ESTABLISHING A FAULT SCARP DEGRADATION MODEL IN JOINTED BEDROCK BASED ON FIELD OBSERVATIONS

The age of fault scarps in fractured bedrock cannot be determined using a standard geomorphic framework of landform degradation: Diffusion models break down as fracture-bounded block size increases relative to the size of the landform, and uniform fault-plane weathering models, such as those used to characterize fault scarps in limestone, do not take into account the significant geomorphic disturbances present in fracture-dominated systems. Our study collects the profile morphologies of normal fault scarps in jointed rock in three locations with different tectonic settings, lithologies, and climates, and describes the processes that act on these scarps, with the ultimate goal of establishing a fault-scarp degradation model for jointed bedrock. In Iceland, we surveyed four, 8-to-21m-high scarps cutting post-glacial lava shields. In northeast California, we surveyed three scarps on the north-flank of Medicine Lake volcano, where 85-to-132 m-high scarps cut basalt flows from the middle to late Pleistocene. In central California, we surveyed four 20-to-60 m-high scarps in the Volcanic Tablelands. We mapped the scarps by overall scarp height, original fault structure, jointing, talus size, and vegetation. We used structure-from-motion photogrammetry along selected fault segments to create surface models and fracture maps.

Field observations of all fault scarps indicate that multiple erosional processes are operating at variable rates and scales on heterogeneous initial conditions, resulting in a range of possible scarp morphologies. The shape of smaller scarps is dominated by variable fault geometry and toppling of cooling-joint-bounded blocks. Since these primary contributors vary along the scarp, small scarps have a range of free face to talus slope ratios, and the slopes themselves show variable geometry (convex to concave). As faults remain active, scarps increase in height, and the initial heterogeneities gradually contribute less to the overall scarp form. Processes such as sporadic downslope movement of blocks or debris flows begin to shape scarps, resulting in more uniform scarp geometry, with a small proportion of free face and large talus piles sloping at 30-40°. We quantify the height-normalized variability in scarp form to show the relationship between scarp age and variability in form.