Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean

J.P. Platt, J.I. Soto, M.R. Whitehouse, A.J. Hurford, S.P. Kelley
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, England


The Alboran Sea formed by late Oligocene to Miocene extension on the site of an earlier contractional orogen, coevally with thrusting in the external zones of the surrounding Betic-Rif orogenic arc. Metamorphic rocks drilled at ODP Site 976 show decompression from about 1050 MPa to 350 MPa while temperature increased from 53050tex2html_wrap_inline32C to 67525tex2html_wrap_inline32C. Muscovite Ar-Ar and apatite fission-track analysis show that final exhumation and cooling occurred between 20.5 and 18 Ma, coinciding with the start of sedimentation in the basin. Thermal modeling of the PT path was carried out to constrain geodynamic models for the formation of the basin. Results suggest that the observed PT path can only be explained by a combination of high radiogenic heat production combined with a significant post- contractional pause (to produce high T in rocks buried to 40 km depth); removal of lithospheric mantle below 62.5 km (to produce further heating during decompression), extension by a factor of 3 in 6 m.y (to delay the attainment of maximum T until the rocks reached shallow depths), and final exhumation and cooling in 3.3 m.y. (to satisfy radiometric and petrological constraints). Exhumation from 40 km depth to the surface therefore took 9.3 m.y. Lithospheric stretching in response to plate-boundary forces, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks.

AGU Index Terms: 8109 Continental tectonics-extensional; 8120 Dynamics of lithosphere and mantle-general; 3660 Metamorphic petrology; 1035 Geochronology
Keywords/Free Terms: Continental extension, Delamination, Thermal modeling, Metamorphic petrology, Radiometric dating.

Tectonics 98TC02204
Vol. 17 , No. 5 , p. 671

© 1998 AGU