Rheology of the Lithospheric Mantle Beneath the Mojave Region from Naturally Deformed Peridotite Xenoliths

本项目的目的是利用一套自然变形的橄榄岩捕虏体来表征岩石圈地幔剖面的流变特性，这些橄榄岩捕虏体来自加利福尼亚莫哈韦沙漠构造活跃的更新世西玛火山场。莫哈韦的西玛捕虏体就是从这个地区衍生出来的，在这个地区，岩石圈的瞬态和稳态流变结构的几种模型都是通过兰德斯（1992）和赫克托尔矿（1999）地震后的地表速度测量得到的。它也是一个地球物理观测（包括地震速度和衰减，以及接收函数研究）强调切变速度下降的区域，这种速度下降在深度上很快，被解释为地震岩石圈-软流圈边界；剪切速度下降的幅度太突然，不能仅用温度对比来解释。因此，地幔岩石圈的成分和流变特性对于理解是什么影响了大波长地震后信号，以及岩石圈盖层向低粘度、低速软流圈地幔的突然转变至关重要。该研究将记录应力、温度、含水量、变形机制、晶格优选方向和定位方式如何随着岩石圈地幔深度的增加而变化（深度至少可达45公里，最多可达70公里）。岩石圈地幔是西马捕虏体的来源。这些自然测量的流变参数将用于a)测试实验推导的流动规律和LPO发展预测对地幔岩石的适用性，b)开发莫哈韦地区上地幔的自然约束应力剖面，以及c)检查测量的流变参数如何影响该地区一系列更大规模的地球物理模型。一些需要解决的关键问题包括：这个地区下面的地幔岩石圈是湿的还是干的？地幔的变形是在狭窄的区域内发生，还是广泛分布？地幔岩石圈是最强的吗？是大陆岩石圈的一部分，是最弱的，还是介于两者之间？这个项目的结果将揭示地球如何？地壳的刚性外壳，即岩石圈，对沿活动大陆板块边界的变形作出反应。PI和学生们将研究岩石圈地幔部分的样本，这些岩石圈是由年轻的火山下面快速上升的岩浆携带并带到地表的。这些被夹带的岩石，被称为捕虏体，提供了一个独特的视角来研究加利福尼亚东部一些主要地震产生走滑断层下的岩石圈的结构和物理特性，包括产生兰德斯（1992年，M=7.3）和赫克托尔矿（1999年，M=7.1）地震的断层。

The aim of this project is to characterize the rheological properties of a lithospheric mantle section using a suite of naturally deformed peridotite xenoliths erupted from the dominantly Pleistocene Cima volcanic field in the tectonically active Mojave desert of California. The part of the Mojave from which the Cima xenoliths are derived is a region in which several models of both the transient and steady-state rheology structure of the lithosphere have been made from surface velocity measurements following the Landers (1992) and Hector Mine (1999) earthquakes. It is also a region where geophysical observations (including seismic velocity and attenuation, and receiver function studies) highlight a drop in shear velocity that is rapid in depth, interpreted to be the seismological lithosphere-asthenosphere boundary; the magnitude of the shear velocity drop is too abrupt to be explained by temperature contrasts alone. The compositional and rheological properties of the mantle lithosphere here are therefore critical to understanding what influences both the large-wavelength post-seismic signal, and the abrupt transition from lithospheric lid to low viscosity, low velocity asthenospheric mantle. The study will document how stress, temperature, water content, deformation mechanism, lattice preferred orientation and style of localization vary with increasing depth (down to at least 45 km and as much as 70 km) in the lithospheric mantle from which the Cima xenoliths are derived. These natural measurements of rheological parameters will be used to a) test the applicability of experimentally derived flow laws and predictions of LPO development to mantle rocks, b) develop a naturally constrained stress profile of the upper mantle in the Mojave region and c) examine how the rheological parameters measured might influence a range of larger-scale geophysical models for the region. Some of the critical questions to be addressed include: is the mantle lithosphere beneath this region wet or dry? Does deformation in the mantle occur in narrow zones, or is it widely distributed? Is the mantle lithosphere the strongest ?load-bearing? part of the continental lithosphere, the weakest, or perhaps somewhere in between?Results from this project will shed new light on how Earth?s rigid outer shell, the lithosphere, behaves in response to deformation along an active continental plate boundary. The PI and students will examine samples of the mantle section of the lithosphere entrained and brought to the surface by rapidly ascending magmas beneath young volcanoes. These entrained rocks, known as xenoliths, provide a unique perspective on the structure and physical properties of the lithosphere beneath some of the major earthquake-producing strike-slip faults in eastern California, including the faults that generated the Landers (1992, M=7.3) and Hector Mine (1999, M=7.1) earthquakes.