The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow

Publication Type  Journal Article
Year of Publication  2006
Authors  Abers, G. A.; van Keken, P. E.; Kneller, E. A.; Ferris, A.; Stachnik, J. C.
Journal Title  Earth and Planetary Science Letters
Volume  241
Issue  3-4
Pages  387-397
Journal Date  Jan 31
ISBN Number  0012-821X
Accession Number  ISI:000235289000004
Key Words  subduction; thermal models; alaska; receiver functions; intermediate-depth earthquakes; mantle wedge; 100-250 km depth; fore-arc mantle; polycrystalline olivine; northeastern japan; wave attenuation; central alaska; northern gulf; rupture zone; heat-flow;
Abstract  

Thermal models of subduction zones often base their slab-wedge geometry from seismicity at mantle depths and, consequently, cannot be used to evaluate the relationship between seismicity and structure. Here, high-resolution seismic observations from the recent Broadband Experiment Across the Alaska Range (BEAAR) constrain, in a rare instance, the subducting slab geometry and mantle wedge temperature independent of seismicity. Receiver functions reveal that the subducting crust descends less steeply than the Wadati-Beinoff Zone. Attenuation tomography of the mantle wedge reveals a high Q and presumably cold region where the slab is less than 80 km deep. To understand these two observations, we generate thermal models that use the improved wedge geometry from receiver functions and that incorporate temperature- and strain-rate-dependent olivine rheology. These calculations show that seismicity within the subducting crust falls in a narrow belt of pressure-temperature conditions, illuminating an effective Clapeyron slope of 0.1 K/N4Pa at temperatures of 450-750 degrees C. These conditions typify the breakdown of high-pressure hydrous minerals such as lawsonite and suggest that a single set of dehydration reactions may trigger intermediate-depth seismicity. The models also require that the upper, cold nose of the mantle wedge be isolated from the main flow in the mantle wedge in order to sustain the cold temperatures inferred from the Q tomography. Possibly, sufficient mechanical decoupling occurs at the top of the downgoing slab along a localized shear zone to 80 km depth, considerably deeper than inferred from thrust zone seismicity. (c) 2005 Elsevier B.V. All rights reserved.

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URL  <Go to ISI>://000235289000004
DOI  DOI 10.1016/j.epsl.2005.11.055