Recurrent Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data

Abstract:

Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ~3.3 Ga, including the Paleoproterozoic Great Falls orogeny, Mesoproterozoic extension associated with formation of the Belt-Purcell basin, and the late Phanerozoic Sevier and Laramide orogenies. We investigated the long-term (>1 Ga), low-temperature (erosion and burial within 10 km of the surface) thermal history of these tectonic events with zircon (U-Th)/He thermochronology. Data collected from n=55 zircon aliquots across 9 sample localities in the northern and southern Madison ranges, the Blacktail-Snowcrest arch, and the Tobacco Root uplift were modeled in a forward and inverse sense with a zircon radiation damage and annealing model (ZRDAAM). Forward modeling with ZRDAAM shows the first-order sensitivity of our zircon (U-Th)/He data to major cooling and reheating episodes associated with block-uplift and sedimentary burial. Further investigation into the thermal histories of these basement ranges using the inverse modeling program HeFTy confirms the thermal history trends illuminated by forward models and previously published thermochronometeric data sets. Our model results for these basement ranges show substantial cooling from temperatures in excess of 400 °C to near surface conditions between 1.7 and 1.4 Ga. Subsequent late Phanerozoic cooling (exhumation) culminated at ~75 Ma. Late Phanerozoic cooling is coincident with thin-skinned thrusting along-strike in the Sevier belt of southeastern Idaho and older than exhumation in basement-involved uplifts in the Wyoming Laramide province. Our long-term, low-temperature thermal record for these southwestern Montana basement ranges therefore shows that: 1) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, which may have influenced Phanerozoic thrust architecture and 2) the late Phanerozoic phase of thick-skinned thrusting recorded by these rocks is among the earliest thermochronologic records of basement-involved shortening within the Laramide structural province and was concomitant with thin-skinned thrusting elsewhere in the Sevier fold-thrust belt.