FARWELL MOUNTAIN SHEAR ZONE: TERRANE BOUNDARY IN THE PARK RANGE OF COLORADO BASED ON STRUCTURAL, PETROLOGIC, AND SEISMIC EVIDENCE

The Farwell Mountain shear zone is here suggested as an important early accretionary boundary within Paleoproterozoic juvenile crust and the first major Proterozoic suture south of the Cheyenne Belt. This zone separates the 1.78 – 1.75 Ga Green Mountain block to the north from the 1.75 – 1.72 Ga Rawah batholith block to the south. The zone contains ultramafic slivers, sillimanite pod-rock, and deformed metasedimentary rocks that may represent accretionary mélange. Preliminary U-Pb zircon, U-Pb sphene, and Th-U-Pb monazite dates have distinguished events in the Park Range involving metamorphism and shortening at 1.75-1.74 Ga and 1.68 Ga. The former is interpreted to record docking of the Rawah block from the south; the latter shortening due to subsequent southern collisions. Metamorphic grade varies sharply across the zone; lower amphibolite grade to the north (500 C, 2-8? kbars), upper amphibolite grade to the south (650 C, 6 kbars). Overprinting of Paleoproterozoic fabrics by 1.42 Ga tectonism and emplacement of the Mount Ethyl Pluton is concentrated near the Soda Creek - Fish Creek shear zone, 30 km south of the Farwell Mountain zone.

Seismic reflection data from the CDROM Cheyenne Belt line reveal two strong, oppositely dipping reflectors that surface near the Farwell Mountain shear zone. A weak 40°-N-dipping discontinuous event changes from multicyclic to a simple reflection and can be traced to a depth of about 18 km. A 20°-S-dipping event is a series of multicyclic reflections that continue to a depth of 25 km. There are several (unresolved) explanations for the oppositely dipping reflections, such as flipping subduction polarity or back thrusts. We tentatively interpret the zone to be Paleoproterozoic based on available timing data and structural style. The overall geometry is consistent with the commonly-discussed hypothesis of formation of juvenile continental crust by thrust-related accretion of arc terranes.