Expedition to the Mariana forearc
March 23 - May 4, 2003
The results of this project will provide a detailed study of the variability of decollement and supra-subduction zone-derived mud flow materials, and slab-derived, pore-fluid geochemistry at known sites of active mud volcanism in a nonaccretionary convergent plate margin. Surveys performed with the DSL 120 side-scan sonar system will produce detailed maps of the summits and a selected flank region of these seamounts that can be used to set seafloor observations and sampling in a tightly controlled geologic context. Use of the upgraded Jason II ROV will permit a level of controlled sampling with respect to stratigraphic exposures and structural features that is not possible from surface ships. The coring and bottom-sampling program will provide samples that will permit the investigators to determine to study fluid egress with respect to structural features on the mud volcanoes and at the base of the edifices, to perform detailed mineralogical and metamorphic studies and to investigate biological activity at the target sites. The data collected will provide critical information to be used in picking deep-ocean drilling sites for a highly ranked drilling proposal and setting those sites in a well-defined geological context for future drilling efforts at these mud volcanoes.
The muds that are erupting from the mud volcanoes are mainly serpentine, but recent sampling efforts show that they contain blueschist minerals and rock, clearly indicating a deep-sourced origin, probably the subducting Pacific ocean plate. The investigators will be able to determine the P/T conditions of the decollement region and forearc wedge where the materials originate by studying the mineral assemblages present in the samples that they will collect and by comparison with samples already obtained in the region. Slab-derived fluids are also rising with the muds. Fluid venting is a critical component of the overall chemical mass balance of convergent plate margins. Chemical mass balance in subduction zones is in turn a critical element of the total chemical mass balance process in plate tectonics. Most of the existing data from subduction margins, where plates are consumed, is from studies of the shallow portions of accretionary convergent margins. In these subduction systems large sediment wedges obscure the over-riding plate and tend to alter by reaction the slab- derived fluids rising through them. A better place to look for the least disturbed signature from slab-generated fluids, is in nonaccretionary plate margins where little, if any, sediment cover obscures the over-riding plate, where the country rock in nearly monolithologic, and where pervasive faulting permits channeling of fluids and the establishment of long-lived conduits for the formation of mud volcanoes. Furthermore, the studies of the shallow part of the subduction zone that have been where most of the accretionary convergent margins studies have been confined tell us nothing about the metamorphic reactions that dominate the intermediate to deep parts of the subduction zone. The studies of the mud and rock materials that comprise the mud flows on the Mariana forearc seamounts derive from depths of 15 to 29 or more kilometers and will contribute information on processes that occur in the deeper portions of the subduction zone.
A comparison of the data from the four mud volcanoes that we will sample will provide a first quantitative look at the variability of slab-related fluids with distance from the trench axis and over several hundred km along strike of the Mariana system. Studies of the mudflow materials (particularly the blueschist materials) of the Mariana forearc have the potential to provide a tracer of P/T conditions at depth. The P/T conditions control the physical properties of the decollement region, thus affect the potential for seismic activity in the subduction zone. The study of isotopic variations in the samples will help to define the provenance (thus the origin) of mantle materials in the overriding plate and will permit the estimation of the degree of slab contribution to that mantle through time and in space. The studies proposed here will obtain data that will permit the investigators to constrain the parameters responsible for the nature of seismicity and other tectonic processes active in the Mariana convergence zone. They will also permit estimating the slab contribution to the supra-subduction zone both along the strike of the forearc and with depth to the slab over ranges of 15 to 29 km.