A STUDY OF THE FOWLER PASS SHEAR ZONE IN THE PROTEROZOIC BASEMENT OF THE CIMARRON MOUNTAINS, NORTHERN NEW MEXICO

The Cimarron Mountains of northern New Mexico, expose Proterozoic basement that records the growth and stabilization of southwestern North America. Proterozoic rocks in the Cimarrons are divided into two tectonic blocks, an upper amphibolite grade block, and a greenschist grade block, separated by the Fowler Pass shear zone (F.P.S.Z). The F.P.S.Z. was reactivated as a brittle reverse fault during Laramide orogenesis. West of the F.P.S.Z. high-grade rocks record at least three phases of deformation as recorded by overprinting foliations. In contrast, the greenschist grade block contains a single cleavage. The differences in metamorphic grade and deformation history demonstrate that the F.P.S.Z. represents a fundamental boundary within the Proterozoic basement. Ductile deformation within the greenschists is variably developed with intensity of deformation ranging from weakly deformed to mylonitic. Foliations dip predominately northeast. Mineral stretching lineations have variable north-northwest plunges, with the average plunge decreasing towards the SW. In contrast, foliations within the high-grade block intersect the trace of the brittle strand of F.P.S.Z. at high angles with E-W strikes and steep dips. This relationship suggests Laramide motion on the F.P.S.Z. significantly modified the Proterozoic structures. However, kinematic indicators within the greenschist grade rocks record sinistral northeast side down sense of motion. This suggests that Proterozoic extensional shearing is responsible for the initial juxtaposition of the two crustal blocks. The timing of this event is poorly constrained, although extant Ar40/Ar39 dates suggest that this deformation event may have occurred between 1450 and 1350 Ma. However, crosscutting relationships around the Cimarron River Pluton suggest that deformation may have occurred in the Early Proterozoic. Regardless of the timing of this event, structural relationships require significant normal shearing during the Proterozoic.