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,I <br />t <br />ACTIVE FAULTING POTENTIAL OF THE SAN LUIS SEGMENT <br />SANGRE DE CRISTO FAULT ZONE, COLORADO <br />INTRODUCTION <br />t This study expands the benchmark seismic hazard assess- <br />ment completed earlier (Glass, 1988). It comprises two parts. <br />First, a literature study provides the tectonic setting of <br />the Battle Mountain Gold property within the Rio Grande <br />Rift, and assesses the potential for future faulting near <br />the mine site. Second, a semi-Markhov analysis provides an <br />' estimate of the probability of occurrence of future surface <br />faulting earthquakes along the San Luis segment of the Sangre <br />de Cristo fault in the next 100 years. The major conclusion <br />is that it is unlikely (probability of less than 12 percent) <br />that a surface rupture along the fault at the mine sire will <br />occur within the next 100 years. <br /> <br />TECTONIC SETTING <br /> San Luis Valley forms part of the northern end of= the <br />f <br />h <br />d <br />60 k <br />d <br /> o. At 1 <br />m long an <br />Rio Grande Ri <br />t in sout <br />ern Colora <br /> approximately 75 km wide, it is the largest topographic <br /> feature of the Rio Grande Rift, a major intracrustal rift <br /> zone extending from southern Colorado on the north to northern <br /> Chihuahua, Mexico on the south. <br /> The tectonic history of the Rio Grande Rift comprises <br /> two distinct episodes of activity. An early phase of rift <br /> .extension occurred during the Oligocene and Miocene (Machette, <br /> 1986) followed by a period of waning activity in the middle <br /> Miocene. A late phase of extension then began approximately <br /> 10 million years ago, and extended to approximately 3 million <br /> years ago. This two-phase history seems also to be shared by <br /> <br />1 <br />i <br />