FAULTS IN BASALTS: HOW FLOOD BASALT CHARACTERISTICS CHANGE NORMAL FAULT SCARP MORPHOLOGY

Fault scarps are surfaces developed by the vertical motion of fault slip. Slopes, orientations, and geomorphologic character of scarps inform our interpretation of subsurface geology, fault geometry, and larger tectonic patterns. Normal fault scarps are often steep and characterized by triangular faces, alluvial fans, slickensides, and mineralization structures. However, few (if any) studies consider how these characteristics are presented in faults cutting flood basalts, rock units pervasive on Venus, Mercury, Mars, and the Moon. We conducted fieldwork in the Steens flood basalt units of southeastern Oregon and the Basin and Range Province to describe the appearance and morphology of normal fault scarps in flood basalts. After observing hundreds of km of fault length, we were surprised to observe shallow slopes, a lack of slickensides or mineralization, and fans crosscut by distinctive ridges of basalt boulders and narrow channels with boulder-built levees as high as the channels were deep. Preferential erosion of the weaker, more fractured flow units (entablature, densely vesicular, and pillow basalt layers) leaves the intact flow units of the footwalls, columnar basalts, unsupported. This leads to collapse of columnar units. Talus slopes consisting of rocks from weaker units are thus capped by fans and fan-cutting channels and ridges composed of the larger columnar talus. Because the fault surface (scarp) slope appears to be driven by erosion, subsurface modeling and fault geometry should not be constrained by the surface slope. We also observed secondary horsts and graben to be typical in fault geometry, suggesting that cooling fractures may have reduced the tensile strength of these rocks promoting multiple scales of failure. Together, these observations suggest that when modeling faulting events in flood basalt units, flows and their diversity of units must be considered as a rock mass with pre-existing weaknesses due to flow characteristics.