Abstract

Volcanoes in rift situations often show distinctive “hour-glass” fault patterns, with increased fault throw as the volcano is approached. We present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass. For faults to be reorientated there must be a ductile layer below the volcano. This can be hot crust at mid-ocean ridges, continental rifts, or weak sedimentary strata. Increased volcano size or mass and lower brittle/ductile ratios lead to increased fault curvature, whereas an increased regional extension rate decreases the effect. Volcanoes on one side of a rift may capture it, forming the axis of a new rift, with curved or en echelon zones on either side. By concentrating extension, magma is more easily erupted, so volcanoes may erupt more magma of less-evolved composition. A positive feedback between increased extension and magma eruption rate will lead to rift narrowing, which can favor the formation of oceanic crust.

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