FIELD AND THERMOCHRONOLOGIC EVIDENCE OF THE EXTENSIONAL EXHUMATION OF THE MID-BOSNIAN SCHIST MOUNTAINS

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. Field observations along the fault bounding the east side of the MBSM, however, support a normal shear sense suggesting an extensional tectonic exhumation. New low temperature thermochronolgic dates along with published Ar/Ar and K/Ar dates reveal an Eocene-Oligocene episode of rapid cooling (~25 ^oC/My) in the MBSM. We propose that extensional exhumation of the MBSM represents the tectonic inversion from shortening to extension in an active fold-and-thrust belt, yet the relatively short duration and geographically restricted extent of exhumation supports a tectonic event rather than an ongoing and migrating process (roll-back) as the extensional driving force. Tomographic imagery across the Dinarides depicts a short segment of downgoing slab (~140 km in length), which is insufficient to account for the total amount of lithosphere consumed during mountain building, and therefore implies previous slab detachment. Slab detachment has been cited as a likely driving force for extension along convergent margins in the central Mediterranean. In the Dinarides, however, no geologic evidence of the crustal response to the apparent removal of the subducted slab has been recognized. We suggest that the extensional exhumation of the MBSM is the crustal response and provides a timing estimate of ~43 Ma of the detachment of the subducted Tethyan lithospheric slab beneath the Dinarides.