Secular change in metamorphism and magma production at active margins: Numerical modelling
|Location||International Geological Congress,oslo 2008|
|Author||Gerya, Taras; Loew, Irena; Sizova, Elena|
|Holding Date||11 October 2008|
Possible secular change in metamorphism and magma production at an active margin is analyzed by using a 2D coupled geochemical-petrological-thermomechanical numerical model of an oceanic-continental subduction process. This model includes spontaneous slab retreat and bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks and melt extraction processes resulting in magmatic arc crust growth. In order to study possible secular change in such system back in time we explored effects of systematic increase in radiogenic heat production and mantle temperature.
In the numerical experiments, many realistic features characteristic for natural arc settings can be observed. One is the construction of an accretionary wedge, including frontal and basal accretion, as well as subduction erosion. The generation of a magmatic arc results in the growth of a new volcanic (mainly basaltic) layer atop the continental crust leading to subsidence and thinning of the underlaying continental crust. The lateral width of the magmatic arc is constricted to 30-70 km due to the limited extent of the melt extraction area in the hydrated mantle wedge atop the slab. In part of the experiments an intra-arc extension is documented. This process is followed by rapid slab retreat triggering the formation of a backarc basin with the new spreading center resulting in dry decompression melting of the mantle and building of new oceanic floor.
Numerical models also predict that active margin processes notably change with higher crustal heat production and hotter mantle temperature which are representative for the early Earth history: (i) higher temperatures (by 100-300oC) are observed in both the subduction channel and in the lower crust of the backarc region corresponding to conditions of HP granulites and UHT complexes, respectively; (ii) hotter continental crust results in notable crustal melting and ductile crust thinning in the backarc region rather then opening of new oceanic floor; (iii) processes of decompression melting of dry asthenospheric mantle become more important and hot partially molten mantle often underplate lower continental crust due to overriding plate extension; (iv) hotter and more ductile subduction channel favor delamination of the slab from the overriding plate and opening gap between two plates which further results in fore-arc extension and underplating of accretion wedge by hot partially molten asthenospheric mantle.