Heat sources for Tertiary metamorphism and anatexis in the Annapurna‐Manaslu Region central Nepal

Abstract

Following the collision of India with southern Tibet, crustal rocks of the leading edge of India (1) underwent regional metamorphism to upper amphibolite grade, (2) melted locally to produce anatectic granitoids, and (3) were sheared and thrust onto lower grade rock along the Main Central Thrust, yielding an inverted metamorphic sequence. This sequence is exemplified in the Annapurna‐Manaslu region. We use simple physical calculations to examine the heat sources involved in the different phases of metamorphism. The regional metamorphism apparently is due to the burial of the northern edge of India beneath the accretionary prism along the southern edge of Tibet. The observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation, during the 10–30 m.y. before slip on the Main Central Thrust began, of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mW m−2. The release of water from the footwall of the Main Central Thrust apparently facilitated melting of the overlying crust in the second phase. Such melting could have occurred in the first million years or so of thrusting, if warm (550–650°C) crust in the footwall contained the necessary water. If melting did not occur in the earliest stages of slip on the Main Central Thrust, dissipative heating, with shear stresses of 10 to 100 MPa, is required for temperatures near the Main Central Thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material. The thickness (6–8 km) of the zone of inverted isograds associated with the fault, if undisrupted and due solely to thermal diffusion, implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30–40 km was 4–8 m.y. The apparent inverted temperature gradients (about 10–25°C/km) in this zone can be understood as the combined result of two processes. Diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10–20°C/km. The peak temperatures in such a zone, however, would not have exceeded about 350°C without an additional source of heat. Dissipative heating at shear stresses of about 100 MPa can account for peak temperatures in excess of 600°C during slip on the fault and would have contributed as much as 13°C/km to the inverse gradient. Although inversion of metamorphic isograds could have occurred as a result of deformation within the Main Central Thrust zone, the high temperatures during slip on this zone still require dissipative heating, unless the duration of slip exceeded 25 m.y.