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I <br />1 Losh (1982) reports the results of mapping within several <br />l <br />V <br />f <br />k <br /> ansas <br />trenches across young <br />au <br />t scarps in Upper Ar <br />alley, <br /> north of San Luis Valley and the northernmost segment of <br />' the Rio Grande Rift. This mapping displays as many as six <br /> faulting events within the past 100,000 to 140,000 years. He <br />`. estimates a return period of from 20,000 to 30,000 years for <br /> surface faulting in the Upper Arkansas Valley, and estimates <br /> that this faulting activity is capable of generating :Large <br /> earthquakes, in the magnitude 7 range. <br />I In a comprehensive and well conceived study of faulting <br /> in the San Luis Valley north of the San Luis fault segment, <br /> McCalpin (1981) estimates that approximately 7 km of ~iplift <br />' has occurred in San Luis Valley since early to mid-Tertiary. <br /> Using scarp morphology and other geomorphic indicatory, <br /> McCalpin characterized the spatial and temporal tectonic <br /> character of San Luis Valley north of Tobin Creek. In his <br /> study he divided northern San Luis Valley into the fo:lowing <br /> four geographic zones: a) Vi11a Grove fault zone, b) iiorth- <br /> west-trending segment of the Sangre de Cristo fault zone, <br /> c) north-trending segment of the Sangre de Cristo fau~t zone <br />' and d) a short east-trending segment near Tobin Creek, <br /> Villa Grove Fault Zone <br /> <br /> The Villa Grove fault zone is a 10 km long discontinuous <br /> zone of en echelon scarps having a maximum scarp height of <br /> 14 m at the southeast end decreasing to from 0.4 m to 1 m at <br />' the northwest end. The latest fault event approximate:y 10,000 <br /> years ago offset Pinedale glacial deposits an amount equal <br /> to approximately 0.8 to 1.4 m. McCalpin has been able to <br /> document the following chronology of faulting on the ~Jilla <br /> Grove fault: <br />' a) Faulting began approximately 300,000 to 250,0()0 years <br />ago. <br />' b) Between 250,000 and 60,000 years ago there were 3 to <br />4 faulting events totalling approximately 5 m. <br />4 <br />