TESTING AN ACCOMMODATION-ZONE HYPOTHESIS IN THE NORTHERN RIO GRANDE RIFT: PRELIMINARY INSIGHTS FROM THE GEOLOGY, DEFORMATION STYLE, KINEMATICS, AND AEROMAGNETIC EVIDENCE IN THE PONCHA PASS AREA, COLORADO

The Rio Grande rift in central Colorado is interrupted by a WNW-trending mountain block of Proterozoic and lesser Tertiary rocks, across which the asymmetry of half-grabens forming the axial rift basins (San Luis to the south and upper Arkansas to the north) reverses. The Poncha Pass block has been interpreted as part of an accommodation zone in the rift – a hypothesis which we are examining through a combination of geologic mapping, detailed structural analyses, and geophysical surveys. Preliminary mapping results show that, internally, this block is much more extensionally faulted than was previously mapped, and is dominated by two intersecting but not strictly conjugate fault sets. A set of generally NW-striking faults apparently formed earlier than a dominant set of N- to NE-striking faults. The large-scale structure of the rift in this area evidently evolved through time, especially in the early phase of rift development.

Bounding the north edge of the block, and forming the south border of the upper Arkansas basin, is an array of WNW- to WSW-striking oblique-slip faults that offset Mio-Pliocene basin fill against Proterozoic rocks. Hydrothermal alteration is common along this range-front zone, but is particularly localized at zones of pervasive fracturing associated with fault intersections. Movement along WNW-striking, aeromagnetically expressed faults appears to have resulted in tilting of Proterozoic foliations. Fault kinematic data from the Proterozoic rocks are complex; faults have myriad orientations and multiple slip modes with significant oblique slip. Fault slip data are organized into contractional versus extensional sets and are related to Laramide (~70 to ~40 Ma) and Rio Grande rift (~26 Ma to present) fault trends, respectively. Mean orientations for each fault set intersect along a nearly vertical line and most extensional slip directions radially diverge from this line, possibly indicative of non-plane strain. Faults cutting both Proterozoic rocks and Dry Union Formation basin-fill in the range-front fault zone have similar W and NW strikes and oblique- to strike-slip, atypical of the rift on a regional scale. Initial results on the geometry and kinematic patterns of structures of the Poncha Pass block thus appear consistent with the concept that it is part of an intrarift strain-transfer zone.