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

北美下地壳热演化：对大陆岩石圈物理性质的U-Pb热年代学制约对地球范围的倡议至关重要的是北美岩石圈的年龄、热演化和物理性质。由地震研究得出的现今岩石圈速度结构的年龄和起源主要受暴露的大陆地壳的年龄、热历史和物理性质的知识制约。然而，下地壳岩石(30-45公里深)具有丰富的历史，这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-温度历史可以通过用放射性时钟测定辅助矿物的年代来限制，放射性时钟开始记录从1000摄氏度到~400摄氏度的不同温度(关闭温度)的时间。这使得能够在大陆范围内绘制从组装到稳定到构造重新激活和在超过30公里深处加热的时间尺度图。下地壳条件下相对较慢的冷却速度(0.5°C/百万年)控制了金红石、磷灰石和钛铁矿等矿物的关闭温度，使得可以在1000°C到400°C的范围内确定下地壳的时间-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关闭温度，这些矿物成为对热结构扰动的非常敏感的监测器，包括玄武岩浆作用(底侵)、岩石圈减薄和/或软流圈上涌，以及记录与远场造山事件有关的流体流动事件。虽然北美相对较深的古代地壳暴露有限，但捕虏体提供了唯一的下地壳物理样品，可以在深部地壳和地幔的地球物理观测与地表地质之间建立直接联系。目前的工作集中在利用地壳捕虏体约束北美下地壳的热历史、岩石学演化和物理性质，这些捕虏体沿着从大陆北部太古宙核向南进入元古界增生地体的N-S走向断面。堪萨斯州和密歇根州的额外样本套件允许与没有被与科迪勒兰边缘相关的年轻构造和热事件叠加的样本进行比较。研究从大陆太古宙核心到更年轻的吸积带的岩石圈的热演化使地球科学家对与大陆组装和稳定有关的速率有了新的理解，并对影响岩石圈及其热结构的热事件和构造事件的时代提供了新的见解。将这些数据与新的高分辨率地震数据相结合，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.