The highly metamorphosed Western Idaho Suture Zone (WISZ; aka Salmon River Suture Zone) marks the contact between northeastern Oregon's Wallowa terrane and the craton. From near Boise to Orofino, ID the WISZ is very well defined and dips steeply as evidenced by lithologic contacts, closely spaced Sr87/Sr86 isopleths, and reasonably sharp Bouguer and aeromagnetic anomalies. To the south the WISZ plunges beneath the Snake River Plain. To the north Sr87/Sr86ratios indicate a sharp turn to the west where the Miocene Columbia River Basalt Group covers the suture. Tracing the suture zone to the north is problematic. Mohl and Thiessen (1995, USGS PP 1438) use Bouguer and aeromagnetic anomalies to map the zone into SE Washington but indicate a need for higher resolution data to trace it to the north. Sporadic outcrops of sub-basalt cratonic rocks and isolated Sr87/Sr86 determinations have lead others to draw the suture into central Washington with tentative connections to British Columbia at around 120^o W.

Where best defined the Western Idaho Suture Zone is only a few kilometers wide and separates lithospheric plates having contrasting histories and mechanical properties. The relationship of gravity to elevation provides a traditional method with which to map such boundaries by using estimates of elastic thickness. However, because plate properties change over a short distance across the suture there is a corresponding rapid change in the relation of gravity anomalies and topography which has proven difficult to locate with spectral methods dependent on long wavelength contributions to the correlation of gravity and elevation signals.