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1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
2 Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
3 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
The Homestake shear zone, one of the principal Precambrian structures within the Colorado mineral belt, has a history of tectonism that extends from the Proterozoic to the Tertiary. New field mapping, microstructural analysis, and electron-microprobe U-Th-Pb monazite dates define the style, kinematics, and timing of multistage Proterozoic ductile deformation. Early subhorizontal shearing (D1) progressed to northwest-southeast crustal shortening (D2) during a protracted ca. 1710–1630 Ma orogeny involving high-temperature metamorphism, partial melting, and emplacement of the ca. 1.7 Ga Cross Creek batholith. The subvertical, northeast trend of the shear zone was established by D2 shortening. A major episode of simple-shear displacement occurred ca. 1.4 Ga during emplacement of the St. Kevin batholith. Complex kinematics are recorded by 1380 Ma, southeast- side-down, dextral mylonite zones (D3) and post–1380 Ma southeast-side-up, dextral ultramylonite-pseudotachylyte zones (D4) that were both localized in D2 foliation domains. The succession of ductile tectonites in the Homestake records >300 m.y. of episodic deformation at progressively shallower crustal levels as Colorado evolved from an active accretionary margin (1.8–1.7 Ga), through crustal assembly (1.7–1.6 Ga), to a tectonically active continental interior (post 1.45 Ga). The Homestake shear zone constitutes a classic example of ductile shear-zone reactivation and the development of persistent tectonic zones in the continental lithosphere.
Key Words: shear zones • mylonite • reactivation • Proterozoic • monazite • geochronology
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