How Hydrothermal Cooling and Magmatic Sill Intrusions Control Flip-Flop Faulting at Ultraslow-Spreading Mid-Ocean Ridges

Abstract

<p>The young oceanic lithosphere created at mid-ocean ridges (MORs) is shaped by a complex interplay of magmatic, tectonic and hydrothermal processes. At decreasing melt budgets and (ultra)slow spreading rates, the tectonic spreading mode changes from normal fault dominated to detachment faulting. We investigate an endmember type of detachment faulting, the so-called “flip-flop” detachment mode, observed exclusively at almost amagmatic sections of ultraslow-spreading MORs (e.g. Southwest Indian Ridge 62°E to 65°E): Without significant melt supply, extension in the hanging wall of active detachments cannot be compensated by magmatic accretion. The fault zone therefore migrates across the ridge axis towards the hanging wall side until it is superseded by a new on-axis fault of opposite polarity. Using a steady-state temperature field, a recent numerical study shows that an axial temperature maximum is essential to trigger flip-flop faults by focusing flexural strain in the footwall of the active fault. However, ridge segments with very low melt budget are more likely to be in a transient thermal state controlled, at least in part, by the faulting dynamics themselves.</p><p> </p><p>In our study, we investigate (1) the processes having first order control on the thermal structure of the lithosphere, (2) their respective feedbacks on its mechanical evolution, and (3) how their interplay can explain observed faulting patterns. We present results of 2-D thermo-mechanical numerical modelling including serpentinization reactions and dynamic grain size evolution. The model features a novel form of parametrized hydrothermal cooling along fault zones as well as the thermal and rheological effects of periodic sill intrusions.</p><p> </p><p>We find that the interplay of both enhanced hydrothermal cooling along the active fault zone and periodic sill intrusions in the footwall are essential for entering the flip-flop detachment mode. Hydrothermal cooling increases the temperature difference between the active detachment and its surroundings. Fault zone cooling thereby pushes the temperature maximum into the footwall, while intrusions near the temperature maximum further weaken the rock and facilitate the opening of new faults with opposite polarity. Our model allows us to put constraints on the magnitude of both processes, and we obtain reasonable melt budgets and hydrothermal heat fluxes only if both are considered. The parameter range, where flip-flop faulting dominates, is bounded by other faulting modes, including symmetric modes which provide alternative explanations for the observed sea floor characteristics.</p>