New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

In the southernmost outcrop of pre-Quaternary rock in the Chupadera Mountains, Socorro County, New Mexico, a down-to-the-south normal fault is exposed in a bedrock quarry just east of New Mexico State Road 1. Physical characteristics of this fault zone grant insight into timing and methods of faulting in the San Marcial basin, a Rio Grande rift sub-basin that contains few fault outcrops. This quarry exposes silty sandstones of the Spears Group and the Andesite of Willow Springs, which are of similar upper Eocene age. The east wall of the quarry exposes the fault, where both the hanging and foot walls comprise andesite in a ~20 m-wide fault zone around a 1.5 m-wide fault core. Outcrop geometry suggests that the Spears Group sedimentary rocks are no more than ~10 m below the fault outcrop. Nearly all fractures in the fault zone are filled with cataclasized sediments showing physical and mineralogical similarities to the sedimentary rocks of the Spears Group. Cataclasites at the study site contain clasts with a smaller average diameter than the nearby Spear Group sediments, and microtextural observations suggest grain-to-grain comminution during faulting likely caused quartz spalling. While the majority of cataclasite in the fault zone appears to have been transported into fractures via particulate flow processes, interpreted here to represent faulting prior to lithification or during poorly-lithified conditions, the presence of angular clasts of sandstones within the fault zone also suggests that portions of the Spears Group also exhibited cohesive behavior during faulting, interpreted to represent relatively well-lithified sands. Other features in the fault zone include zones of oxide clast concentration within cataclasites and post-faulting calcite vein mineralization. Because the San Marcial basin lack a topographically-expressed uplifted footwall block or basin-bounding horsts, little is known about timing and styles of faulting during the basin’s tectonic and sedimentary evolution. This fault is the most basin-central known fault in the San Marcial basin and therefore offers insight into increasing understanding of its tectonic history.