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Palinspastic Reconstruction and Forward Modelling |
Department of Petroleum Geoscience, University of Brunei Darussalam, Gadong 3186, Brunei
Fault heave methods for estimating extension (including balanced cross-sections and the flexural cantilever model) are important because they are the only methods that are universally applicable to estimating extension from the syn-rift stratigraphic section. One of the main criticisms of the methods when based on reflection seismic data is the failure to account for crustal extension along sesimically invisible faults (displacements up to a few tens of metres). The fractal distribution of several fault populations has been cited as evidence for the existance of such minor faults. It has been suggested by several workers that the discrepancies between extension estimates for the North Sea rifts calculated from the syn-rift section and by backstripping (using the McKenzie model) can be explained by a substantial amount of crustal extension having taken place on seismically invisible faults. This paper examines whether it is reasonable to extrapolate such conclusions to other rifts.
Estimates for the contribution of minor faults to crustal extension based on fractal distributions of faults need to be treated with caution for several reasons: (1) in some rifts large boundary faults (several kilometres displacement) may account for 80–95% of the total extension amount. In such rifts minor faults may not contribute significantly to crustal extension (less than 5%). (2) The different distribution of fault size and spacing between different zones within rifts suggests that a single fractal distribution along a regional rift traverse is commonly unrealistic. (3) Fractal fault populations measured in syn-rift sedimentary sequences may be measuring faults involved in gravity tectonics and compaction, not just those faults that contribute to crustal extension.
Regions with very complex pre-rift structural histories (e.g. North Sea) appear to have significant amounts of extension on minor faults. This contrasts with rifts developed on crusts with a relatively stable history (e.g. East African rift), where much of the extension is accommodated on a few large faults. The differences between these two rifts (despite both rifts having undergone similar amounts of extension) illustrate that pre-existing fabrics are extremely significant for understanding fault populations, fault orientations, partitioning of extension between different sizes of fault, the types of linkage between faults (transfer zones), syn-rift basin subsidence and fault lengths. An appreciation of this variability should lead to more accurate prediction of the contribution of seismically invisible faults to extension on balanced cross-sections.
