WIDE-ANGLE SEISMIC IMAGING OF PRISTINE ARCHEAN CRUST IN THE NAIN PROVINCE, LABRADOR

A detailed refraction - wide-angle reflection seismic experiment was c arried out in northern Labrador to determine the velocity structure of relatively unaltered Archean crust in the Nain Province. Six 3-compon ent land seismometers were used to record an airgun source along a pro file parallel to the coast. Forward modeling of traveltimes and amplit udes yields a P- and S-wave velocity model that shows two crustal bloc ks separated by a fault. Magnetic data suggest, but do not prove, that the fault is the offshore continuation of the Handy fault. A southwar ds thickening of the lower crust across the fault indicates that a tra nscurrent component might have been associated with the faulting. The total crustal thickness is 33 km to the north and 38 km to the south o f the fault. The presence of PmS reflections imply a sharp transition at the Moho. Upper crustal velocities of 5.8-6.3 km/s and Poisson's ra tios of 0.20 and 0.24, north and south of the fault respectively, are consistent with a gneissic composition, but suggest a higher quartz co ntent in the northern block. Velocities in the middle crust increase t o 6.5 km/s, where a discontinuity at a depth between 16 and 18 km mark s the transition to the lower crust with velocities between 6.6 and 6. 9 km/s. Poisson's ratios of 0.24 and 0.26 indicate, respectively, a fe lsic middle crust and an intermediate composition for the lower crust. The absence of a high-velocity basal layer is in accordance with othe r examples of Archean crust.