KINEMATIC TESTS FOR THE CA. 1.6 GA MODIFICATION OF PALEOPROTEROZOIC CRUST IN COLORADO AND SOUTHERN WYOMING

Two hypotheses have been proposed as the mechanism for the widespread ca. 1.6 Ga deformation first recognized in the thrust tear/strike slip reactivation of the Battle Lake Fault Zone (BLFZ), Sierra Madre, southern Wyoming of Duebendorfer et. al (2006): (1) far field strain 1500 Km inboard of the accretionary margin of Laurentia at that time, or (2) slab subduction of 200 kilometers of ocean basin associated with a strike-slip fault connecting to the BLFZ. We test these hypotheses by using detailed kinematic studies of the Proterozoic shear zones in the Front Range and Park Range as piercing points on either side of this proposed strike-slip feature. The Soda Creek-Fish Creek shear zone (SCFC) is one case study, where we recognize at least four periods of deformation from 1.8 to 1.4 Ga. The first period of fabric development (D1) is defined by NNW striking moderate- to steep-dipping foliations (S1) provisionally dated at 1702+9 Ma by U-Pb of metamorphic sphene. D2 is defined by a pervasive ~241^o steeply dipping fabric (S2), and is similar in orientation to dominant regional fabrics. This fabric is provisionally dated at 1679+6 Ma. Two newly documented fabrics for the SCFC shear zone are a NNW striking foliation (S3) and NE striking mylonites with a preliminary age of 1623+5 Ma. These fabrics clearly postdate the 1.68 Ga fabric (D2), and support the hypothesis that the ca. 1.6 Ga deformation and modification of Proterozoic lithosphere is the result of a large NNW striking shear system. We will also compare the SCFC kinematic history to those of previously studied shear zones in the Front Range and new studies of lesser documented shear zones, for which detailed kinematic histories are being worked up as part of this study. Deformational histories are being determined by U-Pb dates of syn-deformational mineral growth (e.g. zircon, sphene, epidote, monazite) for these shear zones.