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Abstract:
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Fault scarps cutting alluvial deposits define the present -day eastern boundary of Basin and Range faulting in Trans -Pecos Texas . These faults are found in the Salt Basin graben north of Van Horn , Texas , and its southerly extensions along Lobo Valley and Michigan Draw . Discontinuous and en echelon Quaternary fault scarps quickly die out to the south on the eastern side of the graben but become more continuous and develop larger displacements (up to 6 meters ) along the west side of Salt Basin at the base of Sierra Diablo . The orientation of the more than one hundred Quaternary down -to -the -basin fault scarps and photolineaments appears to be controlled by pre -existing structural zones of weakness . The preferential alignment of Quaternary (Holocene ? ) fault scarps along the western side of the graben , the westward shifting of playa lakes and the preferred orientation of giant desiccation polygons show that the graben floor is subsiding more rapidly along the western margin . These faults also separate fresh from saline ground water . Four transverse structural lineaments crossing the northern portion of Salt Basin graben were mapped by P .B . King (1948 , 1965 ) and M . Wiley (1970 ) . A fifth zone , trending east -west between the Babb flexure and Bitterwell Mountain is proposed in this thesis . 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 earthquake . The frequency of small earth tremors felt by the residents of the basin and recorded by temporary seismograph stations indicate tectonic adjustments are presently occurring . |