Tectonic and gravitational fold and thrust belts: Insights from discrete element simulations and Galicia ocean-continent transition zone: New seismic reflection constraints
Morgan, Julia K.
Doctor of Philosophy
The evolution and formation of tectonically and gravitationally driven fold and thrust belts were investigated with 2D numerical simulations using the discrete element method (DEM). In the tectonic study, the occurrence of triangle zones at the front of thrust belts was investigated, specifically how mechanical stratigraphy affects their formation. Simulations with homogenous stratigraphy deformed predominantly along forethrusts. Adding a weak upper unit caused more forethrusts and popup structures in the upper unit relative to the lower unit. Thicker décollement surfaces and strong upper and lower units enhanced decoupling and formed triangle zones. Results compared favorably with triangle zones in Alberta. Simulations of gravitationally driven thrust belts consisted of an updip extensional zone, syntectonically loaded with sediments, which is connected at depth to a downdip contractional zone. The whole system overlies a mobile shale unit. Our simulations show more diffuse décollements connecting the normal faults in the extensional zone with toe thrusts in the contractional zone, then are interpreted on seismic profiles. We also look at the distributions of stress and strain within out simulations, relating the distributions of σ1 to the vergence of thrust faults. We compare out models to the Niger Delta type locale for shale tectonics. The West Iberia continental margin is a type locale for magma-poor rifting, and has been instrumental in changing the classical view of the ocean-continent transition (OCT) from a discrete boundary juxtaposing continental and oceanic crust, into a more complicated zone of varying width that can include exhumed mantle. This study examines two new seismic lines in the Galicia Bank area extending west of the Peridotite Ridge, showing high resolution images of five new ridges. These ridges could be hyperextended continental crust, exhumed continental mantle, or rough ultra-slow spreading oceanic crust. There are no tilted fault blocks with pre-syn rift stratigraphy that would indicate continental crust. There are also no faults indicating mid-ocean spreading with seismic layer stratigraphy indicating normal oceanic crust. Therefore, it is likely the western ridges are also made of serpentinized mantle, with a wide OCT similar to the Southern Iberia Abyssal Plain.