Syn-eruptive soft-sediment deformation structures in a deep submarine caldera: Havre, 2012

The 2012 rhyolitic eruption of Havre submarine caldera in the Kermadec arc generated an assemblage of new, soft-sediment deformation structures on the caldera floor. These structures are the first documented examples of syn-eruptive, soft-sediment deformation structures within a submarine caldera. They are exquisitely portrayed on a high-resolution (~1 m) bathymetric map made in 2015. Their timing has been resolved by means of stratigraphic relationships with an overlying giant pumice deposit that correlates with the pumice raft first observed on 18th July 2012 UTC. Weeks to months earlier in 2012, the largest lava was erupted from a vent on the southwestern caldera wall, flowed down to the caldera floor and advanced almost 1 km farther. A surficial layer of the caldera-floor sediment surrounding the leading edge of the lava failed, most likely in response to load and heat transfer from the lava. Failure produced a slump sheet comprising folds with arcuate crests at the toe, an undeformed domain in the middle and a basin-and-ridge domain at the head, adjacent to the lava. Subsequently, (1) the lava ruptured on two main faults that propagated across the caldera floor, and (2) strike slip on the faults moved the lava that was in between forward, simultaneously offsetting segments of the slump sheet. These two events were probably triggered by the earthquake swarm recorded on the 17th July 2012 UTC, a day before the pumice raft appeared. The leading edge of the lava then sank slowly into the underlying sediment. Sinking was accommodated by sideways expulsion of sediment from beneath the lava, modifying the topography around the edge of the lava. The caldera-floor sediment was also disturbed when part of the southwestern caldera wall collapsed, generating a small debris avalanche which is represented by a cluster of hummocks on the caldera floor. Collapse was a response to shallow intrusion of a rhyolite dyke into the southwestern caldera wall.