EXHUMATION OF THE MID-BOSNIAN SCHIST METAMORPHIC CORE COMPLEX

Syn or post collisional extension is expressed in several Mediterranean orogens as mid-crustal rocks exhumed along low angle normal faults and has variously been attributed to slab roll-back or break-off. The Dinarides are a late Cretaceous-presently active fold-and-thrust belt driven by the collision between the Adria microplate and Eurasia. The Mid-Bosnian Schist Mountains (MBSM) are a fault bounded body of lower greenschist facies metamorphic rocks in the hinterland of the active Dinaric fold-and-thrust belt. Existing geologic maps and cross sections indicate that the faults bounding the MBSM are thrust faults and interpret the emplacement of these mid-crustal rocks as a compressional event. However, field observations along the fault bounding the east side of the MBSM support a normal shear sense suggesting an extensional tectonic exhumation. New low temperature thermochronolgic dates along with published Ar/Ar dates reveal an Eocene-Oligocene episode of relatively rapid cooling synchronous with shortening in the fold and thrust belt. We propose that the MBSM represent the tectonic inversion from shortening to extension in an active fold-and-thrust belt. The relatively short duration and geographically restricted extent of exhumation supports a slab break-off event rather than an ongoing and migrating process (roll-back) as the exhumational driving force.

Several important considerations remain regarding the implications of tectonic inversion in the Dinaric setting. First, exhumation occurred along the paleogeographic margin of a thick carbonate platform. Second, tomographic images across the Dinarides reveal an intact shallowly dipping slab, the entirety of which may have been subducted following break-off of a pre-existing slab. The questions arise: What is the role of upper crustal lithology on microplate interaction, and what drives subduction in the absence of a negatively buoyant slab?