Active fault mapping and reconnaissance paleoseismic investigations in the La Pine graben, Oregon Cascades, USA

Abstract

Despite geologic and geodetic evidence of ongoing extension across the Oregon Cascades, the late Quaternary structural expression of this deformation is not well constrained. High resolution airborne lidar-derived bare earth topography enables examination of densely forested portions of the Cascade Range to test for the presence of late Quaternary fault-related deformation.We target a portion of this extensional system by mapping evidence for recent faulting across the La Pine graben on the eastern flank of the central Oregon High Cascades. Our mapping documents >20 new Quaternary fault traces across the graben, illustrating that late Quaternary deformation is distributed across a wide zone. Paleoseismic investigations on the Twin Lakes maars fault within the La Pine graben provide evidence for a mid-late Holocene earthquake based on the offset of the 7.6 ka Mazama tephra. Potential temporal and genetic relationships between faulting and volcanic activity remain unclear because of limited age control, but this research provides a critical foundation for future paleoseismic efforts, targeted geochronologic studies, and basin structure and development investigations.