THE RIGGS FAULT, SOUTHEASTERN CALIFORNIA: A BRITTLE, WEST-VERGENT, MESOZOIC LOW-ANGLE NORMAL FAULT

The Riggs fault is a major, well-exposed low-angle fault in the Silurian Hills, eastern Mojave Desert. Along the fault, upper plate Paleozoic miogeoclinal carbonate rocks are emplaced on top of siliciclastic Proterozoic to lower Paleozoic rocks. At many locations, footwall rocks immediately below the main fault surface are intricately faulted forming a series of stacked, wedge-shaped blocks in a zone referred to as “chaos structure.” This zone, up to several hundred meters thick, developed during Riggs fault activity as shown by the presence a few chaos blocks interleaved within the Riggs fault hanging wall. The nature and origin of the Riggs fault has not been well-understood, in part because it occurs in an isolated mountain range affected by multiple deformational events. Recent study has clarified three key aspects of the fault, however.

First, contrary to some previous interpretations (including my own), the Riggs and chaos faults developed under brittle conditions, as evidenced by the presence of slickensided fault surfaces and tectonic breccias associated with the fault deformation. Confusion on this issue of rheology occurred because a ductile deformational overprint in the southeastern part of the Silurian Hills has been misinterpreted as a primary fabric from Riggs fault activity.

Second, evidence on many scales indicates an extensional origin for the fault. On the largest scale, the Riggs fault is a younger-on-older fault, which is most easily explained by normal faulting. Similarly, the chaos fault blocks are mostly stacked younger-on-older. Finally, outcrop-scale, moderately to steeply dipping faults both in the chaos zone and upper plate rocks show normal displacement.

Finally, the third key aspect is that the fault is west-vergent. This is clearly demonstrated by displacement along small-scale, low-angle faults within the chaos zone.

A major remaining challenge is to understand the tectonic origin of the fault, which requires a better understanding of the fault’s age. The best upper age constraint at present comes from recognition that the fault is older than the middle to late Cretaceous Teutonia batholith, which intrudes the upper and lower plates without deformation. The fault is not likely to be older than Mesozoic given the lack of evidence for major tectonism in the region prior to that time.