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The SO312 BRASS cruise, which will take place off the coast of New Zealand in May 2025, will investigate the underwater volcano BROTHERS.

Caldera collapse is an important geological process that shapes many of Earth’s volcanoes. The structures that accommodate the collapse play a primary role in hydrothermal fluid circulation and focus, and related mineralization. In addition, the collapse process and the associated volcanism can pose a significant hazard to society. To understand caldera collapse and related hydrothermal processes, three-dimensional (3D) investigations of the structural architecture and hydrothermal fluid circulation are required. However, no 3D seismic data currently exist for any caldera in the world. Here, we propose to image Brothers volcano— one of the best-studied examples of a submarine caldera volcano in the world—to address three fundamental hypotheses: (1) that the caldera formation was caused by a sudden collapse after an explosive eruption, rather than by incremental subsidence associated with effusive eruptions; (2) that preexisting zones of ‘volcanic weakness’ played a major role in the collapse; and (3) that large-scale, seawater-dominated hydrothermal activity at Brothers is intrinsically linked to caldera collapse structures. We propose to conduct a major, high-resolution 3D seismic experiment that images the entire volcanic edifice of Brothers volcano at a 3 x 3 x 6 m resolution. In addition, we will collect an ocean bottom seismometer dataset that will deliver a tomographic model of the upper 5 km beneath the volcano at 100-200 m resolution. We will augment these seismic data with the simultaneous acquisition of gravity and magnetic data. Ground-truthing will be achieved using video-guided sampling and integration of geological results from recent IODP drilling and earlier ROV and AUV surveys. The study will provide unprecedented insight into the geological processes involved in submarine caldera collapse and will aid hazard assessment as well as mineral resource assessment of similar volcanoes.

To obtain information on the regional geological framework in the beginning of SO312, we will undertake a regional 2D seismic and bathymetry survey. The results from the 2D seismic data will guide us in electing the optimal layout for the subsequent OBS and P-Cable surveys. These OBS instruments will provide important velocity information for migration and depth conversion of the 3D seismic data. After completion of the OBS deployment, we will collect a 3D seismic cube with a volume of approx. 7 x 17 km to image the complex three-dimensional structures inside the volcano at high resolution. After retrieval of the P-Cable system we will deploy a large G-gun array (3000 cubic inch) to shoot 30-km-long radial lines extending out from the volcano. This part of the seismic survey will provide the main dataset for conducting a three-dimensional seismic tomographic inversion for the deep structure of the volcano at about 100-200 m (decreasing with depth) resolution and an accuracy of 0.1 km/s in seismic velocity. This data set will allow us to determine if there still is a hot magma chamber within the top 5 km of the seafloor underneath Brothers volcano. During all seismic operations (P-Cable and OBS profiling) we will collect both gravity and magnetic data.

According to the NZ requirements, visual and acoustic Marine Mammal Observers (PAM and MMO) will be on board during the seismic experiments.