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

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.