TECTONIC "FINGERPRINT" FOR DEFORMATIONAL HISTORY IN THE COLORADO MINERAL BELT: TWO ~100 MA OROGENIC INTERVALS PUNCTUATED BY ~15 MA TECTONIC PULSES

Shear zones of the Colorado Mineral Belt (CMB) have similar orientation, kinematics, deformation textures, and timing of movement, and provide information on the tectonic evolution of an important intracontinental zone of deformation. The CMB is interpreted as a lithospheric zone of weakness that was established in the Paleoproterozoic and reactivated repeatedly since then. Within the CMB, we have found that the Homestake (HS), Gore (GR), St. Louis Lake (SLL), and Idaho Springs-Ralston (ISR) shear zones are all NE-trending, near vertical, with steep mineral stretching lineations. The zones are composed of highly-strained gneisses and schists that are overprinted by lower-temperature S-side down mylonites, S-side up ultramylonites, pseudotachylites, and brittle faults. The complex overprinting of tectonites with progressively lower-temperature and more focused deformation indicates that these shear zones are long-lived zones of weakness in the lithosphere. Monazites from HS and ISR tectonites record deformation and metamorphism at ~1712, ~1700, and ~1680 Ma, and development of S-side down mylonites at ~1380 Ma. We view the CMB shear zones as parts of a system that deformed synchronously in the Paleoproterozoic, during crustal assembly, and in the Mesoproterozoic, as an intracontinental zone of deformation ~1000 km inboard of a convergent margin. Our hypothesis is that repeated reactivation of the CMB was a response to far field stresses, with strain focused in zones that were previously weakened through grain size reduction and reaction softening. The ~1400 Ma intracontinental zone of deformation may have been transpressive in nature, with steep dip-slip movements focused in mylonite and ultramylonite zones and strike-slip movements (still undocumented) distributed over broader areas. The shear zones may represent a flower structure, like that proposed for the northern boundary of the Aegean plate, which steepens at middle-crustal depths.