The thermal response of a metamorphic belt to extension: constraints from laser Ar data on metamorphic micas

Abstract

Metamorphic rocks may be exhumed to the surface either by isostatic rebound, uplift and erosion or tectonically via thrusting or extension. The latter mechanism is particularly important in the Himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at ∼20 Ma. Here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension. The data, when combined with existing constraints on the prograde evolution, lead to a uniquely complete thermal history for a metamorphic belt. Laser argon data are reported for quartz and micas from semi-pelitic metamorphic rocks from the Zanskar Himalayas, NW India. The samples derive from three traverses extending from low-grade rocks near the detachment to high-grade rocks in the core of the belt. Most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment. The ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached. Muscovite and biotite ages from within the shear zone date deformation associated with extension at 20–25 Ma. The remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active. Rocks in the core of the orogen were still heating at this time. Later fast cooling of the high-grade rocks in the core of the belt ensued when the extensional regime became active. Simple heat-transfer calculations based on the distance of the high-grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23–25 Ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt. Our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima.