CLAY-RICH CLASTIC DIKES LOCALIZE FAULT SLIP IN THE RIO GRANDE RIFT

Understanding processes that affect fault core development is essential to interpretations of both fault mechanics and permeability structure. In particular, a well developed clay fault core will decrease cross-fault permeability and the coefficient of friction relative to most earth materials. Clay is generally inferred to be incorporated into faults in sedimentary sequences via clay smear from host beds during slip. However, normal faults of the Rio Grande rift commonly exhibit clay cores with sharp boundaries, lacking evidence of connection to host sediments. Identification of a clay-rich clastic dike and observations of structures in a small fault exposed near Rio Rancho, New Mexico lead us to hypothesize that faults can initiate on clay dikes. The clay-rich dike has curviplanar margins and tapers to a wedge that ends in a fracture. The fault transects beds of both sand and reddish clay, which record 17 cm of throw. Its gray-green core sharply cuts all beds and displays root-like structures at the base of the exposure (similar to ‘bifurcated dynamic fractures’ of Levi et al. 2006). The clastic dike and core exhibit a margin-parallel foliation. Preliminary, qualitative XRD analyses of both core and a clay-rich bed truncated by the core include montmorillonite, clinochlore, and quartz. The fault core contains more layer silicates than the bed, which has more quartz than clinochlore. Field observations and SEM images of hand samples show fine slickenside striae recording slip on foliation surfaces in the core. Observations of the host clay-rich bed, however, indicate slip localized on small-displacement, discrete slickenside surfaces near and at one core margin. The non-foliated clay-rich bed records displacement along the fault and shares similar mineralogy to the dike, and its boundaries aren’t deflected into the fault core. Distributed deformation within the clastic dike and clay fault core, in contrast to their sharp margins, is consistent with liquefaction, mobilization, and injection of clay. We suggest the clay core initiated as a foliated clastic dike, which localized subsequent normal fault slip.

Levi, T., Weinberger, R., Aïfa, T., Eyal, Y., & Marco, S. (2006). Earthquake-induced clastic dikes detected by anisotropy of magnetic susceptibility. Geology, 34(2), 69–72. https://doi.org/10.1130/G22001.1