Investigating the mantle expression of continental strike-slip fault systems with scattered wave imaging of the lithosphere-asthenosphere boundary

利用岩石圈-软流圈边界散射波成像研究大陆走滑断层系地幔表现

• 批准号: 1416753
• 负责人: Karen Fischer
• 金额: $ 23.13万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-07-15 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1416753&HistoricalAwards=false
• 关键词: Investigating; mantle; expression; continental; strike

At continental strike-slip plate boundaries, such as the San Andreas fault system in California, one portion of continental lithosphere translates past another with primarily horizontal motion. This plate motion can be measured at the surface with GPS data and by offsets on shallow faults, including the slip produced by crustal earthquakes. However, the form of this motion in the mantle portion of the lithosphere is much less certain. Models of mantle deformation beneath strike-slip plate boundaries range from narrow shear zones that lie directly beneath the position of the plate boundary at the surface to broad zones of diffuse deformation that are more than ~100 kilometers wide. In this research project, seismic waves from distant earthquakes will be used to measure the properties of the continental lithosphere beneath two major strike-slip fault systems, the Northern Anatolian fault in Turkey and the Southern Alpine fault in New Zealand, and these results will be compared to prior work beneath the San Andreas. In particular, waves that convert from shear motion to compressional motion (or viceversa) at the base of the lithosphere will be used to constrain the thickness of the lithosphere and the gradient in seismic wave velocity between the colder, relatively rigid lithosphere and the warmer asthenosphere. If lithospheric thickness and/or the lithosphere-asthenosphere velocity gradient change over small horizontal length-scales beneath strike-slip plate boundaries, these findings would support the narrow shear zone model. For example, prior work indicates that shear at the base of the lithosphere beneath the central San Andreas is distributed across a zone that is less than 50 kilometers wide. More gradual horizontal changes in the properties of the mantle lithosphere would support the presence of broader plate boundary mantle shear zones. Numerical modeling of mantle deformation beneath strike-slip fault systems will aid in interpreting the seismologically-measured lithospheric properties. The results of this research will provide new insight on the physical properties of plate boundaries and how plate motion is accommodated within the continental lithosphere. We propose to measure lithospheric thickness and the lithosphere-asthenosphere velocity gradient beneath the Northern Anatolian and Southern Alpine faults with seismic imaging primarily based on Sp phases that convert at the lithosphere-asthenosphere boundary (LAB), and we will employ numerical modeling of strike-slip plate boundary deformation to provide a physical framework for our results. This imaging approach directly constrains structure at the base of the mantle lithosphere, including possible variations in LAB structure due to through-going shear zones associated with shallow fault systems. In prior work on the LAB beneath the San Andreas fault system, we found that Sp phase amplitudes and the corresponding drop in shear-wave velocity at the LAB are systematically smaller on the western side of the plate boundary than to its east. In central California, the change in LAB velocity gradient occurs over a horizontal length scale of less than 50 km directly beneath the San Andreas fault. These results are consistent with the juxtaposition of mantle lithospheres with different properties across the central San Andreas fault across a narrow shear zone ( 50 km in width) that extends to the base of the lithosphere. Beneath the Northern Anatolian and Southern Alpine fault systems, Sp receiver functions will be stacked into 3D images to determine LAB discontinuity depths and amplitudes and whether LAB properties vary systematically across each of the faults. Constraints from Ps phases will also be assessed. Modeling based on synthetic seismograms will provide quantitative estimates of the vertical shear velocity gradients associated with the LAB and a robust assessment of how well Sp and Ps phases resolve lateral changes in LAB properties. Numerical modeling of lithospheric and asthenospheric deformation beneath the San Andreas, Northern Anatolian, and Alpine fault systems will allow us to integrate the results of the Sp and Ps imaging with constraints on mantle anisotropy from shear-wave splitting. It will also help us to interpret the seismological results in terms of lithosphere-asthenosphere viscosity contrasts and the role of deformation-induced anisotropy in creating apparent LAB properties. Comparison of results from the three fault systems will address several key questions. How wide are strike-slip shear zones in the deep continental lithosphere? What are the implications of LAB structure across strike-slip fault systems for the rheologies of the lithosphere and asthenosphere? What processes are responsible for the lateral contrast in LAB properties across the San Andreas fault system and potentially the Alpine and Northern Anatolian fault systems?

