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1.
This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well‐exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ～340 °C. Temperatures increase down‐section, reaching ～650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ～310 °C km?1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1‐km‐thick zone of distributed top‐down‐to‐the‐northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo‐isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up‐section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.  相似文献   

2.
I conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid-crustal rocks exposed in the Dolpo region (West Nepal). My results indicate that the Main Central Thrust is located ∼5 km structurally below the previous mapped locations. Deformation temperature increasing up structural section from ∼450 °C to ∼650 °C and overlap with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at “close” to peak metamorphic conditions. I interpreted the telescoping and the inversion of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm = 0.88–1) pervasively distributed through the lower portion of the GHS. My results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectonometamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth.  相似文献   

3.
Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year?1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.  相似文献   

4.
张忠义  肖文进  杨国龙  高军辉 《地质论评》2023,69(3):2023030011-2023030011
东天山晚古生代康古尔塔格构造—金矿带的中段南带开展构造控矿研究,为区域金矿定位预测与勘探提供依据。采用区域构造分析和构造解析方法,在雅满苏北部厘定出一类已发生变形改造的大型面状脆韧性—韧性剪切带,构造恢复表明,其形成于晚古生代造山早期向北的分层剪切或低角度逆冲剪切（S1//S0）作用,并作为区域金矿的一级控矿构造而成为俯冲带深源成矿流体向上运移成矿的主通道。在造山过程中递进变形的分层剪切或低角度逆冲剪切晚期、向南北向横向缩短转换阶段,伴随区域抬升和断褶作用,拆离剪切带分支断裂开始成生并向上突破,导致封闭在拆离带内运移的深源含矿流体以断层阀方式分流排泄,成矿流体沿分支断裂向上运移,在断裂上盘或上盘背斜枢纽处的低序次的断裂、破裂中聚集卸载,形成充填石英脉和交代蚀变岩型（造山型）金矿,并有时限为276. 5±2. 9Ma的石英闪长斑岩侵入产出;晚期褶皱、断裂等叠加构造则对矿床（体）破坏、改造及保存起了重要作用。  相似文献   

5.
东天山晚古生代康古尔塔格构造—金矿带的中段南带开展构造控矿研究,为区域金矿定位预测与勘探提供依据。采用区域构造分析和构造解析方法,在雅满苏北部厘定出一类已发生变形改造的大型面状脆韧性—韧性剪切带,构造恢复表明,其形成于晚古生代造山早期向北的分层剪切或低角度逆冲剪切(S1//S0)作用,并作为区域金矿的一级控矿构造而成为俯冲带深源成矿流体向上运移成矿的主通道。在造山过程中递进变形的分层剪切或低角度逆冲剪切晚期、向南北向横向缩短转换阶段,伴随区域抬升和断褶作用,拆离剪切带分支断裂开始成生并向上突破,导致封闭在拆离带内运移的深源含矿流体以断层阀方式分流排泄,成矿流体沿分支断裂向上运移,在断裂上盘或上盘背斜枢纽处的低序次的断裂、破裂中聚集卸载,形成充填石英脉和交代蚀变岩型(造山型)金矿,并有时限为276.5±2.9Ma的石英闪长斑岩侵入产出;晚期褶皱、断裂等叠加构造则对矿床(体)破坏、改造及保存起了重要作用。  相似文献   

6.
New structural and tectono‐metamorphic data are presented from a geological transect along the Mugu Karnali valley, in Western Nepal (Central Himalaya), where an almost continuous cross‐section from the Lesser Himalaya Sequence to the Everest Series through the medium‐high‐grade Greater Himalayan Sequence (GHS) is exposed. Detailed meso‐ and micro‐structural analyses were carried out along the transect. Pressure (P)–temperature (T) conditions and P–T–deformation paths for samples from different structural units were derived by calculating pseudosections in the MnNKCFMASHT system. Systematic increase of P–T conditions, from ~0.75 GPa to 560 °C up to ≥1.0 GPa–750 °C, has been detected starting from the garnet zone up to the K‐feldspar + aluminosilicate zone. Our investigation reveals how these units are characterized by different P–T evolutions and well‐developed tectonic boundaries. Integrating our meso‐ and micro‐structural data with those of metamorphism and geochronology, a diachronism in deformation and metamorphism can be highlighted along the transect, where different crustal slices were underthrust, metamorphosed and exhumed at different times. The GHS is not a single tectonic unit, but it is composed of (at least) three different crustal slices, in agreement with a model of in‐sequence shearing by accretion of material from the Indian plate, where coeval activity of basal thrusting at the bottom with normal shearing at the top of the GHS is not strictly required for its exhumation.  相似文献   

