Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia

Similar resurgent blocks occur in caldera depressions at the islands of Pantelleria and Ischia. The two blocks, which are tilted, show similar deformation patterns and similar distributions of volcanic vents related to resurgence. The remaining parts of the floors of the two calderas are not deformed. Similarities between the two features are so striking that the resurgence mechanism must have been alike. A dynamic model for resurgence is presented and called simple-shearing block resurgence. The triggering mechanism is an increase of the magmatic pressure in the upper part of a shallow magma chamber, which results in a vertical direction of the maximum stress applied on the roof rocks. The block is first defined by high-angle inland-dipping marginal detachments and then deformed through a simple-shearing mechanism. Reverse faults on its mostly uplifted side determine an horizontal shortening of the block which is balanced on the opposite side by normal faults. Oblique movements take place along the other two sides. Due to this distribution of the stress around the block, magmas can reach the surface only through the extensional faults on the less uplifted side. © 1991.