The impact of subduction initiation on regional to global tectonics and the compositions of major Earth reservoirs are topics of vigorous ongoing research. Here, pristine glasses extracted from ~51.9 Myr old basalts and younger boninites that erupted in the Izu-Bonin Mariana forearc immediately after subduction initiation were analyzed by microbeam techniques, with goals of characterizing the mantle sources and the conditions under which melting occurred to produce nascent arc crust. Forearc basalts (FAB) have relatively differentiated major element compositions. Thus, to determine melting conditions and source compositions, primitive melt compositions were restored through an inferred crystallization history based on melt liquidus associations. Subsequent modeling indicates that they were generated at high temperatures and low pressures relative to a mid ocean ridge basalt (MORB). Incompatible trace element compositions of FAB show that they are similar to MORB in that they were generated largely by decompression melting. Differences in several trace element ratios between MORB and FAB indicate that the mantle sources for FAB were unusually depleted. Differences between FAB sub-units indicate a range of petrogenetic histories. Upper FAB sub-units are weakly enriched in fluid-mobile elements which may indicate that fluids from the subducting Pacific plate contributed to melting. Boninites are separated into high and low silica types based on preexisting whole rock analyses. Glasses separated from these boninites are highly differentiated and thus classify as high-Mg andesites rather than boninites on MgO-SiO2-TiO2 diagrams. These glasses are also enriched in a suite of fluid mobile elements indicating that they are products of flux melting of the mantle involving fluids and melts from the subducting plate. Olivine calcium concentrations are consistent with hydrous parental boninite melts. Aluminum partitioning between olivine and hosted spinel inclusions constrains the temperatures of initial crystallization between 1170 and 1330 degrees Celsius. The change from decompression melting which generated forearc basalts to flux melting which generated high silica boninites illustrates an evolution of the subduction system over the course of the initiation process. Based on trace element ratio plots, mixing relationships between upper forearc basalts and highly enriched fluids probably released by the nascent subducting slab suggest that both decompression melting and fluid fluxing operated to produce low silica boninite during subduction initiation. This melt composition progressively becomes dominated by fluid flux melts with additional components derived from the slab to make high silica boninite. These late volcanic rocks record melting of a highly depleted mantle source. The fact that heavy rare earth element concentrations become increasingly depleted from FAB to low silica boninite to high silica boninite indicates that the mantle source changed in composition over time. The progressive decrease suggests that the initial mantle source for FAB remained the mantle source for the duration of subduction initiation related magmatism.

High-resolution (20-250 Hz) multichannel seismic (MCS) reflection data with record lengths of 4-8 s TWT, totaling approximately 4620 line km on the shelf and slope of the Sandino forearc basin of offshore western Nicaragua, were acquired in November-December 2004 (cruise EW04-12) and subsequently processed at the University of Texas Institute for Geophysics. Seismic sequence interpretation was conducted using these MCS data in conjunction with deeper penetration (16-20. s TWT) MCS profiles from a previous survey (cruise EW00-05). Age estimates were based on cuttings from intersecting industry wells. Structure and isochron maps were created for 16 sequences and used to identify structural and depositional trends within the Sandino basin. The Tectonostratigraphic evolution of the basin varies considerably along-strike and is divided into five general stages from Late Cretaceous to recent. Evidence for multiple episodes of terrane accretion is observed from Late Eocene to Late Oligocene and potentially during Mid- to Late Miocene as well. Stratal stacking patterns suggest the Nicaraguan margin has not been dominated by subduction erosion during its history and extensional features beneath the slope are interpreted to have originated as a result of processes related to collision of allochthonous terrane of the downgoing plate, sediment underplating, and slab roll-back. With more precise age control, the stable northwestern region of the Sandino basin, where sediment is relatively undeformed since Late Oligocene and measures ≥ 16 km thick, offers a unique convergent margin setting for investigations of forcings on sequence development. / text

