“Order from chaos”: A field-based estimate on bulk rheology of tectonic mélanges formed in subduction zones

Abstract

Many tectonic HP–LT mélanges exhibit block-in-matrix structures. In contrast to the pervasively deformed matrix, the blocks of various lithologies and PT-histories usually appear undeformed. If we accept that an HP–LT mélange reflects persistent high strain deformation in a deep (>30km) subduction channel, the record of these rock associations can provide insight into stress state, rheology, and flow patterns along a plate interface. The bulk rheology of a block-in-matrix structure is controlled by (a) the dominant deformation mechanism of the matrix, (b) the volume fraction of blocks engulfed in that matrix and (c) the geometry of the blocks. The microstructures of the matrix indicate distributed deformation by dissolution–precipitation creep (DPC), while crystal plastic deformation is subordinate. DPC is grain size sensitive, controlled by the type of interfaces, and requires an aqueous pore-fluid at quasi-lithostatic pressure. The rheology of a rock body undergoing deformation by DPC is believed to be Newtonian viscous. Matrix viscosities are estimated to be on the order of ~1019Pas or less. Here, we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology. Using stereology, we determine the block size distributions as well as the total block volume fractions for block-in-matrix structures from several regions. The volume fractions are found to range from 2 to 70% with typical values between ~5 and 50%. Using published mixing laws, we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50%, the viscosity increase is less than one order of magnitude. Consequently, the influence of blocks on the bulk viscosity of a subduction channel is minor. Instead, bulk rheology is primarily controlled by the deformation behaviour of the matrix material.