Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 20-7
Presentation Time: 9:00 AM-6:00 PM


ALEXANDER, Katherine A.1, AMOS, Colin B.1, BALCO, Greg2, AMIDON, William H.3, LESNAU, Reyne K.3, CLARK, Doug1 and MEIGS, Andrew4, (1)Geology Department, Western Washington University, 516 High St. MS 9080, Bellingham, WA 98225, (2)Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, (3)Department of Geology, Middlebury College, 14 Old Chapel Road, Middlebury, VT 05753, (4)College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Wilkinson 202D, Corvallis, OR 97331

Active fault systems in the central Oregon Cascades accommodate extensional deformation in response to ongoing rotation of the Oregon Coast block with respect to North America. This study focuses on two active fault systems along the eastern margin of the Oregon Coast block, within the central High Cascades. These fault systems, the NNW-striking White Branch fault (WBF) and N-striking Dilman Meadows fault (DMF), are located on the west and east sides of the modern Cascades volcanic arc, respectively, and contribute to extension within the arc. Airborne lidar spanning these fault zones allows for the identification and mapping of datable, Quaternary geomorphic surfaces and fault scarps previously obscured by young arc volcanism and dense forest cover. Cosmogenic 3He exposure dating of these surfaces relies on olivine and pyroxene-bearing volcanic bedrock derived material from young, proximal Cascade sources. West of the Cascades crest, near Mackenzie Pass, exposure ages of three offset moraines cut by the WBF reveal ages of ~19-21 ka (no correction for snow shielding), broadly consistent with the last glacial maximum (LGM) (~15 – 25 ka). East of the crest in the La Pine graben, we collected two cosmogenic 3He depth profiles from prominent outwash surfaces deformed by the DMF along the Deschutes River. The younger terrace yields an LGM age that is consistent with moraine chronologies to the west when corrected for surface shielding due to deposition of Mazama tephra on these surfaces. An older surface yields an age nearly an order of magnitude older, broadly consistent with glacial outwash during marine isotope stage 5 (~80 -130 ka).This age is also similar to recently published exposure ages from moraines in the Klamath basin to the south. Fault scarp profiles extracted from airborne lidar data combined with these ages suggest maximum vertical separation rates of ~0.6 mm/yr and ~0.2 mm/yr for individual strands of the WBF and DM, respectively. Additional work we will present at the meeting includes comparison of surface roughness of moraines as a proxy for depositional age, refined estimates of snow shielding, and extension rate calculations across both fault zones.