Oblique continental rifting and long transform fault formation based on 3D thermomechanical numerical modeling

Abstract

The length of oceanic transform faults is highly variable in nature and can range from zero-offset fracture zones to longer than thousand kilometers. The initiation and evolution of relatively short (tens of km) transform faults was successfully reproduced by previous thermomechanical models. However, the origin of long (hundreds of km) structures remains more enigmatic and requires further numerical modeling effort. In this study, we use a 3D thermomechanical model of oblique continental rifting and breakup followed by oceanic spreading to study the formation of long transform faults. Our starting model geometry consists of an initial oblique weak zone (mobile belt) located in the mantle lithosphere. The obliquity of the weak zone, the extension rate, the mantle potential temperature, the rheology of the lower crust, and lateral boundary conditions are the main parameters examined by our experiments. By varying these model parameters, mature oceanic spreading with long transform faults was obtained. We track the initiation and evolution of long transform faults from the initiation of oblique rifting to the point where steady-state seafloor spreading is observed. Our model results suggest that (1) long transform faults with a length greater than 200km are established gradually within a several million years long transition from oblique rifting to mature seafloor spreading, (2) the initiation and development of opposite-dipping large-offset normal faults along the strike of a continental margin is a precursor process for the formation of these transform faults, (3) large weak zone obliquity and slow boundary extension are favorable conditions for the establishment of long transform faults, (4) extreme weak lower crust promotes the development of multiple ridge–transform fault systems, (5) other parameters (the mantle potential temperature, the rate of strain softening and lateral boundary conditions) only play a secondary role on long transform faults development, and (6) heat flux across mature long transform faults is relatively low.