Mid‐Jurassic through mid‐Cretaceous extension in the Central Graben of the North Sea ‐ part 1: estimates from subsidence

Abstract

Abstract The post early Carboniferous subsidence history of the Central North Sea basin can be separated into three major periods: Permian, Triassic and post Mid‐Jurassic. Prior efforts to account for this subsidence within an extensional framework have concentrated on the post Mid‐Jurassic. These efforts have assumed that the effects of the previous periods of extension necessary to create the Permian and Triassic subsidence are negligible. We consider the 80‐km value for the Mid‐Jurassic‐mid‐Cretaceous extension from these efforts a reasonable upper estimate of the likely amount of extension. This value has received considerable criticism as it is almost four times as great as that determined by summing the horizontal displacement (heave) on faults observed on industry seismic lines in the area. We treat the two earlier phases of extension as one phase and develop a method to estimate the maximum value of this extension. We use this value, with estimates of the total extension from the early Carboniferous to Present, to determine a likely minimum value for the mid‐Mid‐Jurassic through mid‐Cretaceous extension. After justifying the use of Airy isostasy for the loading response of the lithosphere we show that the observed unloaded basement subsidence history is compatible with the parameters we derive for the pre and post Mid‐Jurassic extension. Our minimum estimate of 38 km is still significantly higher than that: made by summing the heave on the faults active throughout the Upper Jurassic and lower Cretaceous.