The effect of brittle-ductile weakening on the formation of tectonic
patterns at mid-ocean ridges
Abstract
One of the most prominent plate tectonic processes is seafloor
spreading. But its formation processes are poorly understood. In this
study, we thoroughly address how the brittle-ductile weakening process
affects the formation and development of tectonic patterns at spreading
centers using 3D magmatic-thermomechanical numerical models. Grain size
evolution and brittle/plastic strain weakening are fully coupled into
the model. A spectrum of tectonic patterns, from asymmetric long-lived
detachment faults in rolling-hinge mode, short-lived detachment faults
in flip-flop mode, to symmetric conjugate faults in flip-flop mode are
documented in our models. Systematic numerical results indicate that
fault strength reduction and axial brittle layer thickness are two
pivotal factors in controlling the faulting patterns and spreading
modes. Strain weakening induced by localized hydrothermal alteration can
lead to the variation of the fault strength reduction. Strong strain
weakening with large fault strength reduction results in very asymmetric
detachment faults developing in rolling-hinge mode, while weak strain
weakening leads to small fault strength reduction, forming conjugate
faults. Moreover, the thermal structure beneath the ridge is influenced
by spreading rates, hydrothermal circulation, and mantle potential
temperature, which in turn controls the thickness of the axial brittle
layer and results in variation in tectonic patterns. Further, in order
to test a damage mechanism with a physical basis, we investigate grain
size reduction at the root of detachment faults. We found that its
effect in the formation of detachment faults appears to play a
subordinate role compared to brittle/plastic strain weakening of faults.