ADDING “TIME” TO THE EARTHSCOPE IMAGE: PETROLOGIC ANALYSIS, STRUCTURAL ANALYSIS, AND MONAZITE GEOCHRONOLOGY OF THE PROTEROZOIC CRUST

The Earthscope (US-Array) image of the southwestern Lurentian crust will be a composite view of structures related to a series of tectonic events, including: (1) early arc development (1.78-1.72 Ga); (2) accretion of arcs, crustal shortening, and tectonic burial during the Yavapai and Mazatzal orogenies (1.7-1.65 Ga); (3) crustal shortening, metamorphism, and plutonism (1.45-1.35 Ga); and (4) localized thermal, deformational, and plutonic events during the Grenville orogeny (ca 1.3-1.0 Ga); and (5) exhumation and further modification by largely brittle events in the Neoproterozoic and Phanerozoic. In many parts of the orogen, older structures have localized younger deformations, younger events have reactivated older fabrics, and similar structures or assemblages have been produced during distinct tectonic events. Microprobe monazite geochronology combined with structural and petrologic analysis have proven to be extremely valuable for timing deformation and metamorphic events, especially in combination with high-resolution U-Pb isotopic data. In northern Arizona, rocks are dominated by 1.7-1.65 Ga structures and fabrics with little 1.4 Ga overprint, except perhaps for late discrete structures such as near the Mojave-Yavapai terrane boundary. In Colorado, older, high-T, 1.7-1.65 Ga gneissic fabrics have been distinguished from localized 1.4 and post-1.4 Ga shear zone fabrics with distinct styles and kinematics. In New Mexico, 1.4 Ga deformation and metamorphism were intense, but in situ geochronology records a gradient in the 1.4 Ga effects, some cryptic 1.65 Ga fabrics, and several 1.4 Ga shear and fault zones. The 1.4 Ga plutonic/tectonic event has left a dramatic structural and metamorphic mark on the crust. The combination of P-T-t-D analyses, xenolith studies, and Earthscope imaging will illuminate the deep signature of this event (cause or effect), as well as the broader tectonic history of Laurentia and structures involved with its evolution.