by

Platt, J. P.

Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.

Instituto Andaluz de Ciencias de la Tierra & Departamento de Geodinámica, C.S.I.C.-Universidad de Granada, Facultad de Ciencias, Campus Fuentenueva s/n, 18071-Granada, Spain.

Martin J. Whitehouse, Laboratory ofr Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, Sweden.

Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.

Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.

High-grade metamorphic rocks drilled at Ocean Drilling Program Site 976 in the Alboran Sea show a PT path characterized by decompression from about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550±50°C to 675±25°C. Ar/Ar dating on muscovite and apatite fission-track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5-18 Ma, which coincides with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late-Cretaceous? to Paleogene contractional orogen, and extension coincided with thrusting in the peripheral parts of the Betic-Rif arc, 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 (i) the thickness and thermal gradient of the post-orogenic lithosphere; (ii) the radiogenic heat production in the thickened crust; (iii) the time gap (pause) between the end of contractional tectonics and the start of extension; (iv) removal of lithospheric mantle below 125, 75, or 62.5 km; (v) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve high radiogenic heat production combined with a significant post-contractional pause (to produce high temperatures in rocks initially at 40 km depth); removal of lithosphere 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 the maximum temperature until the rocks reached shallow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximum of about 9 m.y. for exhumation from 40 km depth to the surface.
    Models that involve lithospheric stretching in response to plate-boundary forces such as trench rollback, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature. Only a combination of significant post-collisional radiogenic heating, then wholesale removal of lithospheric mantle below the orogenic crust, followed by rapid stretching, can explain the observed PT path. These results appear to support some form of lithospheric delamination as the primary cause for the formation of the Alboran Sea basin.
 

Tectonics, 1998, vol. 15, no. 5, 675-689.

Here I reproduced the principal figures of the paper with a simplified figure caption (for more details see the paper). All the modelled P-T-t paths have been calculated by J.P. Platt.
 
 

Figure 2. Hypotheses for the formation of Mediterranean extensional basins. (a) The negative bouyancy of a subducting slab of oceanic lithosphere induces extension in the back-arc region accompanied by rollback of the subduction hinge. (b) Delamination of lithospheric mantle creates a pattern of induced mantle convection and asymmetrically disposed extension and compression of the overlying crust . (c) Convective removal of a thickened root of lithospheric mantle creates of region of high gravitation potential energy, which then extends; outward motion is taken up by compression in the surrounding regions.
 

Figure 5. Results of thermal modeling showing the modeled PT path for three starting depths: 30, 40, and 55 km. The starting points show the conditions at the end of a contractional episode that is assumed to have double the thickness of crust and lithosphere (to 60 km and 250 km thick respectively). The model which better reproduced the observed P-T path involves a post-contractional pause of 30 m.y., a removal of lithosphere (CRL) below 62.5 km, extension by a factor of 3 (beta) in 6 m.y., and a high radiometric heat production (6.0e-10 W/kg) (assumed constant throughout the crust).

Figure 6. Temperature-time paths (b and c) using the conditions of Run 33 (60 m.y. pause, CRL to 62.5 km, beta = 3 in 6 m.y., heat production = 4.4 x 10-10 W/kg) followed by denudation at 2.2 km/m.y (red line) or 4 km/m.y. (green line).
 

Main Page

Up Page