Investigating fault scarp evolution in jointed bedrock in southwestern Iceland and northern California

Fault scarps in fractured bedrock are common along extensional and transtensional plate boundaries, but their morphological ages have proven difficult to determine. We aim to identify the characteristics of evolving normal fault scarps in jointed basalt, to describe the processes that act on scarps of varying stages and to establish a technique to evaluate relative morphological age.

Field sites include southwest Iceland and northeast California. In Iceland, the divergent boundary creates surface-breaking normal faults. We surveyed four scarps cutting post-glacial basalt flows (younger than 9 ka, maximum height 8-21m). In California, back-arc and basin-and-range extension create scarps in the Modoc Plateau. We surveyed scarps on the north-flank of Medicine Lake volcano, where scarps cut multiple small-volume basalt flows from the middle to late Pleistocene, and have much larger maximum heights (85-132m). We mapped the scarps by geomorphic domain, defined by overall scarp height, original fault structure, jointing, height of the free face relative to talus slope, size of the talus material, and vegetation. We used structure-from-motion (SfM) photogrammetry along selected fault segments to create surface models.

Our field observations do not support a simple conceptual model of parallel scarp retreat with a talus wedge accumulating through time. In both regions, small scarps (<30m) show substantial geomorphic variability along strike. This variability - and the observation that both the young Iceland scarps and older California scarps demonstrate the same variations- suggest that age is not a primary control on the morphology of small scarps in jointed basalt; primary bedrock structure, fracture distribution, and fault-segment geometry are more likely to influence scarp morphology in the early stages of degradation. However, scarps appear to reach a threshold where this variability is no longer present. In California, we observe that scarps higher than 40m are uniform along strike and characterized by large talus piles sloping at 30-40°. We hypothesize that fault scarps in basalt display morphological changes as throw accumulates along a fault. These changes do not conform to a simple model of parallel retreat nor to the diffusion-dominated model often applied to fault scarps in unconsolidated materials.