Subducted slab being torn apart

The Mediterranean area is possibly the most tectonically complicated area there is.  It’s a plexus of microplates, all shuffling and jostling like guilty schoolboys accused of sticking gum under their desks.  That is a result of the misfit between the continental masses carried on the Eurasian and African plates, which was never resolved by the collision between the two that threw up the Alpine chain.  Complex as it is, the region is small enough, close enough to research institutes and pleasant enough to work in for there to have been a great deal of effort to understand its active plate tectonics.

The latest method to be applied is the analysis of seismic waves’ arrivals at seismometers in the manner of body scanning – seismic tomography.  Combining these new 3-D data of deep motions in the mantle with a review of surface geology, M.J.R. Wortel and W. Spakman of the Vening Mensz Research School of Geodynamics at the University of Utrecht build a remarkable picture of what seems to be going on (Wortel, M.J.R. and Spakman, W. 2000.  Subduction and slab detachment in the Mediterranean-Carpathian region.  Science, v. 290, p. 1910-1917).  One of their remarkable conclusions is a suggestion that subducted slabs are becoming detached, thereby changing the configuration of slab-pull forces in the region.  They sketch out how that might happen, by the formation of small ‘nicks’ in the short subducting slabs that focus slab-pull force along the reduced length of intact slab.  Thus focused, the pull more rapidly helps propagate the “nick” into a fully-fledged tear, which will migrate over the remaining length of the subduction zone.

Mechanically, that is interesting enough, but should it happen at a shallow depth influx of asthenosphere would generate magma on a small scale, and perhaps induce hydrothermal activity and unusual sequences of metamorphism in the overlying crust.  Isostatic responses might change depositional process at the surface too.  Wortel and Spakman suggest that there is geological evidence throughout the region for this process having operated in the past, with consequences such as these, as well as going on today.

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