Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb-Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation ⁴⁰Ar/³⁹Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29-32 Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30 km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74-67 Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62-58 Ma), widespread Late Cretaceous-Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22-16 Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basins.     

The clew bay group: A displaced terrane of highland border group rocks (Cambro-ordovician) in Northwest Ireland

Marine turbidites, tuffs, black mudstones and conglomerates of the Cambro-Ordovician Clew Bay Group, were deposited in the E–W elongate transtensional Clew Bay Graben that is centred on Clew Bay, NW Ireland. The group is characterized by extensive sedimentary deformation and mass movement on slides; olistostromes, autoclastic breccias and course proximal turbidites are interbedded with apparently less disturbed but often overturned sediments. The Clew Bay Group lies structurally above serpentinized dunite/harzburgite breccias, schistose carbonate peridotites, and other basic and ultrabasic igneous rocks that have ophiolitic geochemical affinities; the sediments may have been in part deposited upon oceanic crust. Ophiolites and sediments that now rest on the Clew Bay Thrust abut Silurian shallow water strata in which the main tectonothermal history, associated with sinistral transcurrent faulting along the thrust zone, is dated at about 410 Ma. The sole thrust dips northward and coalesces with a major deep structure along the Fair Head-Clew Bay Line (FCL) that is the western continuation of the Highland Border Fault of Scotland. Blueschist relics in the Dalradian immediately to the north of the FCL indicate that subduction was active early in the history of the late Cambrian–early Ordovician Grampian orogeny. The Clew Bay Thrust was a sinistral, transpressional shear zone late in its history, but it probably originated as an obduction complex. The Clew Bay Group cannot be traced into sedimentary, metamorphic or structural continuity with the adjacent Dalradian to the north or Ordovician and Silurian rocks in the South Mayo Trough to the south. It should be considered as a distinct terrane (Clew bay Terrane) or a subterrane of Highland Border-type rocks along the southern margin of the Grampian Terrane.     

沂沭断裂带晚白垩世-早古新世左行走滑的低温年代学约束

沂沭断裂带晚白垩世地层中保留大量的左行压剪的几何学与运动证据,左行活动时限为晚白垩世—古近纪。取自4条主断层的不同部位断层泥及断层碎裂岩的磷灰石裂变径迹年龄,远小于周围未发生变形的地层年龄,构造活动使得断层带物质的磷灰石裂变径迹完全退火。通过相关软件对不同磷灰石裂变径迹数据模拟,显示在70~60Ma前及约10Ma开始出现快速抬升(冷却)。综合分析认为,沂沭断裂在70~60Ma经历过较大规模左行挤压活动。约10Ma的快速抬升事件,具有区域性,可能与青藏高原约10Ma的快速伸展有关。     

莺歌海盆地构造转折界面的确定及其地质意义

中国西南部红河断裂带的活动演化历史长期以来备受国内外学者的关注,该断裂从陆地向海域延伸进入莺歌海盆地,并对莺歌海盆地的形成和演化起重要的控制作用。目前,红河断裂带经历早期的左旋走滑运动和后期的右旋走滑运动已经得到公认,但对于其精细的构造演化历史及其左旋走滑向右旋走滑运动转换的时间还未能达成共识。本文利用构造控制沉积、沉积反映构造的思想,通过对莺歌海盆地三维地震资料的构造解析,从T27界面上下地层厚度存在"跷跷板"式的变化、沉积中心的迁移、沉积速率的变化、陆架-陆坡坡折带的出现、微小断裂的特征以及底辟构造等方面的研究,确定莺歌海盆地红河断裂带的左旋走滑运动停止于T40(10.5Ma);T40~T30(10.5~5.5Ma)是构造变形的平静期;T30~T27(5.5~2.4 Ma)为左旋走滑运动向右旋走滑运动转换时期;T27(2.4 Ma)以后右旋走滑活动开始,并控制坡折带(包括莺歌海盆地和琼东南盆地)和底辟构造等的形成;T20(1.9 Ma)以来,右旋走滑活动逐渐减弱。     

