Behrmann, Jan Hinrich

Stipp, Michael

English

OceanRep

Geophysical Research Abstracts, Vol. 9, 05342, 2007SRef-ID:© European Geosciences Union 2007Subducted crust and sediments – a source forintermediate depth earthquakes?J.H. Behrmann (1), M. Stipp (2)(1) Marine Geodynamics, IFM-GEOMAR, Kiel, Germany, jbehrmann@ifm-geomar.de / Fax:+49-431-6002922, (2) Geologisches Institut, Universität Freiburg, Freiburg, GermanyNucleation of subduction thrust earthquakes is often attributed to dehydration embrit-tlement of ultramafic rocks. Deep seismic imaging indicates that in addition to down-going mantle lithosphere and oceanic crust, continental slivers and marine sedimentsare transported deep into the earth mantle. Also, remnants of deeply subducted crustand sediments are ubiquitous in exhumed ancient subduction complexes, e.g. in theAlps or other orogens. P/T-estimates hint at confining pressures of at least 3-4 GPa,suggesting subduction depths of 100 km or more. Global calculation of sediment bud-gets shows that only about 30% of the sediment income in deep sea trenches is storedin accretionary prisms, whereas as much as 70% is subducted to greater depth and inmost cases does not return to the surface. These materials are preferentially distributedalong the plate boundary between the downgoing and the overriding plates in activesubduction zones. Continental crust and marine sediments, and their metamorphicequivalents are much weaker than basalts or mantle peridotite. So why should defor-mation partition into the much stronger mafic and ultramafic rocks when boundary lay-ers of weak rock are sandwiched between the two convergent plates? We advance thehypothesis that plate boundary deformation is preferentially localized in this boundarylayer, and that strain-hardening may control switches from aseismic creep to brittle,or seismic deformation. Experimental data show that quartz, the mineral controllingbulk rheological behaviour of a wide range of crustal rocks and metasediments, cansuffer dramatic strain-hardening by small parameter changes, e.g. strain rate increasedue to more localized deformation or dehydration at high confining pressures. This inturn forms asperities for earthquakes nucleating near the deep end of the seismogeniczone. The lower seismicity downdip from the seismogenic zone is thought to indicatewidespread aseismic creep in the boundary layer, but at greater depth (70-100 km),comparable effects are also likely to occur in the eclogitized downgoing oceanic slab,accounting for the observed increased seismic activity there. We conclude that strainhardening and rupturing in downgoing sediments and crust is an important additonalmechanism for subduction earthquake nucleation.

Subducted crust and sediments - a source for intermediate depth earthquakes?

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