The influence of extension rate and crustal rheology on the evolution of passive margins from rifting to break-up

Abstract

Extension rate and crustal strength are two important factors that influence the shape of initial rift basins and final passive margin architecture. The extension rate will also affect crustal strength as the passive margin evolves, therefore adding more complexity to the system. We used thermomechanical numerical experiments to systematically explore the relationship of extension rate and initial crustal strength on final passive margin structure. Varying the initial amount of ductile crust from 33% to 70% and extension rates from 2 to 50mmyr−1 results in passive margins of different widths, shapes, and evolution histories. We find four distinct margin types: Type I–narrow symmetric margins, Type II–wide symmetric margins, Type III–asymmetric margins, and Type IV–rift jumping. Experiments with strong initial crust (33% ductile crust) have crust-mantle coupling of strain and produce narrow, symmetric margins with high rift flank uplift (Type I margins). Ultraslow extension experiments (2mmyr−1) undergo thermal cooling and produce wide, symmetric margins with hyperextended crust (Type II margins). Experiments with intermediate to fast extension rates (10–50mmyr−1) and thick layers of ductile crust will produce initial rift basins that are symmetric, but later rifting stages yield asymmetric conjugate margins with hyperextended domains 100–250km wide (Type III margins). In special cases with intermediate extension rates and thick ductile crustal layers, we observe a jump in rift location over 50km and abandoned rifts with isolated crustal blocks (Type IV margins). These results highlight how ductile crustal layers and extension rates influence passive margin formation during different stages of rifting.