Controls of brittle extension on the large scale at mid-ocean ridges

Abstract

Seafloor morphology at Mid-Ocean Ridges is thought to be controlled by the interaction of tectonic and magmatic processes. In turn, these are thought to control the distribution of volcanic and brittle tectonic features on the seafloor over multiple scales; from the placement of individual faults measuring hundreds of metres, to shallow and deep bathymetric swales over tens of kilometres. It is unclear, however, whether there is a systematic relationship between these smaller and larger features, with varying melt supply, spreading rate, and spreading obliquity. Here, to investigate this relationship systematically, I have developed an innovative method to robustly and objectively identify large scale variations in brittle tectonic extension, expressed on the seafloor as faults. This method allows comparison with a number of proxies for melt supply along-strike and orthogonally-away from spreading ridge segments. I objectively identify fault planes by cross-correlating dip, dip direction, and backscatter that might individually suggest their presence. The fault heave is measured to estimate extension over length and temporal scales comparable to those melt-related processes involved in forming second order ridge segments. Regions of high and low melt supply are identified and defined quantitatively from positive and negative anomalies in bathymetry. These indications are cross-validated by comparison to gravity anomalies and other bathymetric features diagnostic of high or low melt supply.
Following analysis of several mid-ocean ridge examples, spanning a range of spreading rates and ridge/spreading orientation, no consistent correlation between large scale extension and melt supply at scales of 1 km, 10 km, or 20 – 50 km was found. This null-result has three potential, contributing, explanations: First, environmental processes erode and mask faults, reducing their apparent heave which then deviates from the theoretical model. Second, fault extension occurs over a broader area surrounding the spreading axis while melt intrusion and delivery into the upper crust remains more focused on a narrow zone within the spreading axis. This would result in a spatial and temporal disconnect between the location and occurrence of brittle faulting and the location and timing of delivery of magma. Third, brittle extension and fault distribution may be locally controlled by dykes and lava domes, whose distribution may not scale-up to the larger magnitude of second order spreading centre features such as elevated and inflated segment centres and depressed and subsided segment ends.

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ORCID for Gavin Haughton: orcid.org/0000-0002-9970-4045

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