PRELIMINARY KINEMATIC OBSERVATIONS OF LATE CENOZOIC FAULTS AT THE EDGE OF AN INTERBASIN MOUNTAIN BLOCK WITHIN THE CENTRAL COLORADO RIO GRANDE RIFT: MANIFESTATIONS OF AN EXTENSIONAL ACCOMMODATION ZONE?

A WNW-trending mountain block of Proterozoic and Tertiary rocks that forms the northern tip of the Sangre de Cristo Range and separates the Upper Arkansas and San Luis rift basins has previously been associated with an extensional accommodation zone. However, little is actually known about the geometry, kinematics, age, and evolution of structures in this block, or their role in accommodating or transferring rift extension between the basins to the north and south.

Preliminary slip measurements (strike, dip, rake, slip sense, and separation) of faults were collected in Mio-Pliocene basin-fill sediments of the Dry Union Formation along the north flank of the mountain block. Most faults can be divided into two sets: (1) NW- to NNW-striking sinistral-normal oblique-slip (10-80^o rake) faults; and (2) WSW-striking dextral strike-slip (0-30^o rake) faults. Some faults of both sets exhibit small to large components of reverse slip, and the majority have moderate dips (40-70^o) irrespective of their slip modes. A few W- to NW-striking faults along the northern flank of the mountain block have aeromagnetic expression that reveals kilometer-scale strike lengths. In a quarry at the north margin of the block, near the mouth of Little Cochetopa Creek, a W-trending zone of faults (0.5 to 3 m offsets) cuts late(?) Pleistocene gravel. These young faults show normal slip, with subordinate components of sinistral slip on WNW- to W-striking faults, and of dextral slip on W- to WSW-striking faults.

The faults that cut Pleistocene and Tertiary sediments exhibit kinematic similarities, suggesting either that much or all of the faulting is young, or that the strain field has remained relatively unchanged since as early as the middle Miocene. The fault datasets from the older and younger sediments were each inverted for best-fit paleostress tensors. Each computation resulted in a similar normal-faulting stress solution with a ~N-trending σ₃ axis, consistent with the similar kinematic aspects of the two datasets. Although preliminary, these results suggest that the interbasin stress field during the main and(or) late stages of rifting was markedly different than the more typical ~W σ₃ trends of the rift reported regionally, which perhaps is a manifestation of intrarift strain transfer.