TIMESCALES OF TRANSIENT BRITTLE DEFORMATION IN SUBDUCTING SLABS: CONSTRAINTS FROM DIFFUSION MODELING FROM THE MONVISO OPHIOLITE

Metamorphic rocks have the potential to record crustal processes that occur over a wide range of both P-T conditions and temporal scales. Currently, there is a general paucity of examples in the rock record of processes that occur on very rapid timescales (< 1 Ma). One location where these short-timescale processes have received recent attention is in subduction zones, where the importance of rapid fluid flow and seismicity during subduction has been highlighted.

Here, we present temporal constraints on the periodicity of brittle deformation as recorded in brecciated mafic eclogites from the Monviso ophiolite. The Monviso ophiolite represents a slice of oceanic lithosphere that was subducted and subsequently exhumed during the Alpine orogeny, reaching peak P-T conditions of 550ºC and 2.6 GPa. Garnet porphyroblasts in breccia clasts display numerous healed fractures which are distinguished into different populations using both textural and chemical criteria. High-spatial resolution EPMA traverses across healed fractures (n = 52) reveal a low-Mn (2 mol %) and a high-Mn (8 mol %) fracture population. Texturally, the high-Mn healed fractures cross-cut the low-Mn healed fractures. A forward diffusion model with four-component (Fe, Mg, Mn, Ca) coupled diffusion is utilized to constrain the timescale near peak P-T for each healed fracture population. Our results suggest that the low-Mn healed fracture population has a best-fit timescale of ~ .85 Ma, while the high-Mn healed fracture population records a timescale of ~.15 Ma. The difference in timescale between these two healed fracture populations (~ .7 Ma) is interpreted to represent a maximum time lapse between two successive fracturing events recorded by these garnets.

This episodic brittle deformation of the Monviso ophiolite at eclogite facies conditions may be related to fluid overpressuring and associated seismic activity deep in the subduction zone (i.e. Intermediate Depth Seismicity) and/or represent the mechanism for detachment of the ophiolite from the down-going slab to the subduction channel.