliao Basin exhibits a relatively high Vp/Vs ratio and a shallow Moho. An obvious velocity discontinuity appears within the crust beneath the Songliao Basin as evidenced by theoretical model tests which managed effects from multiple waves generated by sedimentary layers in the basin. This discontinuity inclines gently and is interpreted to represent the deep extension of the Chifeng-Kaiyuan fault (CKF), which itself serves as the surface boundary between the NCC and CAOB. The NCC feature appears to the south of this boundary, while a nappe structure formed during subduction-collision and composed primarily of a subduction-accretionary complex lies to the north. The Tanlu Fault consists of two branches in the study area, the Yilan-Yitong fault (YYF) and the Dunhua-Mishan fault (DMF). The YYF cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents a crustal-scale normal fault. The DMF gives no obvious indication of vertical offset in the images but may be mainly characterized by sinistral strike slip movement.

Early Cretaceous foreland-like Northeastern Qaidam Basin, Tibetan Plateau and its tectonic implications: Insights from sedimentary investigations, detrital zircon U–Pb analyses and seismic profiling

The Early Cretaceous collision between the Lhasa and Qiangtang terranes is one of the critical tectonic events that shaped the southern margin of the Eurasian continent, and that impacted the formation of the Cenozoic Tibetan Plateau. However, how far north this collision affected the structural and sedimentary geology of the Early Cretaceous remains uncertain. To address this issue, we present results from an integrated study of Lower Cretaceous strata in the Northeastern Qaidam Basin (NEQB), northern Tibetan Plateau, based on field observations, detrital zircon U–Pb analyses, and seismic profile interpretations. The Lower Cretaceous mainly consists of deltaic, fluvial, and alluvial sandstone and conglomerate with a pattern of orogeny-related molasse deposits and a provenance in the nearby South Qilian Shan to the north. A number of south-directed reverse faults developed and formed piggy-back basins in the Early Cretaceous. These reverse faults remained active and affected erosion during the Late Cretaceous, leaving an unconformity above the Cretaceous strata. These observations indicate a foreland-like NEQB due to the uplift and southward thrusting of the South Qilian Shan in the Early Cretaceous. We further interpreted the NEQB and the South Qilian Shan to be the northern edge of the compressional regime associated with the Lhasa-Qiangtang collision in the Early Cretaceous, which contrasts with the contemporaneous extensional setting in the North Qilian Shan and further north. We also observed that some Early Cretaceous faults were rejuvenated as major thrusts in the South Qilian Shan thrust belt in the Cenozoic. For the first time, this study maps the northern boundary of the Late Mesozoic Qiangtang Plateau and highlights a protracted influence on Tibetan Plateau tectonism since the Early Cretaceous.

Structural anatomy of the early Paleozoic Laohushan ophiolite and subduction complex: Implications for accretionary tectonics of the Proto-Tethyan North Qilian orogenic belt, northeastern Tibet