7.
In the Greater Himalayan sequence of far northwestern Nepal, detailed mapping, thermobarometry, and microstructure analysis are used to test competing models of the construction of Himalayan inverted metamorphism. The inverted Greater Himalayan sequence, which is characterized by an increase in peak metamorphic temperatures up structural section from 580 to 720 °C, is divided into two tectonometamorphic domains. The lower domain contains garnet‐ to kyanite‐zone rocks whose peak metamorphic assemblages suggest a metamorphic field pressure gradient that increases up structural section from 8 to 11 kbar, and which developed during top‐to‐the‐south directed shearing. The upper portion of the Greater Himalayan sequence is composed of kyanite‐ and sillimanite‐zone migmatitic gneisses that contain a metamorphic pressure gradient that decreases up structural section from 10 to 5 kbar. The lower and upper portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that spatially coincides with the base of the lowest migmatite unit. Temperatures inferred from quartz recrystallization mechanisms and the opening angles of quartz c‐axis fabrics increase up section through the Greater Himalayan sequence from ～530 to >700 °C and yield similar results to peak metamorphic temperatures determined by thermometry. The observations from the Greater Himalayan sequence in far northwestern Nepal are consistent with numerical predictions of channel‐flow tectonic models, whereby the upper hinterland part evolved as a ductile southward tunnelling mid‐crustal channel and the lower foreland part ductily accreted in a critical‐taper system at the leading edge of the extruding channel. The boundary between the upper and lower portions of the Greater Himalayan sequence is shown to represent a foreland–hinterland transition zone that is used to reconcile the different proposed tectonic styles documented in western Nepal.  相似文献   

8.
The most popular models regarding the exhumation of the Greater Himalayan Sequence (GHS), such as extrusion, channel flow, critical taper and wedge extrusion, require prolonged activity of the two bounding shear zones and faults, the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). We present the crystallization age of an undeformed leucogranite that intrudes both the GHS and the Tethyan Himalaya Sequence (THS). Zircon and monazite U‐Pb ages in the leucogranite give ages between 23 and 25 Ma constraining, at that time, the end of shearing along the STD. Our results limit the contemporaneous activity of the MCT and STD to a short period of time (~1–2 Ma) and thus argue against exhumation models requiring prolonged contemporaneous activity of the MCT and STD.  相似文献   

9.
The Kangra Re-entrant in the NW Himalaya is one of the most seismically active regions, falling into Seismic Zone V along the Himalaya. In 1905 the area experienced one of the great Himalayan earthquakes with magnitude 7.8. The frontal fault system – the Himalayan Frontal Thrust (HFT) associated with the foreland fold – Janauri Anticline, along with other major as well as secondary hinterland thrust faults, provides an ideal site to study the ongoing tectonic activity which has influenced the evolution of drainage and landscape in the region. The present study suggests that the flat-uplifted surface in the central portion of the Janauri Anticline represents the paleo-exit of the Sutlej River. It is suggested that initially when the tectonic activity propagated southward along the HFT the Janauri Anticline grew along two separate fault segments (north and south faults), the gap between these two fault and the related folds allowed the Sutlej River to flow across this area. Later, the radial propagation of the faults towards each other resulted in an interaction of the fault tips, which caused the rapid uplift of the area. Rapid uplift resulted in the disruption and longitudinal deflection of the Sutlej river channel. Fluvial deposits on the flat surface suggest that an earlier fluvial system flowed across this area in the recent past. Geomorphic signatures, like the sharp mountain fronts along the HFT in some places, as well as along various hinterland subordinate faults like the Nalagarh Thrust (NaT), the Barsar Thrust (BaT) and the Jawalamukhi Thrust (JMT); the change in the channel pattern, marked by a tight incised meander of the Beas channel upstream of the JMT indicate active tectonic movements in the area. The prominent V-shaped valleys of the Beas and Sutlej rivers, flowing across the thrust fronts, with Vf values ranging from <1.0–1.5 are also suggestive of ongoing tectonic activity along major and hinterland faults. This suggests that not only is the HFT system active, but also the other major and secondary hinterland faults, viz. the MBT, MCT, SnT, NaT, BaT, and the JMT can be shown to have undergone recent tectonic displacement.  相似文献   

