Brittle behaviour and petrologic change of the subducting oceanic lithosphere

Metamorphism causes major changes in the mineralogy and rheology of the lithosphere. However, without coupled deformation and fluid flow, the unaltered lithosphere remains long time stiff and metastable, thus sustaining large differential stresses. This is relevant to subduction of oceanic lithosphere, where fluid presence vs absence affects seismicity and eclogitization. The subduction-zone behavior of hydrated oceanic slabs has been deeply studied in the recent years; differently, the unaltered lithosphere from the inner slab is much less known, though italso hosts earthquakes and its eclogitization can drive the slab pull.

Aim of this contribution is providing field-based evidence of the main structural and metamorphic changes affecting the dry portions of subducting oceanic slabs. The ophiolitic gabbro-peridotite of the Lanzo Massif (W. Alps) largely escaped Alpine subduction metamorphism due to poor oceanic hydration. This made these rocks dry, stiff asperities in the subduction complex, which locally developed pseudotachylytebearing faults and widespread meso- to micro-faulting at intermediate-depth depths. In the field, thin, flat-lying metric faults cause centimetre-scale offsets of gabbro dykes: such faults contain sub micrometric “annealed” ultracataclasite of fresh olivine and pyroxene locally overgrown by secondary chlorite. Cataclastic plagioclase is progressively altered into high-pressure zoisite + paragonite up to become the most intensively eclogitized mineral domain in the studied samples. The fault planes thus developed at dry conditions in the olivine stability field; localized fluid access promoted fault hydration and massive plagioclase replacement by high-pressure assemblages. By means of LA-ICP-MS element trace analyses, we also identified the internal redistribution of fluid-mobile elements. This implies that the subduction zone eclogitization of the slab mantle is triggered by fluid access along pervasive fault discontinuities and reactive minerals. The faulted Lanzo lithospheric mantle can represent slab domains affected by minor slip events and close to areas of faulting and pseudotachylyte formation during major earthquakes.