Unit Affiliation: Marine and Polar Geophysics, Lamont-Doherty Earth Observatory (LDEO)
The evolution of oceanic lithosphere involves incorporation of water into the physical and chemical structure of the crust and shallow mantle through thermally driven fluid circulation, which initiates at the mid-ocean ridge and continues on the ridge flanks long after crustal formation. At subduction zones, water stored and transported with the descending plate is gradually released at depth, strongly influencing subduction zone processes. Cascadia is a young-lithosphere end member of the global subduction system where relatively little hydration of the down-going Juan de Fuca (JdF) plate is expected due to its young age and presumed warm thermal state. However, numerous observations support the abundant presence of water within the subduction zone, suggesting that the JdF plate is significantly hydrated prior to subduction at the trench. At present the state of hydration of the JdF plate is poorly known with few constraints on crustal and upper mantle structure. In this project an active source seismic experiment was conducted to characterize the evolution of the crust and shallow mantle across complete transects of the JdF plate, from formation at the ridge, through alteration and hydration within the plate interior, to subduction at the Cascadia margin.
OUTCOMES: The largest earthquakes and the most explosive volcanic eruptions on our planet occur at subduction zones where one tectonic plate descends beneath another. A key parameter that affects many aspects of the subduction process is how much water is bound within the downgoing plate and how that water is distributed. At the Cascadia subduction zone, which extends from northern California to British Columbia, the Juan de Fuca plate descends beneath North America giving rise to large earthquakes in the geologic past. Regional variations in subduction zone properties and seismicity are observed along the Cascadia margin and variations in the hydration and structure of the subducting plate could contribute.
In order to further our understanding of the state of hydration of the Juan de Fuca plate prior to subduction, a marine active source seismic study was conducted across the plate and along the continental margin. The data acquired provide the first plate-scale imaging of any oceanic plate as well as new constraints on the structure of the crust and uppermost mantle of the Juan de Fuca plate. Our primary results include first evidence for subduction bend faulting through the crust and into the mantle at Cascadia and the discovery of regional variations in this bend faulting with greater extents of bend related plate hydration inferred for offshore Oregon. The amount of water bound in the incoming Juan de Fuca plate estimated from the seismic data is lower than at other subduction zones with most water bound in the upper crust. However a marked regional change is observed at ~45°45’N with higher water contents to the south of this location. Our seismic reflection images reveal a coincident change in the density and orientation of crustal faults at this location, consistent with more subduction bend faulting extending through the crust offshore central Oregon.
Our characterization of the state of the Juan de Fuca plate prior to subduction bears on studies of earthquake hazards in the heavily populated Pacific Northwest where great Cascadia megathrust and damaging intraslab earthquakes have occurred in the historical past. The new data provide support for the presence of a zone of aseismic slip offshore Oregon and reveal segmentation in down-going plate structure associated with pseudofaults inherited from crustal formation that coincides with previously proposed paleoseismic rupture zones, relevant for predictions of seismic hazard along the margin.
State of the Planet: http://blogs.ei.columbia.edu/tag/cascadia-in-motion/
This project supported an active source marine seismic investigation. The marine seismic and other related geophysical data acquired during the experiment are available through the MGDS data repository:http://www.marine-geo.org/tools/search/entry.php?id=MGL1211
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