253-4 Repeated Reactivation Reveals Persistent Weakness in Colorado's Continental Lithosphere

Abstract:

Northeast-trending Proterozoic shear zones exposed across Colorado are viewed as classic examples of repeated reactivation and progressive strain localization traditionally linked to the Yavapai (1.72–1.68 Ga), Mazatzal (1.66–1.60 Ga), and Picuris (1.47–1.35 Ga) orogenies. These structures have also been suggested to influence much younger deformation and mineralization associated with the late Mesozoic to Cenozoic Colorado Mineral Belt. Linking specific structures or fabrics to individual deformation episodes remains challenging, particularly for lower temperature (ultra)mylonites. Here, we present new geochronology and structural interpretations that directly date mixed plastic–brittle deformation in these shear zones, demonstrating a longer and more complex reactivation history than previously recognized. In-situ U-Pb monazite, xenotime, and titanite ages, combined with in-situ Rb-Sr mica geochronology, document progressive down temperature reactivation of mylonitic, ultramylonitic, and recrystallized pseudotachylite fabrics in the Grizzly Creek, Homestake, and Moose Mountain shear zones at ca. 1.6, 1.4–1.3, 1.2, and 1.1 Ga. We interpret the ca. 1.2–1.1 Ga ages of ultramylonite and recrystallized pseudotachylite to represent previously unrecognized, widespread reactivation more than 1000 km inboard from the active Laurentian plate boundary during the Grenville orogeny.

Our findings support the interpretation that Proterozoic shear zones in Colorado acted as persistently weak structures over much of the Proterozoic and were repeatedly reactivated under evolving thermomechanical conditions. This long-lived weakness has broad implications for lithospheric rheology, the transmission and accommodation of far-field stress, and the potential for seismicity within inherited structures far from active plate boundaries. Further, our results highlight the importance of detailed petrochronologic characterization that integrates mineral ages, textures, and chemistry with microstructural context, particularly in multiply deformed rocks.

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