Time and Space Evolution of Detachment Faulting and Magmatism at the Ultraslow-Spreading Southwest Indian Ridge, 63.5° to 66°E.

Abstract:

It is now generally accepted that about 25% of the seafloor formed at slow spreading mid-ocean ridges (MOR), and along a good proportion of the ocean-continent transition zones, comprises mantle-derived ultramafic rocks and gabbros that are emplaced in the footwall of large offset normal faults (also called detachments). These detachment faults are therefore a fundamental component of plate tectonics. Based on proxies such as axial depth, axial valley relief, and gravity anomalies, our study area qualifies as a worldwide MOR end-member in terms of low melt supply. It is also an end-member in terms of the abundance of mantle-derived rocks exposed at the seafloor. Our study focuses on a corridor of nearly-amagmatic spreading that extend along-axis for up to 70 km, and across-axis on both diverging plates over distances that correspond to 10 myrs-worth of accretion. Asymmetric detachment faults in this corridor accommodate almost 100% of plate divergence, yet the overal pattern of magnetic anomalies is symmetrical, suggesting that detachments have repeatedly flipped polarity.

In this presentation, we use geological observations, geophysical data and plate reconstructions in this magmatically-poor ultraslow (14 mm/yr) MOR end-member region to address key questions about the time and space evolution of mid-ocean ridge detachments. We analyze the lifetime and polarity of successive detachments, we examine their lateral (along-axis) evolution as the nearly amagmatic corridor transitions into adjacent, more magmatic ridge domains. We also use seafloor reflectivity and dredging to test possible links between sparse volcanic events in the nearly amagmatic corridor and the initiation of new detachments. Detachment surfaces exposed in the older parts of our study area (> 6 myrs) are commonly corrugated, yielding significantly more gabbroic samples, mixed with ultramafics in dredge hauls, than younger non-corrugated, smooth detachment surfaces. We analyze the transition between these two types of exposed fault surfaces and outline a conceptual model that could be tested and generalized to more magmatically active slow and ultraslow ridges.