Hydration and Deformation in the Lower Crust of Southern Wyoming: Evidence from Leucite Hills Crustal Xenoliths and Implications for the Evolution of Seismic Structure in the Wyoming Craton

Earthscope's goal of understanding the 4-D evolution of the North American continent requires the novel integration of a broad range of geological and geophysical studies. Crustal xenoliths can commonly provide a crucial link between remote geophysical observations and the composition, age, and structure of the generally inaccessible deep continental crust. In the Rocky Mountain region of Montana and Wyoming, persistent questions include 1) what characterizes the modern lower crustal structure and how much of it represents Archean, Proterozoic, or Cenozoic geodynamic processes? and 2) what is the age and nature of the anomalously thick high-velocity (>7.0 km/s) lower crust that is observed seismically throughout much of the Wyoming Craton?

The Leucite Hills xenoliths, exhumed at ~1 Ma in south-central Wyoming, are dominantly two-pyroxene-hornblende-biotite granulites (~1.0 GPa) with ca. 2.6 Ga protolith ages. The hydrated nature of the xenoliths (published bulk Vp's = 6.5-7.0 km/s) is consistent with the apparent absence of the high velocity lower crustal layer in this portion of the craton. Calculations from modal abundance, single crystal elastic properties, and EBSD analysis of microstructure indicate Vp anisotropy of ~10%, due primarily to the combined effects of strongly aligned hornblende and biotite. This magnitude of anisotropy should be readily observable through seismic techniques if the anisotropy persists to km-scale. Receiver function analysis from seismic stations in Earthscope's USArray and the earlier Deep Probe experiment will address this question. In addition, initial modeling suggests that mineralogical changes due to hydration strongly influence the bulk velocities as well as potential anisotropy of the xenoliths. Thus, the nature and timing of hydration and deformation in the xenoliths have important implications for the temporal evolution of the structure and physical properties of the deep crust in this region as well as for the nature of the enigmatic high-velocity lower crust throughout the craton.