Quaternary faulting in Salt Basin Graben, West Texas
— Lisa K. Goetz

The Salt Basin graben is the easternmost limit of modern Basin and Range tectonism in Texas and is the easternmost structure related to the Rio Grande rift in the United States. Quaternary faulting in Salt Basin appears to be controlled by pre-existing structural zones of weakness. Over one hundred Quaternary fault scarps and photolineaments were mapped in Salt Basin (Goetz, 1977; fig. 1). Three types of fault scarps were recognized: lakeshore scarps, mid-fan breaks, and bedrock boundary faults. The faults, particularly the mid-fan breaks, frequently act as semipervious barriers separating fresh water on the mountain side from saline water on the playa side.

 

A tectonic origin is suggested by the orientation of the scarps and the proximity of the Mayfield fault scarps to the estimated epicenters of the 1931 Valentine earthquakes (intensity VIII on the modified Mercalli scale). It is suggested that these fault scarps have been produced and maintained by intermittent activity rather than by a single event. A Holocene age for some of the scarps is strongly suggested by a comparison of the erosion rate there with that on the 1968 Borrego Mountain, California, scarps (Clark and others, 1972) and by interpolation using the caliche development scale developed by Giles and others (1970) for southern New Mexico.

 

The frequency of small earth tremors felt by the residents of the basin and recorded by temporary seismograph stations indicate that tectonic adjustments are presently occurring (Dorman and others, 1976; see also Dumas, this volume).

 

I suggest that the basement blocks of the graben floor are being actively downdropped to the west-southwest along the eastern hinge of the half-graben. Evidence for this includes the preferential alignment of Quaternary and Holocene fault scarps along the western side of the graben. The westward shifting of the playa lakes and the preferred orientation of giant desiccation polygons also were probably induced by this movement (see second paper by Goetz, this volume).

 

The basement blocks are broken by four transverse structural lineaments crossing the Salt Basin graben mapped by King (1948, 1965) and Wiley (1970). A fifth zone trending east-west between the Babb flexure and Bitterwell Mountain has been proposed by the author (Goetz, 1977).