New Mexico Geological Society Annual Spring Meeting — Abstracts

The Little Hatchet Mountains of southwestern New Mexico expose rocks of Paleozoic to Tertiary overlying Precambrian basement. This area has been affected by at least two deformational events, Late Cretaceous-Paleogene age Laramide contraction and middle Tertiary extension. The range is cut by an east-west striking high-angle strike slip fault known as the Copper Dick fault. South of the Copper Dick fault, a continuous section of Jurassic and Lower Cretaceous sedimentary rocks is exposed. The Jurassic Broken Jug Formation section contains beds of limestone and chert clast conglomerate with a calcite matrix. Clasts within these beds are stretched and have long axis to short axis ratios ranging from 1.5-7. Clasts generally have shallow plunges and trend within 20° of north-south. The chert clasts have been pulled apart in the same direction as the limestone clasts and have a top-to-the-north shear sense. The style of deformation of these units and metamorphic white-mica growth within adjacent units suggests that deformation took place at < ~300°C.

Strain patterns in the range suggest that the conglomerate deformation is not related to Laramide folding. Numerous large-and small-scale Laramide folds in the area have axes that trend east-northeast. The consistent orientation of the elongate limestone clasts suggests that north-south stretching occurred after the folding event. Syntectonic growth strata of the Ringbone and Hidalgo formations indicate the folding event taking place at least as recently as 70 Ma. North-south stretching in the Little Hatchet Mountains is also recorded by numerous east-west striking mafic and rhyolitic dikes that cut Jurassic beds in the range. In addition, Karlstrom et al. (1994) noted calcite veins that may record north-south extension from about 45 to 30 Ma, though the timing of vein formation is not well-constrained.