10.
The Rhodope Metamorphic Province represents the core of an Alpine orogen affected by strong syn- and postorogenic extension. We report evidence for multiple phases of extensional unroofing from the western border of the Rila Mountains in the lower Rila valley, SW Bulgaria. The most prominent structure is the Rila-Pastra Normal Fault (RPNF), a major extensional fault and shear zone of Eocene to Early Oligocene age. The fault zone includes, from base to top, mylonites, ultramylonites and cataclasites, indicating deformation under progressively decreasing temperature, from amphibolite-facies to low-temperature brittle deformation. It strikes E–W with a top-to-the-N-to NW-directed sense of shear. Basement rocks in the hanging wall and footwall both display amphibolite-facies conditions. The foliation of the hanging-wall gneisses, however, is discordantly cut by the fault, while the foliation of the footwall gneisses is seen to curve into parallelism with the fault when approaching it. Two ductile splays of the RPNF occur in the footwall, which are subparallel to the foliation of the surrounding gneisses and merge laterally into the mylonites of the main fault zone. The concordance between the foliation in the footwall and the RPNF suggests that deformation and cooling in the footwall occurred simultaneously with extensional shearing, while the hanging-wall gneisses had already been exhumed previously. The RPNF is associated with thick deposits of an Early Oligocene, syntectonic breccia on top of its hanging wall. Integrating our results with previous studies, we distinguish the following stages of extensional faulting: (1) Late Cretaceous NW–SE extension (Gabrov Dol Detachment), exhumation of the present day hanging wall of the RPNF; (2) Eocene to Early Oligocene NW–SE to N–S extension (RPNF); (3) Miocene to Pliocene E–W extension (Western Border Fault), formation of the Djerman Graben; (4) Holocene to recent N–S to NW–SE extension (Stob Fault), reactivating the SW part of the Western Border Fault.  相似文献   

11.
以西安地铁临潼线穿越骊山山前断裂为研究背景,采用数值模拟方法,通过建立地铁隧道-断裂-地层三维有限元模型,研究了骊山山前断裂错动作用下隧道结构的变形受力特征,以此揭示了骊山山前断裂错动作用对西安地铁临潼线的影响机制以及重点设防位置,确定了地铁临潼线的设防范围,提出了相应防治建议措施。研究结果表明:断裂错动作用造成地铁隧道沿纵向发生弯曲变形,大致可分为3个变形区域:下盘稳定区、剪切拉张区和整体沉降区。断裂附近地层竖向应力和隧道拱底接触压力均表现为上盘减小而下盘增大,而隧道拱顶接触压力在上盘增大下盘减小。同时,沿纵向隧道顶部结构在上盘受压而下盘受拉,底部结构受力刚好相反,在上盘受拉下盘受压;隧道受剪区范围随断裂位错量变化基本保持不变,且最大值均出现在与断裂相交位置处。最后,综合确定了西安地铁临潼线跨越骊山山前断裂的纵向设防长度至少为80 m,并给出了跨断裂西安地铁临潼线的防治措施。研究结果可为西安地铁临潼线跨越骊山山前断裂带设计及其病害防治提供科学参考。  相似文献   

12.
The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc.  相似文献   

13.
The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D1 is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D2 with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D3, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D3 shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D3 deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10−12 to 10−14 s−1. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D3 deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.  相似文献   

