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1.
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   

2.
Structural analysis and new zircon U/Pb geochronology were performed in the Cuonadong Dome in order to test the hypothesis that the top‐to‐North deformation in the North Himalayan Gneiss Domes (NHGD) has been correlated with deformation along the South Tibetan detachment system (STDS). Structural analysis suggests that the X‐axes of sheath folds at the different flanks of the dome are consistent, and show average a trend and plunge of 350° and 18°. Many shear sense indicators are recorded in the ductile shear zone during the top‐to‐north deformation, including the augen structure, σ‐type and δ‐type rotating porphyroclast, S–C fabric and pressure shadow structure. The development of the sheath folds first provide a direct‐field structural evidence that represents a deeper ductile manifestation of the STDS, and further suggests that the top‐to‐north deformation in the NHGD have been correlated with deformation along the STDS. The emplacement of the synkinematic granites at ca. 32 Ma exposed at the core of the sheath folds is also interpreted as the initiation of slip along the STDS.  相似文献   

3.
A specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ～749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ～4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.  相似文献   

4.
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   

5.
In metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down-dip continuation of a detachment fault, or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault. Detailed microstructural, fabric, and strain data from mylonites in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, constrain the structural development of the lower plate shear zone. Widespread exposures of ∼22–21 Ma granitoids of the Swansea Plutonic Suite enable us to separate Miocene strain coeval with core complex extension from older deformation. Mylonites across the lower plate consistently record top-to-the-NE-directed shear. Miocene quartz and feldspar deformation/recrystallization mechanisms indicate ∼450–500 °C mylonitization temperatures that were relatively uniform across a distance of ∼35 km in the extension direction. Quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction. Strain recorded in the Swansea Plutonic Suite is also relatively uniform in the extension direction, which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault. Altogether these data suggest the mylonitic shear zone initiated with a ≤4° dip and was unroofed by a more steeply dipping detachment fault system. Lower plate mylonites in the Buckskin-Rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down-dip continuation of a detachment fault.  相似文献   

6.
Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks. This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust. Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata, juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about 15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades, although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic metamorphism.  相似文献   

7.
The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.  相似文献   

8.
The so‐called Plankogel detachment is an east‐west trending, south‐dipping low‐angle structure that juxtaposes the high‐P rocks of the eclogite type locality of the eastern European Alps against amphibolite facies rocks to the south. It occurs in both the Saualpe and Koralpe Complex in eastern Austria. During Cretaceous intracontinental subduction, the footwall and the hangingwall units of the Plankogel detachment were buried to different crustal levels as inferred by pseudosection modelling and conventional thermobarometry: ~23–24 kbar and 640–690 °C for the eclogite facies units in the footwall of the detachment and ~12–14 kbar and 550–580 °C for the amphibolite facies metapelites in the hangingwall. Despite the different peak metamorphic conditions, both sides of the detachment display a common overprint at conditions of ~10 kbar and 580–650 °C. From this, we infer a two‐stage exhumation process and suggest that this two‐stage process is best interpreted tectonically in terms of slab extraction during Eoalpine subduction. The first stage of exhumation occurred due to the downward (southward) extraction of a lithospheric slab that was localized in the trace of the Plankogel detachment. The later stage, however, is attributed to more regional erosion‐ or extension‐driven processes. Since the Plankogel detachment is geometrically related to a crustal‐scale shear zone further north (the Plattengneiss shear zone), we suggest that both structures are part of the same extraction fault system along which the syn‐collisional exhumation of the Eoalpine high‐P units of the Eastern Alps occurred. The suggested model is consistent with both the mylonitic texture of the Plattengneiss shear zone and the overall ambiguous shear sense indicators present in the entire region.  相似文献   

9.
The Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.  相似文献   

10.
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   

11.
Abstract Geological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge-shaped body bounded below by the N-dipping Main Central thrust system and above the N-dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high- T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between the P–T paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment system.  相似文献   

12.
At the junction of the Atlantic and Arctic margins, the crustal‐scale Keisarhjelmen detachment of north‐west Svalbard records previously unrecognised magnitudes of extension. The detachment separates a corrugated metamorphic core complex in the footwall from a mantling Devonian supradetachment basin in the hangingwall. The detachment has a top‐N displacement of more than 50 km, which is aligned with the map‐scale corrugations, and an upwards ductile to brittle transition with shear related footwall retrogression. This configuration has striking similarities to extensional collapse detachments in the paired Scandinavian–Greenland Caledonides, but orientation and position link the detachment with the Ellesmerian orogen.  相似文献   

