GRAVITY TRANSECTS ACROSS THE SALMON RIVER SUTURE ZONE, WESTERN IDAHO

The Pacific Northwest Cordilleran of the U.S. was constructed through the accretion of complex island arc terranes to Laurentia beginning in the Mesozoic. The boundary between the pre-collisional Laurentian margin and the accreted arc terranes now lies in western Idaho within the Salmon River suture zone. To investigate the crustal structures in the Salmon River suture zone, a detailed gravity survey was constructed along three transects.

Gravity stations were collected using a LaCoste Romberg gravity meter along three transects. Two transects, from 1) French Creek, ID to Seven Devils lookout, ID (62 km) and, 2) Smith Mountain, ID to Ant Basin trailhead, ID (18 km), were collected with ~1 km spacing between stations. A regional 100 km transect from Homestead, OR to New Meadows, ID was conducted at 2 km spacing. A local gravity base station was established in Cascade, ID to tie to the absolute gravity base station at Grangeville, ID in order to merge the new data with pre-existing data. The data were reduced to complete Bouguer gravity anomalies using Hammer charts and 10 meter DEMs for terrain corrections. The transects span across the Seven Devils Group volcanics of the Wallowa island arc terrane in the west, through metasedimentary/metavolcanic rocks of the Riggins Group, and into amphibolites and orthogneisses in the east. The complete Bouguer gravity anomaly data will be used to create two dimensional models constrained by Earthscope broadband seismic models to provide deeper insights into the crustal structures of the Salmon River suture zone. These results will be used to construct models to investigate the subsurface architecture of major thrust sheets in the suture zone that will provide insight into the crustal structure of accretionary orogens.