The Transformation of Sediment Into Rock : Insights From IODP Site U1352, Canterbury Basin, New Zealand

ACKNOWLEDGMENTS We thank the crew of the RV JOIDES Resolution for professional seamanship, excellent drilling, and the scientific support on board. GHB and SCG thank the Australia–New Zealand IODP Consortium (ANZIC), and KMM thanks the Consortium for Ocean Leadership U.S. Science Support Program for partly funding this work. Thanks also to funding agencies of the respective authors, and Mark Lawrence (GNS Science) and Cam Nelson (University of Waikato) for their thoughtful comments on an earlier draft. Karsten Kroeger (GNS Science) helped by providing compaction data for New Zealand basins, and Michelle Kominz (Western Michigan University) provided data on which Figure 8 was developed. Further improvements were the result of thoughtful detailed reviews by Gemma Barrie, Bill Heins, Stan Paxton, Associate Editor Joe Macquaker, and Editor Leslie Melim.Peer reviewedPostprin

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

International Ocean Discovery Program (IODP) Expedition 351 drilled a rear-arc sedimentary succession ~50 km west of the Kyushu-Palau Ridge, an arc remnant formed by rifting during formation of the Shikoku Basin and the Izu-Bonin-Mariana arc. The ~1-km-thick Eocene to Oligocene deep-marine volcaniclastic succession recovered at Site U1438 provides a unique opportunity to study a nearly complete record of intra-oceanic arc development, from a rear-arc perspective on crust created during subduction initiation rather than supra-subduction seafloor spreading. Detailed facies analysis and definition of depositional units allow for broader stratigraphic analysis and definition of lobe elements. Patterns in gravity-flow deposit types and subunits appear to define a series of stacked lobe systems that accumulated in a rear-arc basin. The lobe subdivisions, in many cases, are a combination of a turbidite-dominated subunit and an overlying debris-flow subunit. Debris flow–rich lobe-channel sequences are grouped into four, 1.6–2 m.y. episodes, each roughly the age range of an arc volcano. Three of the episodes contain overlapping lobe facies that may have resulted from minor channel switching or input from a different source. The progressive up-section coarsening of episodes and the increasing channel-facies thicknesses within each episode suggest progressively prograding facies from a maturing magmatic arc. Submarine geomorphology of the modern Mariana arc and West Mariana Ridge provide present-day examples that can be used to interpret the morphology and evolution of the channel (or channels) that fed sediment to Site U1438, forming the sequences interpreted as depositional lobes. The abrupt change from very thick and massive debris flows to fine-grained turbidites at the unit III to unit II boundary reflects arc rifting and progressive waning of turbidity current and ash inputs. This interpretation is consistent with the geochemical record from melt inclusions and detrital zircons. Thus, Site U1438 provides a unique record of the life span of an intra-oceanic arc, from inception through maturation to its demise by intra-arc rifting and stranding of the remnant arc ridge

A record of spontaneous subduction initiation in the Izu–Bonin–Mariana arc

The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age—as constrained by the biostratigraphy of the overlying sediments—to the 52–48-million-year-old basalts in the adjacent Izu–Bonin–Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu–Bonin–Mariana subduction in a mode consistent with the spontaneous initiation of subduction

Mineralogy and petrographic summary of ODP Leg 198 sites and DSDP Hole 62-463, Shatsky Rise

This petrological study of the lower Aptian Oceanic Anoxic Event (OAE1a) focused on the nature of the organic-rich interval as well as the tuffaceous units above and below it. The volcaniclastic debris deposited just prior to the OAE1a is consistent with reactivation of volcanic centers across the Shatsky Rise, concurrent with volcanism on the Ontong Java Plateau. This reactivation may have been responsible for the sub-OAE1a unconformity. Soon after this volcanic pulse, anomalous amounts of organic matter accumulated on the rise, forming a black shale horizon. The complex textures in the organic-rich intervals suggest a history of periodic anoxia, overprinted by bioturbation. Components include pellets, radiolarians, and fish debris. The presence of carbonate-cemented radiolarite under the OAE1a intervals suggests that there has been large-scale remobilization of carbonate in the system, which in turn may explain the absence of calcareous microfossils in the section. The volcanic debris in the overlying tuffaceous interval differs in that it is significantly epiclastic and glauconitic. It was likely derived from an emergent volcanic edifice

Petrographic data for samples from DSDP Holes 63-471 and 65-485

Detrital modes of middle Miocene sandstone recovered at Deep Sea Drilling Project Site 471 on the Magdalena Fan support the hypothesis that the fan has been displaced northward from a source hundreds of kilometers to the south near the present mouth of the Gulf of California. The modes are dissimilar to those of modern sand derived from onshore outcrops of Miocene volcanic and volcaniclastic rocks, Neogene sedimentary rocks, and Mesozoic subduction complex. They most closely match sand associated with the mouth of Gulf of California. The overall stratigraphy, sand composition, and diagenesis at Site 471 are consistent with deposition of the Magdalena Fan on young oceanic crust near a spreading ridge at a triple junction