Superposed reverse and normal faults in the central Florida Mountains, southwestern New Mexico, and their implications for post-Cretaceous crustal deforamtion

A bstrAct — 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–south-east. 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 40 Ar/ 39 Ar 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 reacti- vated 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.