Consequences of progressive eclogitization on crustal exhumation, a mechanical study

The very low water content of the granulitic unit of Holsnoy, in the Bergen Arcs, Norway, caused its partial metastable preservation throughout Caledonian burial end exhumation, leading to the observed mixture of completely eclogitized and uneclogitized rocks. The eclogitization of the granulite-facies protolith led to a density increase similar to 10%, but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction. We address the consequences on subduction dynamics of the concomitant evolution of large-scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel-flow model. Large-scale buoyancy and density are integrated in a single adimensional parameter, the 'exhumation number' alpha, which describes the capacity of the crust to exhume. For a crust whose eclogitization causes a very large viscosity decrease ('burial resistant' end-member), alpha goes through a maximum in the partially eclogitized zone, that is, there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle. For this partially eclogitized crust with alpha max, the maximum possible downward flow is very low, this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming. This return flow zone is bounded downwards in the partially eclogitized zone by alpha max, while its upper limit propagates upwards at a constant rate. The curve alpha(eclogite fraction) controls the maximum proportion of eclogite of the crust that can be exhumed. We also demonstrate that large exhumation rates reached by certain UHP units are incompatible with any stationary flow regime, whatever the nature of subducted crust. The additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones, but does not qualitatively change the dynamics of the subduction channel. This study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large-scale crustal properties with progressive eclogitization.