13.
Intracontinental foreland basins with fold-and-thrust belts on the southern periphery of the Tianshan orogenic belt in China resulted from still-active contractional deformation ultimately cased by the India–Asia collision. To quantify the amounts of shortening distance and the rates of deformation, and to decipher the architectural framework, we mapped the stratigraphy and structure of four anticlines in the Kuqa and Baicheng foreland thrust belts in the central southern Tianshan. In the Baicheng foreland thrust belts, Lower Cretaceous Baxigai and Bashijiqike Formations located in the core of the Kumugeliemu anticline are overlain by the Paleocene to Eocene Kumugeliemu Formation, above which are conformable Oligocene through Pleistocene sediments. A disharmonic transition from parallel to unconformable bedding at the boundary of the Miocene Kangcun and Pliocene Kuqa Formations suggests a change from pre-detachment folded strata to beds deposited on top of a growing anticline. Most of the anticlines have steep limbs (70–90°) and are box to isoclinal folds, suggestive of detachment folding or faulted detachment folding (faults that transect a fold core or limb). Shortening estimates calculated from the cross-sections by the Excess area method indicate that the total shortening for the Kelasu, Kuchetawu, Kezile and Yaken sections are 6.3 km, 6.4 km, 5.8 km and 0.6 km, respectively, and the respective depths of the detachment zones are (2.3 km and 6.9 km), 2.3 km, 2.5 km and 3.4 km. Time estimates derived from a paleomagnetic study indicate that the transition to syn-folding strata occurred at ∼6.5 Ma at the Kuchetawu section along the Kuqa river. In addition, according to our field observations and previous sedimentary rate studies, the initial time of folding of the Yaken anticline was at 0.15–0.21 Ma. Therefore, the average shortening rate that began at ∼6 Ma was ∼2 mm/a for the Kelasu, Kuchetawu and Kezile sections. At 0.15–0.21 Ma, the average shortening rate increased to 3–4 mm/a in the Yaken section. Combined with the recent GPS data, the shortening rate in the central southern Tianshan area increased to 4.7 ± 1.5 mm/a at present. We suggest that there was a linear increase in shortening rate in the southern Tianshan foreland basin, which also indicates that the far field stress increased considerably from the late Miocene to Present in response to the India–Asia collision.  相似文献   

14.
刘江  张进江  郭磊  戚国伟 《岩石学报》2014,30(7):1899-1908
晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。  相似文献   

15.
The recent identification of multiple strike‐parallel discontinuities within the exhumed Himalayan metamorphic core has helped revise the understanding of convergence accommodation processes within the former mid‐crust exposed in the Himalaya. Whilst the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their identification and characterization has become important for potential correlations across regions, and for constraining the kinematic framework of the mid‐crust. The result of new phase equilibria modelling, trace element analysis and high‐precision Lu–Hf garnet dating of the metapelites from the Likhu Khola region in east central Nepal, combined with the previously published monazite petrochronology data confirms the presence of one of such cryptic thrust‐sense tectonometamorphic discontinuities within the lower portion of the exhumed metamorphic core and provides new constraints on the P–T estimates for that region. The location of the discontinuity is marked by an abrupt change in the nature of P–T–t paths of the rocks across it. The rocks in the footwall are characterized by a prograde burial P–T path with peak metamorphic conditions of ~660°C and ~9.5 kbar likely in the mid‐to‐late Miocene, which are overlain by the hanging wall rocks, that preserve retrograde P–T paths with P–T conditions of >700°C and ~7 kbar in the early Miocene. The occurrence of this thrust‐sense structure that separates rock units with unique metamorphic histories is compatible with orogenic models that identify a spatial and temporal transition from early midcrustal deformation and metamorphism in the deeper hinterland to later deformation and metamorphism towards the shallower foreland of the orogen. Moreover, these observations are comparable with those made across other discontinuities at similar structural levels along the Himalaya, confirming their importance as important orogen‐scale structures.  相似文献   

