PALEOPROTEROZOIC SUTURE (?) IN NORTHERN COLORADO: INSIGHTS INTO THE GEOMETRY AND KINEMATICS OF CONTINENTAL ASSEMBLY IN THE SOUTHWESTERN U.S

Geologic and reflection seismic data highlight the "Lester Mountain suture zone" as potentially the most northerly and earliest of a series of sutures south of the Cheyenne Belt that resulted in addition of >1000 km of juvenile Paleoproterozoic crust to southern Laurentia by 1.65 Ma. The northern block contains the 1.78 Ga Green Mountain volcanics and a 1.78-1.77 Ga plutonic complex that includes co-mingled Elkhorn Mountain gabbro and Seven Lakes granodiorite. The southern block contains the 1.75-1.73 Ga Buffalo Pass/Rabbit Ears granodiorite/quartz diorite complex. The intervening Lester Mountain zone contains tectonic slices of both blocks, as well as marble, sillimanite pod rock, and ultramafics that we interpret to represent an accretionary complex between two once-separate arcs. The CD-ROM seismic reflection profile, that crosses N-S through and just west of the exposed Proterozoic rocks in the Park Range, shows N- and S-dipping families of seismic reflectors that extend to depths of 18 and 25 km respectively and project towards the surface in the proposed Lester Mountain suture zone. Structural mapping does not show N-dipping structures that may correspond to the moderately N-dipping reflectors and we speculate that these may represent the base and southern margin of the Elkhorn Mountain gabbro. In contrast, supracrustal rocks near Lester Mountain contain a shallowly S-dipping S2 fabric that is axial planar to N-verging tight to isoclinal folds. These folds deform a composite S0/S1 fabric and are themselves folded and transected by a steeply S-dipping, NE-striking S3 foliation. The S3 upright foliation appears to also be a composite fabric that resulted from several pulses of NW-SE shortening and steepening of the suture zone due to continued tectonism to the south. Although the steeply dipping S3 forms the most obvious regional structural grain, and locally overprints and obscures the more shallowly S-dipping S2 foliation, we interpret the F2 structures to be part of a system of N-verging thrusts and recumbent folds that correlate to the shallowly-S-dipping seismic reflectors. The overprinting and steepening of earlier accretionary structures by subvertical shortening fabrics may be typical of Proterozoic juvenile crust of the Southwest and helps explains the difficulty in identifying suture zones.