Detachment fault growth modulated by brittle softening and ductile flow in amagmatic (ultra)slow-spread oceanic lithosphere

Large-offset detachment faults are common at slow-spreading mid-ocean ridges (MORs). They are typically thought to form in ridge portions that receive a moderate supply of magma. However, they are also found along certain sections of ultraslow-spreading MORs that are largely amagmatic, and feature unusually cold and thick (>15 km) brittle lithosphere. Here we combine geological observations and numerical simulations to assess how these unusual conditions enable and modulate the growth of detachments.

We simulate amagmatic seafloor spreading using 2-D thermo-mechanical models with self-consistent thermal evolution. The brittle lithosphere is modeled as a Mohr-Coulomb elasto-plastic material whose friction decreases with accumulated plastic strain. Ductile deformation is parameterized through experimentally-derived olivine flow laws.

We first investigate how the strength contrast between the fault zone and surrounding lithosphere affects tectonic styles. Geological observations suggest fault zones have lower effective friction coefficients due to serpentinization and fluid circulation.  Evidence for grain size reduction in  ultramafic rocks also suggests additional ductile weakening. In our simulations, varying the strength contrast between faults and lithosphere leads to 3 regimes:  (1) a stable detachment that migrates toward its hanging wall; (2) the sequential growth of horsts bound by two active antithetic faults; and (3) “flip-flopping” detachments that cross-cut each other, comparable to those documented in the natural case. A greater contrast in friction and/or cohesion favors the stable detachment mode, which is consistent with previous studies.

We next focus on the specific effect of a strong, viscous lower lithosphere on brittle deformation in the upper lithosphere. We do so by comparing simulations that use dry olivine flow laws for rocks hotter than ~700ºC with models in which the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. We find that a strong lower lithosphere favors more distributed faulting and shifts the transition to the stable detachment regime to greater strength contrasts.

We also investigate the impact of pervasive fluid circulation in the shallow axial lithosphere, which manifests as active hydrothermal sites. We parameterize its mechanical and thermal effect, i.e., reducing the effective normal stress through a hydrostatic fluid pressure and efficiently cooling young lithosphere. While the latter strongly modulates the depth to the brittle-ductile transition, we find that the former has small effect on tectonic styles, akin to a slight weakening of unfaulted lithosphere.

Finally,  extensive mass wasting is also documented at mid-ocean ridge detachments, but its potential effect on tectonics remains poorly known. We implement diffusive erosion of the model's free surface, which promotes a transition from the stable to flip-flopping detachment regime. This is possibly due to a modulation of topographic stresses.

Overall, because of the delocalizing effect of a strong ductile lithosphere, the growth of detachments at cold, amagmatic MOR sections requires some degree of rheological weakening, both in the brittle and ductile domains. We find, however, that even moderate frictional weakening (e.g., a friction coefficient of 0.4) which can be attributed to serpentinization of the fault zone, can be sufficient to promote large-offset faulting, a process that may be aided by mass redistribution at the seafloor.