Mesoproterozoic reactivation of a Paleoproterozoic transcurrent boundary in the northern Colorado Front Range: Implications for 1.7- and 1.4-Ga tectonism

Abstract

Proterozoic metamorphic and igneous rocks in the northern Colorado Front Range display evidence for significant but localized 1.4-Ga deformation. Initial metamorphism and deformation occurred during crustal assembly ∼1.7 Ga, and the area was subsequently affected by widespread “anorogenic” granitic plutonism at ∼1.4 Ga. Although there is little evidence for penetrative 1.4-Ga deformation in northern Colorado, data from the northeast-striking Moose Mountain shear zone indicate localized 1.4-Ga contractional strain. The shear zone deforms both pre-1.70-Ga supracrustal rocks and the ∼1.4-Ga St. Vrain granite. Kinematic indicators within the deformed granite show south-side-up reverse motion. A continuum from magmatic to solid-state mylonitic fabrics indicates that deformation occurred during the emplacement and cooling of the granite at ∼1.4 Ga. Thrust-sense deformation, coupled with synchronous northeast-southwest extension recorded in dike swarms, is consistent with a model for regional northwest-directed contractional deformation during emplacement of the 1.4-Ga plutonic suite. Paleoproterozoic supracrustal rocks within the shear zone show evidence for the 1.4-Ga shearing as well as for an older phase of deformation apparently dominated by sinistral motion. Supracrustal rocks on opposite sides of the shear zone record different structural histories, and detrital zircon samples differ dramatically in age populations across the shear zone. These data lead us to suggest that the shear zone separates two unrelated packages of rocks that were juxtaposed against one another during ∼1.7-Ga assembly of the region. Thus, original assembly of this region around 1.7 Ga probably involved previously undocumented transcurrent movements that juxtaposed “terranes” with differing Paleoproterozoic histories and resulted in large-scale zones of crustal weakness that localized subsequent deformation.