14.
The Bardoc Tectonic Zone (BTZ) of the late Archaean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is physically linked along strike to the Boulder-Lefroy Shear Zone (BLSZ), one of the richest orogenic gold shear systems in the world. However, gold production in the BTZ has only been one order of magnitude smaller than that of the BLSZ (∼100 t Au vs >1,500 t Au). The reasons for this difference can be found in the relative timing, distribution and style(s) of deformation that controlled gold deposition in the two shear systems. Deformation within the BTZ was relatively simple and is associated with tight to iso-clinal folding and reverse to transpressive shear zones over a <12-km-wide area of high straining, where lithological contacts have been rotated towards the plane of maximum shortening. These structures control gold mineralisation and also correspond to the second major shortening phase of the province (D2). In contrast, shearing within the BLSZ is concentrated to narrow shear zones (<2 km wide) cutting through rocks at a range of orientations that underwent more complex dip- and strike-slip deformation, possibly developed throughout the different deformation phases recorded in the region (D1–D4). Independent of other physico-chemical factors, these differences provided for effective fluid localisation to host units with greater competency contrasts during a prolonged mineralisation process in the BLSZ as compared to the more simple structural history of the BTZ.  相似文献   

15.
In the Caledonides of northwest Scotland, two independent geothermometers (Fe‐Mg exchange and quartz c‐axis fabric opening angle) are used to characterize the thermal structure of the lower part of the Scandian (435–420 Ma) orogenic wedge within the Moine, Ben Hope and Naver‐Sgurr Beag thrust sheets. Traced from west (foreland) to east (hinterland), Fe‐Mg exchange thermometry yields peak or near‐peak temperatures ranging from 484 ± 50 °C to 524 ± 50 °C in the immediate hangingwall of the Moine thrust to 601 ± 50 °C in the immediate hangingwall of the Ben Hope thrust, to 630 ± 50 °C in the Naver thrust sheet. Preserved metamorphic facies and textural relationships are consistent with thermometric estimates. Deformation temperatures calculated from quartz c‐axis fabric opening angles across two similar orogen‐perpendicular transects also yield systematic increases (Glen Golly – Ben Klibreck, 520–630 °C; Ullapool‐Contin, 465–632 °C) traced towards the Naver and Sgurr Beag thrusts. In addition, deformation temperatures show a pronounced increase along the leading edge of the Moine thrust sheet moving south towards the Assynt window, which is interpreted to reflect deeper exhumation of the thrust plane above the Assynt footwall imbricate stack. Because temperatures calculated from metamorphic assemblages are within error of the quartz fabric‐derived deformation temperatures that are of demonstrably Scandian age, the metamorphic sequence between the Moine and Naver‐Sgurr Beag thrusts is interpreted to have developed during the Scandian orogeny. Integration of our results with previous 2D thermal‐mechanical studies allows development of new conceptual thermal‐kinematic models of Scandian orogenesis that may be broadly applicable to other collisional systems. Furthermore, it highlights the critical nature of coupling between orogen kinematic and thermal evolution.  相似文献   

16.
New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.  相似文献   

17.
Determination of the peak thermal condition is vital in order to understand tectono-thermal evolution of the Himalayan belt. The Lesser Himalayan Sequence (LHS) in the Western Arunachal Pradesh, being rich in carbonaceous material (CM), facilitates the determination of peak metamorphic temperature based on Raman spectroscopy of carbonaceous material (RSCM). In this study, we have used RSCM method of Beyssac et al. (J Metamorph Geol 20:859–871, 2002a) and Rahl et al. (Earth Planet Sci Lett 240:339–354, 2005) to estimate the thermal history of LHS and Siwalik foreland from the western Arunachal Pradesh. The study indicates that the temperature of 700–800 °C in the Greater Himalayan Sequence (GHS) decreases to 650–700 °C in the main central thrust zone (MCTZ) and decreases further to <200 °C in the Mio-Pliocene sequence of Siwaliks. The work demonstrates greater reliability of Rahl et al.’s (Earth Planet Sci Lett 240:339–354, 2005) RSCM method for temperatures >600 and <340 °C. We show that the higher and lower zones of Bomdila Gneiss (BG) experienced temperature of ~600 °C and exhumed at different stages along the Bomdila Thrust (BT) and Upper Main Boundary Thrust (U.MBT). Pyrolysis analysis of the CM together with the Fission Track ages from upper Siwaliks corroborates the RSCM thermometry estimate of ~240 °C. The results indicate that the Permian sequence north of Lower MBT was deposited at greater depths (>12 km) than the upper Siwalik sediments to its south at depths <8 km before they were exhumed. The 40Ar/39Ar ages suggest that the upper zones of Se La evolved ~13–15 Ma. The middle zone exhumed at ~11 Ma and lower zone close to ~8 Ma indicating erosional unroofing of the MCT sheet. The footwall of MCTZ cooled between 6 and 8 Ma. Analyses of P–T path imply that LHS between MCT and U.MBT zone falls within the kyanite stability field with near isobaric condition. At higher structural level, the temperatures increase gradually with P–T conditions in the sillimanite stability field. The near isothermal (700–800 °C) condition in the GHS, isobaric condition in the MCTZ together with T–t path evidence of GHS that experienced relatively longer duration of near peak temperatures and rapid cooling towards MCTZ, compares the evolution of GHS and inverted metamorphic gradient closely to channel flow predictions.  相似文献   