肥东花岗质岩脉的变形及年代学特征对郯庐断裂带早白垩世构造活动的指示

郯庐断裂带肥东段位于大别造山带与苏鲁造山带之间。在肥东段西韦采石场内发育了大规模的北北东向左行走滑韧性剪切带和一条低角度的韧性滑脱正断层。走滑韧性剪切带为郯庐断裂带走滑活动的产物,低角度滑脱正断层则代表了伸展背景下的构造活动。低角度滑脱正断层上、下盘发育未变形和变形的岩脉,走滑韧性剪切带内外也发育有受剪切带活动而变形的岩脉。对采石场内岩脉的构造和同位素年代学研究表明,低角度韧性滑脱正断层在129~126Ma之间发生过剪切活动,走滑韧性剪切带的活动时间在125Ma之后。综合研究认为,郯庐断裂带的伸展活动可能开始于早白垩世早期(130Ma),但在早白垩世并非一直处于伸展活动之中,125Ma之后的左行走滑活动很可能发生在早白垩世的晚期。     

Tectonics of the Scandian orogeny and the Western Gneiss Region in southern Norway

Two crust-forming events dominate the Precambrian history of the Western Gneiss Region (WGR) at about 1800–1600 Ma and 1550–1400 Ma. The influence of the Sveconorwegian orogeny (1200–900 Ma) is restricted to the region south of Moldefjord-Romsdalen. A series of anorthosites and related intrusives are present, possibly derived from the now-lost western margin of the Baltic craton that may have been emplaced in the WGR as an allochthonous unit before the Ordovician.The Caledonian development is split into two orogenic phases, the Finnmarkian (Cambrian — Early Ordovician) and the Scandian (Late Ordovician/Early Silurian — Devonian). The lower tectonic units west of the Trondheim Trough may be Finnmarkian nappes ; they were part of the lower plate during the Scandian continental collision. The Blåhö nappe is correlated with dismembered eclogite bodies along the coast. A regional change of nappe transport direction from 090 to 135 marks the initiation of an orogen-parallel sinistral shear component around 425 Ma. The change caused the development of a complex sinistral strike-slip system in the Trondheim region consisting of the Möre-Tröndelag Fault Zone and the Gränse contact. The latter cut the crust underneath the already emplaced Trondheim Nappe Complex, thus triggering the intrusion of the Fongen-Hyllingen igneous complex, and initiating subsidence of the Trondheim Trough, and was subsequently turned from a strike-slip zone into an extensional fault. Minor southward transport of the Trondheim Nappe Complex rejuvenated some thrusts between the Lower and the Middle Allochthon. A seismic reflector underneath the WGR is interpreted to be a blind thrust which subcrops into the Faltungsgraben. During Middle Devonian orogenic collapse, detachment faulting brought higher units, now eroded elsewhere, down to the present outcrop level, such as the Bergen and Dalsfjord nappe and the Old Red basins.     

Palinspastic reconstruction of the Grenville terrane in the Blue Ridge Geologic Province,southern and central Appalachians,U.S.A

Examinations of Grenville massifs in the Blue Ridge Geologic Province of Virginia and North Carolina indicate that the country rocks (∼ 1100–1450 Ma) are layered gneisses that were metamorphosed during Grenville orogenesis (∼ 1000–1100 Ma) to amphibolite to granulite facies and intruded by plutonic suites. Subsequently, the Grenville terrane was intruded by a suite of peralkaline granitic plutons (∼ 700 Ma) and progressively overlapped westward by Upper Precambrian to Cambrian sedimentary and volcanic rocks. Following deposition of Upper Precambrian and Palaeozoic rocks, the Blue Ridge Geologic Province was subjected to Taconic metamorphism (∼ 450–480 Ma) which generally increased in intensity southeastward from greenschist (chlorite grade) to upper amphibolite (sillimanite grade) facies. Large-scale late Devonian thrusting (∼ 350 Ma) along the Fries fault system and the Brevard zone-Yadkin fault system produced the present day distribution of juxtaposed Grenville massifs and Palaeozoic metamorphic zones in the Blue Ridge Geologic Province. Palinspastic restoration of the Taconic metamorphic zones to their pre-late Devonian relative positions yields an ∼ 50 km displacement on the Fries fault system near the Grandfather Mountain window and and an ∼ 80 km displacement on the Smith River allochthon farther east. Restoration of the Grenville massifs to this same palinspastic base shows that Grenville metamorphic grade decreased southeastward from the deeper granulite facies (opx + gar) to the shallower granulite facies (opx ± amp) to amphibolite facies.     

