New Mexico Geological Society Annual Spring Meeting — Abstracts

The Alamogordo fault is a major Rio Grande rift structLJre in southern New Mexico along which the Tularosa Basin has been down-dropped relative to the Sacramento Mountains. This fault has displaced surfaces developed on Quaternary deposits by as much as 10 m. The timing of late Quaternary rupture events along the Alamogordo fault adjacent to the Sacramento Mountains was investigated by: (1) mapping proximal piedmont deposits along the western foot of the Sacramento Mountains, (2) describing exposures of offset Quaternary sediment and collecting 11 samples of dateable material, and (3) measuring fault scarp profiles at 40 localities.

The ages of four late Quaternary surface rupture events are constrained by stratigraphic relationships and C-14 dates. South of the city of Alamogordo, two large rupture events probably occurred within a time span of a few thousand I years. A limiting upper numeric age indicates that both happened shortly before 12.6 ka (radiocarbon years). The estimated average displacement associated with each of these two events is approximately 3-4 m. North of the city of Alamogordo, the youngest interpreted surface rupture event is constrained by C14 dates to have occurred between 10.4 and 11.4 ka (radiocarbon years). South of the city of Alamogordo, the youngest surface rupture very likely occurred in the early Holocene during a period of major alluvial fan aggradation. This event probably had an average displacement of ~1 m. The alluvial fan stratigraphy suggests that these four ruptures belong to a temporal clustering phenomena that occurred over a period of ~8 ka during the latest Pleistocene and early Holocene. These four ruptures probably produced seismic moment magnitudes within the range of 6.8 to 7.3.