Topography growth and sediment fluxes in active subduction margin settings are poorly understood. Geological record is often scarce or hardly accessible as a result of intensive deformation. The Hawke Bay forearc basin of the Hikurangi margin in New Zealand is well suited for studying morphstructural evolution. It is well preserved, partly emerged and affected by active tectonic deformation during Pleistocene stage for which we have well dated series and well-known climate and eustasy. The multidisciplinary approach, integrating offshore and onshore seismic interpretations, well and core data, geological mapping and sedimentological sections, results in the establishment of a detailed stratigraphic scheme for the last 1.1 Ma forearc basin fill. The stratigraphy shows a complex stack of 11 eustasy-driven depositional sequences of 20, 40 and 100 ka periodicity. These sequences are preserved in sub-basins that are bounded by active thrust structures. Each sequence is characterized by important changes of the paleoenvironment that evolves between the two extremes of the glacial maximum and the interglacial optimum. Thus, the Hawke Bay forearc domain shows segmentation in sub-basins separated by tectonic ridges during sea level lows that become submerged during sea level highs. Over 100 ka timescale, deformation along active structures together with isostasy are responsible of a progressive migration of sequence depocenters towards the arc within the sub-basins. Calculation of sediment volumes preserved for each of the 11 sequences allows the estimation of the sediment fluxes that transit throughout the forearc domain during the last 1.1 Ma. Fluxes vary from c. 3 to c. 6 Mt.a⁻¹. These long-term variations with 100 ka to 1 Ma timescale ranges are attributed to changes in the forearc domain tectonic configuration (strain rates and active structure distribution). They reflect the ability of sub-basin to retain sediments. Short-term variations of fluxes (<100 ka) observed within the last 150 ka are correlated to drastic Pleistocene climate changes that modified erosion rates in the drainage area. This implies a high sensitiveness and reactivity of the upstream area to environmental changes in terms of erosion and sediment transport. Such behaviour of the drainage basin is also illustrated by the important increase of sediment fluxes since the European settlement during the 18th century and the following deforestation.

The Xigaze forearc basin in southern Tibet, one of the largest and best-preserved forearc basins on Earth, records upper-plate processes active prior to and following the inter-continental collision between India and Asia. However, the understanding of the timing and mechanisms of forearc development and its evolution following collision is spatially and temporally limited. Fundamental questions remain concerning how the basin formed, its paleogeography prior to collision, its subsidence history and the thermal history of the basin following the initial and ongoing continent-continent collision. Answering these questions is important to reconstructing upper plate dynamics during active subduction of oceanic and continental lithosphere. This dissertation addresses the Early Cretaceous to Pliocene history of the Xigaze forearc, using field mapping, sedimentology, sandstone modal petrography, geohistory analysis, U-Pb detrital zircon geochronology, and low-temperature thermochronology (apatite and zircon (U-Th/He)). Appendix A documents the sedimentology and stratigraphy of Lower Cretaceous to Paleocene strata to identify the relationship between the Yarlung-Tsangpo ophiolite and Xigaze forearc basin, reconstruct the sedimentary environments of the southern margin of Asia during the initial to middle stages of forearc deposition, and use the basin history to evaluate the mechanisms controlling forearc subsidence. In Appendix B, analysis of Eocene sedimentary strata, the youngest preserved in the forearc basin, constrains the timing of collision between Asia and the Tethyan Himalaya (India) to be no later than 58-54 Ma based on the similarity of the U-Pb detrital zircon age spectra and sandstone compositions between the Xigaze forearc and strata deposited atop the passive margin of the Tethyan Himalayan at that time. Apatite and zircon (U-Th)/He thermochronologic results in Appendix C constrain the maximum burial temperature of the basin following collision to ~140-200 °C, which corresponds to depths attainable by sedimentation in the forearc and a Paleogene forearc successor basin. This integrated dataset also identifies the initial stage of post-collisional exhumation during the Early Miocene (~ 20-15 Ma), followed by accelerated cooling during the Late Miocene to Early Pliocene (~ 10-4 Ma). These results suggest the presence of a paleo-Yarlung River and/or intensification of the Asian monsoon during the Early to Late Miocene. The Pliocene cooling signal is the youngest reported to date along the IYSZ and likely reflects increased river incision of the Yarlung-Tsangpo driven by accelerated orogen-parallel extension across structures which cross-cut the IYSZ. Robust interpretation of thermochronologic data requires knowledge about the geologic factors and intrinsic properties of the minerals. Appendix D of this dissertation examines intragranular zonation as a source of anomalously young zircon (U-Th)/He ages from leucogranites that intrude Greater Himalayan Sequence rocks at Ronbguk Valley, north of Mt. Everest. Depth profile laser ablation ICP-MS analysis was used to quantify the U-Th concentration profiles of a series of zoned, single, whole zircon grains and to apply a grain specific zonation-dependent age correction. Zircon grains corrected for zonation yield zircon He ages of 15-17 Ma, in agreement with AFT, ZFT and mica ⁴⁰Ar/³⁹Ar ages from the region. This study highlights the importance of characterizing intragranular zonation, especially in complex, zoned zircons that are typical of crustal melts and high-grade metamorphic rocks. Appendix E synthesis the results from Appendices A-C in the context of a global comparison with other ancient and preserved forearc basins. Results from the Xigaze forearc basin are compared to general models for forearc basin formation, subsidence trends, and preservation. The results show similarities between the Xigaze forearc basin and modern forearc basins, such as the Japan forearc off Honshu Island. The mechanisms driving tectonic subsidence are addressed, but remain an area of frontier research in continental dynamics.

