Paleoseismology of Crack-in-the-Ground Fault, Central Oregon

Central Oregon exhibits both extensive active faulting and Quaternary volcanism, yet the relations between them are complex and poorly constrained. The N15W striking North Christmas Valley fault system is a Basin and Range type structure offsetting High Lava Plains volcanics forming a ~3 km wide graben that bounds Christmas Lake. Several young volcanic vents, including Green Mountain (GM), Four Craters (4C), and East Lava Field, are aligned parallel with and bounded by the graben. We focus on Crack-in-the-Ground (CITG) fault which vertically displaces the GM and FC basalts. The GM basalt is dated at 740 +/- 59 ka by 40Ar/39Ar (Jordan, 2002). Our preliminary dating of the 4C basalt yields an average age of 12 +/- 2 ka, determined by cosmogenic 3He exposure dating of olivine from flow surfaces. Since emplacement of the GM basalts, the fault has developed a ~10 m hanging wall monocline and a vertical hinge crack which yields the 0.013 mm/yr slip rate suggested in Jordan (2002). The 4C basalt has also been cracked by the CITG fault, producing a 30 +/-10 cm vertical offset. A trench excavated across a section of CITG exposed the stratigraphy of the upper 2.25 m. The upper 1.25 m is a modern soil developed on ~7 ka Mt. Mazama ash. The whole unit is offset ~12 cm as the result of a presumed co-seismic graben structure in the crack. The lower one meter is a palesol developed on windblown sandy silt. The 4C cinder cones may have produced a basaltic tephra that is not seen in the trench, therefore we think the lower unit is post-4C. The paleo-surface has a shallow east dip caused by co-seismic deformation. The two events seen in the trench are likely post-4C, suggesting the 4C offset is cumulative between two events, yielding a slip rate of 0.025 mm/yr. Comparison of the two slip rates suggests an increased slip rate post-4C. Our results are consistent with a potential volcano-tectonic relationship in which slip rate increased during the Late Pleistocene associated with volcanism of the High Lava Plains.