SEISMIC-REFLECTION INVESTIGATIONS OF THE MOUNT CAYLEY BRIGHT SPOT - AMIDCRUSTAL REFLECTOR BENEATH THE COAST MOUNTAINS, BRITISH-COLUMBIA

An unusually high-amplitude, midcrustal reflector was discovered by a Lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern British Columbia, Canada. The ''bright spot'' was observed adjacent to Mount Cayley, a large Quaternary volcanic cen ter associated with the Cascadia subduction zone magmatic are. Charact erization of the bright spot may advance our understanding of either t he midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust. The location and three-dime nsional geometry of the reflector were determined using travel time in version. The resulting suite of models consistently describe a northwe st dipping reflector that lies 12.5 to 13 km beneath the Mount Cayley volcanic complex. The areal extent of the reflective surface is 3 km b y 1 km and the structure is less than 1.6 km thick. The transition bet ween the body and the country rock must be rapid (< 200 m) and exhibit a large impedance contrast in order to generate reflections of such h igh frequencies (30 Hz) and relative amplitudes (8.0 dB). Our preferre d interpretation for the reflector is a fossil sill complex associated with the volcanic development of Mount Cayley. However, the data do n ot rule out the possibility that the reflective structure represents m elt lenses or saline fluid produced by the dewatering of the subducted slab. The Mount Cayley bright spot Lies just below the brittle upper crust, as do the midcrustal reflectors detected beneath Japanese subdu ction are volcanoes. This correlation suggests that thermal and rheolo gical properties just below the brittle/ductile crustal transition may be favorable for reflector formation.