THE ROLE OF FLUIDS IN THE COOLING OF METAMORPHIC CORE COMPLEXES

Metamorphic core complexes manifest large-scale lateral displacement of the lithosphere and are characterized by regionally-extensive, low-angle detachment faults which juxtapose high-grade mid-crustal rocks against generally unmetamorphosed upper-crustal lithologies. Hydrothermal alteration and economically-significant mineralizations along these faults result from extensive fault-related fluid flow. Models of lithospheric extension based on metamorphic core complex evolution have relied on quantitative constraints on the cooling, uplift and slip rates derived primarily from thermochronologic data (⁴⁰Ar/³⁹Ar and fission tracks). However, inherent in the interpretation of these data is the assumption that cooling of the footwall occurred via conduction in response to uplift. Petrologic and oxygen isotope data from the Whipple Mountains metamorphic core complex in southeastern, California (USA) documents an unusually steep thermal gradient in the footwall, immediately underlying the fault, of ~80°C over ~40m (or ~2160°C/km). Ambient pre-detachment temperatures were estimated using two feldspar thermometry to be 458 ± 35°C. Values of d¹⁸O for adjacent quartz and epidote grains, measured using a laser extraction system, yield average D_Qtz-Ep temperatures that decrease systematically from 432°C at 50m below the fault, to 350°C at 12m below the fault. This gradient is interpreted to result from the extraction of heat from the lower plate as cold, surface-derived fluids flowed along the detachment fault, in a process termed fault zone refrigeration. Fluid d¹⁸O values were ~2‰ at the fault surface and are interpreted to document exchange with relatively cold, surface-derived basinal brines which circulated down through high angle normal faults in the upper plate. As the fluid flowed through the upper portions of the footwall, heat was advectively removed causing rapid cooling of the lower plate. If this fluid-induced refrigeration of the fault zone is a process inherent to core complex evolution, then quantitative constraints on core complex evolution derived solely from thermochronologic data may not be accurate.