Improved Characterization of Apatite Fission-track Annealing Kinetics, and Application to Core Complex Exhumation, Southern Basin and Range

本项目旨在对磷灰石矿物的裂变径迹退火过程提供更好的理解和表征。当铀通过自发裂变衰变时，裂变轨迹就形成了，由裂变粒子造成的损伤以由环境温度决定的速度退火（修复）。因此，裂变径迹分析已被证明是确定岩体热史的有力工具，并被广泛应用于构造研究、景观演化和石油勘探等领域。这项研究取得的进展将用于辨别导致亚利桑那州南部和中西部山脉隆起最后阶段的断层的性质。尽管磷灰石裂变径迹热年表学是一项成熟而可靠的技术，但我们对退火如何发生以及它如何受到磷灰石成分的影响的理解仍然非常模糊。实验研究表明，氯、铁和锰等杂质可以使退火所需的温度升高几十度，但目前的实验数据库太过稀疏，无法确定杂质是如何单独和/或协同工作以带来这些变化的。本研究将利用一种有效的方法来确定30种不同成分和单位胞尺寸的磷灰石的相对退火性能，从而极大地扩展退火数据库。这些数据将不仅提供对成分效应的洞察，而且提供对退火机制本身的洞察。由此产生的新裂变径迹退火模型将用于研究高角度和低角度正断层在亚利桑那盆地和山脉岩心杂岩体发掘的最后阶段的相对作用。最近的研究表明，低角度断裂可能已经加速到接近板块构造的速度。然而，这些数据也可以用低角度断层向高角度断层的转变来解释。这两种情景对冷却速率在时间和空间上的变化做出了不同的预测，这取决于距离断层的远近。裂变轨迹方法的改进将用于验证这些预测，要么证实可能由北美西部的平板窗迁移引起的移动速率的戏剧性转变，要么阐明在连续的挖掘过程中拆顶的性质如何变化。

This project is designed to provide a much improved understanding and characterization of the process of fission-track annealing in the mineral apatite. Fission tracks form when uranium decays by spontaneous fission, and the damage caused by the fissioning particles anneals (is repaired) at a rate determined by the ambient temperature. Fission-track analysis has thus proven to be a powerful tool for determining the thermal history of rock bodies, and is widely used for tectonic studies, landscape evolution, and petroleum exploration, among other fields. The advances created by this research will be used to discern the nature of faulting that caused the final stages of uplift of mountain ranges in southern and west-central Arizona. Although apatite fission-track thermochronology is a well-established and robust technique, our understanding of how annealing occurs, and how it is affected by apatite composition, remains extremely vague. Experimental studies have shown that impurities such as chlorine, iron and manganese can increase temperatures required for annealing by several tens of degrees, but the experimental database is currently far too sparse to determine how impurities work individually and/or in concert to bring about these changes. This study will vastly expand the annealing database by utilizing an efficient procedure to determine the relative annealing properties of 30 apatites with widely varying compositions and unit cell dimensions. These data will provide insight not only into compositional effects, but into the annealing mechanism itself. The resulting new model of fission-track annealing will be used to examine the relative roles of high-angle and low-angle normal faulting in the final stages of core-complex exhumation in the Arizona Basin and Range. Recent work has suggested that low-angle faulting may have accelerated to near-plate-tectonic rates. However, the data may also be explained by a transition from low-angle to high-angle faulting. These two scenarios make different predictions about how cooling rates vary in time and space depending on proximity to faults. The improvements in the fission-track method will be used to test these predictions, either confirming a dramatic transition in movement rates possibly caused by the migration of the slab window off of western North America, or illuminating how the nature of unroofing can change during a continuous episode of exhumation.