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doi:10.2204/iodp.proc.333.105.2012

Site C0012¹

Expedition 333 Scientists²

Background and objectives

Integrated Ocean Drilling Program (IODP) Expeditions 322 and 333 were designed to document characteristics of incoming sedimentary strata and uppermost igneous basement prior to their arrival at the subduction front (Saito et al., 2010). To accomplish this objective, coring was conducted at two sites on the subducting Philippine Sea plate. IODP Site C0011 is located on the northwest flank of a prominent bathymetric high (the Kashinosaki Knoll; Ike et al., 2008), whereas IODP Site C0012 is located near the crest of the knoll (Fig. F1). Data acquired during Expedition 322 and logging-while-drilling data at Site C0011 acquired during IODP Expedition 319 provide new information on presubduction equivalents of the seismogenic zone (Underwood et al., 2010). After jetting-in to ~70 meters below seafloor (mbsf), rotary core barrel (RCB) coring at Site C0012 penetrated almost 23 m into igneous basement and recovered the sediment/basalt interface intact at 537.81 mbsf. The merger of lithofacies and age-depth models shows how correlative units change from an expanded section at Site C0011 to a condensed section at Site C0012 (Fig. F2). The age of basal sediment (reddish-brown pelagic claystone) at Site C0012 is older than 18.9 Ma. Geochemical analyses of interstitial water on top of the basement high show clear evidence of upward diffusion of sulfate and other dissolved chemical species from the basement (Underwood et al., 2010). The depth of the sulfate reduction zone is also anomalously deep at Site C0012. Chlorinity values increase toward basement because of hydration reactions in the sediment and diffusional exchange with basement fluids. In contrast to Site C0011, where chlorinity decreases with depth, the more saline fluids at Site C0012 appear largely unaffected by focused flow and/or in situ dehydration reactions associated with rapid burial beneath the trench wedge and frontal accretionary prism. Thus, Site C0012 is thought to provide a reliable geochemical reference site, unaffected by subduction processes.

The specific questions addressed by additional drilling at input sites are

• Is fluid circulation in basement and permeable sedimentary layers influencing heat flow and diagenesis at Sites C0011 and C0012?

• How does contrasting interstitial fluid chemistry at Sites C0011 and C0012 relate with in situ diagenesis and fluid flow?

• Can a change of physical properties between 200 and 250 mbsf at Site C0011 be related to lithologic variation or silica diagenesis? Does the same transition occur at Site C0012?

• Was magmatic activity heterogeneous in composition and age on the backarc basin basement high (Kashinosaki Knoll)?

• Is alteration of the upper oceanic basement heterogeneous and how does such alteration influence geochemical and fluid budgets?

The main objectives of returning to Site C0012 were to perform temperature measurements for heat flow determination, to expand the age-depth models into the Pliocene and Quaternary, and to core the basement to at least 100 m below the sediment/basement interface. Knowledge of thermal state, interstitial water geochemistry, hydrologic properties, and basement alteration are needed to characterize the state of the subduction inputs and model their evolution with downdip increases in temperature and pressure.

¹ Expedition 333 Scientists, 2012. Site C0012. In Henry, P., Kanamatsu, T., Moe, K., and the Expedition 333 Scientists, Proc. IODP, 333: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.333.105.2012

² Expedition 333 Scientists’ addresses.

Publication: 18 May 2012
MS 333-105

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