郯庐断裂带构造演化的同位素年代学制约

近年来在郯庐断裂带内获得了大量的同位素年龄,为了解该断裂带的演化规律与相关动力学过程提供了有效的制约。该断裂带早期走滑构造带内给出了238～236 Ma的白云母 40Ar/39Ar 变形年龄,指示其起源于华北与华南克拉通碰撞过程的深俯冲阶段,支持其造山期陆内转换断层成因观点。其晚中生代走滑韧性剪切带内已获得的较大白云母 40Ar/39Ar冷却年龄为162～150 Ma,表明其再次左行平移发生在晚侏罗世初或中 晚侏罗世之交,出现在区域压扭性动力学背景下。这一事件应代表了中国东部滨太平洋构造域的开始时间。已获得的一系列断裂带内岩体与火山岩锆石LA ICPMS年龄显示,该断裂带内伸展性背景下最早的岩浆活动时间为136 Ma。而断裂带所控制的断陷盆地内地层时代表明其伸展活动发生在早白垩世初（约145 Ma）。这应指示了中国东部转变为伸展性动力学背景的时间。该断裂带一系列长石40Ar/39Ar年龄与磷灰石裂变径迹年龄,显示其在晚白垩世与古近纪仍处于伸展活动之中。     

Sedimentological evaluation of Pb-Zn exploration potential of the Precambrian Griquatown Fault Zone in the Northern Cape Province, South Africa

The Griquatown Fault Zone (GFZ) is a major target for Pb-Zn exploration in South Africa. The sedimentary, structural and thermal history of the fault zone are evaluated. The fault zone experienced a synsedimentary period of activity between 2550 and 2500 Ma and a major post-Postmasburg Group (less than 2223 Ma) episode of mainly vertical movements. Possible source rocks for generation of metalliferous brines are abundant along the southwestern margin of the Kaapvaal craton and shales southwest of the GFZ are time correlative to a thick peritidal stromatolitic carbonate sequence in the northeast. Fluids driven by compaction and orogenic pressure migrated across the GFZ , via the carbonates, towards the east. Metamorphic overprint south of the GFZ, based on illite crystallinity, and fluid inclusions north of the fault zone are above the oil window. Metamorphic peaks south of the fault zone are at 1750 Ma and 1213 Ma and the intensity of metamorphic overprint decreases from west to east. Because of high temperatures of metamorphic and orogenic overprint and possible remobilisation of fluids, Mississippi-Valley-Type (MVT) ore deposits are unlikely to be found within the Griquatown Fault Zone, but are expected, rather, to the northeast of it. Higher temperature, remobilised vein-related deposits could, however, occur in the GFZ itself. Received: 11 June 1996 / Accepted: 7 January 1997     

塔里木南缘浅变质岩系形成时代及构造背景

塔里木盆地南缘和田地区，沿西昆仑北缘的铁克力克断裂北侧断续出露一套低绿片岩相变质地层，由于长期以来对其原岩时代、变质时代及成因等存在不同认识，人们常称其为浅变质岩系。岩石经历了多期变质变形作用，片理十分发育，但总体变质程度明显低于铁克力克断裂南侧下元古界埃连卡特群变质岩。浅变质岩之上被上泥盆统奇自拉夫组或上石炭统卡拉乌依组角度不整合覆盖。在基本地质现象观察以及详细的岩石学研究基础上，通过K－Ar和Rb-Sr法同位素年龄测定和对已有年龄数据的具体对比分析，认为本区浅变质岩的原岩年龄在517－448Ma之间，属晚寒武－中奥陶世地层，形成于塔里木古大陆南缘弧后盆地的滨－浅海环境；志留纪－中泥盆世期间，发生在塔里木大陆板块南缘的弧－陆碰撞（440－377Ma)事件不仅使寒武－奥陶系普遍遭受区域低温动力变质作用（早期主要变质作用），还导致了寒武－奥陶系及下伏地层由南而北的逆冲推覆以及韧性剪切变形（晚期韧性动力变质作用）。上尼盆统奇自拉夫组粗碎屑磨拉石建造是陆壳焊接阶段的典型产物，在此之前，本区浅变质岩系已经定型。     

Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at ∼38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between ∼306 and ∼286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to ∼250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.     