Abstract Recognition of accretionary tectonics in ancient orogenic collages is important for reconstructing the long-term subduction, accretion, and erosional history of fossil convergent margins, and for understanding crustal growth and supercontinent assembly. The North Qilian orogenic belt (NQOB), located between the Alxa block and the Central Qilian–Qaidam block in northeastern Tibet, is a typical Phanerozoic accretionary-to-collisional orogenic belt that represents the termination of the northern branch of the Proto-Tethys Ocean. It contains two subparallel ophiolitic belts, arcs, and subduction complexes; the ophiolitic rocks in the northern belt have generally been considered to have formed in a back-arc setting. However, the subduction-accretion-collision history, subduction polarity, and timing of closure of the back-arc ocean remain equivocal. To address these problems, we conducted detailed field, structural, and geochronological investigations of the Laohushan ophiolite–accretionary complex and related sedimentary rocks in the eastern NQOB. The Laohushan Complex is divisible into (1) a northern sedimentary forearc, and a supra-subduction zone-type ultramafic-mafic forearc (ca. 450 Ma) composed of serpentinized harzburgite, gabbro, basalt, and plagiogranite; and (2) a southern accretionary complex, which consists of relatively coherent basalt-chert-mudstone ocean plate stratigraphy that is structurally repeated many times, trench-fill turbidites, mélanges, and widespread thrust imbricates and duplexes, block-in-matrix and asymmetric structures. Kinematic analysis indicates that the accretionary complex underwent southward thrusting and shearing; coupled with the spatial architecture of the different tectonic units, which suggests northward subduction beneath the northern forearc on the southern margin of the Alxa block. Detrital zircon ages of forearc clastic sandstones, pelagic mudstones, trench-fill turbidites, and the matrix of mélanges, together with the zircon ages of igneous ophiolitic rocks and post-accretionary intrusions, indicate that the terminal accretion and tectonic stacking of the Laohushan subduction complex was between ca. 447 and 430 Ma. We propose a geodynamic model involving back-arc basin opening (ca. 517–449 Ma), intra-oceanic subduction-accretion (ca. 449–430 Ma), and final obduction of the northern forearc to account for the evolutionary processes of the North Qilian back-arc basin. The anatomy of the forearc ophiolite and structurally lower accretionary complex indicates the complicated origins and mechanism of emplacement of the ophiolitic rocks. Field-based reconstruction of accretionary complexes and upper plate ophiolites, together with provenance analysis of related sedimentary sequences, provide crucial constraints on the prolonged evolution of paleo-ocean basins and accretionary-to-collisional orogens.

Seismicity around the Mahendragarh–Dehradun basement fault in the western Ganga plain, India: a neotectonic perspective

The NE–SW trending Mahendragarh–Dehradun Fault (MDF) is a basement fault in the western Ganga plain. The earthquake (4.7 mb) epicentered on this fault on 2nd June 2017 was focused around 10 km depth and does not show any surface rupture. Ground Penetrating Radar (GPR) survey around the epicenter detects few shallow-depth subsurface normal faults parallel to the MDF. However, it is unclear whether these normal faults are linked with the 2017 earthquake or other previous earthquakes. Seismic record (USGS) since 1975 to 2017 suggests occurrence of 22 earthquakes (~ 5 mb) around the MDF and majority (~ 63%) of these are clustered around Delhi and Rohtak. Following the 2017 earthquake, frequency of earthquakes occurence sharply increased than previously recorded instrumentation period from 1975 to 1995. Majority of the seismicity is focused around 10 km depths, shallowing to depths of 25–207 km recorded from 1975 to 2004. Earthquakes clustered along the Main Himalayan Thrust (MHT) shows similar temporal trend as near the MDF. Southward thrust movement of the Himalaya orogenic wedge predicts to impose strike-slip motion on the orthogonal-oriented steep basement faults, which is consistent with geologic and geomorphic field relations, and seismicity focal mechanisms along the MDF. The observed upper crustal deformation associated with such fault deformation pattern may be partly influenced by the MDF and partly by the foreland bulge seismicity. Potential tectono-geomorphic parameters and soil-chronosequence may suggest the surface deformation is active through the Holocene.

Tectonochemistry andp‐tConditions of Ramgarh and Almora Gneisses from Askot Klippe, Kumaun Lesser Himalaya

The chemical and petrological correlation of metamorphic nappes and klippes overlying the Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses, not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the Ramgarh granitic gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic gneisses is likely to have been controlled by feldspar crystallization in restites. During the anatexis at > 776°C temperature and >6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The Rb/Sr ratio ∼0.54 and Nb ∼10 ppm is consistent with the granodioritic composition. The negative Sr anomaly in the underlying Ramgarh granitic gneisses indicates a distinct mantle derived source/plagioclase fractionation with a notable correspondence to other late orogenic granites, particularly the basement Ulleri gneisses from the Nepal Himalaya. Ramgarh gneisses plot in the late‐ and post‐COLG field. The Askot ensemble is likely to be the tectonometamorphically reworked basement, viz. the Ramgarh Group along with its metapelitic cover of the Almora Group, together comprising southward thrust remnants of the leading edge of the Indian Plate that collided with Tibet during the Tertiary Himalayan orogeny.

