Testing Models for Subduction Initiation Using the Late Triassic Sedimentary and Magmatic Record of the Talkeetna Arc, Alaska

Plate tectonics is a unifying theory for the Earth sciences and explains how the Earth’s surface and subsurface has changed through time. It proposes that the Earth’s outermost layer, the crust, is composed of multiple rigid plates that move across Earth’s surface. Plate movement is largely driven by subduction, which is the tectonic process by which oceanic crust is pulled into the Earth’s interior.

While decades of research have helped understand how subduction drives plate motion, induces volcanism, and is related to hazardous earthquakes, the processes through which subduction starts remain largely unknown. Without this knowledge, the theory of plate tectonics remains incomplete. This grant is focused on understanding the process of subduction initiation by studying an ancient subduction zone that is exposed in southern Alaska along the Shelikof Strait.

This area preserves sedimentary and volcanic rocks that were deposited prior to, during, and after subduction started. The formation of this new subduction zone in the Late Triassic led to the development of the Talkeetna volcanic arc. The preservation of such a complete record of subduction initiation is rare and will allow the team to better understand the processes that caused oceanic crust to begin to sink into the Earth’s mantle.

The characterization of these rocks will be the focus of an ambitious PhD student’s research under the direction of the two PIs at Purdue University. The grant will also include the production of Earth science classroom materials and the production of popular science education videos that will be posted on YouTube.

To accomplish the proposed research goals, the PIs will conduct a multidisciplinary study of Late Triassic sedimentary and volcanic strata that are exposed along the Shelikof Strait. These rocks are the oldest known portion of the well-studied Talkeetna arc and record the events preceding, during, and immediately following subduction initiation in this area.

The exceptional preservation of these strata will allow the team to test two end-member hypotheses that have emerged as possible subduction initiation mechanisms: forced and spontaneous. Forced initiation occurs when compression leads to crustal shortening and the development of structures along which negatively buoyant oceanic lithosphere can sink into the mantle.

Spontaneous initiation occurs when density unstable oceanic lithosphere begins to sink into the mantle along a pre-existing lithospheric weakness. Both end-member processes make specific predictions for the nature of pre-, syn-, and post-subduction initiation sedimentary and volcanic strata. Forced initiation should lead to uplift and unconformity development prior to initiation.

Spontaneous initiation will lead to sudden extension coeval with initiation. Typically, the record of these processes is obscured by subsequent deformation and magmatism in modern and ancient arcs. However, the Late Triassic strata along the Shelikof Strait remain relatively undeformed and are suitable for testing these models.

To fully understand this record, the PIs will make new geologic maps of the area, produce new geochemical measurements and radiometric age determinations, and measure stratigraphic sections of sedimentary and volcanic strata. Once integrated, these data will provide a remarkably continuous record of subduction initiation processes over geologic time.

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.