EXHUMATION OF DEEP-CRUSTAL METAMORPHIC ROCKS AND CRUSTAL EXTENSION IN ARC AND BACK-ARC REGIONS

We adress the problem of exhumation of metamorphic rocks in arc and back-arc positions based on observations of the Aegean and Tyrrhenian back-arc regions. In both regions back-arc extension has been active for 25 Ma at least, while convergence and crustal thickening was going on at the thrust front. Compressional structures are reworked in the back-arc domain as extensional flat-lying detachments which allow unroofing of the metamorphic pile. Metamorphic rocks suffer first a dynamic burial during nappe stacking and then fast exhumation when they reach the extensional region. We describe a continuous process of burial and exhumation that is contemporaneous with the migration of the thrust front toward external zones. The front of extension follows behind at approximately one hundred kilometers. At the convergence front, continuous subduction of a cold oceanic slab or continuous accretion of continental units keeps a cool P-T gradient and HP-LT metamorphic rocks are formed at depth. The overall cool environment leads to a deep brittle-ductile transition and thus to a resistant crust which can sustain its large thickness. While the thrust front propagates forward, parts of the accretionary wedge which were formed earlier enter the back-arc domain which can be already under extension and which is warmer because of a greater distance to the cold slab and the presence of the volcanic arc. The brittle-ductile transition rises in the thick crust which then becomes unstable and subsequently collapses. The exhumation history of metamorphic rocks below detachments is two-fold. (1) A constantely thickening crust is unroofed by flat-lying detachments which rework early thrusts. This first period is characterized by a good preservation of HP-LT parageneses. (2) The second period is when rocks are in the zone of crustal-scale extension of the back-arc region. Deep crustal metamorphics are exhumed below flat-lying detachements which affect the whole crust.