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

Abstract:

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) bathymetry 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 July 18th, 2012. 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. Compression by the lava caused a surficial layer of the caldera-floor sediment to buckle into folds with arcuate crests in a domain surrounding the leading edge of the lava. This deformation was probably slow, occurring at the same rate as forward propagation of the lava (probably m per day). Three subsequent events occurred suddenly: (1) detachment and sideways movement of the layer of caldera-floor sediment, (2) rupture of the lava on two main faults and forward advance of the middle sector, and (3) propagation of the faults across the caldera floor, offsetting segments of the folded domain. These 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 and the underlying sediment was forced out from beneath the lava, producing a domain of highly irregular 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.