Thermal Evolution of North American Lower Crust: U-Pb Thermochronological Constraints on the Physical Properties of Continental Lithosphere
Crucial to the EarthScope initiative are the age, thermal evolution, and physical properties of North American lithosphere. The age and origin of the present-day lithospheric velocity structure deduced from seismic studies are constrained mostly by knowledge of the age, thermal history, and physical properties of the exposed continental crust. However, lower crustal rocks (from 30-45 km depth) contain a rich history that may be connected more directly to the formation and stabilization of the sub-adjacent lithospheric mantle and the North American craton. Unlike mantle rocks, a time-temperature history of the lower crust can be constrained through dating of accessory minerals with radioactive clocks that begin recording time at different temperatures (closure temperature) from >1000 degrees C to ~400 degrees C. This allows continent-scale mapping of the timescales from assembly to stabilization to tectonic reactivation and heating at a depth of greater than 30 km. Relatively slow cooling rates (<0.5 degrees C/million years) at lower crustal conditions controls the closure temperature for minerals such as rutile, apatite, and titanite, allowing a time-temperature history of lower crustal to be determined over the range from >1000 degrees C to <400 degrees C. Since the temperature at the base of the crust in stable continental lithosphere is very near the closure temperatures for rutile and apatite, these minerals become remarkably sensitive monitors of perturbations to the thermal structure, including basaltic magmatism (underplating), lithospheric thinning and/or asthenospheric upwelling, as well as recording fluid flow events related to far field orogenic events.
While there are limited exposures of relatively deep ancient crust in North America, xenoliths provide the only physical samples of lower crust with which to establish direct links between geophysical observation of deep crust and mantle and surface geology. Present efforts are focused on constraining the thermal history, petrologic evolution, and physical properties of the lower crust beneath North America using crustal xenoliths along a N-S trending transect from the northern Archean core of the continent southwards into the Proterozoic accretionary terranes. Additional sample suites in both Kansas and Michigan allow comparisons to samples not overprinted by younger tectonic and thermal events associated with the Cordilleran margin. Examining the thermal evolution of lithosphere from the Archean core of the continent to younger accretionary belts are giving earth scientists a new understanding of the rates associated with continental assembly, and stabilization, as well as providing new insights into the age of thermal and tectonic events that have affected the lithosphere and its thermal structure. Integration of these data with new high-resolution seismic data has the potential to revolutionize our understanding of formation of the North American continent.
