SUBSURFACE GEOMETRY OF THE SILETZ-KLAMATH SUTURE IN SOUTHWEST OREGON FROM MAGNETOTELLURIC IMAGING

Siletzia, a Paleocene to early Eocene basaltic seamount terrane, was sutured onto western North America by middle Eocene and comprises the modern forearc basement throughout the Pacific Northwest. The suture zone is mostly obscured by overlying rocks of the Cascades volcanic arc. In southwest Oregon, however, it is preserved where previously accreted Paleozoic-Mesozoic terranes in the Klamath Mountains overthrust Siletzia. The geometry of this suture at depth, however, is poorly constrained. In 2006, long-period (deep-crustal) and broadband (shallow-crustal) magnetotelluric (MT) data were collected along a 200-km profile stretching from the Pacific coast north of Coos Bay, OR to the California border south of Medford, OR. South of the suture, the profile crosses Franciscan and Great Valley equivalents, arc lavas and sediments of the Western Klamath terrane, ophiolitic mélange of the Rattlesnake Creek terrane, and the Hayfork volcanic arc terrane, ending within the Condry Mountain Window. The MT data are in general agreement with the NE surface strike of the suture and the accreted Klamath terranes. Two-dimensional inversion of the MT data resulted in a preliminary resistivity model extending to ~50 km depth in accordance with observed surface geology. The obliquity of geologic strike to the modern subduction margin necessitates three-dimensional inversion to properly image deep-crustal resistivity structure. The resulting geometry of the Siletzia-Klamath suture is compared to previous potential-field modeling that suggest a thinning wedge of Siletzia has been thrust within the Klamath terranes.