Slip rates and kinematics of active crustal faults in the central Oregon Cascades

New cosmogenic 3He chronologies and surficial geologic mapping constrain the age of glacial deposits and slip rates of predominantly normal faults in the White Branch and Dilman Meadows fault zones in central Oregon, USA, over the last ca. ~80 kyr. Our mapping of glacial landforms and deposits distinguishes three primary episodes of glacial deposition in the White Branch fault zone. Twenty-two new cosmogenic 3He surface exposure dates indicate that the youngest glacial unit represents a last glacial maximum deposit (ca. 19.4 +10.1/-6.2 kyr). Mapping of outwash terrace surfaces and deposits in the Dilman Meadows fault zone, constrained by two new cosmogenic 3He depth profiles, identifies an older outwash deposit (75.0 +11.3/-9.2 kyr) associated with MIS 5b (~75 ka), as well as a younger last glacial maximum outwash surface (20.8 +0.04/-0.03 kyr). For the first time, glacial chronologies are linked across the Cascades, where west of the crest, where erosion and precipitation rates are high and vegetation cover is dense, and east of the crest, where precipitation and erosion rates are significantly lower and tree cover is sparse. We use offset measurements from fault scarps that show primarily dip-slip motion in combination with the depth profile ages to determine a summed slip rate across the Dilman Meadows fault zone of 0.1 – 0.4 mm/yr since ~75 ka. Surface faulting evident in the White Branch fault zone represents deformation strictly since the last glacial maximum (< 20.3 +6.2/-5.3 ka). Combining offset values from scarp profiles that show purely dip-slip displacement with the cosmogenic 3He surface ages yields a summed dip slip rate across the White Branch fault zone of 0.6 ± 0.5 mm/yr since the last glacial maximum. The White Branch and the Dilman Meadows fault zone both accommodate mostly extension in the Cascade arc and could be mechanisms of Siletzia rotation or volcanic upwelling.