Sierra Negra, Galápagos: A resurgent-block basaltic caldera

Abstract Caldera resurgence is rare at mafic volcanoes. Here we consider the well-exposed resurgence at Sierra Negra caldera (Galápagos) to investigate how resurgence develops at a mafic system. Based on topographic and field analysis, the structure of the resurgence consists of an eastward-tilted block bounded by a fault-propagation fold activated by a steep inward-dipping reverse fault with dip decreasing toward the surface. Extension of the uplifted part of the reverse fault results from gravitational instability and is accommodated by horst-and-graben structures over a zone several hundred meters wide. Extension culminates in the main normal fault responsible for the inward tilt of the lava pile, forming a distinctive ridge. The resurgence results from spatially and temporally distinct unrest episodes, promoted by the shallow accumulation of large volumes of magma. Sierra Negra is the first documented example of piecemeal resurgence, as shown by the recent uplift episodes associated with eruptions in 2005 and 2018. The example of Sierra Negra suggests that the formation of resurgent blocks depends on the initial location of the feeding system, with non-centered feeding systems developing asymmetric (trapdoor) blocks. Finally, Sierra Negra demonstrates that mafic volcanoes without well-developed rift zones may promote resurgence when reaching a mature stage with significant amounts of cumulates, favoring shallow magma accumulation.