PRELIMINARY KINEMATIC ANALYSIS OF MYLONITE ZONES WITHIN THE SOUTHERN RUBY MOUNTAINS, SW MONTANA: IMPLICATIONS FOR REGIONAL PROTEROZOIC(?) TECTONIC ACTIVITY

The Ruby Mountains in southwest Montana are one of several Archean-aged basement cored uplifts exposed during Cretaceous-Tertiary time. Our work has concentrated within the southern portion of the Ruby range and reveals the presence of several ductile shear zones and related granitic and leucogranitic mylonites. These shear zones are laterally extensive for several kilometers and occur within a sequence of biotite and hornblende granitic gneisses, calcitic-dolomitic marble, amphibolite, sillimanite-grade pelitic schists and gneisses, and quartzites Small (tens of meters to 1 km) bodies of ultramafic rock intrude the metamorphic sequence and locally have caused contact zone melting of adjacent country rock.

The shear zones range between 5-10 meters in thickness with mylonitic foliations generally striking to the NE and dipping to the NW. The fabric is parallel to the foliations in the adjacent rocks suggesting mylonitization was concurrent with a period of regional metamorphism. A well-developed mineral stretching lineation records predominantly dip-slip movement, but locally, oblique-slip is also recorded. Sheath-type folds developed in thin calc-silicate layers in calcitic marble units have axial orientations that cluster around the down-dip mineral stretching lineations of the shear zones.

Mesoscopic analysis of the mylonites reveals the presence of sigma-type porphyroclasts that record a strong top-to-the-south sense of shear consistent with regional compression and thrusting. However, in places the opposite shear sense can be identified indicating a period of top-to-the-north or normal sense of motion. The thrust movement is consistent with regional shortening during a period of high-grade metamorphism. The normal sense of movement appears to postdate thrusting, suggesting either a period of late orogenic collapse following the overthickening of the metamorphic pile (similar to the modern-day Himalayas) or a discretely younger tectonic event. The timing of deformation and mylonite formation is unknown, although regional tectonic arguments suggest that the thrusting most-likely occurred during the Early Proterozoic Big Sky Orogeny.