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

这个EarthScope项目，在这里被称为大角项目，是一个综合的地质和地球物理调查收缩的地下室涉及前陆拱。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起来的。在怀俄明州北部和蒙大拿州南部的落基山大角拱的研究结合了地表几何和运动学指标的地质调查，以及来自主动/被动地震实验的三维地壳和上地幔几何的地球物理成像。由此得出的前陆拱变形的4D（三维时空）岩石圈尺度模型，验证了目前关于落基山脉、亚洲和安第斯山脉活动造山带涉及基底的前陆冲断带的假设。这些假设包括：1)由岩石圈穿透逆冲断层定义的断块；2)地壳内亚水平剥离；3)岩石圈弯曲；4)纯剪切岩石圈增厚。我们的研究确定了基底拱形成的驱动机制，促进了我们对大陆岩石圈流变学的认识。这个为期三年（2009-12年）的合作项目通过整合从地表暴露和石油工业地下数据（Eric Erslev, University of Wyoming; Christine Siddoway, Colorado College）获得的弧级上地壳几何数据，以及混合地震实验的结果，确定了岩石圈体积为1.5 x106 km3。该实验的被动部分包括1.25年（2009-10年）部署27个宽带地震仪，使EarthScope可移动阵列更加密集（Megan Anderson，科罗拉多学院），6.5个月部署220个短周期地震仪（Anne Sheehan，科罗拉多大学），以及9天部署800个高频“德克萨斯”地震仪（Kate Miller，德克萨斯大学埃尔帕索分校）。有效成分由上述仪器记录的9次射击（2010年夏季）和额外的1600个“德克萨斯”地震仪组成，部署了5天。这些仪器排列在一个网格中，由三条西南-东北线和两条西北-东南线组成，总线路长度为1000公里。主动和被动结果联合反演确定了大角拱内的地壳和上地幔速度和界面结构。这些新的地震结果（2010- 2012）被整合到一个基于gis的三维地理空间框架中，包括来自研究区域的暴露数据、地质图和工业地下数据。来自断裂断面（Erslev, Siddoway）和重力建模（Miller）的运动学信息用于指导岩石圈尺度的三维结构恢复，以测试3D几何模型中不同组件的兼容性。大角项目更广泛的影响部分在于开发了地质和地震研究定量整合的基本方法。此外，大角项目涉及范围广泛的地球科学家，包括1名博士后，4名研究生和15名本科生，他们将参加IRIS和Keck财团赞助的研究。该结果对能源资源具有重要意义，可以预测对油气生产至关重要的开放裂缝。制作了一个生动的动画，展示了拉腊米造山运动期间大角拱的四维结构发展，为公众提供了重要的宣传。

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.