LITHOSPHERIC ARCHITECTURE OF THE KLAMATH MOUNTAINS PROVINCE, CASCADIA SUBDUCTION ZONE, CALIFORNIA AND OREGON, USA

The Klamath Mountains Province (KMP), a region of elevated topography above the Cascadia subduction zone, occurs at the juncture of four different tectonic regimes. The southwestern margin of the KMP is pinned by the Mendocino Triple Junction; the western margin is the Juan de Fuca-North American convergent plate boundary; the northern boundary is defined by the Columbia embayment; and the eastern margin is a composite boundary defined by the overlapping north-south trending Cascades arc and Basin and Range extensional corridor. The KMP formed by accretion of tectonostratigraphic terranes over 450 m.y. Since Pliocene time, the KMP has undergone uplift from near sea level to elevations in excess of 2000 m within the central region of the orogen, with accompanying glaciation and deep river incision exposing > 6 km of structural relief. Previous workers have suggested that Neogene doming occurred to exhume mid-crustal rocks in the central portion of the KMP and modify the accretionary structure of the orogen. An extensive Pliocene or younger erosional surface has been mapped in the western half of the range where it varies in elevation from < 100 m along the coast to greater than 1500 m inland. We propose two competing models for the evolution of the KMP. Model 1 invokes Neogene serpentinization of the mantle wedge above the subducting Gorda plate, leading to rock uplift and exhumation of the central KMP and adjacent regions. Model 2 invokes an east-dipping crustal ramp beneath the central KMP to accommodate exhumation. The timing is not constrained for Model 2; however, the conventional interpretation is that all thrusting within the KMP is Mesozoic in age. Previous seismic work near the Mendocino triple junction and southern Klamath mountains and to the north in the Cascades indicates a weak seismic signal from the Moho in regions thought to be affected by serpentinization of the upper mantle. Our preliminary receiver function images indicate a window with a weak or absent Moho Ps phase beneath the central KMP which may constrain the location of the serpentinized mantle. The Moho Ps phase from either side of this region is relatively sharp. Gravity and magnetic observations will be combined to interpolate between Moho depth estimates and amplitude anomalies to better constrain Moho geometry and the distribution of serpentinized mantle.