Multiple rupture and healing events along the Plate Interface at Ultra-High-pressure depth. Insights from the Lower Shear Zone, Monviso Massif, Italy

The processes that rule coupling/decoupling and rupture mechanisms along the plate interface in the deep portions of active subduction zones are largely inferred from geophysical observations. These observations highlight that a wide range of rupture and deformation mechanism may coexist, such as: aseismic slip, episodic non-volcanic tremor and slip (ETS), and regular earthquakes. Despite the high amount of data obtained through indirect approaches, our comprehension of the processes occurring along the plate interface is still limited. In particular, processes occurring at great depth along the subduction interface are difficult to interpret solely based on flow laws and rheological properties of rocks also due to the scarcity of direct geological observations.

Exhumed ultra-high pressure (UHP, > 90 km of depth) rocks represent a natural laboratory to investigate the interplay of metamorphic reactions and fluids, both affecting slab rheology, at great depth. The Lower Shear Zone (LSZ) from the Monviso massif (W Alps) represents a fossil plate interface accreted within the Western Alpine chain and constitutes the one-off example of an oceanic plate interface that reached coesite stability field at UHP depth., i.e., ca. 90-100 km, and was then exhumed^[1,2]. The LSZ preserves snapshots of the different stages of deformation and metamorphism along the subduction plate interface shear zone, testifying the coexistence of brittle (brecciation of rigid eclogite-facies gabbroic mylonites) and ductile behaviour (shearing along weak, serpentinite-rich shear zone) at eclogite-facies depth^[1,3].

Our new field, micro-structural and petrographic observations extend the existing record of brittle features along the LSZ and show that brecciated blocks of mylonitic eclogites are systematically traceable for almost 25 km, i.e. the entire length of the exposed LSZ. These blocks are embedded within a highly deformed serpentinitic matrix. The brecciated fabric is defined by a mosaic breccia texture with randomly distributed clasts cemented by a polyphasic omphacite-rich matrix. The matrix is locally brecciated and sealed again, highlighting a cyclic rupture and healing mechanism promoted by fluid pulses and consequent dehydration embrittlement. These features are comparable to the classical geological observations of structures attributed to ETS described from shallower region of the plate interface. The similarity suggests that ETS may transiently occur even at greater depths than those at which they are currently recorded by seismometers and GNSS stations. Our observations imply that decoupling at great depth along the plate interface could be favoured by embrittlement of the plate interface.

1: Angiboust, S., Agard, P., Yamato, P., Raimbourg, H., 2012. Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes? Geology 40, 707-710.

2: Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F. A. 2023. A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy). American Mineralogist, 108(7), 1368-1375.

3: Locatelli, M., Verlaguet, A., Agard, P., Federico, L., Angiboust, S., 2018. Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps). Lithos.