Low-Temperature Thermochronology of the Little Belt Mountains of Central
Montana with Implications for Models of Laramide Tectonism
Abstract
The “Laramide-style” uplifts of North America—characterized by
blocks of Proterozoic-Archean basement that were exhumed along reverse
faults within the Cordilleran foreland basin—are widely interpreted to
be a result of flat-slab subduction of oceanic lithosphere beneath the
continent. Despite this general consensus, the causal mechanisms of
basement-cored uplifts remain unclear. Assessment of the hotly debated
geodynamic models that relate flat-slab subduction and upper crustal
deformation hinge on the availability of accurate estimates for the
timing of exhumation of Laramide uplifts. A major problem with current
models is that they do not incorporate timing constraints from the
Laramide region of central Montana. This region represents the
northernmost extent of Laramide deformation and timing constraints are
critically needed to enhance our understanding of how stress is
transmitted inboard during flat-slab subduction. We present the first
low-temperature thermochronological ages from the Little Belt Mountains
(LBM) of central Montana, which is the northernmost Laramide-style
uplift with exposed basement gneisses. Apatite fission track ages
ranging from ca. 90-70 Ma suggest that the core of the LBM was exhumed
through the 120-60°C apatite partial annealing zone in the Late
Cretaceous. These ages corroborate recent low-T thermochronology and
sedimentological work that propose an earlier than previously recognized
onset of Laramide deformation in southwestern Montana and eastern Idaho
(>80 Ma), but additional data are needed to reduce the
uncertainty between ages and ascertain the exhumation history of the
LBM. To this end, we will integrate new low-temperature thermochronology
(<150°C) and associated thermal history models in order to
constrain the Cretaceous tectonic evolution of the LBM and the extent of
Laramide deformation in the western USA.