Southeastward Propagation of Thermotectonism within the Paleoproterozoic Big Sky Orogeny: New Constraints from the N. Madison Range, SW Montana
Cailey Condit, University of Colorado
Kevin Mahan, University of Colorado
Alexis Ault, University of Arizona
Spatial and temporal patterns of deformation and metamorphism can yield insight into the kinematics, rheology, and overall tectonic significance of exhumed paleo-orogens, and allow better understanding of the processes at work in active orogens of similar scales. The Big Sky orogen in SW Montana is recognized as a late Paleoproterozoic convergent belt with a general NW-SE shortening direction. Previous work in the Highland range, the farthest NW exposure of known Big Sky affected rocks, document zircon growth and peak P-T conditions at ~1810-1780 Ma. Farther to the SE, the Tobacco Root Mountains and northwest N. Madison Range experienced conditions of ~1.0-1.2 GPa and 800 °C between 1775 and 1750 Ma. These data suggest a potential pattern of younging thermotectonism from NW to SE that we have tested by analyzing basement rocks along a continued transect to the southeast.
New geochronological, petrological and structural data for the SE-central portion of the N. Madison range indicates that these rocks were also extensively reworked during the Big Sky orogeny. Multiple samples constrain peak metamorphism and deformation occurring at conditions of ~0.85 GPa and 715 °C. Both SIMS U-Pb zircon and U-Th total-Pb monazite EMP geochronology constrain the timing of this metamorphism and deformation to between ~1750-1720 Ma. Thus, peak metamorphic conditions appear to progressively young from ~1810-1780 Ma in the NW to ~1750-1720 Ma in the SE. We interpret this space-time pattern to represent propagation of shortening and regional metamorphism within the Big Sky hinterland from NW to SE over 50-90 m.y., period. The high-grade metamorphic core of the Big Sky Orogen extends over a minimum length scale of ~100 km from the Highland Mountains SE to at least the central N. Madison range. This extended core encompasses half of the exposed hinterland-foreland length of ~200 km as defined by discrete Paleoproterozoic, but otherwise poorly dated, greenschist-grade structures in ranges further to the southeast. This space-time pattern of propagation and 100 km orogenic length scale provide better context for understanding the significance of Big Sky Orogen, one of North America's most recently recognized major collisional belts.