The Alboran Sea is an extensional basin of late Tertiary age that formed in close association with the Betic-Rif thrust belt in adjacent regions of southern Spain and Morocco. Leg 161 of the Ocean Drilling Program established that the basement underlying the Neogene sediments of the basin is made up of high-grade metamorphic rocks very similar to those in nearby regions of the Betic Cordillera (Shipboard Scientific Party, 1996). This appears to confirm the hypothesis that the Alboran Sea formed by the extensional collapse of a collisional orogen that previously occupied the region (Platt and Vissers, 1989). The exhumation history of the basement is therefore of crucial importance to understanding the timing and mechanism of basin formation. This paper presents the results of fission-track (FT) analyses of apatite from basement rock that was cored at Site 976. The primary objective of the analyses was to determine the low-temperature thermal history of the basement, as an aid in determining the timing and rate of exhumation.
Site 976 is located at 36°12.3´N, 4°18.8´W, in the Alboran Sea, 60 km south of Málaga in southern Spain (Shipboard Scientific Party, 1996). It lies on a basement high in the West Alboran Basin that was created by normal faulting, probably in early to middle Miocene time (Comas et al., 1992; Watts et al., 1993), but which is now completely buried beneath Miocene-Holocene sediment. The crustal thickness in the area is probably between 15 and 20 km (Working Group, 1978). In Hole 976B, the basement is covered unconformably by 670 m of sediment of Serravallian (nannoplankton zone NN7, middle Miocene) to Pleistocene age (Shipboard Scientific Party, 1996), which places a younger age limit of 11-12 Ma on the exhumation of the metamorphic rocks.
The top 124 m of the basement core from Hole 976B consists of high-grade pelitic schist with interlayers of carbonate and calc-silicate rocks. This passes down into a massive pelitic gneiss, which is locally migmatitic, with irregular veins and segregations of granitic material. The petrology of both rock types suggests that they reached peak temperatures above 650°C, and remained at high temperature to very low pressures, of the order of 4 kbar or less (Platt et al., 1996; Shipboard Scientific Party, 1996; Soto et al., Chap. 19, this volume; Soto et al., in press).