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

必须知道像圣安德烈亚斯断层这样的大断层的强度，以便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，但这些故障的强度仍然不清楚。 支持弱断层的最广泛接受和经常引用的论点之一是在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大量的热量;然而，对于弱断层，这一影响被称为“应力/热流”悖论。这项研究通过提供有关综合时间的信息，为解决应力/热流悖论提供了直接相关的新数据。温度的历史（最终，强度）在圣安德烈亚斯断层附近帕克菲尔德。 这些数据来自于从SAFOD钻孔的岩屑和岩心中收集的锆石。 从第一阶段钻井的垂直钻孔中收集了四个均匀分布的样本;在第二阶段的倾斜孔中收集了十三个样本。联合学院的裂变径迹实验室正在分析一份锆石分离物，以确定裂变径迹年龄和径迹长度分布。 耶鲁大学的地质年代学实验室正在分析另一份样品，以确定锆石的U-Th/He年龄。 第三份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环带。 这些分析的结果提供了我们的有限元计算机模型的圣安德烈亚斯故障的约束。 该模型综合考虑了断层的孕震上地壳和蠕滑下地壳的强度，以及由锆石裂变径迹和U-Th/He数据所揭示的时温历史。 建模的结果提供了深入了解帕克菲尔德附近的圣安德烈亚斯断层的强度。 这项研究和SAFOD项目的成果将造福社会，因为我们将更好地了解与这个和其他活动断层系统相关的地震危险。 我们的研究结果将有助于地震科学家更好地了解断层的力学，从而更好地预测破坏性地震的发生沿着圣安德烈亚斯系统。

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.