Collaborative Research: From subduction to suture: testing collisional stage and lithospheric strength as controls on orogenic structure in the Caucasus

在构成地球最外层(地壳)的两个板块相撞的地方，形成了山脉带。这些碰撞是由板块构造旋回驱动的。其中一个碰撞板块由致密的海洋地壳组成，另一个板块则由更浮力的大陆地壳组成，大陆地壳构成了地球上的大部分陆地，海洋地壳通过俯冲作用下沉到地球内部深处，将表面的物质循环回到地球地幔。然而，当两个碰撞的板块由大陆地壳组成时，它们都抵抗俯冲，从而形成了地球上最大的山带。形成这些山脉带的碰撞产生了毁灭性的地震，集中了自然资源，并通过地形的快速增长/衰退推动了生物进化。大陆碰撞经常终止大洋板块俯冲的时期，但关于这两个过程之间的过渡，包括俯冲带如何转化为大陆碰撞以形成山链，是什么控制了大地震的大小和位置，或者是什么调节了经济关键矿石的分布，人们仍然不太清楚。加州大学戴维斯分校、密歇根大学和乔治亚共和国伊利亚州立大学的研究人员开展了为期三年的国际合作，将研究大高加索山脉从俯冲到大陆碰撞的转变，大高加索山脉是阿拉伯-欧亚大陆碰撞的主要构造要素，也是当今地球上仅有的几个可以观察到这种转变的地方之一。该项目将通过培训博士生来促进科学知识并为社会做出贡献，这有助于培养一支具有全球竞争力的STEM劳动力队伍，并扩大STEM教育者队伍。该项目将吸引和指导来自未被充分代表的少数民族的本科生，提供专业发展和更多地参与STEM，并将通过吸引美国学生参与合作的国际工作，为伊利亚州立大学的研究生提供培训，深化美国和格鲁吉亚研究人员之间的合作，以及支持美国国内的新研究合作，增加国内和国际伙伴关系。最后，该项目将通过支持美国研究大学的分析能力来加强研究和教育基础设施。从海洋俯冲到大陆碰撞的转变定义了沿板块边界的力量平衡和动力学的深刻变化。因此，确定从俯冲到碰撞的转变过程中应变的时空分布，对于将造山带的变形响应与驱动变形的远场板块运动联系起来是至关重要的。在转换过程中，俯冲大陆边缘的浮力地壳进入海沟，减少了俯冲巨型逆冲的倾角，增强了板块-边界耦合，并使上覆板块变形。从俯冲到碰撞的转变触发了沿会聚边缘的变形、折返和沉积的空间分布和速率的根本变化，因为增长的楔形构造演化为碰撞大陆块体之间的缝合。在活动大陆碰撞带内从俯冲到碰撞的转变过程中，形变的空间和时间模式的地质观察仍然非常难以捉摸，主要是因为缺乏研究这一转变的地点。这个项目将检验大高加索作为研究俯冲到碰撞过渡的天然实验室的想法，通过确定地质、构造、地震活动和大地缩短方面的基本沿走向二分法是否是从山脉东部的活动俯冲过渡到西部的大陆碰撞和末端缝合的结果。具体地说，该项目将检验这样的假设，即在东经约45°E以西的大高加索地区存在一条末端缝合线，它代表了一个现在被俯冲的大洋弧后盆地，以前将欧亚大陆向北与向南的大陆带隔开。在45°E以东，尚未发生碰撞，范围以海洋活动俯冲为主。该项目有两个目标：(1)通过描述和量化动态中这一根本变化的地质表达、持续时间、运动学和力学，阐明识别古代大陆碰撞中缝合所需的地质观测；(2)捕捉造山带对与从俯冲到碰撞相关的板块边界力的演变变化的变形响应，从而量化板块强度和板块边界耦合在控制造山构造中的相对作用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，认为值得支持。

Mountain belts form where two plates that make up the outer-most shell of the Earth (the crust) collide. These collisions are driven by the plate tectonic cycle. Where one colliding plate is composed of dense oceanic crust and the other is more buoyant continental crust, which makes up most of the landmasses on Earth, the oceanic crust sinks back in the Earth’s deep interior via subduction to recycle material from the surface back into Earth’s mantle. However, when the two colliding plates are composed of continental crust, they both resist subduction, resulting in the formation of the largest mountain belts on Earth. The collisions that form these mountain belts produce devastating earthquakes, concentrate natural resources, and drive biological evolution via the rapid growth/decay of topography. Continental collisions often terminate periods of oceanic plate subduction, but much is still not understood about the transition between these two processes, including how a subduction zone converts to a continental collision to produce a mountain chain, what controls the size and locations of large earthquakes, or what modulates the distribution of economically critical ores. This 3-year, international collaboration between researchers at the University of California, Davis, the University of Michigan, and Ilia State University in the Republic of Georgia will investigate the transition from subduction to continental collision in the Greater Caucasus Mountains, a major tectonic element of the Arabia-Eurasia continental collision, and one of the only places Earth where this transition can be observed today. This project will advance scientific knowledge and contribute to society by training PhD students, which contributes to the preparation of a globally competitive STEM workforce and expands the pool of STEM educators. The project will engage and mentor undergraduate students who are from underrepresented minorities, providing professional development and increased participation in STEM, and will increase domestic and international partnerships by engaging US students in collaborative, international work, providing training to graduate students from Ilia State University, deepening collaborations between US and Georgian researchers, and supporting new research collaborations within the US. Finally, this project will enhance research and educational infrastructure by supporting analytical capacities at US research universities.The transition from oceanic subduction to continental collision defines a profound change in force balance and dynamics along the plate boundary. Thus, determining the spatial and temporal distribution of strain during the transition from subduction to collision is critical for relating the deformational response of the orogen to the far-field plate motions driving that deformation. During the transition, buoyant crust of the subducting continental margin enters the trench, reduces the dip of the subduction megathrust, increases plate-boundary coupling, and deforms the overriding plate. The transition from subduction to collision triggers fundamental changes in the spatial distribution and rates of deformation, exhumation, and deposition along the convergent margin as the accretionary wedge evolves into the suture between colliding continental blocks. Geologic observations of the spatial and temporal patterns of deformation during the transition from subduction to collision within an active continental collision zone remain highly elusive, primarily because of a paucity of localities to study this transition. This project will test the idea that the Greater Caucasus serve as a natural laboratory to study the subduction to collision transition by determining if a fundamental along-strike dichotomy in the geology, structure, seismicity, and geodetic shortening results from a transition from active subduction in the eastern part of the range to continental collision and terminal suturing in the west. Specifically, this project will test the hypothesis that there is a terminal suture in the Greater Caucasus, west of ~45°E longitude, that represents a now-subducted oceanic back-arc basin that formerly separated Eurasia to the north from a continental ribbon to the south. East of 45°E, collision has not yet occurred, and the range is dominated by active oceanic subduction. The project has two goals: (1) To illuminate the geologic observations necessary to recognize suturing in ancient continental collisions by describing and quantifying the geologic expression, duration, kinematics, and mechanics of this fundamental change in dynamic state and (2) to capture the deformational response of an orogen to the evolving shift in plate boundary forces associated with the transition from subduction to collision, and thus quantify the relative roles of plate strength and plate boundary coupling in controlling orogenic structure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.