Modelling palaeostress magnitude and age in extensional basins: a case study from the Mesozoic Bristol Channel Basin, U.K.

Analysis of normal faults and extensional veins developed in the Upper Triassic and Lower Jurassic succession exposed along the Bristol Channel coast indicate a protracted period of rift-related deformation, from Late Triassic to Early Cretaceous. During this period it is demonstrated that a consistently oriented stress system operated with σ3 oriented NE-SW, but with stress ratios (φ = (σ2 − σ)(σ1 − σ3)) varying from 0.9-0.1. proaches are described to estimate rifting stress magnitudes. The first involves data from synsedimentary faults, and yields, for Late Triassic rifting, σ1 = 0.714 MPa, σ2 = 0.169 MPa and σ3 = 0.033 MPa. The second is developed for faulting in the Lower Jurassic section, where no direct evidence of age is available. The method calculates stress magnitudes, with the rifting stress ratio of 0.9, for varying increments of overburden load. Each increment represents the possible magnitude of tectonic stresses at the time indicated by the amount of burial. By incrementally adding the estimated remaining Lower Jurassic-Lower Cretaceous overburden load to the stress magnitudes of each case, a plot of reducing stress ratio for each age of faulting is determined, and that which best reproduces the range of stress ratios calculated represents the modelled estimate of stress magnitude and timing. The results suggest an end Early Jurassic onset of faulting, with principal stress magnitudes of σ1 = 12.98 MPa, σ2 = 12.56 MPa, σ3 = 8.80 MPa. Several simplifying assumptions used in the analysis are discussed.