Salisbury, Matthew H. et al. (2006): Leg 195 synthesis; Site 1201; a geological and geophysical section in the West Philippine Basin from the 660-km discontinuity to the mudline

ODP 195
ODP 195 1201


Salisbury, Matthew H.
Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, NS, Canada

Shinohara, Masanao
Geological Survey of Canada, Canada

Suetsugu, Daisuke
Ocean Drilling Program, United States

Arisaka, Michio
Japan Agency for Marine-Earth Science and Technology, Japan

Diekmann, Bernhard
University of Leicester, United Kingdom

Januszczak, Nicole
Universita di Napoli "Federico II", Italy

Savov, Ivan P.
University of Southampton, United Kingdom

Leg 195 synthesis; Site 1201; a geological and geophysical section in the West Philippine Basin from the 660-km discontinuity to the mudline
In: Shinohara, Masanao (editor), Salisbury, Matthew H. (editor), Richter, Carl (editor), Araki, Eiichiro, Barr, Samantha R., D'Antonio, Massimo, Dean, Simon M., Diekmann, Bernhard, Edwards, K. Michelle, Fryer, Patricia B., Gaillot, Philippe J., Hammon, William S., III, Hart, David, Januszczak, Nicole, Komor, Stephen C., Kristensen, Mette B., Lockwood, John P., Mottl, Michael J., Moyer, Craig L., Nakahigashi, Kazuo, Savov, Ivan P., Su Xin, Wei, Kuo-Yen, Yamada, Tomoaki, Proceedings of the Ocean Drilling Program; scientific results; seafloor observatories and the Kuroshio Current; covering Leg 195 of the cruises of the drilling vessel JOIDES Resolution; Apra Harbor, Guam, to Keelung, Taiwan; Sites 1200-1202; 2 March-2 May 2001
Texas A&M University, Ocean Drilling Program, College Station, TX, United States
The installation of a borehole seismic observatory 500 m below the seafloor at Site 1201 in the West Philippine Basin has created a unique listening post for long- and short-term seismic studies of the Earth's asthenosphere and lithosphere, as well as the crust and upper mantle near one of the most active subduction complexes in the world. Broadband waveforms recorded by ocean bottom seismometers across the Philippine Sea and the borehole seismometer at Site 1201 show that the 410- and 660-km discontinuities are located at depths of 377 and 669 km, respectively, below the site, whereas controlled-source experiments over the site show the crust to be abnormally thin (3.8 km). Layer 3, which is only 2.3 km thick, is strongly anisotropic, with VP = 6.7 km/s in the east-west direction, but only 6.3 km/s in the north-south (paleospreading) direction, probably due to faults and fissures. Coring at Site 1201 shows the upper crust to be transitional in composition between arc tholeiites and mid-ocean-ridge basalt (MORB), which is consistent with the formation of the West Philippine Basin by backarc spreading between two opposed subduction zones beginning in the Eocene. After the site drifted northwest away from the spreading axis and volcanism ceased, 0.5 km of sediments was deposited in three stages: (1) quiescent marine sedimentation in deep water into the late Eocene; (2) pelagic sedimentation mixed with, and finally overwhelmed by, volcaniclastic turbidites from the Palau-Kyushu Ridge from the late Eocene through the early Oligocene; and (3) waning turbidite deposition, followed by barren, deep-sea pelagic sedimentation below the carbonate compensation depth (CCD) from the early Oligocene to the early Pliocene, when sedimentation ceased altogether. Extensive low-temperature (100 degrees -150 degrees C) zeolite facies hydrothermal alteration occurred in many of the basalts under oxidizing open-circulation conditions, which became more restricted as sedimentation proceeded. In the overlying sediments, the deeper levels of the turbidites have undergone profound low-temperature (<100 degrees C) diagenesis, causing the vitric shards in the turbidites to be completely replaced by smectite, mixed-layer clays, and zeolites and the interstitial seawater to be replaced by pore water that is extremely enriched in Ca and depleted in K and Mg, consistent with alteration of basaltic glass to zeolites and clays. High geochemical gradients in the pore waters above 300 meters below seafloor (mbsf) suggest that alteration is continuing at present in the upper turbidites but that below 300 mbsf the pore water has reached equilibrium with the products of alteration.
Coverage:Geographic coordinates:
West:135.0500East: 135.0600

Solid-earth geophysics; Applied geophysics; 670-km discontinuity; boreholes; crust; geophysical methods; geophysical profiles; geophysical surveys; Leg 195; lithosphere; mantle; North Pacific; Northwest Pacific; Ocean Drilling Program; ocean floors; oceanic crust; oceanic lithosphere; ODP Site 1201; Pacific Ocean; Philippine Sea; sedimentary cover; seismic methods; seismic profiles; surveys; waveforms; West Pacific; West Philippine Basin;