A NORTHWEST-STRIKING, PRECAMBRIAN SHEAR ZONE IN THE NORTHERN MEDICINE BOW MOUNTAINS, SOUTHEASTERN WYOMING

A northwest-striking, southwest-dipping Precambrian shear zone (here named Rock Creek shear zone) is exposed at the northern end of the Medicine Bow Mountains, southeastern Wyoming. The shear zone is approximately 2 km wide, and it can be traced along strike for approximately 4 km before it is unconformably overlain by Phanerzoic sedimentary rocks. The shear zone is chiefly developed in the Arlington granite, a mylonitic two-mica granitic gneiss. An early set of mafic dikes (now L-S or L amphibolite tectonites) intruded the Arlington granite prior to the development of the shear zone, and these mafic rocks were metamorphosed under amphibolite-facies conditions synchronous with the shear-zone deformation. Thus rocks with the shear zone (both granitic and mafic) are penetratively deformed with a strong northwest-striking foliation and commonly a southwest-trending mineral elongation lineation. Shear strain gradually decreases to the southeast where it is partitioned into discrete, small-scale shear zones (m-scale rather than km-scale) and eventually the granitic rocks are only weakly strained or virtually undeformed. To the west-southwest, the shear strain is minimal as indicated by the excellent preservation primary sedimentary structures in the Conical Peak Quartzite of the Phantom Lake Metamorphic Suite. Scarce kinematic indicators (asymmetric mantled porphyroclasts and shear bands) are in the mylonitic granitic gneiss. However, the dominant feldspar porphyroclast is a phi-type mantle porphyroclast. The few definitive kinematic indicators thus recognized suggest a sense-of-shear down the lineation; i.e., normal-sense in present orientation. The northwest-striking shear zone is at a high angle to northeast-striking Early Proterozoic shear zones related to the Medicine Bow orogeny. Thus the shear zone may predate the Medicine Bow orogeny and be related to the Late Archean tectonic development of southeastern Wyoming. In progress, U-Pb radiometric dating of metamorphic sphene will test this hypothesis.