The origin and tectonic history of the Alboran Basin: insights from Leg 161 results

Abstract

The Ocean Drilling Program (ODP) Leg 161 drilled in the Western Mediterranean Sea (May‐June, 1995) to investigate outstanding processes in the origin and structural evolution of the Alboran Basin. Studies at the Site 976 basement high, in the West Alboran Basin, demonstrate that the basin is floored with metamorphic rocks of continental origin (high-grade schist, migmatitic gneiss, marble, and calc-silicate rock, cross-cut by granite dikes) from the Alpujarride Complex of the Alboran Crustal Domain. Sediments encountered at the tops of the basement high are middle Miocene in age (about 11‐12 Ma, late Serravallian). The pressure-temperature (P-T) evolution of the high-grade schist shows heating during decompression, from pressures of up to 10.5 kbar at about 500 °C (corresponding to a burial depth of around 40 km) to final temperature conditions of 650o‐700°C at a pressure of 3‐4 kbar. The decompression was followed by cooling to T < 500o‐600 °C and P < 2‐3 kbar. Ar/Ar dating on coexisting muscovite and biotite shows that this cooling occurred from 426 °C at 20.0 ± 0.2 Ma to 330°C at 19.2 ± 0.7 Ma. Apatite fission-track shows that final cooling of the basement below 60 °C took place at 18.3 ± 1.0 Ma (Burdigalian). Site 976 reveals two periods of subsidence, in the middle Miocene (synrift subsidence, at 11‐10.7 Ma; rate of 3 km/m.y.) and since the latest Pliocene (postrift transtensive? subsidence, at 2.5‐0 Ma earliest rate of 0.5 km /m.y.), and uplift at 5‐2.5 Ma (ear ly Pliocene). Conspicuous lateral variations in rift-related subsidence phases in the basin suggest migration of the locus of exte nsion, and probably also changes in tectonic-transport direction during the Miocene rifting. Miocene crustal thinning in the Alb oran Domain is related to low-angle intracrustal detachments and shallow normal faulting. Since the latest Tortonian, relativel y uplifted and subsided areas coexist, but a generalized, continuous, and slower subsidence developed within the marine basin. Drilling results in the East and South Alboran basins (Sites 977, 978, and 979) define the timing of the later (post-Messinian) tectonic reorganization of the Alboran Basin into the present-day Alboran Sea basin. Ar/Ar dating of reworked but representative volcanic pebbles from tholeiitic, calc-alkaline, and shoshonitic series (Sites 977 and 978) constrains episodes of middle- tolate Miocene volcanism in the eastern Alboran region at 12.1 ± 0.2 Ma, 9.90 ± 0.4 Ma, 9.29 ± 0.02 Ma, and 6.1 ± 0.3 Ma. Incompatible elements in these rocks include both depleted (MORB-source) component and an ocean-island or plume-type component (“arc signature”). Calc-alkaline magmatic affinities and incompatible-element systematics were considered to support a Miocene oceanic subduction zone (from 6 Ma to at least 12 Ma) beneath the eastern Alboran region. However, these magmatic signatures prove to have also originated from variable crustal-contamination of MORB-type melts, produced by shallow melting during asthenosphere-mantle upwelling. Alternatively, modeled P-T paths in basement rocks suggest removal of mantle lithosphere below about 60 km depth (to explain further heating during decompression at shallow depth) and rapid exhumation during lithosphere stretching (4.5 km/m.y.), followed by final cooling with a minimum exhumation rate of 4 km/ m.y. It took ~9 m.y. for the exhumation of the basement rocks from 40 km depth to reach the surface at about 18 Ma, thus suggesting that extension in the Alboran Domain started at about 27 m.y. (late Oligocene), before the marine transgression occurred. We consider that strong evidence from rocks at the Site 976 basement high favors models that use the removal of mantle lithosphere as the driving force for extension generating the Alboran Basin. Further geological and geophysical data from the westernmost Mediterranean region are needed to corroborate our conclusions to define a fully satisfying model for the Neogene evolution of the lithosphere beneath the Alboran Basin.