GRAVITY MEASUREMENTS ALONG THE IDOR SEISMIC TRANSECT

We conducted a detailed gravity transect from eastern Oregon (Prairie City, OR) to central Idaho (Lemhi Range), along the 430 km-long, EW-oriented IDOR seismic line. Stations were taken at a spacing of 0.5-1.0 km, with the station density highest near the center of the gravity transect. The gravity transect crosses the western Idaho shear zone (WISZ) near its center, which is the current boundary between the accreted terranes to the west and cratonic North America to the east.

Several trends are observable, which are consistent with the pre-existing and regional gravity data. The section of the transect that crosses the Idaho batholith exhibits a low Bouguer gravity signal (~-220-200 mgal), resulting from the low-density granitic rocks. The Sawtooth and Lost River basins toward the eastern end of the transect exhibit local gravity lows, associated with low-density basin fill. The highest gravity values are found near the ID-OR border. This gravity high appears to be in continuity with the gravity high that corresponds to the western Snake River plain. The Bouguer gravity decreases again near the western end of the line, to a low near -170 mGal, associated with the Blue Mountain terranes. This result is consistent with preliminary active seismic data, which indicate that faster seismic velocities (inferred to result from dense rocks) occur in the upper 5-15 km to the west of the WISZ. This result contrasts with slower seismic velocities to the east of the WISZ, corresponding to continental-affinity batholith and craton. Finally, a steep gradient in Bouguer gravity occurs at the western Idaho shear zone, with the eastern (North America) side having a lower gravity signal. The modeled gravity data is consistent with a significantly shallower Moho west of the surface expression of the WISZ, which was determined from preliminary interpretation of active seismic data. This gravity study, combined with ongoing IDOR geological and seismic studies, will provide insight into the tectonic development of this region.