Proterozoic Farwell Mountain-Lester Mountain suture zone, northern Colorado : subduction flip and progressive assembly of arcs
The mechanisms by which juvenile Paleoproterozoic continental crust was derived from the mantle, accreted to southwestern Laurentia ca. 1.78-1.68 Ga, and stabilized as new continental lithosphere have been much debated. This study addresses the problem by assessing the proposed Paleoproterozoic "Farwell Mountain–Lester Mountain suture zone," a broad, complex zone of deformation that may contain the oldest accretionary structures within Paleoproterozoic crust south of the Cheyenne belt. The proposed "suture zone" is mapped in the Park Range of northern Colorado as a northeast-trending belt of imbricated tectonic slivers between ca. 1.79-1.77 Ga Green Mountain rocks to the north and ca. 1.76-1.72 Ga rocks to the south. Marble, chert, mafic to ultramafic bodies, deformed pillow basalts, and sulfide deposits all outcrop along the zone and may represent dismembered ophiolites. One of the major contributions of this study is the correlation of metamorphic and deformational fabrics among four field subareas (Farwell Mountain, Lester Mountain, Mica Basin, and the Soda Creek–Fish Creek shear zone), in an attempt to investigate the history and character of the proposed "suture zone." Steepening and overprinting by an east-west-striking, subvertical fabric is attributed to post-accretionary shortening ca. 1.68 Ga. Also documented is an earlier Paleoproterozoic ductile fabric ("S2/F2"), which may preserve evidence of a north-vergent fold and thrust system. This is consistent with the hypothesis that the region has a collisional accretionary history. Seismic data from the CD-ROM experiment reveal north-dipping reflections that are tentatively associated with a 3-5 km offset of the Moho and a north-dipping, high velocity body and are together referred to as the Farwell Mountain backthrust. Another set of reflections dip southward and project to the surface near Lester Mountain; these are tentatively correlated with the "S2" fabric. Together, these features may record the juxtaposition of two tectonic blocks along south-dipping structures in the proposed suture zone ca. 1.74 Ga. A tectonic model is proposed to explain the geometries outlined by combined surface geologic and seismic data from the Cheyenne belt and Farwell Mountain–Lester Mountain "suture zone," allowing some investigation of the early tectonic history of the region despite overprinting by subsequent tectonism.
Paleoproterozoic, Farwell Mountain-Lester Mountain suture zone, suture zone
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