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Priority:
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Primary
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Position:
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31°47.3874’N, 139°01.5786’E
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Water depth (m):
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2114
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Target drilling depth (mbsf):
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2350
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Approved maximum penetration (mbsf):
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2100
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Survey coverage:
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• Cruise KR07-09, Line IBr5, CDP 34436 (Figs. F8, F9, F11, F13, F14)
• Cruise KR08-04, Line IBM3-NE5, CDP 9383 (Figs. F9, F10, F13)
• Cruise KR08-04, Line IBM3-NW5, CDP 8409 (Figs. F9, F12, AF5, AF6, AF7)
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Objective(s):
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The primary objective at proposed Site IBM-3C is to test three pairs of alternative hypotheses about crustal genesis and mantle evolution:
1. Geochemically asymmetric crust, which is most like “average continent” in the rear arc, is either (a) a fundamental trait of crust in oceanic arcs that is produced in the steady state throughout arc history from Paleogene inception or (b) a secondary trait that develops only after backarc spreading.
2. Intracrustal differentiation amplifies this asymmetry (a) continuously as a steady state process or (b) mostly during nonsteady state events such as arc rifting.
3. After or near the cessation of the Shikoku back-arc basin opening, rear-arc magmatism either (a) started from the western end of the rear-arc seamount chains and migrated east or (b) started at the same time along the length of the rear-arc seamount chains but ended from west to east.
Testing these hypotheses requires obtaining a temporal record of across-arc variation in magma composition from Eocene to Neogene time. This information is in hand for the volcanic front but missing for the rear arc, which overlies the majority of “continent-type” crust. Specifically, our objectives are to establish the temporal history of across-arc variations during five time periods that stand out in the rear-arc evolution: 3 Ma to present, 9–3 Ma, 17–9 Ma, 25–17 Ma, and >25 Ma. We will determine whether there were across-arc variations even at the initial stage of arc development.
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Coring program:
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APC/XCB and RCB coring; casing as necessary
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Wireline logging program:
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Standard downhole logging in one or two phases, depending on hole conditions and need for casing
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Anticipated lithology:
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Volcaniclastic sediment and rock, perhaps with lava, perhaps crystalline igneous rock near bottom of hole
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