Collaborative Research: RUI: Testing the Stress/Heat-Flow Paradox of the San Andreas Fault with Fission-Track and U+Th/He Data from Zircon from the SAFOD Drill Hole

必须了解像圣安德烈亚斯断层这样的大断层的强度，才能更好地了解它们的行为，并有望预测未来的破坏性地震。然而，关于圣安德烈亚斯系统中大断层的强度，一直存在着一种长期的争论：它们的强度是正常的还是异常低的？已经有许多基于现场和实验室的研究集中在这个问题上，但这些断层的强度仍然不是很清楚。支持异常弱断层的最被广泛接受和经常引用的论点之一是，这些断层附近没有高热流。在地壳典型的应力水平上，正常强度断层上的孕震滑动由于摩擦滑动而产生显著的热量；然而，对于弱断层，摩擦产生的热量将少得多。这项研究通过提供有关Parkfield附近圣安德烈亚斯断层完整的时间-温度历史(以及最终强度)的信息，为解决应力/热流悖论提供了直接相关的新数据。这些数据来自从SAFOD钻孔的岩屑和岩芯中收集的锆石。在第一阶段钻井的垂直钻孔中采集了4个均匀分布的样品；在第二阶段的斜井中跨越断层采集了另外13个样品。联合学院的裂变径迹实验室正在分析一份等分的锆石分离物，以确定裂变径迹年龄和径迹长度分布。耶鲁大学地质年代学实验室正在对另一份等分进行分析，以确定这些锆石的U-Th/He年龄。第三等分的锆石正在用电子探针进行化学分析，以确定锆石的化学和化学分带。这些分析的结果为我们的圣安德烈亚斯断层的有限元计算机模型提供了约束。该计算机模型结合了断层的孕震上地壳和蠕动下地壳的强度，以及由裂变径迹和锆石U-Th/He数据破译的时间-温度历史。模拟的结果提供了对帕克菲尔德附近的圣安德烈亚斯断层强度的洞察。这项研究和SAFOD项目的成果将使社会受益，因为我们将更好地了解与该活动断裂系统和其他活动断裂系统相关的地震危险。我们的结果将帮助地震科学家更好地了解断层的机制，从而更好地预测圣安德烈亚斯系统沿线破坏性地震的发生。

The strength of large faults such as the San Andreas fault must be known to better understand their behavior and hopefully to predict future damaging earthquakes. There has been a longstanding debate, however, regarding the strength of large faults in the San Andreas system: Do they have normal or anomalously low strengths? There have been numerous field- and laboratory-based studies focused on this problem, but the strength of these faults is still not well known. One of the most widely accepted and oft-quoted arguments in support of anomalously weak faults is the absence of high heat flow in the vicinity of these faults. At stress levels typical of the earth's crust, seismogenic slip on normal-strength faults generates significant heat due to frictional sliding; however, for weak faults, the amount of frictionally generated heat will be much less. This affect has been coined the "stress / heat-flow" paradox.This research provides new data directly related to resolving the stress / heat-flow paradox by providing information regarding the integrated time-temperature history (and ultimately, the strength) in the San Andreas fault near Parkfield. The data are from zircons collected from cuttings and cores of the SAFOD drill hole. Four evenly spaced samples are being collected from the vertical drill hole of phase one drilling; thirteen more samples are being collected across the fault in the inclined hole of phase two. One aliquot of the zircon separates is being analyzed in the fission-track laboratory at Union College to determine fission-track ages and track length distribution. Another aliquot is being analyzed in the geochronology laboratory at Yale University to determine the U-Th/He ages of the zircons. A third aliquot of the zircons are being chemically analyzed with an electron microprobe to determine the chemistry and chemical zonations of the zircons. The results of these analyses provide constraints for our finite-element computer model of the San Andreas fault. The computer model incorporates the strengthes of the seismogenic upper crust and creeping lower crust of the fault with the time-temperature history deciphered from the fission-track and U-Th/He data of the zircons. The results of the modeling provide insight into the strength of the San Andreas fault near Parkfield. Outcomes of this research and the SAFOD project will benefit society because we will better understand the seismic hazard associated with this and other active fault systems. Our results will help earthquake scientists gain a greater understanding of the mechanics of faulting and thus better predict the occurrence of damaging earthquakes along the San Andreas system.