Collaborative Research: Contrasting Styles of Ca. 1.4 Ga Tectonism in the Southern Rockies: Evidence for a Fossil Rheologic Transition in a Deeply Exhumed Intracontinental Orogen

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Karlstrom 0003500Connelly 0003528Williams 0003569Siddoway 0101314Proterozoic rocks in central Colorado preserve a transition, from semi-brittle upper crust to pervasively ductile middle crust, that was frozen in at about 1.4 Ga. This rheologic transition is expressed as a regionally significant change in the style of ca. 1.4 Ga intracratonic tectonism. Field observations suggest that a subhorizontal boundary separates a domain in northern and central Colorado (exemplified by the Colorado Mineral Belt), where 1.4 Ga deformation is characterized by discrete mylonite zones, from a domain in southern Colorado and northern New Mexico dominated by penetrative ductile deformation. Geodynamic models developed for intracontinental orogens such as Tibet, Altiplano-Puna, and the Sevier-Laramdie indicate that such mechanical decoupling may be a critical factor in controlling the tectonic evolution of intracontinental deformation. Like some of its younger analogs, the structural transition of central Colorado apparently corresponds with a magma- and fluid-rich, high-strain layer. These deeply exhumed rocks present intriguing opportunities to directly investigate deep-crustal processes that are inaccessible in younger orogens. Issues of general impact include characterization of: (1) the controls on the depth and thickness of the brittle/ductile transition (e.g. temperature, lithology, fluids), (2) the kinematics and dynamics of deformation near the rheological transition, (3) mechanisms of accommodating strain incompatibility between different deformation regimes, and (4) the role of granitoid melts and/or a widespread melt-fluid layer, in localizing middle crustal flow and structural decoupling of upper and lower crust. In addition to exploiting the deep crustal exposures of central Colorado to study general processes of intracontinental deformation, this study is contributing to the understanding of the Proterozoic tectonic evolution of the southwestern United States. In particular, alternate models for "anorogenic" (extensional) versus "orogenic" (transpressional) tectonic settings for 1.4 Ga tectonism can perhaps be reconciled in an orogenic plateau model where regional horizontal contraction is balanced by more localized gravitational collapse. The study uses an integrated data set involving structural, petrologic, and U-Pb geochronologic data to evaluate the kinematics and evolution of this important structural transition. Focus is on a well exposed transect across the structural transition. Specifically, studies of the Black Canyon of the Gunnison and adjacent areas within the Colorado Mineral Belt are characterizing the style, extent, and P-T conditions of mylonite shear zones and emplacement of syn-kinematic 1.4 Ga plutons above the transition. Work in the Wet Mountains and Sangre de Cristo Range is characterizing pervasively ductile deformation and fabric reactivation within and below the transition. The geochronology is emphasizing both U/Pb zircon dating of critical syn-kinematic plutons in central Colorado and microprobe monazite dating of high-grade metamorphism along the transect.