Shear zone reactivation during multiple phases of Proterozoic orogeny: evidence from in situ monazite dating of the Slide Lake Shear Zone, Colorado

Abstract

Shear zone systems exposed in the crystalline rocks of Colorado record Proterozoic deformation episodes documenting strain partitioning, reactivation and the progressive tectonic evolution of SW Laurentia. These structures, and their equivalents throughout the southwestern USA, provide insight into the structural evolution of the North American craton, and inform tectonic models for an active, but challenging to decipher period in Earth's history. NE–SW-trending mylonitic bands record regional fold formation and establishment of sub-vertical D1/D2 shear zones occurred during the successive c. 1720–1600 Ma Yavapai and Mazatzal orogenies, and D3/D4 reactivation of these structures during the c. 1450–1300 Ma Picuris Orogeny. The shallowly dipping Slide Lake Shear Zone (SLSZ), in Colorado's Sawatch Range, is adjacent to the steeply dipping Homestake Shear Zone (HSZ), allowing for comparison between high- and low-angle faulting in mid-crustal positions associated with the long-lived Laurentian orogenic margin throughout the Proterozoic. Both structures record similar deformational conditions and bulk NW–SE-verging kinematics. Newly obtained in situ electron microprobe monazite U–Th–Pb geochronology and trace element geochemistry constrain the timing and nature of SLSZ activity, allowing direct comparison with established HSZ activity. Core domains in a steeply dipping segment of the SLSZ record mineral growth during the Yavapai Orogeny (D1) at c. 1700 ± 14 Ma, and support continued SLSZ activation during the Mazatzal Orogeny (D2). Low-angle segments preserve deformation associated with the Mesoproterozoic Picuris Orogeny (D3/D4) at c. 1450–1300 Ma. The age progression of the SLSZ parallels the structural evolution of the HSZ, indicating that sub-horizontal, transtensional, D3/D4 slip on the SLSZ was coeval with sub-vertical, transpressional, D3/D4 slip on the HSZ. Cumulatively, the existing structural and newly established temporal history of the HSZ–SLSZ system is interpreted to indicate that oblique extensional and compressional strain were competing in the middle crust within the Laurentian interior during the Picuris Orogeny, lending credence to models of plateau-style crustal thickening and collapse during this event.