Strain Accommodation in the Walker Lane: Understanding the Evolution of a Diffuse Plate Boundary with Geochronology and Geodesy

这项研究汇集了来自佐治亚理工学院的一组科学家，调查了加州东部和内华达州西部沃克莱恩地区短期和长期应变率之间的差异；太平洋-北美板块边界的一个演化部分。为了实现这一目标，现有的大地测量基础设施，包括区域运动和连续的GPS数据，在该地区增加了10个新的纪念碑。所有的活动地点将在2010年至2012年期间进行三次年度活动。对GPS数据的分析将得出当前应变场的详细图像。结合航空摄影评估、野外测量和偏置冲积扇宇宙核素年代学的地质制图被用来确定穿越正断层的日期，从而确定长期运动速率。综合长期地质资料和短期大地测量资料，将对这一重要板块边界断裂系统晚更新世至近代时间尺度上的分布应变场有一个全面的认识。最终，这将有助于我们更好地理解结构复杂的岩石圈是如何沿着不断演化的板块边界运动的。沿着一些板块边界系统，包括圣安德烈亚斯断层和北安那托利亚断层，构造变形率在很长的时间尺度上似乎是恒定的。然而，在其他板块边界带的部分地区，由GPS测量确定的短期滑动速率与长期地质推断的滑动速率并不一致。例如，从构造地貌断层研究中确定的莫哈韦沙漠东部加利福尼亚剪切带和沃克巷中部晚更新世的变形率，仅为从GPS数据中确定的短期速率的一半到三分之一。Walker Lane的短期和长期速率之间的不匹配与目前的认识相反，并对岩石圈如何适应构造变形以及最终这种变形与地震危险的关系具有重要意义。通过确定沃克湖在每年到10万年的时间尺度上的构造活动速率，将为断层系统在多个地震周期中的行为和演化提供新的见解，并解决有关地壳应变分布的几个基本问题：应变积累和释放的速率在时间和空间上有多恒定？多个地震周期的平均地质滑动率与GPS数据确定的短期变形率是否可比较？如果在短期和长期变形率之间存在差异，那么这种差异是在什么样的时间和空间尺度上发生的？这些差异是与特定区域的结构复杂性有关，还是整个板块边界的特征？

This study brings together a team of scientists from Georgia Institute of Technology to investigate the discrepancies between short- and long-term strain rates in the Walker Lane region of eastern California and western Nevada; an evolving segment of the Pacific-North America plate boundary. To accomplish this objective, existing geodetic infrastructure, including regional campaign and continuous GPS data, is augmented with 10 new monuments in the region. All campaign sites will be measured in three annual campaigns between 2010 and 2012. Analysis of the GPS data will yield a detailed image of the present-day strain field. Geologic mapping incorporating evaluation of aerial photography, field surveying and cosmogenic nuclide geochronology of offset alluvial fans are used to determine the dates, and hence the long-term rates, of motion across normal faults. The integration of the long-term geologic and short-term geodetic datasets will yield a comprehensive view of the distributed strain field over late Pleistocene to Recent time scales along this important plate boundary fault system. Ultimately, this will lead to improved understanding of how the structurally complex lithosphere behaves along evolving plate boundaries.Along some plate boundary systems, including the San Andreas and North Anatolian faults, rates of tectonic deformation appear to be constant over a wide range of time scales. However, on parts of other plate boundary zones short-term rates determined from GPS measurements do not coincide with longer-term geologically inferred slip rates. For example, late Pleistocene rates of deformation across the eastern California shear zone in the Mojave Desert and in the central Walker Lane, determined from tectono-geomorphic fault studies, yield rates that are only one-half to one-third the short-term rate determined from GPS data. The mismatch between short- and long-term rates in the Walker Lane is contrary to current understanding, and has important implications for how tectonic deformation is accommodated in the lithosphere, and ultimately the relationship of this deformation to earthquake hazards. By determining rates of tectonic activity in the Walker Lave over annual to 100,000 year timescales will provide new insights into how fault systems behave and evolve over multiple earthquake cycles, and address several fundamental questions about crustal strain distribution: How constant are rates of strain accumulation and release in time and space? Are geologic slip rates averaged over multiple earthquake cycles comparable with short-term rates of deformation determined from GPS data? If discrepancies between short- and long-term rates of deformation exist, over what temporal and spatial scales does the discrepancy occur? Are these differences related to the structural complexity of a specific region or are they characteristic of entire plate boundaries?