Topography growth and sediment fluxes in active subduction margin settings are poorly understood. Geological record is often scarce or hardly accessible as a result of intensive deformation. The Hawke Bay forearc basin of the Hikurangi margin in New Zealand is well suited for studying morphstructural evolution. It is well preserved, partly emerged and affected by active tectonic deformation during Pleistocene stage for which we have well dated series and well-known climate and eustasy. The multidisciplinary approach, integrating offshore and onshore seismic interpretations, well and core data, geological mapping and sedimentological sections, results in the establishment of a detailed stratigraphic scheme for the last 1.1 Ma forearc basin fill. The stratigraphy shows a complex stack of 11 eustasy-driven depositional sequences of 20, 40 and 100 ka periodicity. These sequences are preserved in sub-basins that are bounded by active thrust structures. Each sequence is characterized by important changes of the paleoenvironment that evolves between the two extremes of the glacial maximum and the interglacial optimum. Thus, the Hawke Bay forearc domain shows segmentation in sub-basins separated by tectonic ridges during sea level lows that become submerged during sea level highs. Over 100 ka timescale, deformation along active structures together with isostasy are responsible of a progressive migration of sequence depocenters towards the arc within the sub-basins. Calculation of sediment volumes preserved for each of the 11 sequences allows the estimation of the sediment fluxes that transit throughout the forearc domain during the last 1.1 Ma. Fluxes vary from c. 3 to c. 6 Mt.a⁻¹. These long-term variations with 100 ka to 1 Ma timescale ranges are attributed to changes in the forearc domain tectonic configuration (strain rates and active structure distribution). They reflect the ability of sub-basin to retain sediments. Short-term variations of fluxes (<100 ka) observed within the last 150 ka are correlated to drastic Pleistocene climate changes that modified erosion rates in the drainage area. This implies a high sensitiveness and reactivity of the upstream area to environmental changes in terms of erosion and sediment transport. Such behaviour of the drainage basin is also illustrated by the important increase of sediment fluxes since the European settlement during the 18th century and the following deforestation.

The Scotland Formation onshore Barbados is often called the only example of a successful hydrocarbon producing accretionary prism reservoir. In spite of this, the hydrocarbon system elements of the BAP have nevertheless not been well studied. Seven outcropping locations of the Scotland were examined to document stacking patterns, key surfaces, depositional element geometries, facies occurrences their vertical and lateral extent, and the unit’s gamma response. Six facies were identified in outcrop: silty muds; laminated, centimeter-scale sandstones interbedded with silts and muds; cross-stratified sandstones; massive, medium to coarse-grained sandstones; very coarse grained sands with gravel or pebbles; and rare conglomerates. These facies combine to form architectural elements—channels, levees, and depositional lobes. Observations from petrographic, outcrop and seismic data suggest that the Scotland Formation was never deeply buried within the prism proper and was possibly deposited within the much larger proto-Tobago Basin. / text

