Active dextral strike-slip faulting records termination of the Walker Lane belt at the southern Cascade arc in the Klamath graben, Oregon, USA

Abstract

Whether or not northward growth of the Walker Lane belt relates to northward motion of the Mendocino triple junction remains an open question in the study of western North American plate deformation. Uncertainty about the potential linkage arises from an incomplete understanding of how and where dextral transtension in the Walker Lane belt terminates on its northern end. To address this issue, we used bare-earth airborne light detection and ranging (LiDAR) topographic data to reveal the geomorphic record of active dextral transtensional deformation in the Klamath graben in the backarc of the southern Cascade arc, south-central Oregon, USA. Fault scarps in late Pleistocene glacial deposits indicate that at least 0.3 mm/yr of NW-SE dextral shear characterizes slip on the NW-striking Howard Bay fault system within the Klamath graben. Dextral slip on the Howard Bay fault is transferred northwestward into extension on N-striking normal faults within the Cascade arc. Thus, active faults in the Klamath graben mark the northern terminus of a dextral transtensional fault system spanning the southern Cascadia backarc from Crater Lake southward to the northern Sierra Nevada. A review of published geologic slip rates throughout the region of the southern Cascade arc suggests that dextral slip from the Walker Lane belt is transferred both into contractional structures in the northern Sacramento Valley and transtensional structures in the southern ~200 km section of the Cascadia backarc. Termination of transtensional deformation into the southern Cascade arc suggests that the Walker Lane belt propagates independently of the Mendocino triple junction and that, instead, expansion of the Basin and Range Province plays a central role in development of the Walker Lane belt.