The Marquesas archipelagic apron: Seismic stratigraphy and implications for volcano growth, mass wasting, and crustal underplating

Abstract

Multichannel seismic lines, sonobuoy and gravity data across the Marquesas Islands are used to study volcano growth, island mass wasting, and crustal underplating at island chains with overfilled moats. The Marquesas bathymetry reflects the changing thickness of the sedimentary infill rather than the basement topography. The moat contains two major regions of differing seismic stratigraphy: (1) the moat edges, where a unit of continuous layered reflectors is present containing minor lenses of chaotic diffractors and, (2) the central moat, where the deep moat basins are overfilled by an acoustically opaque unit of discontinuous reflectors of up to 2 km thickness, in places capped by a ponded unit. Plate flexure models require broad underplating of the crust by low‐density (crustal?) material at the Marquesas Islands to explain the depth to volcanic basement and gravity observations. The seismic velocities and seismic stratigraphy, as well as the general structure of the islands and surrounding seafloor, indicate the apron is mostly debris from island mass wasting. Reflectors of the outermost moat generally onlap the flexural arch in the lower section and offlap and overfill it in the upper section. In the central moat, reflectors change shape from concave up in the lower section to convex in the upper section. Three‐dimensional diffusion models of sedimentation, which incorporate a time‐dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste, suggest that three main factors make the moat stratigraphy at the Marquesas different from Hawaii: (1) the Marquesas moat is overfilled, while the Hawaiian moat is underfilled, (2) sediment diffusivities are lower at the Marquesas, and (3) the Marquesas islands are separated by deep sedimentary basins, in contrast to Hawaii, where islands are separated by a shallow ridge. The lower sediment diffusivity at the Marquesas may reflect a larger proportion of “blocky”, massive material in the central Marquesas moat or alternatively a change in the dominant process of sediment transport. While there is similar sediment supply for a given along‐moat distance at both the Marquesas and Hawaii, the underfilled moat at Hawaii is apparently a consequence of greater moat volumes due to the larger size of the Hawaiian volcanoes, and possibly variations in underplating, that load the plate. The difference in sediment/edifice ratios is likely related to the larger eruption rates at Hawaii and different styles of volcano construction between Hawaii and the Marquesas.