Zircon (U‐Th)/He Thermochronometric Constraints on Himalayan Thrust Belt Exhumation, Bedrock Weathering, and Cenozoic Seawater Chemistry

AbstractShifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible for CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U‐Th)/He thermochronometric and detrital zircon U‐Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in‐sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ∼16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os‐enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global‐scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.

Provenance evolution of the Jurassic northern Qaidam Basin (West China) and its geological implications: evidence from detrital zircon geochronology

The Jurassic system is the major hydrocarbon source rock and of crucial importance for understanding the Mesozoic intra-continental tectonics in West China. This paper presents systematic detrital zircon geochronology of the Jurassic outcropping at the Dameigou locality in the northern Qaidam Basin, and reports ~1000 single-grain U-Pb zircon ages that have implications for the provenance, the corresponding basin property as well as the tectonic setting of West China during Jurassic. Zircon ages exhibit two major clusters at ~250 and ~2400 Ma whereas two minor clusters at ~450 and ~850 Ma, suggesting primary sources from the East Kunlun Shan and Oulongbuluke Block, secondary sources from the North Qaidam UHP belt and South Qilian Shan. Combined with observation of lithology and sedimentary facies, two rifting periods were inferred in the earliest Jurassic and the early stage of the Middle Jurassic, respectively, accompanied by further extension throughout the Jurassic. Our results do not support a foreland basin related to the Jurassic southward thrusting of the South Qilian Shan, but favor that the Mesozoic intra-continental tectonics in West China were characterised by pulsed responses to specific collisions rather than a persisting contractional setting during Jurassic period.

Pliocene–Pleistocene sedimentary–tectonic development of the Mesaoria (Mesarya) Basin in an incipient, diachronous collisional setting: facies evidence from the north of Cyprus

AbstractThe Mesaoria (Mesarya) Basin exemplifies multi-stage basin development within a regional setting of diachronous continental collision. The Plio-Pleistocene represented a period of major sediment accumulation between two topographic highs, the Kyrenia Range in the north and the Troodos Massif in the south. During Pliocene time, open-marine marls and chalks of the Nicosia (Lefkoşa) Formation accumulated in a shelf setting. The Early Pleistocene period was characterized by a relative fall in sea level and a change to shallower-water bioclastic deposition of the Athalassa (Gürpınar) Formation. The northern margin of the basin was approximately delineated by the E–W neotectonic Ovgos (Dar Dere) fault zone. A carbonate ramp system formed directly to the south of this structural feature. During Early Pleistocene time, the basin evolved from an open-marine shelf to semi-enclosed lagoons with deltaic deposits, and finally to a non-marine aeolian setting, flanked by the rising Kyrenia Range to the north. Synthesis of geological evidence from the Mesaoria (Mesarya) Basin as a whole, including outcrop and borehole evidence from the south, adjacent to the Troodos Massif, indicates that the Pliocene – Early Pleistocene represented a relatively quiescent period. This intervened between Late Miocene – earliest Pleistocene southward thrusting–folding of the Kyrenia Range and Pleistocene intense surface uplift of both the Kyrenia Range and the Troodos Massif. The basin development reflects flexurally controlled collapse during Late Miocene – earliest Pliocene time related to southward thrusting, followed by strike-slip during westward tectonic escape of Anatolia, and finally regional uplift controlled by under-thrusting of continental crust from the south, as collision progressed.

Dynamic processes from plate subduction to intracontinental deformation: Insights from the tectono-sedimentary evolution of the Zhaosu–Tekesi Depression in the southwestern Chinese Tianshan

