THE MARQUESAS ARCHIPELAGIC APRON - SEISMIC STRATIGRAPHY AND IMPLICATIONS FOR VOLCANO GROWTH, MASS-WASTING, AND CRUSTAL UNDERPLATING

Multichannel seismic lines, sonobuoy and gravity data across the Marqu esas Islands are used to study volcano growth, island mass wasting, an d crustal underplating at island chains with overfilled moats. The Mar quesas bathymetry reflects the changing thickness of the sedimentary i nfill 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 le nses of chaotic diffractors and, (2) the central moat, where the deep moat basins are overfilled by an acoustically opaque unit of discontin uous 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 th e depth to volcanic basement and gravity observations. The seismic vel ocities and seismic stratigraphy, as well as the general structure of the islands and surrounding seafloor, indicate the apron is mostly deb ris from island mass wasting. Reflectors of the outermost moat general ly onlap the flexural arch in the lower section and offlap and overfil l it in the upper section. In the central moat, reflectors change shap e from concave up in the lower section to convex in the upper section. Three-dimensional diffusion models of sedimentation, which incorporat e a time-dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste, sugges t 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 se dimentary basins, in contrast to Hawaii, where islands are separated b y a shallow ridge. The lower sediment diffusivity at the Marquesas may reflect a larger proportion of ''blocky'', massive material in the ce ntral Marquesas moat or alternatively a change in the dominant process of sediment transport. While there is similar sediment supply for a g iven along-moat distance at both the Marquesas and Hawaii, the underfi lled moat at Hawaii is apparently a consequence of greater moat volume s due to the larger size of the Hawaiian volcanoes, and possibly varia tions in underplating, that load the plate. The difference in sediment /edifice ratios is likely related to the larger eruption rates at Hawa ii and different styles of volcano construction between Hawaii and the Marquesas.