RESPONSE OF THE LITHOSPHERE TO IN-PLANE FORCE VARIATIONS

Abstract

The Earth's lithosphere deforms when subjected to vertical and horizontal tectonic forces because it is not perfectly rigid. The deformation has both elastic (flexural) and inelastic (brittle and ductile creep) components. We use the yield strength envelope formulation of a depth-dependent rheology to predict how the lithosphere will deform in response to applied in-plane force. Brittle deformation of the upper lithosphere is influenced by the distribution of existing faults and fractures in the crust and their orientation with respect to the applied tectonic force. The sense of fault reactivation indicates the sign of the applied in-plane force. The induced flexural deformation is governed by the pre-existing lithospheric deflection, the effective elastic thickness of the lithosphere when the force was applied, and the magnitude of the applied force. Using this framework, we compare the predicted response of the lithosphere to in-plane force variations with that observed in the Central Indian Ocean and the Jeanne d'Arc basin, offshore Newfoundland. The brittle deformation in both regions is readily observed in seismic reflection data. The upper oceanic crust in the Central Indian Ocean is broken into blocks bounded by high-angle reverse faults. The dip of these basement faults and their orientation parallel to the marine magnetic lineations suggest that they represent a reactivated crustal fabric imparted during the formation of oceanic crust. The reverse sense of fault reactivation is consistent with the north-south compression determined from earthquake focal mechanisms. In the Jeanne d'Arc basin, episodes of northwest-southeast extension reactivated the basin-bounding faults in a normal sense during late Barremian, early Aptian, and late Aptian time. In the late Albian, the same crustal blocks were reactivated and uplifted in response to north-northeast in-plane compression. Flexural deformation due to applied in-plane force is observed only in the Central Indian Ocean case, where the oceanic crust and most of the overlying Bengal Fan sediments were deformed into broad, east-west trending undulations with wavelengths of ≈ 100–300 km and amplitudes up to ≈ 2 km. In the Jeanne d'Arc basin, the flexural deformation which should accompany the fault reactivation is difficult to recognize from the preserved stratigraphy. Consequently, brittle deformation within extensional basins, which appears to be the most recognizable response of the lithosphere to in-plane force variations, obscures the effects of the flexural deformation. However, it is this flexural deformation that has been postulated as a tectonic alternative to glacial-eustasy in explaining the 3rd-order sea level cycles. Because it is difficult to recognize the effects of the flexural deformation in the preserved stratigraphy, we surmise that it is not a viable alternative to glacial-eustasy in generating either regional or global unconformities. Instead, we propose that the flexural and brittle deformation due to applied in-plane force may locally enhance or diminish the effects of other processes responsible for unconformity generation.