Mantle potential temperature controls crustal thickness and spreading pattern at slow-ultraslow mid-ocean ridges

Abstract:

Plate separation at slow-ultraslow spreading mid-ocean ridges is accomplished by both poor magmatic intrusion and slips by detachment faults with variations in mantle sources and magmatic processes. The exposures of mantle and lower crustal rocks at these ridges is very different from the situation at faster spreading ridges characterized by relatively stable thickness of the oceanic crust. Previous studies showed that variation in axial depth and crustal thickness may be correlate with each other and with mantle temperature anomaly. Here, we investigate numerically potential effects of spreading rate and thermal structure on the dynamics and crustal thickness evolution at slow-ultraslow mid-ocean ridge through 3D coupled magmatic-thermomechanical numerical models. Numerical results show that (1) at ultraslow spreading rates, conductive cooling will affect the temperature gradient withing the upwelling mantle and the assumption of adiabatic ascent may no longer be valid; (2) detachment fault at slow-ultraslow ridges can induce asymmetric spreading and form an axial rift; (3) transform faults can greatly affect the thermal structure of ridges, inducing the exhumation of mantle rocks and the formation of thinned oceanic curst; (4) mantle potential temperature affects depth and composition of melts and amount of mantle rocks exhumation through detachment faults. Cold thermal anomaly can lead to abundant mantle rocks exhumation into the shallow depth, forming thin crust. In addition, we demonstrate that transform faults are intrinsically tectono-magmatic structures showing complex pattern of shear zones and igneous rocks.