The influence of pre-existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust (e.g. Nanga Parbat Massif, NW Himalayas), where earlier brittle structures ...have been annealed. The kinematics of young faults, formed in a single, continuing tectonic regime (NNW compression), are distinctly different, depending upon the orientation of the early ductile foliations around them. Faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping. Thus structural anisotropy does control faulting in the continental crust, a result with important implications for seismogenesis, fluid flow and basin evolution.
Globally significant interactions between climate, surface processes, and tectonics have recently been proposed to explain climate change and mountain building. Assessing climate-driven erosion ...processes and geomorphic change in high-mountain environments, however, is notoriously difficult. In the western Himalaya, the coupling of climate, surface processes, and tectonics results in complex topography that frequently records the polygenetic nature of topographic evolution over the last ∼100 ka. Depending upon the erosional history of a particular landscape, temporal overprinting of geomorphic events can produce unique topographic properties which define the spatial complexity of the topography. Field work coupled with analysis of the topography using digital elevation models (DEMs) enable low- and high-frequency spatial patterns and scale-dependent properties of the topography to be detected and associated with geomorphic events caused by climate and tectonic forcing. We conducted spatial analysis of the topography at Nanga Parbat in northern Pakistan to demonstrate the utility of geomorphometry and to characterize dramatic geomorphic change over the past 100 ka. Results indicate rapid river incision, extensive glaciation, and variable denudation rates by mass movement, glaciation, and catastrophic flood flushing. Furthermore, topographic and chronologic evidence indicate that glaciation is strongly controlled by the southwestern monsoon, and that modern fluvial systems are still responding to tectonic forcing and deglaciation. Scale-dependent analysis of the topography revealed that different erosion processes uniquely alter the spatial complexity of the topography, such that the greatest mesoscale relief appears to be caused by glaciation. Collectively, our results indicate that topographic development in the western Himalaya is inherently polygenetic in nature, with glaciation, fluvial and slope processes all playing important roles at different times, and that they can do so sequentially on the same portion of the landscape. Given the rapidity of major changes in climate and glaciation over the last ∼100 ka, the landscape cannot be in steady-state.
U‐Pb zircon isotopic ages of seven rock samples are combined with field data to provide constraints on deposition, intrusion, and metamorphism in the northwestern part of the Indian plate (Pakistani) ...hinterland south of the Indus suture zone in Pakistan. The results suggest deformation, intrusion, and regional metamorphism at ca. 2174 Ma, an event that may correlate with granulite facies metamorphism in the Nanga Parbat area. This was followed by erosion and deposition prior to a second Proterozoic deformation at ca. 1850 Ma, which was associated with widespread intrusion and possibly with low‐grade regional metamorphism. Metasedimentary and intrusive rocks of this age are present within the Lesser Himalaya of Nepal but are apparently absent in the High Himalayan crystalline slab north of the MCT. This intrusive/deformational event was followed by erosion and Late Proterozoic (?) deposition with development, in the Cambrian, of an epicontinental, shallow marine shelf. Intrusion in Late Cambrian‐Middle Ordovician resulted only in minor uplift/erosion and development of a disconformity. Marine shelf conditions were reestablished in the Middle Paleozoic prior to a widespread intrusive event in the Carboniferous‐Permian and normal faulting, erosion, and syndeformational deposition and volcanism in the Late Permian. Marine shelf conditions were reestablished in the Triassic prior to the Himalayan orogeny. Zircons with a concordant age of 89 Ma coupled with field and published isotopic age data suggest that Himalayan deformation and metamorphism in the Pakistani hinterland began between 90 and 75 Ma due to subduction of the Indian plate beneath Indus ophiolitic melange and reached peak amphibolite facies conditions between 70 and 48 Ma. This metamorphism precedes Eocene (54–50 Ma) collision of India with the Kohistan arc complex. Field data suggest that the presently exposed Pakistani hinterland from Afghanistan to Babusar was never significantly overthrust or buried by the Kohistan arc. Rather than initiating the metamorphism, the collision of Kohistan with India resulted in uplift, exhumation, and cooling of the metamorphic pile.
The metamorphic evolution of granulitized eclogites recently discovered in the Eastern Himalaya compared to that of the eclogites of the Northwestern Himalaya (upper Kaghan Nappe and Tso Morari Dome) ...suggests the possibility of a Himalaya-wide eclogitic metamorphism of Early Tertiary age. Eclogites from the Northwestern Himalaya record peak metamorphic temperatures of 580–600°C at metamorphic pressures in excess of 23–24 kbar. They have glaucophane as a retrograde phase and followed a nearly isothermal decompression path into the field of epidote amphibolite facies. In contrast, the Eastern Himalaya eclogites have a strong granulite-facies overprint at metamorphic temperatures of about 750°C and pressures of 7–10 kbars, and followed a clockwise decompression path strongly convex towards high metamorphic temperatures. The main difference between the crystalline nappes of the Northwestern Himalaya and those of the East Himalaya appears to lie in the different
P–
T path they followed during exhumation. In particular the Northwestern Himalaya crystalline nappes lack the Miocene high temperature and low pressure overprinting which is characteristic of the Eastern Himalaya, where thermal relaxation of the thickened continental crust erased almost completely the mineralogical record of the early stages of continental collision.
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