The Zhaosu–Tekesi Depression, located to the northern margin of the South Tianshan orogenic belt (STB), is an ideal region for investigating Paleozoic orogenic processes and later modifications of the southwestern Chinese Tianshan. In this study, we employ 2D seismic and borehole data revealing the geological structure and tectonic evolution of the Zhaosu–Tekesi basin, and we report new zircon U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results of the Late Silurian magmatic rocks, as well as the whole-rock trace element data of the Early Carboniferous carbonate rocks. Borehole data (wells ZC1 and ZC2) and seismic imaging suggest that the Zhaosu–Tekesi Depression is composed of Silurian, Lower Carboniferous, Permian, Jurassic, Upper Cretaceous and Paleogene strata, with six unconformity surfaces (C1a/S3, P1/C1a, P2t/P1, J/P2t, K2d/J and E/K2d). The Silurian strata exhibit southward thrust structures and consist of basalt, andesite, dacite, tuff, minor muddy limestone and diorite intrusions. Zircon SHRIMP U–Pb dating for the well ZC1 diorites yield a crystallization age of ∼422Ma, with positive εHf(t) values (3.3–10.6). The diorites and andesites are calc-alkaline and show enrichment in LILEs (such as Th and U) and depletion in Nb, Ta and Ti, relatively high initial 87Sr/86Sr ratios (0.703376–0.706737) and low εNd(t) values (−0.5 to 1.4), and high Th/Yb (0.29–0.35) and Ta/Yb (1.31–1.58) ratios, indicating their formation in an active continental margin setting. The Lower Carboniferous strata mainly comprise oolitic limestone, bioclastic micritic limestone and interbedded calcareous siltstone. High Sr/Ba (1.31–2.33), V/Sc (10.4–18.6) and low Th/U (0.01–0.23) ratios suggest that they were potentially deposited in the mixing near-shore marine environment. These results, in combination with previous data, indicate that the southwestern Chinese Tianshan is controlled by northward subduction of the South Tianshan Ocean during the Late Silurian to Early Carboniferous. The presence of the Lower Permian terrigenous strata and rift basin structure constrains the timing of collision between the CTB–KYB and the Tarim Craton as the late Carboniferous. Thrusting reactivation and syn-tectonic growth strata in the basin suggest that the southwestern Chinese Tianshan experienced Late Cretaceous compression and Neogene modifications after the Late Permian post-faulting thermal subsidence, Triassic peneplanation and Jurassic regional subsidence. Intracontinental subduction due to distant effect is the main tectonic control on the Miocene rapid uplift of the Tianshan. The Zhaosu–Tekesi basin evolution sheds light on dynamic processes of the southwestern Chinese Tianshan from plate subduction to intracontinental deformation.

Structural development of the central Kyrenia Range (north Cyprus) in its regional setting in the eastern Mediterranean region

A detailed structural analysis of the Mesozoic–Cenozoic geological development of the central segment of the Kyrenia Range in its regional tectonic context is given here. The structural evidence comes from five structural traverses, outcrop observations, small-scale structures and related regional evidence. The majority of the structures are fault planes, of which a subordinate number exhibit slickenlines (fault plane data, n = 2688; with kinematics, n = 537). Additional kinematic data were obtained from C–S fabrics and folds. Small-scale structures in each stratigraphic unit were ‘backstripped’ to reveal relative chronology. Synthesis of the structural information indicates three phases of convergence-related deformation: (1) Late Cretaceous, associated with greenschist facies metamorphism, followed by exhumation that was probably associated with WNW–ESE to ENE–WSW-trending high-angle faulting; (2) Mid-Eocene, associated with southward thrusting, coupled with ~N–S strike-slip (transfer faulting) and oblique faulting in an overall sinistral transpressive stress regime; (3) Late Miocene-earliest Pliocene, involving southward thrusting and folding, localised back-thrusting, extensive fault reactivation and large-scale segmentation of the range. Intense uplift of the Kyrenia Range took place during the Plio-Pleistocene, possibly related to the collision of the Eratosthenes Seamount with the Cyprus trench to the south of the island. The three main convergent phases relate to stages of northward subduction and diachronous continental collision affecting the northerly, active continental margin of the Southern Neotethys.

Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

The central Southern Alps retrobelt was built trough a polyphase tectonic evolution that developed in the hangingwall of the SE-directed Alpine Tethys subduction. In the northern sector of the central Southern Alps the earliest stages of shortening resulted in the inversion of structures inherited from the Permian and Triassic rifting phases, together with southward thrusting and large-scale folding. In the San Marco Pass area (N Italy) the Variscan basement overrides the Permian-Triassic sedimentary cover along regional thrusts. Here the E-W trending Orobic Thrust joins the NE-SE Porcile Thrust. Initial thrusting phases formed greenschist facies mylonites in the basement and S-verging folds in the sedimentary covers. The Porcile line, a feature likely inherited from the Permian extension, was re-activated as a steep mylonite shear zone. Fault activity continued at brittle conditions: cataclastic shear zones overprinted the mylonites. During this phase, shortening was preferentially accommodated along the Orobic Thrust. A final transpressive re-activation observed along both thrusts is linked to the Oligocene activity of the Insubric Fault.

Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland, across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30km length N–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour External Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dominant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10km (21%) whereas on the easternmost one shows ~5km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback.

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310-295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.

Emplacement and cooling of the Dien Bien Phu granitic complex: Implications for the tectonic evolution of the Dien Bien Phu Fault (Truong Son Belt, NW Vietnam)

The Dien Bien Phu (DBP) Fault strikes nearly perpendicular to the general tectonic framework, separating northwest Vietnam into two compartments. The DBP and Muong Lay granites are exposed respectively east and west of the fault. The U/Pb age of 277±2Ma obtained for the Muong Lay granite is similar to the age of the Chieng Khuong complex exposed within the Song Ma structure east of the DBP Fault. These two magmatic intrusions are considered as belonging to a same tectonic structure related to the subduction of the Song Ma Ocean under the Indochina Block during the Indosinian orogeny. The DBP Fault with a total post Permian dextral offset of 30–40km shifts the Song Ma Suture. It could then be connected to the Ailao Shan–Shuanggou suture to the north. The undeformed DBP granite gave a U/Pb age of 230±1Ma sealing the Indosinian deformation on the fault. The Ladinian–Carnian activity of the fault is confirmed by the 40Ar/39Ar (biotite) age of 228±4Ma obtained from the paragneiss forming the fault zone. A geodynamic model is proposed to describe the various tectonic phases recorded by on the DBP Fault. It associates field data, stratigraphic observations, and geochronology analyses (U/Pb, Ar/Ar, fission tracks). Following the Late Triassic exhumation of the granite and the end of the Indosinian orogeny marked by the Norian unconformity, the granite was buried under Jurassic and Lower Cretaceous sediments. A strong cooling event during the early Late Cretaceous (100Ma) is associated with the reactivation of the DBP Fault and the accommodation of a southward thrusting of the Song Da terrane onto the Truong Son Belt. This newly recognised tectonic phase could be associated to the subduction of the Paleo-Pacific plate under the Indochina Block. No data are available yet to constrain the Tertiary evolution of the DBP system. However, the Tertiary deformation is likely similar to the Cretaceous one.

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif, NE China and its bearing on the evolution of the Yinshan–Yanshan orogenic belt: Part I: Structural analyses and geochronological constraints

With a cratonic nucleus, the North China Craton (NCC) experienced a complex tectonic evolution with multiphase compressional and extensional events during Mesozoic times. Along the northern part of the NCC, the Yinshan–Yanshan fold and thrust belt was a typical intraplate orogen. Jurassic and Cretaceous continental sedimentation, magmatism, widespread intraplate characterize the Yinshan–Yanshan orogenic belt. The geodynamic significance of these tectonic events is still in dispute. In the western part of the Liaoning province, the Yiwulüshan massif crops out at the eastern end of the Yinshan–Yanshan orogenic belt. The Yiwulüshan massif presents an elliptical domal shape with a NE–SW striking long axis. The structural evolution of this massif brings new insights for the understanding of the Mesozoic plutonic–tectonic history of the NCC. A multidisciplinary study involving structural geology, geochronology, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have been carried out. The presentation of the new results splits into two parts. Part I (this paper) deals with field and laboratory structural observations, and presents the main geochronological results. The AMS, gravity modeling data will be provided in a companion paper (Part II). The early compressional deformation (D1) corresponds to a Late Jurassic to Early Cretaceous southward thrusting. The subsequent deformation is related to the Early Cretaceous exhumation of the Yiwulüshan massif. A detailed structural analysis allows us to distinguish several deformation events (D2, D3, and D4). The Cretaceous extensional structures, such as syntectonic plutons bounded by ductile normal faults, metamorphic core complexes, and half-graben basins are recognized in many places in East Asia. These new data from the Yiwulüshan massif constitute a link between Transbaikalia, Mongolia, North China and South China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.

