OLIGOCENE SHORTENING IN THE LITTLE BURRO MOUNTAINS, SW NEW MEXICO

In SW New Mexico, Larmaide-style shortening structures have been observed in several locations but the youngest deformed rocks in many parts of this region are Cretaceous, making estimation of the cessation of the Laramide orogeny in this region (generally thought to be after 40 Ma) problematic. Recent observations by Copeland et al. (2011) show that shortening is younger than 34.6 Ma in the Silver City Range of SW New Mexico.

In the Little Burro Mountains, 10-20 km SE of the Silver City Range, lying unconformably on Proterozoic granite is the Beartooth ss (K), succeeded by the Colorado Sh (K), andesite lavas and breccias (K-T), the tuff of Indian Peak (T) and the tuff of Wind Mountain (T).

Field mapping in the Little Burro Mountains shows a monoclinal fold with an axial trace trending from NW to SE. The Little Burro monocline displays modestly dipping beds of 12ᵒ in the backlimb, which steepen to approximately 30ᵒ in the forelimb. Normal faults run orthogonal to the axial trace of the fold with low displacement (10s of meters). They are interpreted to have formed synchronous with the monocline to accommodate variations in shortening along strike. This corresponds to a NE propagation of shortening, which is a well-known characteristic of the Laramide Orogeny (Bird, 1988). Fault-propagation-fold modeling and the interlimb angle of the Little Burro monocline support the formation of this structure above a deeply rooted blind thrust fault.

The Wind Mountain ash-flow tuff, was dated by U-Pb on zircons using LA-ICPMS. Using the FC5Z standard, the ²⁰⁶Pb/²³⁸U age is 27.8 ± 0.4 Ma.

These results indicate this Laramide-style shortening was active in the late Oligocene. Examples of rift-related extension and arc volcanism are known to pre-date this age (Mack, 2004; Chapin et al., 2004; McIntosh et al., 1991), suggesting that compressional shortening, arc volcanism and extension were all active in this region during the late Eocene and Oligocene. This complex regional tectonics can be explained by an intermittent break-off of the Farallon slab followed by renewed under-thrusting.