NEOGENE DEVELOPMENT OF THE PLEASANT VALLEY GRABEN (HOWARD, CO) BASED ON THE STUDY OF MESOSCOPIC FAULTS
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. 525. 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.