在大陆走滑板块边界处，例如加利福尼亚州的圣安德烈亚斯断层系，大陆岩石圈的一部分主要以水平运动穿过另一部分。 这种板块运动可以通过 GPS 数据和浅层断层的偏移（包括地壳地震产生的滑动）在地表进行测量。 然而，岩石圈地幔部分的这种运动的形式还不太确定。 走滑板块边界下的地幔变形模型范围从位于地表板块边界位置正下方的狭窄剪切带到宽度超过 100 公里的广泛扩散变形带。 在该研究项目中，来自遥远地震的地震波将用于测量两个主要走滑断层系统（土耳其的北安纳托利亚断层和新西兰的南阿尔卑斯断层）下方的大陆岩石圈特性，这些结果将与之前在圣安德烈亚斯地下的工作相比。 特别是，在岩石圈底部从剪切运动转换为压缩运动（或反之亦然）的波将用于约束岩石圈的厚度以及较冷、相对刚性的岩石圈和较温暖的软流圈之间的地震波速度梯度。 如果岩石圈厚度和/或岩石圈-软流圈速度梯度在走滑板块边界下方的小水平长度尺度上发生变化，这些发现将支持窄剪切带模型。 例如，先前的研究表明，圣安地列斯中部下方岩石圈底部的剪切力分布在宽度不足 50 公里的区域。地幔岩石圈性质的更渐进的水平变化将支持更广泛的板块边界地幔剪切带的存在。 走滑断层系统下地幔变形的数值模拟将有助于解释地震测量的岩石圈特性。 这项研究的结果将为板块边界的物理性质以及大陆岩石圈如何适应板块运动提供新的见解。我们建议通过主要基于在岩石圈-软流圈边界 (LAB) 处转换​​的 Sp 相位的地震成像来测量北安纳托利亚断层和南阿尔卑斯断层下方的岩石圈厚度和岩石圈-软流圈速度梯度，并且我们将采用走向-软流圈边界的数值模拟。滑移板边界变形为我们的结果提供了物理框架。 这种成像方法直接限制了地幔岩石圈底部的结构，包括由于与浅断层系统相关的贯穿剪切带而导致的 LAB 结构可能的变化。 在之前对圣安德烈亚斯断层系下方 LAB 的研究中，我们发现板块边界西侧的 Sp 相位振幅和相应的剪切波速度下降系统地小于其东侧。 在加利福尼亚州中部，LAB 速度梯度的变化发生在圣安德烈亚斯断层正下方不到 50 公里的水平长度范围内。 这些结果与具有不同性质的地幔岩石圈并置在圣安德烈亚斯中央断层上横跨延伸至岩石圈底部的狭窄剪切带（宽 50 公里）的情况相一致。 在北安纳托利亚和南阿尔卑斯断层系统下方，Sp 接收器函数将叠加到 3D 图像中，以确定 LAB 不连续深度和幅度，以及每个断层上的 LAB 属性是否系统变化。 还将评估 Ps 阶段的限制。 基于合成地震图的建模将提供与 LAB 相关的垂直剪切速度梯度的定量估计，并对 Sp 和 Ps 相位解决 LAB 特性横向变化的能力进行稳健评估。 圣安地列斯、北安纳托利亚和阿尔卑斯断层系统下方岩石圈和软流圈变形的数值模拟将使我们能够将 Sp 和 Ps 成像的结果与剪切波分裂对地幔各向异性的约束相结合。 它还将帮助我们根据岩石圈-软流圈粘度对比以及变形引起的各向异性在产生明显的 LAB 特性中的作用来解释地震学结果。 三个断层系统结果的比较将解决几个关键问题。大陆岩石圈深层走滑剪切带有多宽？跨走滑断层系统的 LAB 结构对岩石圈和软流圈的流变有何影响？哪些过程导致圣安德烈亚斯断层系统以及潜在的阿尔卑斯和北安纳托利亚断层系统 LAB 特性的横向对比？