INSIGHTS INTO THE STRUCTURAL DEVELOPMENT OF THE UPPER ARKANSAS VALLEY: THE INFLUENCE OF PRE-EXISTING STRUCTURE AND IMPLICATIONS OF BASIN GEOMORPHOLOGY

An arcuate concave-to-the-west zone of faults and fractures occurs in the Precambrian granitic terrain east of the upper Arkansas Valley. It is ~10km wide and extends ~30km south from Buena Vista, influencing topography and controlling the curving course of the Arkansas River through Browns Canyon. Its size and shape are analogous to the zone of concentric faulting around the Grizzly Peak caldera to the north, and suggest that the development of this part of the upper Arkansas Valley results from the encounter of Neogene ENE-directed crustal extension with a ring fracture zone around the pre-existing Mt. Aetna caldera, in the Sawatch Range west of the basin. Both the eastern and western boundary faults of the basin curve from a NNW trend near Buena Vista to a NNE trend in the Browns Canyon area, mimicking the shape of the eastern caldera margin and the arcuate fault zone east of the basin. Intra-basin horsts are also affected by the pre-existing structure. The most prominent intra-basin horst (informally, the Cogan horst) forms the NNE-trending west side of Browns Canyon, brings Precambrian rocks and Dry Union sediments to the surface, and extends over 10 km NNW into the basin. Its eastern fault guides the river into Browns Canyon and provides lesser resistance to river incision.

Greater Neogene and Quaternary uplift and erosion rates of the Sawatch Range in comparison to those of the southern Mosquito Range have maintained eastward-sloping pediment surfaces and glacial outwash plains in the basin, keeping the river mostly pinned against the east edge of the basin. A dissected bedrock bench east of Browns Canyon at elevation intermediate between current river level and the crest of the southern Mosquito Range is the remnant of a beveled bedrock straith. Subsequent extension and down-cutting left the river stranded east of the Precambrian and Neogene rocks of the Cogan horst, forming Browns Canyon. Browns Canyon provides a temporary base level, 550 ft higher than that at Salida, for the northern part of the basin, preserving Pleistocene sediments capping the Dry Union formation and protecting the area from deep erosion characteristic of areas farther south and west.