Record of intermediate-depth subduction seismicity in a dry slab from an exhumed ophiolite

G. Pennacchioni*, M. Scambelluri, M. Bestmann, L. Notini, P. Nimis, O. Plümper, M. Faccenda, F. Nestola

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

The ophiolitic peridotite and gabbro of Moncuni (Southern Lanzo Massif, Western Alps) retain pre-subduction mantle-to-oceanic, high-temperature (>700C∘) ductile fabrics. These fabrics are overprinted by seismic fracturing and faulting associated with pseudotachylytes. Within the gabbro, the pseudotachylytes preserve dry glass and pristine microlites. The occurrence of rare, minute garnet and the static development of eclogite-facies assemblages in local hydrated domains indicate that pseudotachylytes experienced subduction conditions of 600C∘ and 2.1 GPa. The exceptional survival of glass and the absence of post-oceanic ductile deformation demonstrate prevailing dry conditions during the entire Alpine subduction and exhumation path. Dry conditions inhibited reaction kinetics and viscous flow. In contrast, the majority of the Alpine ophiolites, derived from the upper hydrated portions of the oceanic lithosphere, show pervasive fluid-assisted metamorphism and ductile deformation. The Moncuni body can, therefore, be regarded as representative for the rheological behaviour during subduction of seismic, dry, deeper oceanic lithosphere that is rarely exhumed to the Earth's surface. In Moncuni, the brittle-ductile transition of dry oceanic rocks is constrained to be between 600 and 750C∘. This temperature range corresponds to the observed cut-off of intermediate-depth seismicity within subducting slabs. We infer that the base of the seismic layer corresponds to the brittle-ductile transition of a dry slab rather than the locus of antigorite breakdown triggering earthquakes by dehydration embrittlement.

Original languageEnglish
Article number116490
Pages (from-to)1-14
Number of pages14
JournalEarth and Planetary Science Letters
Volume548
DOIs
Publication statusPublished - 15 Oct 2020

Funding

G.P. acknowledges funding from the University of Padova ( BIRD175145/17 ). M.S. acknowledges the Italian MIUR ( 2017ZE49E7 ) and the University of Genoa for funding. M.B. acknowledges financial support from the Deutsche Forschungsgemeinschaft DFG ( BE 2413/3-1 ). Also, this work benefited of EPOS TCS MSL TNA access to Electron Microscopy Facilities, (Utrecht University, The Netherlands), supported by European Community HORIZON 2020 research and innovation program under grant agreement N 676564, and Dutch national funding; Internationalisation in the Earth and Life Sciences – NWO ALWIN.010 – “Implementing the European Plate Observing System (EPOS): Optimizing Dutch leadership of the multi-scale laboratories component”. The data presented in this paper are publicly available via the data publication platform of Utrecht University ( https://public.yoda.uu.nl/geo/UU01/RVRHHE.html ) We are grateful to two anonymous reviewers for their comments.

Keywords

  • earthquake
  • eclogite
  • fluids
  • ophiolite
  • pseudotachylyte
  • subduction

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