[ghsc-seminars] GHSC Seminar - Darcy Cordell, University of Alberta, April 4, 2023

Wed Mar 29 16:00:35 UTC 2023

Links between slab dehydration and forearc seismogenic zone structure from magnetotelluric imaging

Speaker: Darcy Cordell
                 University of Alberta
Location: online (while the speaker will be virtual, we will still be projecting the talk in the seminar room so come join us!)
Date/Time: April 4, 2023, 10-11AM MDT

Abstract: Fluids in subduction zones influence many important geological processes as the hydrated incoming plate lithosphere (i.e. slab) releases pore-bound and chemically-bound water into the over-riding crust and mantle wedge. In particular, fluids released from the slab beneath the forearc seismogenic zone are thought to create conditionally-stable regions which have strain-dependent slip behavior and may control megathrust rupture. The Shumagin Gap is unique along the Aleutian arc for its paucity of historical megathrust (>8 Mw) rupture nucleating in the gap. Geodetic data suggest that gap is weakly-coupled relative to adjacent segments. The reasons for the weak coupling and lack of megathrust rupture are unclear. Furthermore, the adjacent Semidi segment ruptured in July 2020 with Mw 7.6 and propagated into the Shumagin Gap. To examine the role of fluids, marine magnetotelluric (MT) data were collected along a 250 km trench-perpendicular profile in 2019 as part of the Electromagnetic Alaskan GeoPRISMS Experiment (EMAGE). The resulting 2-D resistivity model contains a conductor near the plate interface and is correlated with a wide-band of reflectors from seismic studies, seismicity, and the July 2020 rupture patch. This suggests that fluids play an important role in controlling the slip behavior in the Shumagin Gap. The source of the fluids at the plate interface is commonly thought to come from the downgoing slab crust. However, the MT results indicate a significant slab mantle conductor >15 km below the slab Moho, directly beneath (and connected to) the conductor at the plate interface. This suggests that deeper slab mantle serpentinization may reach pressure-temperature conditions conducive to dehydration beneath the forearc seismogenic zone. This provides an additional source of forearc fluids not previously considered in subduction zone water budgets which can modulate slip behaviour.

Bio: Darcy Cordell is a post-doc at the University of Alberta working with Ian Mann using magnetotelluric data for space weather hazards prediction. He completed his PhD in 2020 working with Martyn Unsworth at the University of Alberta imaging volcanoes and subduction zones in central Chile, and completed a post-doc in 2022 at Georgia Tech working with Samer Naif imaging the Alaska subduction zone and incorporating petrological constraints on MT interpretations. His main research interests focus on using magnetotellurics for a wide array of applications including imaging magmatic systems, imaging subduction zone fluids, and estimating geoelectric fields during space weather events. He recently returned from field work in Costa Rica trying to image subduction zone fluids beneath the Nicoya Peninsula.

[cid:ef5a8ec1-4741-40b7-ace2-68a8bfb39f2a]

For questions, comments, or speaker suggestions, please contact the GHSC Seminar Committee: Andrea Llenos <allenos at usgs.gov>, Benjamin Murphy <bmurphy at usgs.gov>, Lauren Schaefer <lschaefer at usgs.gov>

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