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 53050°C to 67525°C. 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.
Reprinted by permission of the American Geophysical Union.
The metamorphic basement beneath the Alboran Sea, a Neogene extensional basin drilled during Ocean Drilling Program (ODP) Leg 161 (Site 976), consists of high-grade pelitic schist overlying migmatitic pelitic gneiss. Inferred assemblages in the high-grade schist evolved from garnet + staurolite + biotite + muscovite + plagioclase (Assemblage 1) to biotite + sillimanite + K-feldspar + plagioclase ± garnet (Assemblage 2) to andalusite + biotite + K-feldspar + plagioclase (Assemblage 3). In the gneiss, which shows abundant migmatitic textures, the present assemblage is biotite + andalusite + sillimanite + muscovite + cordierite + K-feldspar + plagioclase ± garnet. Phase relations and thermobarometric calculations suggest that the high-grade schist experienced decompression accompanied by heating from ~500°C at 10.5 kbar through 600 ± 30°C at 6-7 kbar (Assemblage 1), and thence to 650-700°C at 3-4 kbar (Assemblage 2), followed by cooling through 500-600°C at 2 kbar or less (Assemblage 3). Peak temperature and melting occurred under low-pressure conditions. The widespread disequilibrium and overstepping of metamorphic reactions suggests that decompression was rapid. The P-T evolution is consistent with metamorphism in a late orogenic extensional basin, and suggests the existence of an external source of heat, probably indicating the complete removal of lithospheric mantle beneath the extending region.
Reprinted by permission of the author and Oxford University Press (Journal of Petrology).
Low-temperature experiments document changes in magnetic mineralogy in Pleistocene hemipelagic sediments at ODP Holes 976D and 977A. The Verwey transition was observed only for samples below ~1.3 mbsf, which suggests depth-limited appearance of stoichiometric magnetite in the sediment column. Primary magnetite is interpreted to be covered with a maghemite skin as a result of in situ low-temperature oxidation on the sea floor. The oxidized maghemite skin gradually dissolves with depth, and the Verwey transition is observed below ~1.3 mbsf. This depth matches the iron redox boundary inferred on the basis of a sediment color change from tan to green.
Reprinted by permission of the Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).