New Mexico Geological Society Annual Spring Meeting — Abstracts

The Laramide orogeny was a major mountain-building event in the western United States that occurred from latest Cretaceous to Eocene time. In Arizona and New Mexico, Laramide-age structures are generally obscured by Tertiary volcanism and extension. The age and style of Laramide deformation is thus much more controversial than in areas further north. The rocks in the Little Hatchet Mountains, southwestern New Mexico have escaped pervasive overprinting by magmatism and extension and thus preserve Laramide structures. This area has the potential to clarify some of the remaining questions concerning this major tectonic event.

The purpose of this study is to determine the direction, magnitude, and timing of Laramide deformation as recorded in the Little Hatchet Mountains, and to compare structures developed at this time to other Laramide structures observed in the southwestern United States. South of the Copper Dick fault, a continuous Jurassic section of sedimentary rocks is exposed. Numerous NE-striking mafic dikes cut Jurassic beds. Large-scale and small-scale folds are common, as well as thrust faults that may have reactivated as normal faults during Basin and Range extension. Quantitative strain analysis of stretched pebbles, vesicles, and burrows, as well as analysis of folds and faults has been initiated. Field work has revealed the presence of a spectacular stretched-pebble conglomerate within the Jurassic Broken Jug Formation. The elongation direction of limestone clasts within this conglomerate ranges from N 10E to N10W. Long axis to short axis ratios of clasts range from 2 to 10, and chert clasts within the same unit appear to be pulled apart in the same elongation direction. As Basin and Range extension directions were generally east-west in this region, these structures may record ductile deformation associated with the Laramide event. The relationship of these structures to large-scale thrust faults in the Little Hatchet Mountains has not yet been determined. Apparent new mica growth within impure dolostone in the Broken Jug Formation may provide a means of dating this deformation more precisely.