The FT evidence for rapid cooling to 60°C at around 18-19 Ma is consistent with the Ar/Ar data reported by Kelley and Platt (Chap. 22, this volume) for cooling of the basement rocks through the 300°-450°C range at virtually the same time: the two sets of data suggest a very rapid exhumation and cooling event in the early Miocene. The dates are also very similar to those from similar high-grade metamorphic basement in the Betic Cordillera: Zeck et al. (1992), for example, report Rb/Sr, K/Ar, and Ar/Ar dates in the range 18-20 Ma; Monié et al. (1994) report Ar/Ar ages in the range 19-20 Ma; and Andriessen and Zeck (1996) report FT ages in the range 13-20 Ma. The metamorphic basement onshore is overlain in places by Aquitanian-Burdigalian sediment that is approximately coeval with these cooling ages (Bourgois et al., 1972, 1978; Aguador et al., 1990; Durand-Delga et al., 1993). The oldest sediment in adjacent parts of the West Alboran Basin may also be as old as late Aquitanian (Comas et al., 1992). It therefore appears very likely that the rapid exhumation of the basement coincided with the formation of the Alboran Sea basin, and that a primary mechanism of exhumation was tectonic extension and associated normal faulting.
Rapid exhumation results in significant advection of heat toward the earth's surface, steepening the geotherm, which complicates the calculation of exhumation rates from cooling rates. Bearing this in mind, cooling of Site 976 basement through the PAZ probably occurred within the top 1-2 km of the crust, and represents perhaps 1 km of exhumation. The site is located on a horst block with a vertical relief >2000 m (see Watts et al., 1993, for a seismic profile across the block), thus it is likely that this phase of exhumation was directly related to the creation of the present basement morphology in the area. It also means that calculation of a precise exhumation rate from the FT data would be further complicated by lateral heat flow in both horizontal dimensions, and for this reason we have not attempted it.
The absence of sediment older than Serravallian at Site 976 suggests that the horst block may have been emergent until that time, which leaves open the possibility that at least part of the final exhumation of the basement to the pre-Serravallian surface was accomplished by erosion.