ISOTHERMAL DECOMPRESSION OF MIGMATITES: DIAPIRISM OR NORMAL FAULTING?

Pressure-temperature-time (P-T-t) paths recorded by many high-grade metamorphic terrains suggest that deep, hot crust undergoes near-isothermal decompression. In the case of migmatite-cored gneiss domes, partially molten crust with > 10% melt ascends at rates fast enough to retain temperatures of 700 - 800 °C within the domes. What are the mechanisms by which crust can rise from > 25 km to < 10 km without significant cooling? We apply numerical modeling to evaluate the relationship between unroofing and cooling rates resulting from diapiric ascent of partially molten crust vs. exhumation by low-angle normal faults. Input parameters for both models are based on observed P-T conditions and timing of exhumation of the Thor-Odin dome, Shuswap metamorphic complex, BC. The model fault geometry is based on the Columbia River detachment fault of the Shuswap complex. Fault dips vary from 10° to 30°, and the fault flattens at 15 km depth.

P-T-t paths calculated for diapiric ascent rates of 2-20 km/m.y. show that isothermal decompression is possible for rocks within a diapir. For an ascent rate of 20 km/m.y., a rock at the top of the diapir (30 km starting depth) loses heat to the country rock during decompression and records a linear cooling rate from 775 °C to 380 °C in 1 m.y. However, a rock initially at 39 km (9 km below the top of the diapir) and 775 °C maintains a T greater than 750 °C during decompression. In contrast, P-T-t paths calculated for rocks in the footwall of low-angle normal faults suggest these rocks lose heat during the entire exhumation process. Given a fault displacement rate of 20 km/m.y., cooling rates for a point at 39 km depth range from 40 to 120 °C/m.y. for fault dips of 10° and 30°, respectively. To test whether isothermal decompression can result from motion along a low-angle normal fault, we input a displacement rate of 40 km/m.y. along a 30°-dip fault, consistent with 20 km/m.y. vertical motion. A rock at 39 km depth is exhumed to 19 km and cools from 840° to 650 °C in 1 m.y. This calculated decompression/cooling path suggests rapid, but not isothermal decompression. Therefore, we propose that there must be some element of diapirism to retain isothermal decompression paths in exhumed deep crust.