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

自然变形橄榄岩包体中莫哈韦地区下方岩石圈地幔的流变学

• 批准号: 1251621
• 负责人: Whitney Behr
• 金额: $ 27.8万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-06-01 至 2019-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1251621&HistoricalAwards=false
• 关键词: Rheology; Lithospheric; Mantle; Beneath; Mojave

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.

该项目的目的是利用一套从加州构造活跃的莫哈韦沙漠中的更新世奇马火山岩场中喷发的天然变形橄榄岩捕虏体来表征岩石圈地幔部分的流变学性质。莫哈韦的一部分，从Cima捕虏体是一个地区，在该地区的岩石圈的瞬态和稳态流变结构的几个模型已经从地表速度测量后的兰德斯（1992年）和赫克托矿（1999年）地震。这也是一个地区，地球物理观测（包括地震速度和衰减，和接收器功能的研究）突出了剪切速度的下降，是快速的深度，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，不能单独用温度对比来解释。因此，地幔岩石圈的组成和流变特性是至关重要的，了解是什么影响了大波长的地震后信号，从岩石圈盖的突然过渡到低粘度，低速度的软流圈地幔。这项研究将记录应力、温度、含水量、变形机制、晶格择优取向和局部化方式如何随着Cima捕虏体所源自的岩石圈地幔深度的增加（至少下至45公里和多达70公里）而变化。这些自然测量的流变参数将用于a）测试实验推导的流动定律和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的一系列大规模地球物理模型。一些关键问题需要解决包括：这一地区下面的地幔岩石圈是湿的还是干的？地幔中的变形是发生在狭窄的区域内，还是广泛分布？地幔岩石圈是最坚固的吗？承重？大陆岩石圈的一部分，最弱的，或者介于两者之间？这个项目的结果将为地球如何？岩石圈是地球坚硬的外壳，它的行为是对沿着活动大陆板块边界变形的反应。PI和学生将检查岩石圈地幔部分的样本，这些样本是由年轻火山下快速上升的岩浆携带并带到地表的。这些夹带的岩石，被称为捕虏体，提供了一个独特的视角下的岩石圈的结构和物理特性的一些主要的地震发生在加州东部的走滑断层，包括故障，产生的兰德斯（1992年，M=7.3）和赫克托矿（1999年，M=7.1）地震。