Geophysical investigations of the cratonic margin in the Pacific Northwest (USA)

Abstract

The cratonic margin of North America in west-central Idaho has abeen delineated by strontium isotope and field studies. Outboard is the Blue Mountains Province, a complex amalgamation of arc-related terranes. Gravity studies indicate a classic suture-style paired anomaly with a gradient into a low over North America and isolated maxima within and just outboard of the Mesozoic suture. The latter appear to be due to mafic and ultramafic pods. Magnetic anomalies change from relatively quiet over the continent to noisy over the Blue Mountains Province. These geophysical features change character along the length of the suture and do not trend exactly parallel to it. Near Orofino, Idaho, the suture changes from a N-S-trend to due west and extends under the Miocene Columbia River Basalts into southeastern Washington. The geophysical signature parallels this trend. Near Pomeroy, Washington, this signature again turns northward, but the anomalies split. The magnetic boundary and the minor gravity maxima trend northward into the western edge of the Kootenai Arc — the surface boundary between miogeoclinal North American lithologies and accreted terranes. The gravity gradient trends northeastward and extends far into the craton in northern Idaho, where it corresponds to a change in lower crustal character as observed on COCORP reflection seismic data. In northeastern Washington and southeastern British Columbia, the gravity and magnetic signatures appear to follow the suture, but the gravity effects of local geologic features are overprinted on them.KeywordsMagnetic AnomalyGravity AnomalyGeological SocietySuture ZoneBouguer GravityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.