KEYNOTE SPEAKER: DEEP CRUSTAL STRUCTURE, PROCESSES, AND PROPERTIES FROM XENOLITHS AND SEISMIC OBSERVATIONS, MONTANA AND WYOMING, USA (Invited Presentation)

Geophysical studies, xenoliths, magmatic records, and rare exposures of once deep rocks provide fundamental perspectives with which to investigate the structure, composition, and properties of lower continental crust. Each has its own inherent biases and/or limitations, making integrated approaches particularly valuable in furthering our understanding of lithospheric evolution. The cratonic core of Montana and Wyoming, USA, contains some of the most enigmatic lower crust known in North America, with a high seismic velocity (>7.0 km/s P-wave velocity) “layer” contributing to as much as half of the crustal column. We compared the metamorphic, geochronological, and petrophysical record from crustal and upper mantle xenoliths hosted by ca. 50 Ma volcanic rocks from a range of localities across central Montana to seismic observations from the active source Deep Probe/SAREX experiment and several other passive source studies in the region, including EarthScope’s USArray. Samples record diverse and commonly polymetamorphic P-T histories including prograde burial and episodes of decompression in the 1.8-1.7 Ga interval that are interpreted to be related to Great Falls Tectonic Zone collisional activity. U-Pb geochronology and thermochronology indicate igneous, metamorphic and/or fluid flow events at >2.6 Ga, 2.2-2.0 Ga, 1.83-1.68 Ga, and 1.5-1.3 Ga. These characteristics along with heterogeneity in calculated seismic properties from xenoliths interpreted to have been derived from depths below ~30 km support a composite origin for the high seismic velocity layer in the Wyoming craton and southern Medicine Hat block. Heterogeneity of physical properties within the lower crustal layer (e.g., velocity steps) and a locally reduced contrast in properties across the crust/mantle boundary owing to upper mantle metasomatism may help explain contrasting seismic interpretations of crustal thickness in the region. Xenolith compositions indicate that the high velocity lower crust is mafic and potentially stronger than its upper mantle counterpart if metasomatic effects indicated by mica-rich peridotite and pyroxenite xenoliths from the Bears Paw Mountains are widespread.