Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya
Published:January 01, 2006
C. Annen, B. Scaillet, 2006. "Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya", Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones, R. D. Law, M. P. Searle, L. Godin
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The crustally derived High Himalayan leucogranites (HHL) are characterized by strong isotopic heterogeneity and occurrence of magmatic muscovite. Such attributes indicate that the HHL were non-convecting magma bodies and crystallized at pressure-equivalent depths of more than 8.5 km. We have performed one-dimensional thermal modelling in order to simulate the process of incremental growth of a laccolith whose roof is tectonically removed during intrusion, in a context of crustal exhumation due to channel flow. The objective is to define under what conditions HHL laccoliths emplaced close to active normal faults may be built without convecting while crystallizing muscovite. The results indicate that for a HHL thickness in the range 5–10 km, denudation rates cannot be higher than 4 mm a-1, and are more likely below 3 mm a-1. At such denudation rates, the intrusion process needs to start at depths of c. 22 km, except when the final laccolith thickness is 10 km, in which case the depth of first-emplaced magmas cannot exceed 18 km. Thick HHL laccoliths (>7 km) may require a minimum denudation rate, on the order of 1 mm a-1, to prevent wholesale convection and allow muscovite crystallization. Yet, emplacement of such thick HHL laccoliths during normal faulting implies that the top part of the leucogranite nearly reaches the surface while its base is still fed by active intrusions. Overall, such relatively low denudation rates suggest that, when HHL were intruded, the overlying crustal column was not undergoing vigorous erosion. Within the framework of a crustal channel flow, this suggests that the zone of focused erosion during the Miocene was located to the south of the current exposures of the HHL belt. Our results also show that to explain the steep cooling histories documented in many HHL, denudation must have been active after HHL solidification, especially when they were intruded close to their source region. However, to preserve the HHL from exhumation and erosion until the present time, the average denudation rate after emplacement cannot have exceeded 0.5 mm a-1.
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Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones
This collection of 27 review and research papers provides an overview of the geodynamic concepts of channel flow and ductile extrusion in continental collision zones. The focal point for this volume is the proposal that the middle or lower crust acts as a ductile, partially molten channel flowing out from beneath areas of over-thickened crust, such as the Tibetan plateau, towards the topographic surface at plateau margins. This controversial proposal explains many features related to the geodynamic evolution of the plateau and, for example, extrusion and exhumation of the crystalline core of the Himalayan mountain chain to the south. In this volume thermal-mechanical models for channel flow, extrusion and exhumation are presented, and geological and geophysical evidence both for and against the applicability of such models to the Himalayan-Tibetan Plateau system, as well as older continental collision zones such as the Hellenides, the Appalachians and the Canadian Cordillera, are discussed.