DEXTRAL SHEAR ALONG THE EASTERN CALIFORNIA SHEAR ZONE IN NORTHEAST CALIFORNIA AND SOUTHERN OREGON

The southern back-arc of the Cascadia subduction zone is located at the convergence of several tectonic provinces, including the Cascade volcanic arc, the Basin and Range, and the possible northward continuation of the Eastern California Shear Zone (ECSZ). As a consequence, deformation in the region is complex and in some cases may obscure displacements due to magmatic activity beneath Cascade volcanoes (including known subsidence at Medicine Lake, CA, and inflation near South Sister, OR). Deformation patterns in both the Basin and Range and Cascadia have been described by several authors using GPS data from campaign and continuous networks collected over the past decade. In contrast, the geometry and dynamics of the northern ECSZ are not well known.

The ECSZ is manifested in eastern California (latitude ~36N) by NW-striking right-lateral faults with cumulative displacements of up to 12 mm/yr. Dextral slip has been identified as far north as latitude 40N, where ~8 mm/yr occurs on the Honey Lake and Mohawk Valley fault zones, but the continuation of the ECSZ to the north is not well constrained. A series of en echelon, NW-striking dextral faults occur north of the Honey Lake fault zone, including (from south to north) the Likely, Mount McLaughlin, Eugene-Denio, and Brothers lineaments. Recent GPS measurements in the Medicine Lake region of NE California suggest approximately 6 mm/yr of right-lateral displacement on the Likely Fault. Additional dextral shear may be accommodated by faults in southern Oregon.

Combining GPS results from northeast California and southern Oregon (obtained from campaign surveys by the US Geological Survey and permanent stations of the BARD and PANGA networks) with existing data from the Medicine Lake and Basin and Range regions may provide enough information to characterize the northward extent of the ECSZ. We will present GPS-derived crustal displacements for the region and investigate the contribution of dextral shear to the overall pattern of deformation, with the goal of developing a kinematic model for the northern ECSZ and its transition to back-arc Cascadia deformation.