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Geological Society, London, Special Publications; 2002; v. 200; p. 387-407;
DOI: 10.1144/GSL.SP.2001.200.01.22
© 2002 Geological Society of London

Crust and Lithosphere Tectonics

Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins

M. R. Handy1 & H. Stünitz2

1 Geowissenschaften, Freie Universität Berlin, D-12249 Berlin, Germany
2 Geologisch-Paläontologisches Institut, Universität Basel, CH-4056 Basel, Switzerland

Rift-related strain localization in spinel lherzolite from an exhumed passive continental margin in the Southern Alps involved two stages. (1) Critical fracturing coincided with heterogeneous nucleation of plagioclase, olivine, and hornblende aggregates to form discrete, ultrafine-grained (0.5–0.6 µm) shear zones oriented at high angles to the pre-existing foliation in the host rock. The syntectonic replacement of spinal lherzolite by lower pressure, plagioclase-hornblende lherzolite documents extensional exhumation under high temperature (700–900 °C) conditions accompanied by limited fluid infiltration. Deformation involved a combination of dislocation creep (ol) and diffusion-accommodated viscous granular flow (plag, ol, hbl aggregates). (2) Hydrous deformation at lower temperatures (200–400 °C) involved the formation of serpentine-chlorite mylonite and cataclasite along discrete, anastomozing shear zones oriented at low angles to the pre-existing foliation. Both stages involved drastic weakening, particularly once the shear zones coalesced subparallel to the extensional shearing plane. The top of the lithospheric mantle was initially strong, but is inferred to have become weaker than both the underlying mantle and the overlying mafic lower crust. The interconnection of such strong-then-weak delamination zones to form trans-lithospheric extensional shear zones accelerated rifting and led to the exhumation of subcontinental mantle during the late stages of continental breakup.





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