Pulsating subsidence during a rift episode:: stratigraphic and tectonic consequences (Aptian-Albian, northern Spain)

Backstripping analysis has been carried out on nine outcrop sections of the Basque-Cantabrian Aptian and Albian of northern Spain which consist of shallow-water marine deposits up to 7000 m thick. It reveals four main pulses of common accelerated subsidence: I (Early Aptian), II (Early Albian), III (lower Late Albian), and IV (uppermost Late Albian) separated by tectonic quiescent intervals. Tectonic subsidence rates of 70 m Myr(-1) for the Aptian and 101 m Myr(-1) for the Albian were recorded (Sopuerta section). The calculated stretching factor referred to a pre-rift continental crust of 35 km is (beta(s)=1.32) and represents approximately 8.5 km of crustal thinning. Points of change towards lesser subsidence rate in the curves are correlated with major unconformities in the sections, in the Basque-Cantabrian Basin and in Boreal Europe, suggesting geodynamic changes related with the opening of Bay of Biscay. Minor variations of spreading direction and intensity are proposed to explain the subsidence pulses.