Conical Seamount: Seamarc II, Alvin Submersible, and Seismic-Reflection Studies

Abstract

Conical Seamount, the edifice drilled at Ocean Drilling Program (ODP) Sites 778 through 780, is a large seamount on the outer half of the 200-km-wide Mariana forearc in the western Pacific. It is forming by the protrusion of cold, unconsolidated serpentine mudflows and debris flows and by vertical tectonic activity. Dredging on the flanks of this seamount recovered rocks and serpentine muds that are similar to sedimentary serpentinite deposits found in subaerially exposed convergent margin terranes worldwide. The dredged samples were formed by different mechanisms from those previously proposed for sedimentary serpentinite deposits. The dredged samples from Conical Seamount are primarily serpentinized harzburgite. However, serpentinized dunite, metamorphosed gabbro, and basalts have been retrieved from similar seamounts on the Mariana forearc. SeaMARC II imagery and bathymetry of Conical Seamount revealed sinuous flow forms on the flanks of the seamount. Conical Seamount also has both concentric ridges and radial fractures indicative of tumescence. Alvin submersible studies showed these flows to be composed of unconsolidated serpentine muds, containing clasts of serpentinized ultramafic and metamorphosed mafic rocks and authigenic carbonate and silicate minerals. Near the summit of one of the seamounts, chimney structures less than 150 yr old and composed of carbonate and silicate were sampled using Alvin. During sampling of a silicate chimney, cold fluids seeped from numerous orifices in the chimney. The fluids associated with the chimney are unique in composition among fluids collected in the oceans and point to a deep source, probably the subducted Pacific lithospheric slab. Small limpets and gastropods and bacterial mats were collected from the chimneys.