Research of Drilling Complexities in Dabei Belt of Tarim Basin

Kuche depression of Tarim Basin went through tectogenesis many times, due to the southward thrust compressional stress from Tianshan fold uplift, the geology condition of this area is extraordinarily complex, there are problems such as depth difference in geological horizons, abnormal pressure formations and irregular distribution of special lithologies, etc. All of these problems have brought even more difficulties in engineering design and construction. Accidents such as well leakage, drilling sticking and complex situations happened frequently. All of those have influenced the engineering progress seriously and lowered the actual effect of drilling. In order to resolve the problems and complex situations above, we did detailed researches on accidents in the Kuche piedmont belt of Tarim Basin and studied reasons for complex situations, and then we proposed engineering resolutions and evaluated them.

Hot metamorphic core complex in a cold foreland

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Visean/Namurian boundary (≤320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D1), all structural levels shared the same tectonic evolution: D2 (extension and exhumation), D3 (refolding) and post-D3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M2). M2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D2 effects defines a bulk prolate strain in a “pinched pull-apart” setting. Ductile D2 deformation during M2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D1 is only represented by the inverted position of bedding (overturned limbs of recumbent D1 folds) and by refolded D1 folds. U–Pb monazite and zircon ages and K–Ar muscovite ages are in accord with Ar–Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at ≥330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shear-zone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward propagating rift of the Palaeotethys ocean, which helped to destroy the Variscan orogen.

Structural development of the Lower Paleozoic belt of South China: Genesis of an intracontinental orogen

An extensive structural study of Wuyishan and surrounding areas (South China) brings data on the structures formed prior to the Devonian unconformity, building the Lower Paleozoic belt. An Ordovician tectonic event was responsible for: (i) south-directed structures in the Wuyishan proper and its southern border, related to both thin and thick-skinned tectonics, accompanied by metamorphism and crustal thickening; (ii) north-directed structures to the west of Ganjiang Fault and north of Jiangshan–Shaoxing Fault, where only thin-skinned tectonics is visible. The southward deformation accommodating the shortening includes emplacement of thrust sheets, involving deep crustal material and some mantle peridotite, and likely repetition of continental material, both responsible for crustal thickening. The orogeny was due to the underthrusting of the southern part of the South China Block beneath the northern part of this block, closing the pre-existing Nanhua rift, created at around 850–800 Ma, and involving the rift sedimentary fill in the southward thrusting. The syn-metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440–390 Ma. Our new U–Th–Pb EPMA monazite dating gives ages of 433 ± 9 Ma and 437 ± 5 Ma (Early Silurian) for the main anatectic event, and a younger age of 412 ± 5 Ma (Late Silurian–Early Devonian) for a late re-heating process. The Lower Paleozoic belt is an intracontinental orogen, without oceanic suture record. It shows some similarities with the Cenozoic European Pyrénées Chain, with a strong asymmetry.

三波川変成帯中の新たな独立した広域変成帯の存在―白亜紀から第三紀の日本における造山運動―

The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance > 800 km. However, Aoki et al. (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after ca. 90-80 Ma. Furthermore, Aoki et al. (2008) showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional Sanbagawa belt; the structurally upper part is the Sanbagawa metamorphic belt (sensu stricto). It formed as an accretionary complex at ca. 140-130 Ma and subsequently experienced BS-EC facies metamorphism at ca. 120-110 Ma (Okamoto et al., 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after ca. 90-80 Ma and experienced peak metamorphism at ca. 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age (Tsutsumi et al., 2009), clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (Otoh et al., 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (ss) is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shimanto metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama et al., 1996). In SW Japan, the Sanbagawa belt (ss) is paired with the Ryoke low-P/T metamorphic belt and with the ca. 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea ca. 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (ss) and the Ryoke belts.