THERMAL EVOLUTION, RATE OF EXHUMATION, AND TECTONIC SIGNIFICANCE OF METAMORPHIC ROCKS FROM THE FLOOR OF THE ALBORAN EXTENSIONAL BASIN, WESTERN MEDITERRANEAN

High-grade metamorphic rocks drilled at Ocean Drilling Program Site 97 6 in the Alboran Sea show a PT path characterized by decompression fro m about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550 degrees+/-50 degrees C to 6 75 degrees+/-25 degrees C. The Ar/Ar dating on muscovite and apatite f ission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5-18 Ma, which coinci des with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late Cretace ous? to Paleogene contractional orogen, and extension coincided with t hrusting in the peripheral parts of the Betic-Rif are, which surrounds the basin on three sides. Thermal modeling of the PT path was carried out with the aim of constraining geodynamic models for the formation of the basin. Variables considered in the modeling included (1) the th ickness and thermal gradient of the postorogenic lithosphere; (2) the radiogenic heat production in the thickened crust; (3) the time gap (p ause) between the end of contractional tectonics and the start of exte nsion; (4) removal of lithospheric mantle below 125, 75, or 62.5 km; a nd (5) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve hig h radiogenic heat production combined with a significant postcontracti onal pause (to produce high temperatures in rocks initially at 40 km d epth), removal of lithosphere below 62.5 km (to produce further heatin g during decompression), extension by a factor of 3 in 6 m.y. (to dela y the attainment of the maximum temperature until the rocks reached sh allow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximu m of about 9 m.y. for exhumation from 40 km depth to the surface. Lith ospheric stretching in response to plate-boundary forces such as trenc h rollback, without removal of lithosphere, cannot explain the late on set of heating and the high temperatures reached by these rocks.