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doi:10.14379/iodp.pr.352.2015 Additional resultsSedimentology and biostratigraphyEocene to recent deep-sea sediment was recovered from above the Izu-Bonin fore-arc basalts and boninites (the upper part of a putative SSZ ophiolite complex). The sedimentary record provides an excellent reference for modern and ancient sedimentation in an intermediate-latitude, intraoceanic fore-arc setting. Three of the drill sites (Sites U1439–U1441) are located in fault-controlled sediment ponds up to several hundred meters thick, whereas one site (U1442) was positioned on thin sediment overlying a fault-controlled basement high (Figure F25). The overall sediment sequence reflects the interplay of 5 main types of control:
The basal sediment, typically overlying a manganese crust (Eocene–Oligocene), is pelagic carbonate mixed with detritus that was eroded from underlying, variably altered igneous crust. Above the basement, pelagic carbonate predominates in the Oligocene to earliest Miocene and also in the late Pliocene to Pleistocene. In contrast, relatively noncalcareous, radiolarian-rich silty clays dominated during the early Miocene. In addition, volcaniclastic sediment is relatively abundant during the early Oligocene to Eocene and also in the middle Miocene to lower Pliocene. Three phases of highly explosive volcanism (latest Pliocene to Pleistocene, late Miocene to earliest Pliocene, and Oligocene) are represented by 132 graded air fall tephra layers, which are likely to be correlative between the 4 drill sites (Figure F26). Felsic ash layers (tephra) appear to be relatively abundant at Sites U1439 and U1440 (larger sediment ponds) compared to more mafic ash layers at Sites U1441 and U1442 (smaller basins). The switch to mixed clastic and siliceous ooze sedimentation (with well-preserved radiolarian and siliceous sponge spicules) during the Miocene reflects a relative rise in the CCD, which could have been either tectonically or paleoceanographically controlled. The carbonate-poor interval is more extensive in the deeper water sites (U1440 and U1441) compared to the shallower sites (U1439 and U1442). At Sites U1440 and U1441, sediment was affected, to different extents, by local gravity redeposition and current reworking, which was probably tectonically triggered. Pumice-rich volcaniclastic sediment was reworked from the fault-controlled margins of the sediment ponds, especially at Sites U1440 and U1441. There is evidence of tectonic tilting, both prior to and during sediment accumulation (e.g., Site U1439). Postdepositional tilting of Oligocene sediment and associated fracturing was locally exploited by fluid flow and manganese oxide precipitation (Site U1442). Diagenetic effects include carbonate recrystallization, zeolite growth, and upward mobilization of manganese from the igneous crust into the sediment column. Finally, the probable difference in age between the oldest sediment recovered (Eocene; ~35 Ma), and the igneous basement based on comparisons with comparable well-dated fore-arc basalts and boninites in the region, indicates that a 7–15 My hiatus may exist between cessation of volcanism and covering of the seafloor by pelagic carbonates in different areas. The likely explanation of such a hiatus is that the outer part of the fore arc that encompasses the four drill sites existed as a submerged bathymetric high that remained free of sediment accumulation until the tectonically controlled formation of the sediment ponds that still remain today. StructuresPostmagmatic extension of the outer IBM fore arc resulted in the formation of asymmetric sedimentary basins such as, for example, the half-grabens at Sites U1439 and U1442. Along their eastern margins, these basins are bounded by westward-dipping normal faults. Sedimentation was mainly syntectonic. The lowermost sequence of the sedimentary units was tilted eastward by ~20°. These tilted bedding planes were subsequently covered by subhorizontally deposited sedimentary beds. Based on biostratigraphic constraints, the minimum age of the oldest sediments is ~35 Ma; the timing of the sedimentary unconformities lies between ~27 and 32 Ma. At Sites U1440 and U1441, postmagmatic deformation resulted mainly in strike-slip faults possibly bounding the sedimentary basins. The sedimentary units within these basins were not significantly affected by postsedimentary tectonic tilting. Based on biostratigraphic ages, the minimum age of the basement-cover contact lies between ~29.5 and 32 Ma. Overall, the postmagmatic tectonic structures observed during Expedition 352 reveal a multiphase tectonic evolution of the outer IBM fore arc. At Sites U1439 and U1442, shear with dominant reverse to oblique reverse displacement was localized along distinct subhorizontal cataclastic shear zones as well as steeply dipping slickensides and shear fractures. These structures, forming within a contractional tectonic regime, were either reactivated as or cross-cut by normal faults as well as strike-slip faults. Extension was also accommodated by steeply dipping to subvertical mineralized veins and extensional fractures. Faults observed at Sites U1440 and U1441 show mainly a strike slip sense of motion. Physical properties and loggingBasement rock P-wave velocities of 2.5–4.0 km/s are observed in the sonic logs in Holes U1439C and U1440B. These velocities are ~1.5 km/s slower than observed logging velocities of normal ocean crust sites such as Holes 504B and 1256D (Becker et al., 1989; Swift et al., 2008). In contrast, porosities of the IBM and ocean crust sites are similar. Postcruise research will investigate these results, and the implications for IBM fore-arc crust and SSZ ophiolites as analogs for oceanic crust. |
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