Strata in the Cook Inlet forearc basin in south-central Alaska record the effects of tectonic events related to normal subduction and two flat-slab subduction events. Through detrital zircon geochronology we track provenance changes of strata deposited in a forearc basin in conjunction with these different subduction processes. Our data from strata deposited concurrent with normal subduction help to confirm previous provenance models of forearc basins that suggest provenance is sourced primarily from a proximal, coeval arc. However, compared to these models, our data from strata deposited coincident to flat-slab events show markedly different provenance signatures dependent upon: (1) geographic position relative to the flat-slab event; (2) pre-established, or lack thereof, topography; and (3) type of flat-slab event. Detrital zircon signatures of strata deposited in the Cook Inlet after flat-slab subduction of a mid-ocean ridge diversify to include older detritus found in the distal inboard region. This distal signature is then incrementally cut-off in younger strata due to deformation of the upper-plate from progressive insertion of a shallowly subducted oceanic plateau. Detrital zircon signatures for strata associated with each flat-slab event are largely older than depositional age due to the lack of coeval arc activity. Our data may help to improve the ability to recognize other flat-slab events through detrital zircon geochronology. In particular, changes in detrital zircon signatures found in strata deposited during flat-slab subduction of an oceanic plateau correlate well with the exhumation of rocks associated with the propagation of deformation in the over-riding plate due to plate coupling.

Subduction zones are places where one hydrated oceanic plate goes underneath another plate, and releases its fluids into the overlying mantle wedge. Slab-derived fluids play a key role in subduction zone processes. They serpentinize the cold forearc mantle at shallow depths; and deeper, they trigger hydrous mantle melting beneath the arc volcanoes and sometimes at backarc basin (BAB) spreading center. Examining the composition of arc and BAB magmas helps understanding genesis of subduction-related magmas, nature and composition of their mantle sources and slab-derived fluids. However, investigating such processes at shallow subduction zones is challenging, because the cold forearc mantle generally does not melt. Here, I investigate an unusual region in the southernmost Mariana convergent margin in the Western Pacific, near the Challenger Deep. The SE Mariana forearc stretched to accommodate opening of the southernmost Mariana Trough ~5Ma ago, opening the SE Mariana forearc rift (SEMFR) and causing seafloor spreading ~2.7-3.7Ma ago. The subducted slab beneath SEMFR deepens from <50km to ~100km, thus studying SEMFR lavas provides a unique opportunity to understand shallow subduction processes. By examining the major and trace element composition, the Pb-Nd-Sr isotopic ratios and the volatile contents (H2O, CO2, Cl, S, F) of SEMFR basalts, associated glassy rinds and olivine-hosted melt inclusions (Ol-MI) collected during three cruises (YK08-08, YK10-12, TN273), I show that: (i) SEMFR lavas were produced by adiabatic decompression melting of depleted asthenospheric BAB-like mantle at ~30±6.6 km depth and 1224±40oC; (ii) Ol-MI represent hydrous melts trapped by forearc mantle olivines. Xenocrysts were entrained with SEMFR basalts during ascent; (iii) SEMFR mantle flowed from the forearc towards the arc volcanoes and was metasomatized by shallow aqueous fluids; (iv) SEMFR shallow fluids are more aqueous than the fluids released beneath the Mariana arc and Mariana BAB; (v) the aqueous slab-derived fluids and the volatile fluxes are greatest at ~50-100km slab depth, suggesting that the minerals from the subducting plate mostly broke down beneath the arc to release their fluids. Such results provide new insights into shallow subduction processes, as previous studies showed that volatile fluxes and aqueous slab-derived fluids should increase toward the trench.

Thesis (M.S.)--Miami University, Dept. of Geology, 2004. / Title from first page of PDF document. Includes bibliographical references (p. 126-129).

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22424号 / 工博第4685号 / 新制||工||1731(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 林 為人, 教授 小池 克明, 准教授 村田 澄彦 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM