ALBERT

Description: The preferred model for the extension of brittle crust involves the rotation of planar fault blocks. We show that in general the distortion at the ends of the blocks does not affect the measurement of extension. The horizontal displacement on a normal fault, the heave, is observed with little distortion on a seismic reflection time‐section. It can be used to estimate the amount of extension. We demonstrate that the sum of the heaves is not equal to the actual elongation if the blocks have rotated. However, the error in the extension factor, β, introduced by equating elongation with the sum of the heaves is small. It increases with the amount of rotation from 0 for no rotation to 13% for the maximum observed angle of rotation of 30o. We compare this value with the practical error introduced by uncertainties in seismic velocities when the elongation is measured from a depth‐converted seismic section. This latter error is significantly smaller being approximately 5% for an error in velocity of 20% when the rotation angle is less than 30o. Copyright © 1988, Wiley Blackwell. All rights reserved

Description: 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. Copyright © 1988, Wiley Blackwell. All rights reserved

Description: 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. Copyright © 1988, Wiley Blackwell. All rights reserved