NEOGENE DEVELOPMENT OF THE PLEASANT VALLEY GRABEN (HOWARD, CO) BASED ON THE STUDY OF MESOSCOPIC FAULTS

The Pleasant Valley graben in central Colorado formed in Miocene time as an isolated depocenter immediately east of the northern Rio Grande rift and Sangre de Cristo Mountains. It is a rhomb-shaped, asymmetrical basin that lies along the trend of the Poncha Pass transfer zone (Chapin & Cather, 1994), and is bounded by NW-trending, NE-side down faults flanking the northern Sangre de Cristo range. Mesoscopic faults cut the Oligocene and younger basin deposits, including 27.8 and 27.3 Ma ignimbrites correlated to Fish Creek and Carpenter Ridge Tuffs by MacIntosh & Chapin (in press). This study uses kinematic information from the brittle structures to assess the Neogene history of the Pleasant Valley graben.

Faults cutting the Tertiary volcanic rocks exhibit highly polished, red-brown, grooved slickensides, or appear as rough surfaces with patchy preservation of striae. They range in trace length from 0.5 to 13 m. Reliable offset markers are generally absent; therefore, such textural features as reidel fractures, positive steps, grooves, and mineral streaking are used for interpretation of kinematic sense. Preliminary data show two dominant sets of brittle structures: 1) prevalent steeply dipping faults with a mean strike of N24W exhibiting low-raking striae, and 2) moderately dipping faults oriented ~N60E, with down-dip striae. The faults are considered to be contemporaneous, since all cut mid-Tertiary volcanic rocks, and to record Neogene tectonism associated with northern Rio Grande rift development. The prevalence of subvertical, NW-striking, dextral faults together with the rhombohedral basin geometry suggest that the Pleasant Valley graben developed in a releasing bend and that the basin-bounding faults accommodated dextral oblique strike slip movement. The results are consistent with results from the upper Arkansas valley, interpreted as a pull-apart basin by Hubbard & Wawrzyniec (2000). Thus, the Pleasant Valley graben may be one in a series of en echelon, strike slip basins.

References cited: Chapin, C. & Cather S., 1994, in GSA Special Paper 291, p. 5–25. McIntosh, W. & Chapin, C., in press, in NM Bureau of Mines & Mineral Resources Bulletin 160. Hubbard & Wawrzyniec, 2000, GSA Abstracts with programme, v.32, p. A-42.