INVERSION OF 3-DIMENSIONAL WIDE-ANGLE SEISMIC DATA FROM THE SOUTHWESTERN CANADIAN CORDILLERA

Seismic refraction/wide-angle reflection data were recorded on a trian gular array in southwestern British Columbia centered on the boundary between the Coast Belt to the southwest and the Intermontane Belt to t he northeast. The experiment, part of the Lithoprobe Southern Cordille ra transect, enabled determination of the three-dimensional (3-D) velo city structure of the crust and upper mantle. An algorithm for the inv ersion of wide-angle seismic data to determine 3-D velocity structure and depth to reflecting interfaces is developed. The algorithm is base d on existing procedures for the inversion and forward modeling of fir st arrival travel times and forward modeling of reflection travel time s, including (1) forward modeling using a 3-D finite difference algori thm; and (2) a simple velocity model parameterization for the inversio n which eliminates the need to solve a large system of equations. The existing procedure is extended to allow (1) the inversion of reflectio n times to solve for depth to a reflecting interface and/or velocity s tructure; (2) the inversion of first arrival travel times to solve for depth to a refracting interface; and (3) layer stripping. Application of the algorithm to southern Cordillera data uses P-g to constrain up per crustal velocity structure, PmP to constrain lower crustal velocit y structure and depth to Moho, and P-n to constrain upper mantle veloc ities and depth to Moho. The 3-D velocity model for the southwestern C anadian Cordillera is characterized by (1) significant lateral velocit y variations at all depths that do not, in general, correlate with sur face geological features or gravity data; (2) a relatively high veloci ty middle and lower,crust in the southwestern part of the study area w hich correlates with a Strong relative gravity high and outlines the e astern extent of lower Wrangellia, an accreted terrane forming the Ins ular Belt to the west; (3) a narrow zone of slower velocity in the low er crust and change in crustal thickness associated with the Fraser Fa ult system, lending additional support to the view that it is a crusta l penetrating fault; (4) an average upper mantle velocity of 7.85 km/s ; and (5) a depth to Moho of 33-36 lan in the Intermontane Belt and 36 -38 lan throughout most of the Coast Belt, decreasing in the west to 3 3 lan near the Insular-Coast contact. Horizontal velocity structure sl ices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region.