New Mexico Geological Society Annual Spring Meeting — Abstracts

The Socorro Canyon fault (SCF) is a major active normal fault of the central Rio Grande rift. This north-northwest trending, high-angle, down-to-the-east normal fault can be traced for 40 km along the western flank of the Socorro Basin. North and west of Socorro, the SCF has the appearance of a large displacement range bounding fault where it transects the eastern toes of west-tilted blocks that form the Lemitar and Socorro Mountains . Southwest of Socorro the SCF horsetails and splays into a 7 km wide zone of distributed Quaternary scarps. These scarps locally outline closely spaced horsts and grabens that cut across the piedmont slope below the east-tilted Chupadera Mountains. The intrabasinal southern segment of the SCF (most active Quaternary trace) dies out near San Antonio, about 11 km south of Socorro Canyon.

Recent mapping near the mouth of Socorro Canyon shows that the "main" (most active) trace of the SCF locally displaces 4 geomorphic surfaces of middle to late Pleistocene age in addition to the early Pliocene basalt of Socorro Canyon. Recurrent displacement is demonstrated by progressively increasing offset of Quaternary surfaces with increasing age. The oldest Quaternary surface is capped by a stage IV laminar calcrete that projects 98 m above the Rio Grande . It is correlative with the Las Canas surface. which marks the top of the Santa Fe Group (maximum level of aggradation) in the Socorro Basin. Correlative surfaces in the Albuquerque and Mesilla basins have been dated at 0.5 and 0.7 Ma, respectively. The main trace of the SCF displaces the 4s Canas surface 21-24 m, thereby yielding an average upper Pleistocene displacement rate of 39 ±9 m/Ma. Preliminary estimates based on younger surfaces suggest the "short term" displacement rate has been relatively constant since 0.7 Ma. The longer term displacement rate, however. was clearly not constant. The main trace of the SCF displaces the basalt of Socorro Canyon (4.1 ±0.3 Ma) approximately 46-49 m, which yields an average Plio-Pleistocene displacement rate of 11 .7 ± 1.2 m/Ma. This significant increase in average displacement rate, from Pliocene to Pleistocene time, could be interpreted as the result of episodic rifting or accelerated rifting. Available map data, however, indicate the most probable scenario is one of constant rifting (in Plio-Pleistocene time) associated with non-uniform partitioning of strain within the adjacent 4 strands (splays) of the SCF. Three strands of the SCF that lie west of the "main" trace show a cumulative post 4.1 Ma displacement of approximately 162 m, which adequately compensates for the "missing" 144 m of Pliocene displacement that should be present on the main trace (projecting present ratt backward over long term). These Pliocene splays of the SCF, observed north of Socorro Canyon, show only minor displacement where they project across the middle Pleistocene piedmont slope south of Socorro Canyon.