Flexarray 3D Passive Seismic Imaging of Core-complex Extension in the Ruby Range, Nevada

大陆地壳最高度伸展的区域，被称为核杂岩，及其边界结构，通常被绘制为低角度断层，尽管这些与地壳力学行为的预测相悖，是大陆伸展最具争议的方面之一。这些特征很难解释，因为科学家们缺乏其深层基础的清晰图像。Earthscope的FlexArray使新一代成像程序成为可能。传统上，研究岩石圈的地震学家受到现有设备的限制，只能部署单线接收器，以成像地壳的二维横截面。Earthscope的FlexArray最终提供了足够的仪器，可以在地面部署二维阵列，从而可以在深度重建三维图像。内华达州的盆地和山脉省提供了无与伦比的各种类型的伸展，包括变质核杂岩，这些杂岩将延展性变形的中地壳岩石与未变质的脆性变形的上地壳岩石并列。这种关系意味着显著的水平伸展，在整个地壳尺度上由大幅度的简单剪切或显著的下地壳横向和垂直流动来补偿。阐明上下地壳同时变形之间的关系需要有足够分辨率的地球物理数据，以令人信服地显示地表地质是如何与深部特征联系在一起的——或者与之分离的。总拉伸的不确定性是构造重建中一个主要的未知因素，无论在世界各地发生这样的核心杂合体。国家科学基金会？美国宇航局的“地球科学”项目正与美国化学会石油研究基金合作，赞助对经典的红宝石山变质核杂岩及其邻近盆地（拉莫伊尔-亨廷顿谷和红宝石谷）进行三维研究，以寻找地壳流入变质核的证据。该地区之所以成为目标，是因为围绕红宝石山脉的现有数据可以以更大的3D体积详细解释新的地球物理结果。FlexArray对地壳和上地幔进行了详细的地球物理研究，利用40个站点的被动宽带地震仪阵列，将为可传输阵列数据和地表地质数据之间提供重要的联系，以了解大陆结构、变形和演化，以及涉及伸展造山带的岩石圈尺度过程。已有的被动记录技术（环境噪声层析成像、接收函数速度反演、共转换点成像、地壳各向异性分析）正被应用于3D，而不是更常见的二维，而且分辨率（5公里间距）比这类研究的典型分辨率更高。该实验的结果将有助于评估扩展区域下方深度的地壳流动模型，从而有助于了解扩展区域下地壳应变在空间和时间上的进展。更详细地描述这些过程是提高我们对大陆裂谷作用和世界范围内沉积盆地演化认识的基本步骤。研究结果也会引起活跃在内华达州的金矿开采公司的兴趣：红宝石山脉可能会暴露出毗邻的卡林趋势的基础，卡林趋势是美洲最大的黄金产区，同时也有助于了解导致内华达州50多年来最具破坏性的地震（2008年2月的韦尔斯6级地震）的地质框架。

The most highly-extended regions of continental crust, known as core-complexes, and their bounding structures, typically mapped as low-angle faults even though these defy predictions of the mechanical behavior of the crust, are among the most controversial aspects of continental extension. These features have defied easy explanation because scientists have lacked clear images of their deep underpinnings. Earthscope's FlexArray is allowing a new generation of imaging procedures. Traditionally, seismologists studying the lithosphere have been restricted by limitations on available equipment to deploying single lines of receivers, allowing imaging of two-dimensional cross-sections through the crust. Earthscope's FlexArray finally provides enough instrumentation that a two-dimensional array at the surface can be deployed, allowing reconstruction of a three-dimensional (3D) image volume at depth. The Basin-and-Range Province in Nevada offers unparalleled exposure of diverse styles of extension, including the metamorphic core complexes that juxtapose ductilely deformed mid-crustal rocks with unmetamorphosed, brittly-deformed supracrustal rocks. This relationship implies significant horizontal extension, compensated at the scale of the whole crust either by large-magnitude simple shear or significant lateral and vertical flow of the lower crust. Elucidating the relationship between simultaneous deformation of the upper and lower crust requires geophysical data of sufficient resolution to convincingly show how the surface geology is linked to - or disconnected from - features at depth. Uncertainty in the total stretching involved is a major unknown in tectonic reconstruction of extensional provinces wherever such core complexes occur around the world. The National Science Foundation?s Earthscope Science program is teaming with the Petroleum Research Fund of the American Chemical Society to sponsor a 3D study of the classic Ruby Mountain Metamorphic Core Complex and its adjacent basins (Lamoille-Huntington Valley & Ruby Valley) to look for evidence of crustal flow into the metamorphic core. This region is being targeted because the existing data that surround the Ruby Mountains allow detailed interpretation of the new geophysical results in a larger 3D volume. The detailed FlexArray geophysical studies of the crust and upper mantle, using a 40-station array of passive, broadband seismometers, will provide an important link between Transportable Array data and surface geologic data to develop understanding of continental structure, deformation and evolution, and the lithospheric-scale processes involved in extensional orogens. Established passive-recording techniques (ambient-noise tomography, receiver-function velocity inversions, common-conversion point imaging, crustal anisotropy analyses) are being applied in 3D instead of the more commonplace two dimensions, and at a higher resolution (5-km spacing) than is yet typical for this class of studies.Results of this experiment will allow evaluation of models for crustal flow at depth beneath extending regions, and thereby help understand the progression of lower-crustal strain in space and time across extending regions. Characterizing these processes in more detail is a fundamental step towards advancing our knowledge of continental rifting and the evolution of sedimentary basins world-wide. The results should also be of interest to gold-mining companies active in Nevada: the Ruby Range may expose underpinnings of the adjacent Carlin Trend, the largest gold-producing region in the Americas, as well as helping understand the geologic framework that gave rise to Nevada's most destructive earthquake for over 50 years (the February 2008 Wells magnitude 6 earthquake).