Flipping detachments: The kinematics of ultraslow spreading ridges.

Abstract Although the seafloor spreading Hess initially proposed was a virtually amagmatic process, little attention has been paid to that possibility since. We construct a kinematic framework for virtually amagmatic and magma-poor Hess-style seafloor spreading, and successfully apply it to processes operating at the Southwest Indian Ridge (SWIR). The kinematic model is based on symmetric divergence about a rift axis at depth, with a repeating cycle in which a fault propagates up from the rift axis, develops into a detachment fault accommodating the plate divergence, migrates beyond the rift axis and is abandoned when a new fault propagates up through the footwall from the rift axis. We rigorously explore the controls on the depth, dip and timing of fault initiation and abandonment and use the kinematic framework to reconstruct the evolution of smooth mantle-dominated seafloor at the SWIR through symmetric divergence about a fixed rift axis. The model predicts the development of successive detachments of flipping polarity, as observed, each rooting along a narrow and fixed rift axis at 20 km depth, the base of the seismically defined brittle lithosphere. The detachments root at 80° (consistent with constraints on seismicity-defined detachment orientation at oceanic core complexes), and exhume mantle. Based on the continuity of basement ridges, of magnetic anomalies and of the seismic activity at the base of the lithosphere, it appears that these exhumation detachments transition laterally into rafting detachments , transporting fault-bounded volcanic slices up and away from the spreading axis to form the rougher volcanic seafloor found between mantle-dominated domains. The kinematic framework shows that increased magmatic divergence requires the detachments to root at shallower depths, consistent with the seismicity-defined shallowing of the base of the brittle lithosphere moving along the ridge axis towards the volcanic centres. Only in the immediate vicinity of volcanic centres, where the seismicity dies out, may magmatism dominate. We conclude that detachment tectonics dominate the process of ultraslow seafloor spreading as well as much of slow seafloor spreading, totalling about one third of the global ridge system, and present the first 3D tectonic model for ultraslow seafloor spreading. Graphical abstract Highlights • Rigorous kinematic model for both amagmatic and magma-poor seafloor spreading. • Explains a wide range of apparently contradictory observations along the Southwest Indian Ridge. • Explains the lateral transition from mantle unroofing to rough, volcanic seafloor. • Explains features of oceanic core complexes and hence of slow spreading ridges. • Provides first 3D tectonic model of ultraslow spreading. [ABSTRACT FROM AUTHOR]