Revealing the subsurface structure of deep-water volcanoes using 3D seismic reflection data

Abstract:

Most of submarine volcanoes emplaced in deep-water (>1.0 km). Whilst the external morphology of deep-water volcanoes can be mapped using bathymetric surveys, their internal structure, basal surface, and true volume remain enigmatic. We investigate Late Miocene-to-Quaternary extrusions that are imaged in 3D seismic reflection data from the northern South China Sea. We quantify the external morphology of the volcanoes and lava flows, and examine their internal structure. The volcanoes were emplaced in water depths >1.5 km, are relatively small (<3.0 km diameter, <0.56 km tall), and have steep slopes (up to 42º). Most of the volcanoes have erosional, 'crater-like' bases, infilled with sub-horizontal seismic reflections. These crater-like bases are overlain by downward-converging, conical seismic reflections delineating the classical volcano morphology. We suggest the crater-like bases formed by excavation of cold, wet, and poorly consolidated near-seabed sediment during expulsion of hydrothermal fluid, and not by explosive magmatic eruptions or gravitational subsidence. Erupted igneous material likely infilled the precursor craters with the observed sub-horizontal layers probably comprising hyaloclastites. After this initial phase of volcanism, the buildup of volcanic material produced layers that are now represented by the flank-parallel or downward-converging, conical seismic reflections. Lava flows associated with the volcano edifices can be >9 km long and contain sinuous lava tubes, which have rugged basal contacts defined by erosional ramps. We calculate that long run-out lava flows account for 50–97% of the total erupted volume, with a surprisingly minor component being preserved in the main volcanic edifice. Our results show accurate estimates of erupted volumes therefore require knowledge of volcano and lava basal surface morphology. Overall, we suggest high hydrostatic pressures of >15 MPa, which are typical of water depths >1.5 km, inhibited degassing and fragmentation of ascending magma and thus erupted lava. This lack of degassing and fragmentation permitted effusive eruptions during the latter stages of volcanism. Our studies of volcano structure and growth demonstrate the power of using 3D seismic reflection data to understand deep-water volcanism.