WIDE-ANGLE REFLECTION STUDIES OF THE CRUST AND MOHO BENEATH THE ARCHEAN GNEISS TERRANE OF SOUTHERN MINNESOTA

Densely spaced wide-angle reflection data from oldest Archean crust in southern Minnesota were processed and modeled to place constraints on average crustal structure and the nature of the Moho. A preliminary 1 -D extremal inversion of tau(p) arrivals extracted from vibroseis and quarry blast recordings covering offsets between 70 and 108 km suggest s a crustal thickness between 45 and 51 km. Slowness-depth models corr esponding to extremal depths have average velocities ranging from 6.5 to 7.0 km/s, with velocities at the base of the crust ranging from 6.8 to 7.5 km/s. Estimates of V(p)/V(s) based on travel time ratios of P- and S-wave arrivals show an increase from 1.71 +/- 0.02 in the near-s urface to an average of 1.76 +/- 0.03 for the whole crust, which is co nsistent with an increasingly mafic or plagioclase-rich composition wi th depth. Although the data are sparse, the occurrence of broad-band P (m)P, S(m)S, and P(m)S/S(m)P arrivals at slightly precritical offsets combined with sporadic multicyclic reflections observed in coincident normal-incidence CDP sections suggests that the Moho beneath this terr ane is not a simple velocity gradient, but rather a layered zone invol ving small velocity contrasts.