Superposed reverse and normal faults in the central Florida Mountains, southwestern New Mexico, and their implications for post-Cretaceous crustal deforamtion
T.F. Lawton and C.A. Clinkscales
At Mahoney Park in the Florida Mountains of southwestern New Mexico, field relations demonstrate several generations of faults that offset basement and the Paleozoic section. The oldest faults have reverse separation; they strike east–west and northwest–southeast. The reverse-separation faults emplaced basement granite over Ordovician strata and Silurian dolostone over Devonian Percha Shale. Younger normal faults with approximate east-west strikes cut basement and strata as young as middle to late Eocene. A major NW-trending, moderately dipping normal fault, termed here the Mahoney Park fault, is either contemporary with the steep faults or post-dates them. The steep faults do not cut the Mahoney Park fault, which emplaces Paleozoic sedimentary rocks on footwall basement syenite and offsets the trace of a major reverse-separation basement fault, the South Florida Mountains fault. The Mahoney Park fault is not expressed in surficial deposits flanking the range, whereas north-trending, range-bounding faults form subdued scarps of several meters on the northwest flank of the range. Regional geologic relations suggest that reverse-separation faults are Late Cretaceous to middle Eocene, both predating and postdating deposition of the middle Paleocene to early Eocene Lobo Formation. Rhyolite and granite dikes with U-Pb and 40Ar/39Ar ages on groundmass, biotite, and hornblende that range from ~32–25 Ma are emplaced along east-west faults in the Little Hatchet Mountains of southwestern New Mexico and the northern part of the Florida Mountains. The dike ages indicate that the east-west normal faults were active in the Oligocene prior to development of north-trending Basin and Range faults. Some of the extensional faults demonstrably reactivated Laramide reverse faults. The Mahoney Park fault may be time equivalent with the Oligocene faults or alternatively may represent an intermediate phase of post-Oligocene, pre-Basin and Range faulting. Exploitation of some Laramide shortening structures by the east-west faults suggests that north-south extension resulted from gravitational collapse of a high-elevation plateau in Oligocene time.
- Lawton, T.F.; Clinkscales, C.A., 2018, Superposed reverse and normal faults in the central Florida Mountains, southwestern New Mexico, and their implications for post-Cretaceous crustal deforamtion, in: Las Cruces Country III, Mack, Greg H.; Hampton, Brian A.; Ramos, Frank C.; Witcher, James C.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 69th Field Conference, pp. 119-125.