Collaborative Research: Comparing Deformation Rates in Wrench Borderlands from Geodetic and Geologic Data to Evaluate the Permanent and Recoverable Components

利用全球定位系统(GPS)进行的大地测量很好地描述了与加州西部板块构造运动有关的当前速度场。在活动变形带中大地测量记录的确切性质仍然是一个重要的问题，因为这项技术记录了扳手边界(与主要走滑断层相邻的区块)的可恢复(弹性)和永久(准塑性)应变。这一建议的目的是通过同时研究圣安德烈亚斯断裂系统的两个不同部分，评估扳手边界地区变形中可恢复分量和永久分量的相对大小。加利福尼亚州中部爬行段：由于断层上的滑移幕之间的间隔很长(50年)，仅凭大地测量通常无法区分可恢复的形变和永久形变。相比之下，在圣安德烈亚斯断层缓慢的中央部分，滑动幕之间的间隔很短(几周到几个月)。通过在爬行段的整个短地震周期中收集大地测量数据，PI将能够评估断层附近位移场的可恢复和永久分量的相对大小。他们建议根据地质和古地磁数据确定的长期平均值来核对目前累积的永久测量值的估计。先前的工作表明，扳手边缘地区中新世和年轻沉积物的古地磁信号是顺时针旋转的。记录整个区域的面积范围、区域分布和旋转量，将使他们能够计算永久变形的长期平均值。提案的这一部分涉及永久性GPS测站和监测、运动式GPS、古地磁学和地质制图。南加州索尔顿海槽的杜米德山：在这一地区，以前的工作已经确定了永久性菌株的数量。PI‘s建议对变形的沉积岩进行逐步的、三维的逆向变形。为了将这些增量应变数据转换为变形速率的估计，必须对变形时间有精确的了解。在这个曝光极好的区域，变形计时的一个方面是与0.76 Ma毕晓普凝灰岩相关的火山灰层的存在。其他信息将来自古地磁技术，它记录了沉积过程中的磁场。利用不同沉积层的不同年龄来提供不同的垂直轴旋转，PI可以通过确定哪些单元受到影响来确定微小构造(例如节理、裂缝和小褶皱)的时间。通过将大地速率(包括可恢复应变分量和永久应变分量)与地质速率(仅记录永久应变分量)进行比较，他们可以评估该地区可恢复应变的累积量。该提案的这一部分涉及地质测绘、古地磁学和运动式GPS。通过结合我们两个领域的地质、大地测量和古地磁调查的结果，PI将评估扳手边界地区可恢复(弹性)和永久(准塑性)应变的相对贡献。这一问题的解决对地震力学、边界形变的地质意义以及主要断层上的潜在滑动量具有基本意义。

The current velocity field associated with plate tectonic motion in western California is well characterized by geodetic measurements using Global Positioning Systems (GPS). The exact nature of what geodetic measurements record in actively deforming zones remains a significant problem, as this technique records both recoverable (elastic) and permanent (quasi-plastic) strains in wrench borderlands (blocks adjacent to major strike-slip faults). The goal of this proposal is to evaluate the relative magnitudes of the recoverable and permanent components in the deformation of wrench borderlands, by simultaneously studying two different sections of the San Andreas fault system. Creeping segment, central California: Geodetic measurements alone are generally unable to distinguish between recoverable and permanent deformation, because of the long (50 yr) interval between slip episodes on faults. In contrast, intervals between slip episodes are short (weeks to months) in the creeping central section of the San Andreas fault. By collecting geodetic data throughout the short seismic cycle in the creeping segment, the PI's will be able to evaluate the relative sizes of the recoverable and permanent components of the displacement field in the vicinity of the fault. They propose to check the estimates of currently accumulating permanent measurements against long-term averages determined from geologic and paleogmagnetic data. Prior work suggests clockwise rotation of the paleomagnetic signal of Miocene and younger sediments in the wrench borderlands. Documenting the areal extent, regional distribution, and amount of rotation throughout this area will allow them to calculate a long-term average for permanent deformation. This part of the proposal involves permanent GPS stations and monitoring, campaign-style GPS, paleomagnetism, and geologic mapping. Durmid Hill, Salton Trough, southern California: In this area, previous work has determined the amount of permanent strain. The PI's propose to complete a step-wise, three-dimensional retro-deformation of deformed sedimentary rocks. In order to convert these incremental strain data into estimates of deformation rates, one must have precise knowledge of time of deformation. In this region of excellent exposure, one aspect of deformation timing is provided by the presence of an ash layer correlated with the 0.76 Ma Bishop tuff. Additional information will result from paleomagnetic techniques that record the magnetic field during deposition. Utilizing the variety of ages of the different sedimentary layers to provide differential vertical axis rotations, The PI's can determine the timing of minor structures (e.g., joints, fractures, and small folds) by determining which units are affected. By comparing geodetic rates (which include both recoverable and permanent strain components) with geologic rates (which record only a permanent strain component) they can assess the amount of recoverable strain accumulation in this area. This part of the proposal involves geologic mapping, paleomagnetism, and campaign-style GPS. By combining the results of geological, geodetic, and paleomagnetic investigations from our two field areas, the PI's will assess the relative contribution of recoverable (elastic) and permanent (quasi-plastic) strains in wrench borderlands. Resolution of this issue has fundamental implications for earthquake mechanics, geological implications of borderland deformation, and potential slip magnitudes on major faults.