GEOLOGY OF THE SILVER CITY RANGE, GRANT CO. NM: IMPLICATIONS FOR THE TIMINING OF LARAMIDE DEFORMATION IN THE SOUTHERN ROCKY MOUNTAINS

The Silver City Range, in Grant County, New Mexico contains Proterozoic basement rocks, upon which were deposited a sequence of marine Paleozoic (Bliss, El Paso, Montoya, Cutter-Fussleman, Percha, and Lake Valley-Oswaldo Formations), Mesozoic (Beartooth Ss and Colorado Sh), and Tertiary strata. These rocks were folded in a broad doubly-plunging, NW-SE trending monocline. This fold is cut by a series of NE-SW trending normal faults, which we interpret to be contemporaneous with the folding. The monocline and early normal faults are cut by NW-SE Basin and Range normal faults.

In SW-to-NE the trending cross-sections in the southeast part of the range, the Paleozoic-Mesozoic section goes from dips of ~10° to 70-80°. In the northwest part of the range, in sections with a similar orientation, post-Colorado rhyolites dip from 15° to 45° and back to 15°. The tilted rhyolites are capped by sub-horizontal basalts, similar to others in the Mimbres Valley dated at ~ 6 Ma. The similarity of structural style throughout the range leads us to conclude that a compressional (Laramide-type) deformation affected the section up to and including the thick sequence of rhyolites. These rocks are similar to other Oligocene volcanic rocks of the Mogollon-Datil volcanic field. If the rhyolites in the NW part of the Silver City Range are also Oligocene, this would suggest Laramide-style deformation was active in the southern New Mexico after 35 Ma but not since ~30 Ma (the beginning of RGR extension). This is in line with previous estimates of the timing of the transition from Laramide compression to younger extension in the Datil region (~36 Ma) and the Big Bend region of west Texas (~32 Ma).