摘要 :
Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over 30km of crustal section during the India-Asia collision. In the Central Pami...
展开
Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over 30km of crustal section during the India-Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7-10km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90km. Two thrust sheets, comprising Cambro-Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite-facies metamorphic rocks within the domesequivalent to lower grade rocks outside the domesform fold nappes with dome-scale wavelengths. E-W stretching occurred contemporaneously with top-to-N imbrication and folding. At 22-12Ma, bivergent (top-to-N and top-to-S), normal-sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at 12Ma with opposite-sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant N-S shortening interacted with E-W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibetan Plateau and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India-Asia collision, and the Miocene-Recent dextral shear and E-W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.
收起
摘要 :
Neogene, syn-collisional extensional exhumation of Asian lower-middle crust produced the Shakhdara-Alichur gneiss-dome complex in the South Pamir. The <1 km-thick, mylonitic-brittle, top-NNE, normal-sense Alichur shear zone (ASZ) ...
展开
Neogene, syn-collisional extensional exhumation of Asian lower-middle crust produced the Shakhdara-Alichur gneiss-dome complex in the South Pamir. The <1 km-thick, mylonitic-brittle, top-NNE, normal-sense Alichur shear zone (ASZ) bounds the 125 x 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km-thick, mylonitic-brittle, top-SSE South Pamir normal-sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22-17 Ma Alichur-dome-injection-complex leucogranites transition from foliation-parallel, centimeter- to meter-thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization-temperature constraints suggest Alichur-dome exhumation from 10-15 km depth. Thermochronologic dates bracket footwall cooling between 410-130 degrees C from 16-4 Ma; tectonic cooling/exhumation rates (42 degrees C/Myr, 1.1 km/Myr) contrast with erosion-dominated rates in the hanging wall (2 degrees C/Myr, <0.1 km/Myr). Dome-scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E-W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between 20-4 Ma to: (i) Mostly northward, foreland-directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non-underthrust Central Pamir.
收起
摘要 :
Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before 35Ma and peaked at 23-20Ma, ref...
展开
Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before 35Ma and peaked at 23-20Ma, reflecting top-to-N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper 7-10km of the Asian crust. Multimethod thermochronology traces cooling through 700-100 degrees C between 22 and 12Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at 22-17Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at 3km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7-31 degrees C/Myr) starting between 23 and 12Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at 23-21Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at 12Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g.,slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.
收起
摘要 :
Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 ...
展开
Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U-Th)/He (AHe), 15 fission track (AFT), and four zircon (U-Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3km high Jabal Akhdar culmination AHe single-grain ages range between 392 Ma and 101 Ma (2 sigma errors), AFT ages range from 518 Ma to 324 Ma, and ZHe single-grain ages range from 62 +/- 3Ma to 39 +/- 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 +/- 4 to 12 +/- 4 Ma, AFT ages from 73 +/- 19Ma to 57 +/- 8 Ma, and ZHe single-grain ages from 81 +/- 4 Ma to 58 +/- 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N-S convergence between Arabia and Eurasia.
收起
摘要 :
A balanced cross-section spanning the Eastern Cordillera and Subandean Zone of southern Peru (13-15 degrees S) constrains similar to 130 km (38%) of Cenozoic orogen-normal SW-NE Andean deformation accommodated by thick- and thin-s...
展开
A balanced cross-section spanning the Eastern Cordillera and Subandean Zone of southern Peru (13-15 degrees S) constrains similar to 130 km (38%) of Cenozoic orogen-normal SW-NE Andean deformation accommodated by thick- and thin-skinned retro-arc fold-thrust belt shortening that overprinted pre-Andean Triassic normal faults. Zircon and apatite (U-Th)/He ages demonstrate continuous Oligocene to Miocene cooling of the Permo-Triassic Coasa pluton in the Eastern Cordillera. Zircon (U-Th)/He ages (similar to 34-18 Ma) are reset and define a steep age versus elevation relationship. Apatite (U-Th)/He results reveal reset ages that define two spatially separated groups with ages of similar to 30-26 Ma and similar to 17-11 Ma. Detrital zircon U-Pb geochronologic results from Cretaceous-Cenozoic siliciclastic rocks from the Altiplano/Eastern Cordillera record Andean fold-thrust belt and magmatic-arc sediment sources. Correlative Subandean Zone rocks preserve a cratonic sediment contribution, with minor Andean sediment appearing in some Cenozoic rocks. We propose that earliest Andean deformation and structural compartmentalization of the Eastern Cordillera was linked to selective inversion of inherited Permo-Triassic basement-involved normal faults that guided subsequent thick- and thin-skinned deformation. Provenance variations between the hinterland and foreland depocentres reveal competing eastern and western sediment sources, reflecting an axial zone in the Eastern Cordillera that coincided with the inherited Triassic graben and impeded sediment source mixing. Our zircon and apatite (U-Th)/He ages are consistent with published constraints along strike and support pulses of Eocene to late Miocene exhumation that were likely driven by normal fault reactivation and protracted Eastern Cordillera deformation.
收起
摘要 :
Large domes of crystalline, middle to deep crustal rocks of Asian provenance make the Pamir a unique part of the India-Asia collision. Combined major-element and trace element thermobarometry, pseudosections, garnet-zoning deconst...
