New Mexico Geological Society Annual Spring Meeting — Abstracts

Intricately faulted Paleozoic strata and subjacent syenite basement in the central Florida Mountains have been interpreted to record low-angle thrust faults that commonly emplaced younger strata over older strata or basement. Faults near Mahoney Park on the west range flank were re-examined in November 2008 in order to test hypotheses regarding fault origin and timing. Both normal- and reverseseparation faults are present in the study area.

Reverse separation faults strike E-W to NW, have moderate to steep southerly dips, and emplace Cambrian syenite on Ordovician strata and Ordovician to Silurian strata over Devonian strata. They are cut by normal separation faults that trend NW, NE and E-W, with moderate dips of about 55º to both NW and SE. Both sets of faults are truncated by a single normal-separation fault, termed the Mahoney Park fault (MPF), which strikes 322 and dips 41-45º SW; the fault extends through the study area, juxtaposes Ordovician through Mississippian hanging-wall strata above syenite, and merges to the southeast without deflection into the trend of the South Florida Mountains fault (SFMF; Clemons, 1998), with a fault plane attitude of ~320 75º SW. The SFMF emplaces syenite over Devonian strata near its juncture with the MPF, and thus preserves reverse separation, but it parallels a nearby normalseparation fault that is a splay or southeastward continuation of the MPF and may have reactivated the SFMF.

Cross-cutting relations indicate that reverse separation faults preceded most normal separation faults. Reverse separation faults have long been regarded as a product of Late Cretaceous-Paleogene Laramide orogenesis. These faults are cut by the moderately dipping normal faults, which are in turn truncated at the MPF. The age of the MPF is not constrained, but its trend parallels the strike of the upper Eocene Rubio Peak Formation, which constitutes most of the central range, but not more northerly strikes of Miocene strata to the north. We postulate that Rubio Peak tilting represents footwall uplift coordinate with offset on the MPF, suggesting that MPF offset is early Miocene, older than northsouth range-bounding faults that tilted the Miocene strata. Our preliminary findings indicate that complex fault relationships in the central Florida Mountains evidently resulted from multiple episodes of offset.