Modes of Extension, Magmatism and Crustal Scale Fluid Pathways in the Great Basin province, USA, from a Magnetotelluric Transect

犹他大学的Phil Wannamaker正在制作和解释穿过大盆地中西部和莫多克高原东南部的高质量电阻率结构剖面图。这是通过在现有的179个站点的样带上增加55个宽带大地电磁测深，以及安装35个长周期大地电磁测深来实现的，以创建一个横跨内华达州和加利福尼亚州东北部到犹他州最西端的约650公里长的样带。大地电磁站的数据将跨越0.005到10000秒的周期，相应的成像深度从几百米到大约300公里，甚至更大。该剖面覆盖了整整100公里的莫多克高原、活跃的西北大盆地、中内华达地震带、相对平静的较高海拔的中东部大盆地，并与犹他州西部活跃的东部大盆地重叠。这些数据正在进行现代处理、趋势分析和正则化反演，以产生一个具有最小伪影的分辨率良好的模型。该项目的总体目标是提高我们对分布的大陆裂谷过程的理解。裂谷的后果可能是深远的，包括可能发生大地震、沉积盆地的发育，最终导致大陆分裂形成新的海洋盆地。许多重要的碳氢化合物、地热和矿产资源赋存可能来自伸展过程中涉及的构造、岩浆和热液过程。具体而言，该项目正在解决尚未解决的垂直应变分布、岩浆地壳生长、地壳尺度流体流动区域以及岩石圈和软流圈之间的耦合等一级问题。这些问题包括：最大地震所在的地壳规模断裂带的普遍存在；上、下地壳减薄在什么情况下解耦；裂谷过程中地幔岩石圈在哪里弱或强；在大陆裂谷的早、中期是否有大量的岩浆输入；以及地幔来源的流体如何进入浅层地下水系统。该项目还涉及社会效益，包括更广泛地了解对断层强度的控制、地壳尺度的水文以及建立破坏性压力的能力。深部地热系统与浅成热液金矿之间的联系也被阐明。一名研究生正获得资助，学习最先进的地球物理仪器、野外勘测设计参数、偏远环境中的实际后勤问题、现代反演方法以及地球电阻和物理化学状态之间的关系。相互关联的配套项目有可能提供接近区域到国家规模的第一批结果，从加利福尼亚州太平洋海岸附近到犹他州S东部边界科罗拉多高原内陆的潜在大断面提供的不同区域的横断面图中，公众兴趣是预期的。除了这项研究，该项目还支持对一名来自地球科学中代表性不足的群体的博士生进行大地电磁方法的研究生培训。这项研究的资金由美国国家科学基金会地球物理和构造计划提供。

Phil Wannamaker from the University of Utah is producing and interpreting a high-quality section view of electrical resistivity structure through the central and western Great Basin, and the southeastern Modoc Plateau. This is being achieved by the addition of 55 wideband magnetotelluric soundings to an existing transect of 179 sites, plus installation of 35 long-period magnetotelluric soundings, to create a transect some 650 km in length span-ning Nevada and northeastern California into westernmost Utah. The magnetotelluric station data will span the range of 0.005 to 10,000 second wave periods, with corresponding imag-ing depths from a few hundred meters to about 300 km, and perhaps greater. The profiling covers fully 100 kilometers of the Modoc Plateau, the actively extending northwestern Great Basin, the Central Nevada Seismic Belt, the relatively quiescent east-central Great Basin of higher elevation, and onlaps the active eastern Great Basin of western Utah. These data are undergoing modern methods of processing, trend analysis, and regularized inversion to pro-duce a well-resolved model with minimal artifacts. The overall goal of the project is to im-prove our understanding of the process of distributed continental rifting. The consequences of rifting can be profound, including the potential for large earthquakes, development of sedimentary basins and, ultimately, breakup of continents to form new ocean basins. Many important hydrocarbon, geothermal and mineral resource occurrences can arise from the structural, magmatic and hydrothermal processes involved in extension. The project specifi-cally is addressing first-order problems regarding vertical strain distribution, magmatic crustal growth, domains of crustal scale fluid flow, and coupling between the lithosphere and asthenosphere remain unsolved. These include the prevalence of crustal scale fault zones which host the largest earthquakes; under what circumstances upper and lower-crustal thin-ning are decoupled; where mantle lithosphere is weak or strong during rifting; whether volumetrically significant magmatic input occurs during the early to middle stages of conti-nental rifting; and how mantle-sourced fluids make their way to the shallow ground water regime. The project also addresses societal benefits including broader understanding of the controls on fault strength, crustal scale hydrology, and the ability to build up damaging stresses. The link between deep geothermal systems and epithermal gold deposits is being illuminated as well. A graduate student is being supported for education into state of the art geophysical instrumentation, parameters of field survey design, practical logistical matters in remote settings, modern methods of inversion, and the relationship between electrical resis-tivity and physico-chemical state in the Earth. Linked companion projects have the potential to provide the first results approaching regional to national scales, and public interest is ex-pected in section views through diverse regions provided by a potential megatransect from near the California Pacific coast to the Colorado Plateau interior at Utah?s eastern border. In addition to the research, the project is supporting the graduate training of a Ph.D. student from an underrepresented group in the earth sciences in magnetotelluric methods. Funding for the research is being provided by the NSF Geophysics and Tectonics programs.