18.
南阿尔金断裂的韧性剪切作用时代及其构造意义   总被引：6，自引：5，他引：1  
崔军文 《岩石学报》2011,27(11):3422-3434
位于阿尔金山腹地古元古界阿尔金群深变质岩系与中-新元古界浅变质岩系间的南阿尔金断裂,是1条以近于E-W走向,微向S高角度倾斜的大型逆冲断裂,它经历了韧性变形和脆性变形2个构造演化阶段.韧性剪切带的形成始于晚寒武世,强烈活动期为中奥陶世-志留纪(468.4～412.2Ma),早-中泥盆世、早石炭世、晚二叠世和早侏罗世时期,剪切带进入以高、中温为主的韧性变形期,随着时间推移,变形温度不断降低,剪切作用的强度明显减弱,早侏罗世后,南阿尔金断裂完全进入以脆性变形为主的构造演化阶段.南阿尔金断裂以北广泛分布的年龄区间为491.3 ±4.6～413.8±8.0Ma的钙碱性系列花岗岩和断裂南侧出露的时代为519±37～500±10Ma的榴辉岩和角闪糜棱岩,表明在发生极性向北的逆冲型韧性剪切作用前,沿南阿尔金断裂曾发生过自南而北的岩石圈尺度的俯冲作用.因此,南阿尔金断裂是阿尔金山腹地的1条重要的早古生代板块汇聚、碰撞带.  相似文献   

19.
盐井?五龙断裂是龙门山中央断裂北川?映秀断裂的南延部分,也是龙门山南段的三大控制性主干断裂之一。为了详细认识盐井?五龙断裂的构造几何学、运动学特征,在野外构造研究的基础上,运用磁组构方法对盐井?五龙断裂105块构造岩定向样品进行深入研究。野外构造解析表明断裂至少发生了韧性挤压剪切、脆?韧性逆冲和脆性挤压碎裂三期构造变形。磁组构研究显示构造岩磁组构样品的平均磁化率k_m值具有强磁化率和弱磁化率两种特征。磁组构形状参数T、磁面理F值、磁线理L值和T-P_J图解显示磁化率椭球体主要为压扁型,磁面理较磁线理发育,局部发育较强磁线理,进一步表明盐井?五龙断裂以挤压、剪切为主,伴有拉伸变形的整体特征,样品的P_J整体较大,显示出构造强变形磁组构特征。最小磁化率主轴Kmin方位表明盐井?五龙断裂北段和南段分别受到了NW-SE向和NEE-SWW向的挤压作用;Kmin方位和倾伏角表明北段晚一期的脆韧性变形主体为自NW向SE的较高角度的挤压逆冲剪切变形,局部伴有极小量的左行走滑特征。断裂南段早期韧性变形整体以自SWW向NEE的挤压逆冲剪切变形为主,上盘(西盘)远离主干断裂表现为左行走滑兼逆冲的运动学特征,下盘变形主要以逆冲剪切变形为主,走滑分量极小,并且自西向东韧性剪切变形具有相对强弱相间的特征。  相似文献   

20.
The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.  相似文献   

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