Proterozoic tectonic evolution of southern Laurentia: new constraints from field studies and geochronology in southern Colorado and northern New Mexico, U.S.A.
Jones, James V.
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New field studies and geochronology from southern Colorado and northern New Mexico constrain the Proterozoic growth and modification of southern Laurentia. The Sangre de Cristo Mountains of southern Colorado preserve evidence for three episodes of Proterozoic magmatism, deformation, and metamorphism. Early deformation produced penetrative, NW-striking fabrics and occurred in an arc setting between 1750 – 1730 Ma. Post-Yavapai granitoid magmatism occurred at 1695±2 Ma and 1682±3 Ma and was contemporaneous with regional quartzite deposition. Deformation at 1637±6 Ma produced localized NE-striking, subvertical fabrics with dextral shear sense. Granitic magmatism at 1434±2 Ma and 1407±6 Ma was accompanied by NW – SE shortening between 1420 – 1412 Ma that produced subvertical, NE-striking fabrics. Thick sequences of quartz arenite were deposited across the region between the Yavapai and Mazatzal orogenies (ca. 1.70 Ga and 1.65 Ga). New geochronology reveals that deposition occurred on exhumed, Yavapai-aged basement (1706±5 Ma and 1698±4 Ma) with detritus dominated by Paleoproterozoic sources only slightly older than the quartzites themselves. Regional quartzite sedimentation was contemporaneous with nearly continuous magmatism in the region at deeper crustal levels. The first-cycle, synorogenic character of quartzites contrasts with their extreme compositional maturity, requiring perhaps anomalous environmental influences that enhanced chemical weathering during deposition. New geochronology and structural studies from the Wet Mountains, Colorado, reveal contrasting structural styles during widespread Mesoproterozoic A-type granitic magmatism. At shallower crustal levels, strongly localized deformation at 1430+5/-3 Ma produced subvertical fabrics throughout the N-striking Five Points shear zone. At deeper crustal levels, penetrative deformation accompanying granitic magmatism at 1435±5 Ma and 1390±10 Ma produced moderately- to shallowly-dipping fabrics. Regionally consistent fabric orientations and kinematics are interpreted to represent an intracontinental response to convergent tectonism, and contrasting, yet coeval, styles of deformation require a structural discontinuity in the middle crust between ca. 1430 – 1360 Ma. Weak, flowing lower crust is consistent with models for intraplate orogenesis and the development of orogenic plateaus, and the southern Wet Mountains might represent an exhumed analog for mid-crustal, low-viscosity layers inferred beneath modern intracontinental orogenic systems such as Tibet and the Altiplano.