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<br />stresses. The Gore, Mosquito, Frontal and Weston faults may have <br />developed or been reactivated at this time. <br />By Eocene time, crustal compressional stress lessened and was <br />followed by a period of tectonic quiescence. The Eocene erosion <br />surface developed and was covered locally by Oligocene volcanic <br />rocks. By mid-Miocene time, compressional forces of the Lazamide <br />orogeny were replaced by east-west directed extension resulting from <br />subduction of an oceanic spreading center off the coast of western <br />North America. At the same time, the Rio Grande rifr began to <br />develop. High-angle normal and reverse faults in north-central <br />Colorado including the Gore, Mosquito, Weston and Frontal faults and <br />other major faults in the Rio Grande rift experienced differential <br />displacement. The Eocene erosion surface and overlying volcanics <br />were faulted and displaced. Faults in the Buena Vista area experienced <br />significant displacements as the result of rifting. <br />As differential uplift and rifting continued, basins between the uplifts <br />including the axial rift graben began to receive rapid fluvial sedimenta- <br />tion including those deposits now mapped as the Dry Union Formation <br />in the Buena Vista-Salida azea. As development of the rift system in <br />north-central Colorado continued, north-striking normal faults <br />displaced the Dry Union Formation. Fault activity in the Rio Grande <br />rift system continued sporadically into the Quaternary. Evidence <br />suggests azeas of the rift system in Colorado south of about 39° 30' N <br />latitude experienced movement in late Pleistocene to Holocene time. <br />seismogenic surface faulting has been identified in the upper Arkansas <br />Valley near Buena Vista, neaz Spinney Mountain in South Pazk, and in <br />the San Luis Valley. To the best of our knowledge, no surface fault <br />movement of late Pleistocene to Holocene age has been documented in <br />Colorado north of latitude 39 30' N. <br />Strong geologic and physiographic evidence suggests rifting in <br />Colorado decreased in intensity from south to north along the axis of <br />the graben. The same evidence along with studies of seismogenic <br />surface faulting cited above also suggests the rate of activity increases <br />to the south. Available geological and geophysical evidence suggests <br />contemporary crustal stresses in north-central Colorado aze extensional <br />and oriented in a generally east-west direction (Zoback and Zoback, <br />1991). Accordingly, high-angle faults oriented favorably to this stress <br />field, i.e., striking close to due north, aze most likely to be seismogenic <br />in the contemporary tectonic setting. <br />IS <br />