Paleoseismology of the Tijeras fault near Golden, New Mexico

Abstract

The 100-km-long Tijeras-Cafioncito fault system is a potential seismogenic source that extends between the Santa Fe and Albuquerque metropolitan areas of New Mexico. The fault system accommodates east-west crustal extension and provides structural linkage between the Espanola and Albuquerque basins of the active Rio Grande rift. At the Adobe Camp site northeast of Albuquerque, we conducted detailed trenching to assess the near-surface location, timing, and style of late Quaternary deformation on the southern Tijeras fault, a major component of the Tijeras-Caftoncito fault system. At Adobe Camp, the fault consists of two main strands that may represent a positive half-flower structure. The eastern fault strand is vertical and coincides with a leftdeflection in a west-flowing arroyo. The western fault strand has reverse fault geometry and dips moderately eastward; it likely merges with the eastern vertical strand in the shallow subsurface (about 30 to 40 m depth). The western strand has placed bedrock over late Quaternary deposits and coincides with northwest-facing scarps that truncate piedmont surfaces. Field relations suggest that vertical separation was produced along the western fault strand, whereas lateral offset occurred primarily on the eastern fault strand. Trench and arroyo-wall exposures show that two colluvial deposits were shed from the fault scarps along the western fault strand. The lower colluvial deposit is faulted, whereas the upper colluvium is not, thereby indicating two faulting events. The ages of these earthquakes are poorly constrained because of uncertainties in the ages of the faulted and unfaulted colluvia. However, based on the relative degree of soil development and stratigraphic position, we estimate that the colluvia most likely are late Pleistocene (11130 ka). Our interpretation is that the scarp-derived colluvia were deposited following two surface-rupturing earthquakes along the southern Tijeras fault during the late Pleistocene. If these earthquakes ruptured the entire 41-km-long southern part of the Tijeras fault, they may have been as large as moment magnitude (Mw) 7.0.