展开
Large domes of crystalline, middle to deep crustal rocks of Asian provenance make the Pamir a unique part of the India-Asia collision. Combined major-element and trace element thermobarometry, pseudosections, garnet-zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15-20km at 37-27Ma. The Central Pamir was then heated at similar to 10-20 degrees C/Myr and buried at 1-2km/Myr to 600-675 degrees C at depths of 25-35km by 22-19Ma. The Shakhdara Dome in the South Pamir was heated at similar to 20 degrees C/Myr and buried at 2-8km/Myr to reach 750-800 degrees C at depths of 50km by similar to 20Ma. All domes were exhumed at >3km/Myr to 5-10km depths and similar to 300 degrees C by 17-15Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large-scale, normal-sense shear zones, and with advection of near-solidus or suprasolidus temperatures into the upper crust, triggering exhumation-related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large-scale thickening and coincident similar to 20Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate-scale events.
收起
摘要 :
Upper crustal extensional structures range from steep normal faults to shallow-dipping detachments. The relationship between extension and formation of synkinematic hanging wall basins including their relative timing is not well u...
展开
Upper crustal extensional structures range from steep normal faults to shallow-dipping detachments. The relationship between extension and formation of synkinematic hanging wall basins including their relative timing is not well understood. The South Rhodope core complex, Southern Balkans, has experienced extension for >40Ma leading to a number of extensional structures and Cenozoic sedimentary basins. We present new bedrock and basin detrital zircon and apatite (U-Th-Sm)/He ages from the Pirin and Rila Mountains and the Sandanski basin. Results identify three episodes of Cenozoic extension in SW Bulgaria accommodated by (1) the Eocene/Oligocene Mesta detachment; (2) the early to middle Miocene Gorno Spanchevo fault (circa 18-15Ma), which is the northern prolongation of the Strymon low-angle detachment; and (3) the late Miocene West Pirin fault (10Ma). Detachment faulting on the Strymon fault accommodated tens of kilometers of ENE-WSW extension and created similar to 1500m topographic relief, but because the resulting hillslopes were gentle (10 degrees), extension did not lead to enhanced footwall erosion or formation of a hanging wall basin. In contrast, the West Pirin normal fault resulted in mostly vertical motion of its footwall causing steep topography, rapid erosion, and formation of the synrift Sandanski basin. Digital topographic analysis of river channel profiles identifies the latest episodes of deformation including westward tilting of the Sandanski and Strymon basins and Quaternary N-S extension. This study demonstrates that basin formation in the South Rhodope core complex is related to normal faulting postdating the main episode of crustal stretching by detachment faulting.
收起
摘要 :
We investigate the spatiotemporal evolution of exhumation in the ice-covered St. Elias syntaxis area, southeast Alaska, using multiple thermochronometers and geochronometers from cobble-sized glacial detritus. Multiple thermochron...
展开
We investigate the spatiotemporal evolution of exhumation in the ice-covered St. Elias syntaxis area, southeast Alaska, using multiple thermochronometers and geochronometers from cobble-sized glacial detritus. Multiple thermochronometers reveal the cooling histories from 500 to 60 degrees C of 27 glacially transported cobbles from the two largest catchments of the syntaxis. Cobble lithologies and 21 zircon U-Pb ages (277-31Ma) were examined to determine sample provenance. Furthermore, eight amphibole and seven biotite Ar-40/Ar-39 ages (276-16Ma and 50-42Ma, respectively), four zircon and six apatite (U-Th)/He ages (35-4.8Ma and 4.2-0.6Ma, respectively), and four apatite fission track ages (17-1.6Ma) were used to reconstruct the individual cobble cooling histories. An additional four bedrock samples from the Fairweather Range yielded three biotite Ar-40/Ar-39 ages between 42 and 5Ma. A compilation of published bedrock and new cobble cooling histories from the St. Elias Mountains and Fairweather Range reveals the regional Cenozoic cooling and exhumation history, emphasizing the position of the St. Elias syntaxis as a transitional zone between transpression and subduction settings. The new cobble and bedrock data indicate an onset of rapid exhumation at 5Ma that was limited in duration (2-3 Myr) and amount (10km) in the syntaxial region. This study also demonstrates the usefulness of cobbles for revealing thermal histories of otherwise inaccessible regions as cobble analysis combines advantages of bedrock and detrital thermochronology.
收起
摘要 :
Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the prograde metamorphic history of Eohimalayan crustal thickening, which has been erased by Oligo-/Miocene migmatization elsewhere in the...
展开
Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the prograde metamorphic history of Eohimalayan crustal thickening, which has been erased by Oligo-/Miocene migmatization elsewhere in the Himalaya. Our zircon and monazite U/Th-Pb data unravel a multi-stage prograde metamorphic evolution. The earliest evidence of prograde Barrovian metamorphic monazite growth is similar to 41 Ma. Peak metamorphic conditions (similar to 8-8.5 kbar, similar to 600-700 degrees C) were attained at 37-36 Ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth, which may be related to the formation of the northern Himalayan nappes (e.g., Shikar Beh nappe, Nyimaling nappe). Rapid exhumation of the crystalline started at similar to 26 Ma and resulted in cooling through the muscovite Ar-40/Ar-39 closure temperature by 21.8 Ma. Although a local continuation of the South Tibetan detachment is not unambiguously identified in central Himachal Pradesh extrusion was likely facilitated by a system of several minor late Oligocene/early Miocene top-to-the-N to NE shear zones. In contrast to the crystalline of Zanskar and eastern Himachal Pradesh, extrusion was not accompanied by widespread decompression melting. (C) 2014 Elsevier B.V. All rights reserved.
收起
摘要 :
The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust...
展开
The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since similar to 10-15Ma. In contrast, the area north of the fault has experienced steady cooling since similar to 25-35Ma. We attribute this cooling contrast to similar to 2km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an similar to 200km wide block, which should be incorporated in future studies on the region's deformation, and in both above-mentioned deformation models.
收起
原文获取