Collaborative Research: Formation of Basement-involved Foreland Arches: An Integrated EarthScope Experiment

这个EarthScope项目，在这里被称为Bighorn项目，是对与基底有关的收缩前陆拱门进行的综合地质和地球物理调查。它阐述了这些前陆拱是如何形成的，以及它们是如何与板块构造过程联系在一起的。这项研究在怀俄明州北部和蒙大拿州南部的落基山大角拱进行，将对地表几何形状和运动学指标的地质调查与来自主动/被动地震实验的三维地壳和上地幔几何形状的地球物理成像相结合。由此产生的前陆拱形变的4D(3D空间和时间)、岩石圈尺度的模型检验了当前关于落基山脉以及亚洲和安第斯山脉活动造山带中涉及基底的前陆冲断带的假设。这些假设包括：1)由穿透岩石圈的逆冲断层定义的断块；2)地壳内的亚水平拆离；3)岩石圈的屈曲；4)纯剪切的岩石圈增厚。我们的研究确定了驱动基底参与的拱形形成的机制，促进了我们对大陆岩石圈流变学的理解。这一为期三年(2009-2012年)的合作项目将地表暴露和石油工业地下数据(怀俄明大学Eric Erslev；科罗拉多学院Christine Siddoway)得出的拱形上地壳几何形状与混合地震实验结果相结合，确定了1.5x106千米的岩石圈体积。这项实验的被动部分包括1.25年(2009-10年)部署27台宽带地震仪以增密EarthScope可移动阵列(Megan Anderson，科罗拉多学院)，6.5个月部署220台短周期地震仪(Anne Sheehan，科罗拉多大学)，以及9天部署800台高频“德克萨斯”地震仪(凯特·米勒，德克萨斯大学埃尔帕索分校)。活动部分包括上述仪器记录的9次炮击(2010年夏季)和另外部署了5天的1600个“德克萨斯”地震仪。这些仪器排列在由三条西南-东北向线路和两条西北-东南向线路组成的网格中，总线路长度为1000公里。主动和被动结果的联合反演确定了大角拱内的地壳和上地幔速度以及界面结构。这些新的地震结果被整合到一个基于地理信息系统的三维地理空间框架内(2010-2012年)，该框架包括研究区域的曝光数据、地质图和工业地下数据。来自断裂断面(Erslev，Siddoway)和重力建模(Miller)的运动学信息被用于指导岩石圈规模的3D结构修复，以测试我们3D几何模型中不同组件的兼容性。Bighorn项目更广泛的影响部分在于开发了定量整合地质和地震研究的基本方法。此外，Bighorn项目涉及范围广泛的地球科学家，包括1名博士后研究员、4名研究生和15名本科生，他们将参加IRIS和Keck联盟赞助的研究。这一结果对能源资源具有关键影响，可以预测对碳氢化合物生产至关重要的开放裂缝。开发一部戏剧性的动画，展示在拉莱姆造山运动期间大角兽拱门的4D结构发展，提供了重要的公众宣传。

This EarthScope project, herein called the Bighorn Project, is an integrated geological and geophysical investigation of contractional basement-involved foreland arches. It addresses how these foreland arches form and how they are linked to plate tectonic processes. The research in the Rocky Mountain Bighorn Arch of northern Wyoming and southern Montana combines geological investigations of surface geometries and kinematic indicators with geophysical imaging of 3D crustal and upper mantle geometries from an active/passive seismic experiment. The resulting 4D (3D spatial and temporal), lithospheric-scale model of foreland arch deformation tests current hypotheses for basement-involved foreland thrust belts both in the Rockies and in active orogens of Asia and the Andes. These hypotheses include: 1) fault blocks defined by lithosphere-penetrating thrust faults, 2) subhorizontal detachment within the crust, 3) lithospheric buckling, and 4) pure shear lithospheric thickening. Our investigation to determine the mechanism driving basement-involved arch formation is advancing our understanding of continental lithospheric rheology. This three year (2009-12) collaborative project defines a lithospheric volume of 1.5 x106 km3 by integrating arch-scale upper crustal geometries derived from surface exposures and petroleum industry subsurface data (Eric Erslev, University of Wyoming; Christine Siddoway, Colorado College) with the results of a hybrid seismic experiment. The passive component of this experiment consists of a 1.25 year (2009-10) deployment of 27 broadband seismometers that densify the EarthScope transportable array (Megan Anderson, Colorado College), a 6.5 month deployment of 220 short period seismometers (Anne Sheehan, University of Colorado), and a 9 day deployment of 800 high frequency "Texan" seismometers (Kate Miller, University of Texas at El Paso). The active component consists of 9 shots (summer 2010) recorded by the above instruments and an additional 1600 "Texan" seismometers deployed for 5 days. These instruments are arrayed in a grid consisting of three SW-NE lines and two NW-SE lines with a total line length of 1000 km. Joint inversion of active and passive results defines crustal and upper mantle velocities and interface structures within the Bighorn Arch. These new seismic results are integrated (2010-12) within a GIS-based, 3D geospatial framework including data from exposures, geologic maps and industry subsurface data for the study area. Kinematic information from fracture transects (Erslev, Siddoway) and gravity modeling (Miller) is used to guide 3D, lithosphere-scale structural restorations to test the compatibility of different components in our 3D geometric model. The broader impact of the Bighorn Project partially lies in the development of fundamental methods for quantitative integration of geological and seismological research. In addition, the Bighorn Project involves a broad range of geoscientists, including one post-doctoral fellow, 4 graduate students, and 15 undergraduates, who will participate in IRIS and Keck Consortium sponsored research. The results have key implications for energy resources, allowing the prediction of open fractures that are critical to hydrocarbon production. The development of a dramatic animation showing the 4D structural development of the Bighorns Arch during the Laramide Orogeny provides important public outreach.