GORE RANGE, BLUE RIVER VALLEY, AND WESTERN FRONT RANGE, COLORADO – ADVANCES IN UNDERSTANDING THE GEOLOGIC FRAMEWORK BASED ON NEW MAPPING AND GEOCHRONOLOGIC STUDIES

Recent geologic mapping and geochronologic studies in the eastern half of the Vail 1:100,000 quadrangle have significantly improved our understanding of (1) Paleoproterozoic history of the basement rocks of the Gore Range and Williams Fork Mountains (western margin of the Front Range), (2) Late Paleozoic history of the Gore fault system, (3) Laramide contractional tectonism, including deformation along the Gore fault and Williams Range thrust, (4) Oligocene and younger extensional history of the Blue River half graben (northern extent of the Rio Grande rift), and (5) late Neogene and Quaternary surficial history. Marine sediments and mafic volcanic rocks deposited between about 1,740 Ma and 1,780 Ma were generally metamorphosed to amphibolite grade (mostly migmatitic biotite-±sillimanite gneiss, quartz-feldspar gneiss, hornblende-±biotite gneiss, and amphibolite) and intruded and deformed by rocks ranging from granite to gabbro between about 1,760 and 1,700 Ma. The distribution of Upper Cambrian to thick Upper Cretaceous platform sediments is now better understood, which allows a refined definition of the late Paleozoic uplift, erosion, and flanking sedimentation of the ancestral Front Range. Detailed mapping has also better defined the geometry of Late Cretaceous to early Tertiary Laramide deformation along the Gore fault system and Williams Range thrust, as well as increased understanding of Oligocene and younger extension along the Blue River normal fault system (the western margin of the Blue River half graben). Scarps along the latter fault system, imaged by LIDAR, indicate latest movement may be as young as Holocene. Detailed mapping of surficial deposits has defined and described (1) 6 ages of terrace alluvium, (2) 3 general ages of landslides, (3) glacial and periglacial deposits, and (4) fan, pediment, talus, and debris-flow deposits.