Models of permeability contrasts in subduction zone mélange: Implications for gradients in fluid fluxes, Syros and Tinos Islands, Greece

Geological and geochemical evidence from previous studies suggest limited fluid flow through the interiors of many blocks of subducted oceanic crust within meta-ultramafic or metasedimentary mélange matrix on the islands of Syros and Tinos, Cyclades, Greece. This finding is provocative because metamorphic devolatilization of igneous and sedimentary rocks during subduction should have liberated substantial volumes of fluid. In an effort to address this problem, two-dimensional numerical models incorporating permeability anisotropy and devolatilization are used to investigate spatial patterns of steady-state fluid flow through subduction zone mélange under high-pressure/low-temperature (HP/LT) conditions. The modeling shows that as the permeability of the mélange blocks decreases relative to the mélange matrix, more flow is diverted around the blocks into the matrix. The ratio of the maximum flux (in matrix) relative to the minimum flux (in blocks) increases with the permeability contrast between the matrix and the less permeable blocks. Order-of-magnitude variations in permeability produce order-of-magnitude spatial variations in fluid fluxes between block interiors and surrounding matrix; flux variations of a factor of 10 or more can be present over very short length scales (as little as meter scale). The largest fluxes are produced in the matrix adjacent to block margins lying sub-parallel to regional foliation and flow direction. Fluid–rock reaction on block margins would be greatest in these areas, leading to asymmetrical distributions of reaction progress around blocks (illustrated herein by field mapping of a mélange block on Syros). However, syn-metamorphic deformation can rotate blocks and strip off reaction zones at their margins, so it is likely that reaction zone asymmetries are commonly disrupted in nature. The inferred absence of strong fluid–rock reaction within mélange blocks on Syros and Tinos could reflect low permeabilities in block interiors which acted to divert flow into mélange matrix. Consequently, fluxes of subduction zone fluids through the mélange matrix could have easily been very large, while low permeabilities shielded block interiors from the regional flow to varying degrees.