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; 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?

在大陆滑雪板边界，例如加利福尼亚州的San Andreas断层系统，大陆岩石圈的一部分主要以水平运动为主要的。 该板运动可以用GPS数据和浅断层上的偏移（包括地壳地震产生的滑动）在表面进行测量。 但是，岩石圈地幔部分中这种运动的形式要确定得多。 地幔变形的模型下方的滑动板边界下的范围从直接位于板边界位于表面的位置下方的狭窄剪切区到宽度大约100公里的弥漫变形区域。 在该研究项目中，将使用遥远地震的地震波来测量两个主要的滑滑断层系统下的大陆岩石圈的性质，土耳其的北部安纳托利亚断层和新西兰的南部阿尔卑斯山断层，这些结果将与先前的San Andreas下班相比。 特别是，从岩石圈底部转变为压缩运动（或Viceversa）的波浪将用于限制岩石圈的厚度以及在冷，相对刚性的岩石圈和温暖的肌磷酸层之间的地震波速度中的梯度。 如果岩石圈的厚度和/或岩石圈 - 亚索速度梯度在小型水平长度尺度上发生变化，则这些发现将支持狭窄的剪切区模型。 例如，先前的工作表明，在中央圣安德烈亚斯（San Andreas）下方岩石圈底部的剪切分布在小于50公里的区域。地幔岩石圈的性质的逐渐逐渐变化将支持较宽的板块边界地幔剪切带的存在。 地幔变形系统下面的地幔变形系统的数值建模将有助于解释地震学测量的岩石圈特性。 这项研究的结果将为板边界的物理特性以及如何在大陆岩石圈内容纳板板运动的新见解。我们提议测量岩石圈的厚度以及岩石圈 - 亚翼圈速度梯度梯度下方，主要基于SP相位，基于SP相位的SP相位，这些SP相位是基于在岩石圈 - 心脏圈边界上转换的SP相（LAB），我们将采用sooke-Slip板板板边界的数字模型来提供物理框架的数字模型，从而为我们提供了一个结果。 这种成像方法直接限制了地幔岩石圈底部的结构，包括由于与浅层断层系统相关的通行剪切区而导致的实验室结构的可能变化。 在San Andreas断层系统下方的实验室的先前工作中，我们发现SP相位幅度和实验室剪切波速度的相应下降在板边界的西侧系统地比东方更小。 在加利福尼亚州中部，实验室速度梯度的变化发生在直接在圣安德烈亚斯断层下方小于50公里的水平长度尺度上发生。 这些结果与地幔岩石圈的并置在狭窄的剪切区（宽度为50 km）的中央圣安德烈亚斯断层的岩石岩的并置，该特性延伸到岩石圈的底部。 在北部Anatolian和南部高山断层系统下，SP接收器功能将堆叠到3D图像中，以确定LAB的不连续性深度和振幅，以及实验室属性是否在每个故障上系统地变化。 PS阶段的约束也将被评估。 基于合成地震图的建模将提供与实验室相关的垂直剪切速度梯度的定量估计，并对SP和PS相的良好评估能够解决实验室性能的横向变化。 岩石圈和低移动层变形的数值模型在圣安德烈亚斯，北安纳托利亚和高山断层系统下方，将使我们能够将SP和PS成像的结果与剪切波散布的地幔各向异性的限制结合在一起。 它还将帮助我们从岩石圈 - 跨层粘度对比以及变形诱导的各向异性在创建明显的实验室特性中的作用来解释地震学结果。 比较三个故障系统的结果将解决几个关键问题。深色大陆岩石圈中的走滑剪切带的宽度有多宽？跨走滑断层系统对岩石圈和软圈的流变学的实验室结构的含义是什么？哪些过程负责整个San Andreas断层系统的实验室属性以及Anatolian断层系统的横向对比度？