A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly

The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.     

A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly

The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.     

The pre-Caledonian Inishkea Division of northwest Co. Mayo,Ireland: Its geochemistry and probable stratigraphic position

The dominantly metasedimentary schists and gneisses of the Inishkea Division of the Erris Complex form a distinct rock group, structurally overlying the Grenvillian Annagh Division orthogneisses and underlying Dalradian metasediments, in the northwest County Mayo metamorphic inlier. A regression line for the Inishkea Division schists, defined by Rb–Sr whole rock analysis, suggests a metamorphic age of about 800 Ma, with a provenance age of about 1000–1300 Ma. The major and trace element chemistry is distinct from both the Annagh Division and Dalradian rocks, although sometimes similar to that of the Erris Group, and it suggests that the Inishkea Division originated as greywackes with associated intrusives. Local variations of the chemistry exist, but no systematic subdivision of the Division has been made. Amphibolite pods within the schists are metamorphosed and deformed tholeiite dykes, which are similar in chemistry to the younger metadolerites seen in the Annagh Division. While the geochemistry suggests that the Inishkea Division rocks are similar to both the Moine Assemblage and Grampian Group rocks of Scotland, the isotopic data suggest that they may have been deposited and then initially metamorphosed during the interval between the Grenville metamorphism of the Moine Assemblage and the onset of Dalradian and Erris Group deposition.     

Fluid-driven low-grade metamorphism in polydeformed rocks of Avalonia (Arisaig Group, Nova Scotia, Canada)

The Lower Silurian??Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite?Csmectite and chlorite?Csmectite mixed-layers are common but Na?CK mica and kaolinite occur only in some samples. The identification of illite?Csmectite mixed-layers in diagenetic samples, with Kübler Index >0.50 ??°2?? and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW?CSE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1?km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200?oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.     

Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ ¹⁸O_(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault. Received: 26 February 1999 / Accepted: 8 November 1999     

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted ⁴⁰Ar-³⁹Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a ⁴⁰Ar-³⁹Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma.     

LARGEST ALTYN TAGH LITHOSPHERIC SHEAR FAULT IN CENTRAL ASIA

LARGEST ALTYN TAGH LITHOSPHERIC SHEAR FAULT IN CENTRAL ASIA     

Ar/Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

⁴⁰Ar/³⁹Ar dating and estimates of regional metamorphic P–T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.     

Main Uralian Thrust and Main Uralian Normal Fault: non-extensional Palaeozoic high-P rock exhumation, oblique collision, and normal faulting in the Southern Urals

The crustal architecture of the Southern Urals is dominated by an orogenic wedge thrusted westward upon the subducted East European continental margin. The N–S trending wedge constitutes an antiformal stack composed mainly of the high-P Maksyutov Complex, the overlying Suvanyak Complex and the allochthonous synformal Zilair flysch further west. These tectono-metamorphic units are separated by tectonic contacts and record discontinously decreasing metamorphic conditions from bottom to top. In the east, the E-dipping Main Uralian Normal Fault cross-cuts the metamorphic footwall and juxtaposes the non metamorphic Magnitogorsk island arc. This syncollisional normal fault compensated crustal thickening and exhumation of the high-P rocks. Orogenic shortening was accommodated by the Main Uralian Thrust, a W-vergent crustal-scale shear zone at the base of the wedge. Geological investigations and reflection seismics (URSEIS '95) argue in favour of a geodynamic evolution integrating subduction and basal accretion of high-P rocks during sinistral oblique thrusting along the Main Uralian Thrust and coeval normal-faulting along the Main Uralian Normal Fault.