1680 TO 1580 MA, 400 KM LONG, NORTH-SOUTH MEGASHEAR IN CENTRAL COLORADO PUTS A NEW SPIN ON THE MAZATZAL OROGENY

New structural mapping and U-Pb geochronological data from syn-deformational minerals establish that the Proterozoic of the SW US was modified by a >400 km long, N-S trending megashear/transform fault ca. 1.6 Ga. The megashear runs from northern New Mexico to southern Wyoming. Basement terranes west of the megashear moved 150-200 km north during progressive shortening and subduction from south to north (1.68 Ga in Wet Mtns., CO, 5 Points shear zone to 1.58 Ga in Sierra Madre, WY, Battle Lake Fault Zone). Evidence supporting this tectonic model includes: 1) widespread ca. 1.6 Ga deformation from Mexico to Wyoming (Duebendorfer et al., 2015), 2) north-directed, thrust-tear style reactivation of the Cheyenne belt, southern Wyoming, with at least 30km offset at 1.62-1.59 Ga (Duebendorfer et al, 2006), 3) 150-200 km long, north-dipping, subducted oceanic slab of Yuan and Dueker (2005) west of the trace of the megashear, 4) occurrence of crustally-derived, 1.63 Ga magmatism above the subducted slab (Jones et al., 2013), 5) rectification of misfit of shear zones across the Front Range and back ranges of Colorado (Chamberlain and Duebendorfer, 2005), 6) contrast in fault plane solutions of the ca. 1.6 Ga brittle, greenschist grade reactivation in the Sierra Madre (west of transform: north-directed thrust fault planes) compared to those in the Medicine Bow Mtns (east of transform: east-west strike-slip fault planes due to N-S shortening) (Strickland, 2004), 7) 400 km-long aeromagnetic low between the front and back ranges of Colorado (Finn and Sims, 2005), and 8) reinterpretation of the timing of formation of the N-S 5 Points shear zone in the Wet Mtns., with major s-s deformation ca. 1.68 Ga followed by minor deformation, but significant mineral growth and heating ca. 1.4 Ga. New evidence comes from extensive dating of shear zones throughout northern Colorado (Cubrich, 2017) from the Soda Creek-Fish Creek shear zone (Park Range), the 5 Points shear zone (Wet Mtns.), the Kings Canyon mylonite (southern Medicine Bows), the Skin Gulch and Idaho Springs-Ralson Creek shear zones (Front Range). The megashear/transform fault model not only refines the Proterozoic tectonic history of the SW US but may also explain the offset in the Jemez lineament in New Mexico, and the shift to NNW trend of the northern extension of the Tertiary Rio Grande Rift.