Abstract - Analysis of faults in offshore sedimentary basins is often concerned with geometric aspects and with the general timing of fault movement. Quantitative analysis of the growth of individual faults has rarely been undertaken and little is known of either the nature or the rates of fault displacement changes through time. Here we estimate fault displacement rates on faults from the Northern North Sea and show how the relationship between fault displacement rates and sedimentation rates strongly influences syn-rift sedimentary architectures.
Several Northern North Sea faults with displacements greater than ca 350m provide estimates of fault displacement rates ranging from ca 0.03mm/year to 0.2mm/year (over several million years), consistent with nominal earthquake repeat times between ca 5,000 and 30,000 years. Syn-rift sedimentation rates of generally less than 0.05mm/year account for the starved nature of hangingwall basins associated with most large faults. Only at the earliest stages of rifting in the Northern North Sea, and more extensively in some restricted areas (e.g. Inner Moray Firth) are sedimentation rates greater than fault displacement rates, providing hangingwall and footwall sequences which blanketed active fault scarps. Insufficient data are available to demonstrate whether, on time scales greater than several seismic cycles, fault growth is continuous or is punctuated.
Data for Middle-Upper Jurassic rifting of the Northern North Sea indicate a basinal stretching rate of ca 1mm/year and a strain rate of 1.5x10-16s-1. These values, and those of fault slip rates and repeat times, are similar to those of the Basin and Range Province. By contrast, post-Miocene extension within the Aegean is characterised by high stretching rates and strain rates (ca 60mm/year and 4x10-15s-1 respectively). The Aegean strain rates are ca 1.5 orders of magnitude greater than those of the Northern North Sea and of the Basin and Range, and are accommodated by proportionally shorter earthquake repeat intervals of ca 100-1000 years. That a high regional strain rate is accommodated by an approximately proportional increase in slip rates on individual faults, as opposed to an increase in fault density, is consistent with the qualitative observation that many extensional basins have broadly similar structural profiles. Our observations suggest that the range of fault densities of large faults (>300m) between basins is relatively small and not more than 1 order of magnitude.
Abstract of talk given to:
Tectonics of the British Isles: onshore and offshore geology TSG Meeting, Durham, March 1995