Structural geology and Laramide tectonics of the Little Hatchet Mountains, southwestern New Mexico
Scott A. Hodgson
More than 3500 m of synorogenic elastic and intermediate volcanic rocks in the northern Little Hatchet Mountains were deposited unconformably upon Lower Cretaceous rocks and deformed both syn- and post-depositionally within a complex tectonic regime during latest Cretaceous to early Tertiary time. Synorogenic rocks arc divided into three formations, the Ringbone (late Campanian–Maastrichtian), Hidalgo (Maastrichtian–Paleocene), and Skunk Ranch (Paleocene–early Eocene) formations. The Ringbone and Skunk Ranch formations were deposited in alluvial-fan, fluvial, and lacustrine environments, and the Hidalgo Formation represents a coeval stratovolcano complex. Synorogenic deposits are exposed in four blocks separated by high-angle faults associated with northeast-vergent thrust faults that deformed each of the synorogenic units. Thrust faults typically dip shallowly west-southwestward, pass laterally northwestward into steeply dipping left-lateral oblique-slip faults, and are flanked on the south by a high-angle right-lateral strike-slip fault. The high angle faults that bound thrusts represent pre-existing structures that were suitably oriented to accommodate displacement during shortening. Laramide stratigraphy and structural geology suggest a two-stage Laramide event. An early Laramide (Late Cretaceous) stage consisted of basement-cored block uplifts along preexisting northwest-trending reverse faults in the southern Little Hatchet Mountains. The Ringbone Formation was deposited in a complementary basin to the north. This was followed by late Laramide (Paleocene–Eocene) left-lateral convergent wrenching along the same northwest-trending faults. The Skunk Ranch Formation was deposited in a wrench basin formed early in this stage of deformation. The system of northeast-vergent thrusts represents a flower structure associated with wrenching. The two Laramide stages correlate with changes in direction and convergence rates of the North American and Farallon plates between 66 and 61 Ma.
- Hodgson, Scott A., 2000, Structural geology and Laramide tectonics of the Little Hatchet Mountains, southwestern New Mexico, in: Southwest passage. A trip through the Phanerozoic, Lawton, Timothy F.; McMillan, Nancy J.; McLemore, Virginia T., New Mexico Geological Society, Guidebook, 51st Field Conference, pp. 109-116.