Quantitative estimates of the pressure-temperature (PT) history of the cores have been made by several authors using geobarometric and geothermometric methods (Table T1) (Cornen et al., 1999; Gardien et al., in press; R. Rubenach, N. Froitzheim, P. Wallace, M. Fanning, and R. Wyzsoczanski, unpubl. data). When combined with the results of isotope geochronology (Table T2) (Féraud et al., 1996; Manatschal et al., in press; Rubenach and Wysoczanski, 1998), a relatively precise understanding begins to emerge of the evolution of the margin.
The in situ mafic rocks encountered at Sites 900, 1067, and 1068 were emplaced in late Hercynian time in a pressure range of 0.6-0.8 MPa and at temperatures between 600° and 740°C. Assuming that pressures were purely lithostatic, the pressure estimates imply depths of 21-29 km (given a continental crust density of 2.8 Mg/m3) and (assuming continental crust thermal gradients of 25°-30°C/km) the temperature estimates imply depths of 20-30 km. Therefore, these rocks were emplaced either near the base of a slightly thinned crust (as suggested by their apparent association with the Moho) (Whitmarsh et al., 2000) or within the crust at lower/middle crustal levels.
The three-stage metamorphic history of the Site 1067 amphibole-plagioclase samples investigated by Gardien et al. (in press) (Table T1) reveals that they experienced a systematic elevation toward the surface, but the metamorphic stages have not been dated.
Dates are available for other hornblendes and plagioclases from Sites 900 and 1067 (Table T2), which indicate passage through the hornblende closure temperature (~500°C) around 161 Ma and through the plagioclase closure temperature (200°-250°C) around 137 Ma (Féraud et al., 1996; Manatschal et al., in press). The final date of exhumation at the seafloor is unknown. Apatite fission-track ages obtained by Gardien et al. (in press) are hard to interpret, but if exhumation continued at the same rate as between the hornblende and plagioclase closure temperatures, then exhumation would be expected around 117 Ma, which is some 7-17 m.y. earlier than predicted by Gardien et al. (in press). Active tectonic exhumation of Hobby High, tentatively during Valanginian-Barremian time (121-137 Ma), is probably indicated by the oldest fossiliferous mass-wasting deposits encountered on its west flank at Site 1068. It can be concluded, therefore, that exhumation of the Hobby High basement rocks continued over several tens of millions of years as they were brought to the surface from depths of at least 20 km.
Seafloor spreading in the southern Iberia Abyssal Plain is estimated to have been under way at a rate of ~10 mm/yr in the vicinity of the peridotite ridge by the time of anomaly M3, ~126.5 Ma (Whitmarsh and Miles, 1995; Whitmarsh et al., 1996). The hornblende gabbro pegmatite at Site 1070, situated 20 km west of the peridotite ridge, passed through the hornblende closure temperature at 119 ± 0.7 Ma and the plagioclase closure temperature at 110.3 ± 1.1 Ma (Table T2). Even allowing for ~2 m.y. (20 km) of seafloor spreading between the peridotite ridge and Site 1070, the pegmatite was clearly exhumed at least 14 m.y. after the magnetic isochron age of the subjacent oceanic crust. Active tectonic exhumation of Site 1070 is probably indicated by a late Aptian (112.2-119.0 Ma) (W. Wise, pers. comm., 2000) mass-wasting deposit that overlies the igneous basement. It is difficult at present to reconcile the >14-m.y. difference between the predicted ~124.5-Ma age of the oceanic crust and the later exhumation of basement rocks (<110.3 Ma) at Site 1070. It might appear that magmatism and tectonism, perhaps when associated with the onset of seafloor spreading, are distributed over a broad region many tens of kilometers wide.
It is also evident that, given a constant geotherm during exhumation, exhumation of the pegmatite at Site 1070 proceeded either almost three times faster than the exhumation of continental crustal rocks at Hobby High or that more likely the geotherm between 250° and 500°C was three times steeper under Site 1070 or that a combination of these factors was present. The second explanation is consistent with the ascent of an asthenospheric diapir, which lay close to the surface once seafloor spreading had begun.