MODELING PALEOSTRESS MAGNITUDE AND AGE IN EXTENSIONAL BASINS - A CASE-STUDY FROM THE MESOZOIC BRISTOL CHANNEL BASIN, UK

Analysis of normal faults and extensional veins developed in the Upper Triassic and Lower Jurassic succession exposed along the Bristol Chan nel coast indicate a protracted period of rift-related deformation, fr om Late Triassic to Early Cretaceous. During this period it is demonst rated that a consistently oriented stress system operated with sigma(3 ) oriented NE-SW, but with stress ratios (phi = (sigma(2) - sigma(3))/ (sigma(1) - sigma(3))) varying from 0.9-0.1. Two approaches are descri bed to estimate rifting stress magnitudes. The first involves data fro m synsedimentary faults, and yields, for Late Triassic rifting, sigma( 1) = 0.714 MPa, sigma(2) = 0.169 MPa and sigma(3) = 0.033 MPa. The sec ond is developed for faulting in the Lower Jurassic section, where no direct evidence of age is available. The method calculates stress magn itudes, with the rifting stress ratio of 0.9, for varying increments o f overburden load. Each increment represents the possible magnitude of tectonic stresses at the time indicated by the amount of burial. By i ncrementally adding the estimated remaining Lower Jurassic-Lower Creta ceous overburden load to the stress magnitudes of each case, a plot of reducing stress ratio for each age of faulting is determined, and tha t which best reproduces the range of stress ratios calculated represen ts the modelled estimate of stress magnitude and timing. The results s uggest an end Early Jurassic onset of faulting, with principal stress magnitudes of sigma(1) = 12.98 MPa, sigma(2) = 12.56 MPa, sigma(3) = 8 .80 MPa. Several simplifying assumptions used in the analysis are disc ussed. Copyright (C) 1996 Elsevier Science Ltd