Site 453: west side of the Mariana trough ( DSDP).

Abstract

Site 453 is the W-most of the three drilling targets in the Mariana Trough (SP4-D), located in a sediment pond approx 20km long and 3-4km wide striking approx 030o. The site is approx 10km E of the E edge of the W Mariana Ridge. A downhole heat flow of 2.4HFU was obtained on the first use of the Uyeda/Kinoshita heat probe. The hole was not logged because the drill pipe was bent after taking the final core. The sediment section consists of 455.5m of muds, silts, and sands composed mainly of volcanic debris. The oldest fossils are early Pliocene from the T. rugosus nannofossil subzone. The latter provides a minimum 'basement' age of over 4.7-5.0Ma which is substantiated by paleomagnetic evidence in the sediments. The sediment age implies a maximum half-spreading rate of 2.7cm/yr in the Marian Trough. Below sediments, 85.5m of coarse, poorly sorted igneous polymict breccia containing mainly large angular gabbro and metagabbro clasts up to 0.5m in size with minor basalts overlie 28.5m of metamorphosed (lower greenschist facies) polymict breccia containing mainly metavolcanic clasts with no large cobbles and rare gabbros. Beneath these are sheared meta-igneous rocks, including a continuously recovered 1.5m interval of highly sheared serpentinite noritic gabbro cataclastite cored before the bit wore out. All of these rocks are geochemically related to arc magma series.-Authors