Geology of Proterozoic outcrops in Dead Man and Little San Nicolas Canyons, southern San Andres Mountains, New Mexico

Proterozoic outcrops in the southern San Andres Mountains generally consist of large granitic plutons. However, isolated areas, such as Dead Man and Little San Nicolas Canyons, contain other rock types. A sequence of interbedded schists and fine-grained quartzites overlain by a crossbedded, micaceous quartzite is exposed in Dead Man Canyon. Three generations of folding have been identified: (1) early isoclinal folds perhaps synchronous with the metamorphism; (2) open to tight folds with a well-developed axial planar cleavage that strikes northwest; and (3) east-trending reclined folds that reorient the earlier structures into broad kinks. A foliated pluton dated at 1632 ± 24 Ma (U-Pb zircon) defines a maximum age for the second phase of deformation. Little San Nicolas Canyon, approximately 32 km south of Dead Man Canyon, contains highgrade gneiss, amphibolite and non-foliated granitic rock. The gneiss displays a foliation, defined by bands of dark and light minerals, that has been folded into tight to isoclinal east-trending folds. This unit is exposed along high-angle reverse faults. Zircons from the gneiss are highly discordant, but yield an age of 1730± 130 Ma. The protolith of this gneiss is unknown, but is most likely plutonic. One post-orogenic granite has been dated at 1460 ± 65 Ma. Although both study areas display similar east-west structural trends, they are markedly dissimilar.