Mid‐Jurassic through mid‐Cretaceous extension in the Central Graben of the North Sea ‐ part 2: estimates from faulting observed on a seismic reflection line

Abstract

Abstract We present an interpretation of the structure and faulting of an industry multichannel line across the Central North Sea Graben. We observe substantial faulting between the mid‐Jurassic and mid‐Cretaceous and on the base Zechstein (salt) reflector. To estimate the extension from these faults we consider movement along both planar and curved faults. We demonstrate that summing the heave (the horizontal displacement) overestimates the time measure of elongation for planar, ‘domino‐type’, faulting. However, for high‐angle normal faults and up to 70% extension (β= 1.7) the heave only overestimates the extension by 13%. In the absence of other information, summing the heave provides a useful estimate of extension in the case of domino‐type faulting. For curved ‘listric’ faults the heave is only a true measure of the elongation if the antithetic faulting which removes the voids is vertical. Antithetic movement along inclined shear planes implies significantly more extension. We used the two models; of faulting to introduce progressively greater amounts of internal deformation in the crustal rocks and sediments to attempt to reconcile the estimate of extension necessary to give the observed subsidence and that given by analysing the faults visible on the seismic line. Estimates of extension obtained by allowing antithetic faulting along inclined shear planes are consistent with the range of estimates necessary to account for the post‐mid‐Jurassic subsidence. The estimates for the prior mid‐Jurassic faulting are still substantially less than those necessary to explain the subsidence. However, this is not of major concern as there are many reasons as to why analysis of the faulting should underestimate the pre mid‐Jurassic extension. Our interpretation of the seismic line implies curved faults bottoming in the lithologically weak Zechstein salt. These faults are decoupled from the region below and, hence, do not reflect the geometry of the faulting in the basement.