Tectonics of slow spreading mid‐ocean ridges and consequences of a variable depth to the brittle/ductile transition

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Abstract
Geophysical evidence, especially microearthquakes that extend into the upper mantle beneath the inner floor of the Mid‐Atlantic and Gorda Ridges, indicates that the axial magma chamber episodically freezes beneath slow spreading ridges. Freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences. Most importantly, large near‐axis faults will intersect as the brittle/ductile transition deepens, causing some of the faults to become locked. The topographic effect of fault locking will be a variation in the width of the inner floor from narrow (< 2 km) when a magma chamber is present to wide (>10 km) when a magma chamber is absent. The deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle. The nature of these faults at depth is unknown, but the following hypotheses are presented: (1) the faults may be planar and pass downward into aseismic ductile shear zones, and (2) the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow. Such low‐angle normal faults are common in continental extensional domains. Large‐scale tilting (50°–70°) is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks (similar to toppling dominoes). Calculations of temperatures at the brittle/ductile transition are made assuming olivine and diabase rheologies for the mantle and crust, respectively. The temperature is approximately 800°C when the transition is at 8 km depth, and approximately 600° when the transition is at 2 km depth. The strength of the lithosphere varies dramatically with the rise and fall of the brittle/ductile transition as magma chambers form and subsequently freeze. Strong lateral temperature gradients will be present when a magma chamber is formed, resulting in localization of faulting within thin weak crust above the chamber. When the magma chamber freezes, lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area; this results in active faulting occurring over a width of as much as 60 km. Some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges. The entire crustal section of the Josephine ophiolite was tilted by approximately 50° prior to deposition of overlying pelagic sediments. This tilting probably took place by faulting, suggesting that oceanic faults extended into the upper mantle. Possible oceanic faults have been identified in the Josephine ophiolite, but their oceanic origin cannot be established beyond doubt.