THE SODA CREEK FISH CREEK SHEAR ZONE: USING MULTIPLE GEOCHRONOLOGIC METHODS TO UNRAVEL THE PROTEROZOIC STRUCTURAL HISTORY OF POLYPHASE DEFORMATION IN HIGHLY DEFORMED ROCKS

The Soda Creek-Fish Creek shear zone (SCFCSZ) in Routt National Forest, Colorado, is a poly-deformed, foliated, folded, and lineated zone of deformation with a mean foliation attitude of 238°, 85° and a width of up to 5 km. It is characterized by steeply dipping foliations with steeply plunging but variably oriented stretching lineations and fold axes. It is bounded by the 1.42 Ga Mount Ethel pluton to northwest, and less intensely deformed Paleoproterozoic metamorphic rocks to the southeast. The shear zone records at least two periods of deformation: 1) a higher temperature ductile phase up to feldspar plasticity, and 2) a lower temperature brittle phase with epidote growth and formation of cataclasite. The two dominant fabrics within the shear zone (locally S1 and S2) have been loosely constrained to have formed after the ca. 1.78 Ga Proterozoic arc accretion and before the emplacement of the 1.42 Ga Mount Ethel pluton.

This study presents U-Pb data from multiple periods of metamorphic growth of titanite and monazite that directly date SCFCSZ activity/deformation from 1.77-1.57 Ga. Our data suggest at least 5 periods of tectonism for the region: 1.77 Ga regional accretionary fabrics, an event at 1.72-1.70 Ga with unknown tectonic significance, at least two periods of high strain ca. 1.66 and 1.60 Ga (SCFCSZ), and 1.42 Ga plutonism associated with the Mount Ethel pluton. Although we tightly constrain the formation of the first fabric (D1) at 1.77 Ga, there appear to be multiple pulses of metamorphism and metamorphic mineral growth ca. 1.67-1.60 Ga associated with the formation of the high-strain SCFCSZ. It may have formed either during several discrete events or over a relatively continuous, protracted 70 million year period. We present structural data compiled from new and previous studies, as well as thermobarometry (garnet-biotite, GASP, pseudosection analysis), which also confirms multiple periods of metamorphism and deformation in the SCFCSZ. The new geochronology and structural data from the SCFCSZ add to a growing body of data that document a major period of regional tectonism at 1.65-1.55 Ga, well inboard from the active accretionary margin.