The structure and origin of hydrothermal vent complexes in volcanic basins

Intrusion of magma into sedimentary basins leads to devolatilization of the host rock in contact metamorphic aureoles. Hydrothermal vent complexes are formed by fracturing the overburden sediments if sufficient overpressure is developed in the aureoles, releasing hot fluids and gases into the hydrosphere and atmosphere. We have mapped the structure and distribution of hydrothermal vent complexes using extensive 3D seismic reflection surveys (c. 40,000 km²) in the Møre and Vøring basins offshore mid-Norway by a combination of seismic horizon and attribute mapping. The seismic horizons have been tied to exploration wells to constrain the timing of their formation. A shallowly buried vent complex, the Modgunn Vent, was subsequently imaged by high-resolution P-Cable 3D seismic data collected using the R/V Helmer Hansen. The upper part of this vent complex was recently drilled by five holes during IODP Expedition 396. In total, more than a thousand hydrothermal vent complexes have been identified in the two basins. A typical vent complex has a diameter of between a few hundred meters and five kilometers and extends from the tip of a sill intrusion to the paleosurface. The upper part of the vent complexes are commonly eye-shaped, where the lower surface represents the base of a crater and the upper dome-shaped surface represents the top of the crater infill. Overlying reflections are sub-parallel to the upper vent surface, locally associated with discontinuous high-amplitude reflections and minor faulting. The chimney-shaped lower part of the vent complexes are characterized by disrupted reflections, sometimes including bulbous-shaped transparent bodies with high-amplitude reflections at the top and base. Surrounding reflections are often dipping towards the center of the chimneys. The structure of the vent complexes suggest they were dominantly formed by erupting fluids and sediments during the Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago. The craters were subsequently rapidly infilled by sediments, and later inverted forming domes above the craters. High-amplitude discontinuous reflections above some vent complexes are interpreted as evidence of long-term fluid flow, sometimes lasting until recent times.