16.
Understanding convergent margin processes requires determination of the onset and the termination of subduction, the duration of subduction‐zone metamorphism, and the subduction zone polarity. Garnet growth and intracrystalline zonation can be used to constrain the timing, duration and kinetics of tectonometamorphic processes. An eclogite from the Huwan shear zone in the Hong'an orogen was investigated with combined pseudosection analysis and multiple geochronologies. The pseudosection analysis illustrates that garnet growth is continuous and along an early near‐isothermal trajectory followed by a near‐isobaric heating path from 1.9 GPa/500 °C to 2.4 GPa/575 °C and subsequent near‐isothermal decompression. 40Ar/39Ar dating of an amphibole inclusion in garnet from the eclogite yielded an age of 310 ± 5 Ma, which is consistent with a U–Pb age of 305 ± 3 Ma for the metamorphic zircon within uncertainty. Garnet core and rim material produced Lu–Hf ages of 296.9 ± 3.8 and 256.9 ± 3.9 Ma respectively; the latter is consistent with its Sm–Nd age of 254.3 ± 4.6 Ma for the same aliquots. Similarly, limited zircon U–Pb ages of c. 257 Ma were obtained in zircon rims with garnet inclusions. These ages were interpreted to bracket the period of garnet growth and the difference of up to c. 40 Ma is best explained by protracted garnet growth. We propose that the rocks represent detachment of part of the downgoing slab and remained free of significant compression/decompression or heating/cooling close to the subduction channel, most likely underplating the mantle wedge, for a long time. These rocks were incorporated into the following subduction channel due to the successive entry of the buoyant materials, and exhumed at some time later than c. 254 Ma. The increasing observations of protracted garnet growth and long‐lived subduction in various orogens worldwide demand more sophisticated geodynamic models.  相似文献   

17.
The Tongbai orogenic belt has an overall antiformal geometry and the hinge of the antiform is sub-horizontal and trends NW–SE. The Tongbai complex (TBC) in the core of the antiform is bounded by the S-dipping Yindian–Malong shear zone in the south, the sub-horizontal Taibaiding shear zone at the top and the N-dipping Hongyihe–Tongbai shear zone in the north. The three shear zones have dextral, top-to-NW and sinistral movement, respectively. They are parts of a single shear zone, termed the Tongbai shear zone, that has a uniform top-to-NW sense of shear. Three samples of deformed granitoid (mylonite or protomylonite) from the shear zone have U–Pb zircon ages of 145 ± 6 Ma, 142 ± 2 Ma and 131 ± 6 Ma, respectively. An L-tectonite in the TBC yielded a metamorphic age of 137 ± 8 Ma and a migmatite an age of 137 ± 1 Ma. The Tongbai shear zone is intruded by undeformed Early Cretaceous granite and dykes and deformation in the shear zone is constrained to ca. 140–135 Ma, synchronous with metamorphism and migmatization in the TBC. Early Cretaceous magma emplacement and the associated uplift modified the TBC into a gentle antiform and the uplift may have continued to ca. 102–85 Ma. Similar geometry and kinematics have been documented in the Dabie orogenic belt to the east, which suggests that the Central Orogenic Belt in China probably experienced a uniform orogen-parallel extension and top-to-NW shearing in the ductile lithosphere in the Early Cretaceous.  相似文献   

18.
We constrain the timing and kinematics of the Serifos detachment in the southwestern Cyclades, Greece, using low-temperature thermochronometry. Fission-track dating shows that the Serifos detachment was active between ~13 and 6 Ma and that the Serifos granodiorite in its footwall intruded at or before ~12–11 Ma into the extensional shear zone and initially cooled very rapidly at rates >180°C per million year. The mylonite zone at the top of the granodiorite and mylonitic structures in its country rocks record a consistent top-SSW shear sense in the ductile crust. In the brittle regime top-NNE shear-sense indicators occur as well. Conjugate top-SSW and top-NNE high-angle normal faults are the youngest deformational features and cut across the detachment. Age–distance relationships for the fission-track data display a relatively flat pattern. We discuss a model advocating initial top-SSW movement on the Serifos detachment before and during emplacement of the granodiorite. Updoming of the detachment during exhumation and cooling caused subsequent bivergent extension in the brittle crust.  相似文献   

19.
The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite 40Ar/39Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.  相似文献   

20.
纪沫  刘俊来  胡玲  关会梅  G DAVIS  张维 《岩石学报》2009,25(1):173-181
辽南变质核杂岩形成于晚中生代华北岩石圈伸展和减薄背景下,其演化过程分为三个阶段：第一阶段（约130Ma）,在伸展作用下辽南地区发育拆离断层及其下伏韧性剪切带;第二阶段（130~120Ma）,同构造花岗质岩体的主动侵位影响了拆离断层的演化;同时在124Ma变质核杂岩的构造-岩浆活动达到峰值;第三阶段（113Ma之后）花岗质岩体在较浅层次侵位于停止活动的拆离断层带。辽南变质核杂岩发育与演化过程在年代学意义上揭示了华北板块晚中生代时期的区域性岩石圈伸展与减薄过程。本文认为由伸展后期侵位的赵房岩体所记录的拆离断层停止的时限（113Ma）可以作为华北岩石圈强烈构造岩浆活动的转捩点,113Ma之前华北板块广泛发育变质核杂岩、拉分盆地等一系列伸展构造,之后则以平稳的隆升和冷却过程为主。  相似文献   

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