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

构成地球最外层（地壳）的两个板块碰撞形成了山脉带。这些碰撞是由板块构造周期驱动的。其中一个碰撞板块由致密的海洋地壳组成，另一个是更具浮力的大陆地壳，它构成了地球上大部分的陆地，海洋地壳通过俯冲下沉到地球的深内部，将物质从表面回收到地幔。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，导致地球上最大的山脉带的形成。 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。大陆碰撞通常会终止大洋板块俯冲的时期，但人们对这两个过程之间的过渡仍有很多不了解，包括俯冲带如何转变为大陆碰撞以产生山脉，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上重要的矿石的分布。这项为期3年的国际合作研究由加州大学戴维斯分校、密歇根大学和格鲁吉亚共和国的伊利亚州立大学的研究人员组成，他们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，也是当今地球上唯一可以观察到这种转变的地方之一。该项目将通过培养博士生来促进科学知识并为社会做出贡献，这有助于培养具有全球竞争力的STEM劳动力，并扩大STEM教育工作者的数量。该项目将吸引和指导来自代表性不足的少数民族的本科生，提供专业发展和增加对STEM的参与，并将通过让美国学生参与合作，国际工作，为伊利亚州立大学的研究生提供培训，加深美国和格鲁吉亚研究人员之间的合作，并支持美国境内的新研究合作，以增加国内和国际合作伙伴关系。 最后，该项目将通过支持美国研究型大学的分析能力来加强研究和教育基础设施。从大洋俯冲到大陆碰撞的转变定义了板块边界沿着力平衡和动力学的深刻变化。因此，确定从俯冲到碰撞的过渡期间应变的空间和时间分布对于将造山带的变形响应与驱动该变形的远场板块运动联系起来至关重要。在过渡期间，俯冲大陆边缘的浮力地壳进入海沟，减少俯冲巨型逆冲断层的倾角，增加板块边界耦合，并使覆盖板块变形。从俯冲到碰撞的过渡触发了变形，折返和沉积的空间分布和速度的根本变化，沿着会聚边缘的增生楔演化为缝合线之间的碰撞大陆块。 在活跃的大陆碰撞带内从俯冲到碰撞的过渡期间，变形的空间和时间模式的地质观测仍然非常难以捉摸，主要是因为缺乏研究这种过渡的地点。该项目将检验大高加索作为研究俯冲向碰撞过渡的天然实验室的想法，确定地质、结构、地震活动和大地缩短的基本走向二分法是否是从山脉东部的活跃俯冲向西部的大陆碰撞和终端俯冲过渡的结果。具体而言，该项目将检验这样一种假设，即在东经45°以西的大高加索地区存在一条终端缝合线，这是一个现在俯冲的海洋弧后盆地，以前将欧亚大陆从南部的大陆带分离到北部。45°E以东尚未发生碰撞，范围以活跃的大洋俯冲为主。 该项目有两个目标：（1）通过描述和量化这种动力学状态的基本变化的地质表现、持续时间、运动学和力学，阐明认识古代大陆碰撞中的碰撞所必需的地质观测;（2）捕捉造山带对板块边界力演变的变形响应，这种变化与从俯冲到碰撞的转变有关。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知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.