CONTRASTING STRUCTURAL DOMAINS IN BASEMENT BLOCKS OF THE SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO: EVIDENCE FOR DISCRETE PALEO- AND MESOPROTEROZOIC DEFORMATION EVENTS

The Gallinas Canyon region in the southern Sangre de Cristo Mountains of New Mexico exposes Proterozoic rocks of similar lithologies but dissimilar structures that provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. In the Upper Gallinas Canyon, quartzofeldspathic gneiss and amphibolite occur as discrete units that show a consistent NNW-striking and moderately SW-dipping foliation in a series of asymmetric E-verging antiforms and synforms. The units are intruded by Kfs megacrystic granitic pegmatites, many of which cross-cut the regional fabric. Deformation (D1), responsible for the penetrative NNW-striking fabrics (S1), and associated metamorphism (M1) are interpreted to have occurred during arc formation and accretion across NW-striking tectonic boundaries during formation of the Yavapai province. The age of D1-M1 is constrained by the 1736±14 Ma Evergreen Valley tonalite and the 1700±15 Ma Hermit’s Peak granite, syntectonic granitoids with NW-striking fabrics. Amphibolite host rock yielded an age of 1724±14 Ma that we ascribe to growth of metamorphic (M1) zircon concomitant with ca. 1.7 Ga magmatism and deformation.

The main Proterozoic massif is separated from isolated smaller basement blocks in the Lower Gallinas Canyon to the east by the Hermit’s Peak thrust fault, one of several Laramide thrusts that bound the east flank of the southern Rocky Mountains. These smaller Proterozoic blocks are characterized by m- scale tectonically interlayered quartzofeldspathic gneiss, amphibolite, and Kfs megacrystic granitic pegmatite. The Lower Gallinas Canyon gneisses display a strong ENE-trending nearly vertical foliation (S2) and meso- to macroscale isoclinal folds which plunge shallowly to moderately SW. Isolated amphibolite boudins preserve an earlier (D1?) fabric. The Cañon del Agua block is intruded by Kfs megacrystic granite; parallelism between the granite and country rock fabric and an abundance of granite pegmatite sills in the contact zone suggest syntectonic emplacement. Geochronology studies are currently underway to constrain the timing of magmatism and metamorphism that accompanied this younger deformation event. Our observations support models for partitioned intracratonic deformation associated with Mesoproterozoic accretionary events.