The Role of Extensional Tectonics on Landsliding, Summer Lake Basin, Lake County, Oregon

Summer Lake basin is located in the northern Great Basin, a physiographic province characterized by closed, down-dropped basins and N-S trending mountain ranges produced by regional extension ongoing since the late Cenozoic. Summer Lake is bound on the west by Winter Rim, a 35 km long escarpment, and by Slide Mountain in the south. Pervasive, gigantic landslides dominate the nearly 1000 meter high escarpments along the southwestern part of the basin. These landslides are characterized by an apparent, large rotational component within the steep bedrock escarpment and relatively long runout on the basin floor. Miocene-age basalt and andesite flows and breccias comprise the upper rim. Basin subsidence has exposed weak, underlying tuffaceous sedimentary rocks and tuffs within which landslides initiate. On the southern end of Winter Rim, landslide morphology is subdued, and propagation of the range front, normal fault through the slide debris may exceed 200 meters of offset. Hummocky topography and deranged drainage patterns increase northward in adjacent landslides, and fault offset within the landslide deposits diminishes. In addition to these observations, greater subsidence in the SW corner of the basin supports a theory for increasing recency of landsliding northward. Further, it suggests that continued subsidence threatens new instability to the north in the presently stable portion of the escarpment.