Collaborative Research: Thermal Evolution of North American Lower Crust: U-Pb Thermochronological Constraints on the Seismic Properties of the Lithosphere

北美下地壳的热演化：U-Pb 热年代学对大陆岩石圈物理性质的限制 对于 EarthScope 计划至关重要的是北美岩石圈的年龄、热演化和物理性质。从地震研究中推断出的当今岩石圈速度结构的年龄和起源主要受到暴露大陆地壳的年龄、热历史和物理性质的了解。 然而，下地壳岩石（深度为 30-45 公里）包含丰富的历史，可能与次相邻岩石圈地幔和北美克拉通的形成和稳定有更直接的联系。与地幔岩石不同，下地壳的时间-温度历史可以通过使用放射性时钟对副矿物进行测年来限制，放射性时钟在 1000°C 至 ~400°C 的不同温度（闭合温度）下开始记录时间。 这使得能够在超过 30 公里的深度绘制从组装到稳定再到构造重新激活和加热的时间尺度的大陆尺度地图。 下地壳条件下相对较慢的冷却速率（0.5°C/百万年）控制着金红石、磷灰石和钛矿等矿物的闭合温度，从而可以确定下地壳在 1000°C 至 400°C 范围内的时间-温度历史。 由于稳定大陆岩石圈地壳底部的温度非常接近金红石和磷灰石的闭合温度，这些矿物成为热结构扰动的非常敏感的监测器，包括玄武岩岩浆作用（底板作用）、岩石圈减薄和/或软流圈上涌，以及记录与远场造山活动相关的流体流动事件。虽然北美相对较深的古代地壳暴露有限，但捕虏体提供了下地壳的唯一物理样本，可以在深部地壳和地幔的地球物理观测与地表地质学之间建立直接联系。目前的工作重点是利用地壳捕虏体沿着从大陆北部太古代核心向南进入元古代增生地体的南北向横断面来约束北美下地壳的热史、岩石演化和物理性质。堪萨斯州和密歇根州的其他样本套件可以与未受到与科迪勒拉边缘相关的较年轻构造和热事件影响的样本进行比较。 研究岩石圈从太古宙核心到较年轻的增生带的热演化，使地球科学家对与大陆聚集和稳定相关的速率有了新的认识，并为影响岩石圈及其热结构的热和构造事件的年龄提供了新的见解。 这些数据与新的高分辨率地震数据的整合有可能彻底改变我们对北美大陆形成的理解。

Thermal Evolution of North American Lower Crust: U-Pb Thermochronological Constraints on the Physical Properties of Continental LithosphereCrucial 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°C to ~400°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 °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°C to 400°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.