Thick Paleogene sequences occur in the southern deformed fold and thrust belt of the Himalayas. In this thesis I describe detailed litho- and biostratigraphy from ten key stratigraphic sections in the Kohat Basin, the Potwar Basin and the Trans Indus Ranges (TIR). These stratigraphies combined with microfacies analysis resulted in a new interpretation of the tectono-stratigraphic history of the area, which is dominated by India-Asia collision but where eustatatic effects can also be identified. Of particular interest is documenting the timing of the final closure of the northern rim of the Tethys caused by this collision. The Kohat and Potwar Basins represent foreland basins within the collision zone. Their stratigraphies document effects of local tectonics and eustatic sea level. The biostratigraphy is based on occurrences of larger benthic foraminifera. Taxonomy of the species is included in the thesis. The Paleogene rocks of the study area are divided into local larger benthic forminiferal biozones: BFZK 1- BFZK 6 in the Kohat Basin and BFZP 1-BFZP 3 in the Potwar Basin and the TIR. These local biozones are correlated to the global standard biozonation schemes of Höttinger (1960), Schaub (1981), and Serra Kiel et al. (1998). The ages of the sequences are Late Paleocene (Thanitian) to Middle Eocene (Upper Lutetian) in the Kohat Basin, and Late Paleocene (Thanitian) to Early Eocene (Middle Cuisian) in the Potwar Basin and the TIR. The sediments were deposited along a carbonate ramp platform in both areas (sensu Read, 1982, 1985). The sequence stratigraphic histories of the two basins are described as follows. In the Kohat Basin, Thanitian to Middle Cuisian strata record the first Transgressive-Regressive cycle (TRK 1). The first sequence boundary (SBK 1) is followed by Middle Cuisian-Upper Cuisian lowstand progradational deposition that marks the end of TRK 1 cycle. Middle Lutetian 1-Upper Lutetian strata represent the second Transgressive-Regressive cycle (TRK 2). The second sequence boundary (SBK 2) ends TRK 2 deposition, after which no more deposition took place. In the Potwar Basin and the TIR, Thanitian strata comprise the first Transgressive- Regressive cycle (TRP 1), whilst Lower Lllerdian-Middle Lllerdian 1 strata constitute the second Transgressive-Regressive cycle (TRP 2). Middle Lllerdian 2 to Middle Cuisian strata mark the third Transgressive-Regressive cycle (TRP 3). Three sequence boundaries (the SBP 1, the SBP 2 and the SBP 3), marked by exposure surfaces, separate the three depositional cycles. The SBP 1 and SBP 2 sequence boundaries are controlled by local tectonics. In contrast the SBP 3 and SBK 1 sequence boundaries are synchronous at 49.5 Ma, and represent a phase of significant relative sea level fall, possibly driven by the combined effect of uplift (collision tectonics) and eustatic sea level fall (e.g. Haq et al., 1987). This implies that proto-closure of the northern rim of the Tethys occurred around 49.5 Ma. Reestablishment of marine conditions in the Kohat Basin occurred in the Middle Lutetian 1 around 45.8 Ma, possibly caused by a combination of flexural loading of the Indian plate (Pivinik & Wells, 1996) and eustatic sea-level rise (e.g. Haq et al., 1987). The final closure of the Tethys, marked by the end of marine sedimentation in the Kohat Basin, occurred in the Upper Lutetian (41.2 Ma). Finally, Himalayan foreland molasses sedimentation occurred during Miocene to Pliocene.

The Beyşehir-Hoyran-Hadim Nappes crop out over 700km, from east to west in the Pisidian and Central Taurus Mountains of southern Turkey. During this study, field obsevations of lithological, structural and sedimentological features are combined with igneous geochemical data derived from samples collected to help redefine a series of tectono-stratigraphic units and also determine the origin of the Beyşehir-Hoyran-Hadim Nappes. Above a regionally autochthonous Tauride carbonate platform, the Beyşehir-Hoyran Nappes begin with Ophiolitic Melange, consisting of blocks of neritic and pelagic limestone, basalt, serpentinite, radiolarian chert and, in places, amphibolite-grade metamorphic sole-type rocks, together set in a highly sheared siltstone and mudstone matrix. Locally, large slices of serpentinized harzburgite are incorporated in the melange. The peridotite sheets include lenses of chromitite and dunite and are cut by a series of dolerite dykes. The higher thrust sheets in the Hadim area begin with the Korualan Unit; a thrust sheet (ca. 400m thick) of mainly redeposited carbonates, quartzose sandstones and mudstones of Mid-Late Triassic age, interpreted as a proximal slope/base-of-slope succession. Regionally above is the Huğlu-type Unit; a thrust sheet (ca. 1 km thick) of Mid-Late Triassic intermediate-acidic extrusives, volcaniclastics and minor pelagic carbonates, interpreted as a continental rift. Post-rift subsidence in this thrust sheet is recorded by thin (<100m thick) Upper Triassic-Upper Cretaceous pelagic carbonate and radiolarian chert, depositionally above. The uppermost thrust sheet, the Boyali Tepe-type Unit, comprises broken formation and melange, including Jurassic shallow-water carbonate, Ammonitico Rosso condensed pelagic limestone, radiolarian chert and Upper Cretaceous pelagic limestone, representing a Bahaman-type carbonate platform which subsided in Early Jurassic time. Anastomosing zones of tectonic-sedimentary melange separate these higher units. The Beyşehir-Hoyran Nappes document Triassic rifting and Jurassic-Cretaceous passive margin subsidence bordering the Northern Neotethyan Ocean. The Late Cretaceous harzburgitic ophiolite probably formed above a northerly dipping subduction zone within the Neotethyan ocean basin. Ophiolitic melange formed along the leading edge of the overiding plate. The ophiolite was emplaced southwards onto the northern margin of the Tauride platform in latest Cretaceous time, probably during collision of the passive margin with a trench. The nappe pile and underlying platform (Hadim Nappe) were thrust ca. 150km further south in Late Eocene time during regional continental collision and suture zone tightening. Several alternative palaeo-tectonic models are considered and tested in the light of data presented from this study. Assuming ‘in-sequence’ thrusting, the Beyşehir-Hoyran Nappes restore to a location north of a northerly Neotethyan spreading axis. More probably, they originated near the south margin of the northern Neotethys, but reached their position by ‘out-of-sequence thrusting’. Formation within a localised southerly strand of the northern Neotethys (Inner Tauride ocean) is more probable than within the main Neotethys further north. Wider implications for the Tethyan ocean as a whole and several other orogenic belts are also considered.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22265号 / 理博第4579号 / 新制||理||1657(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 山路 敦, 教授 田上 高広, 教授 生形 貴男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Sediment deposited in back-arc basins preserves a record of the extensional, volcanic and tectonic history of the arc-backarc systems. Back-arc sedimentation is of particular interest as seafloor massive sulfide deposits may be preserved in back-arc basin sediments. The study of back-arc sedimentation using acoustic data, such as high-resolution sub-bottom profiling data (Parasound) and seismic reflection data, can be a much more cost effective approach than analysis of sediments recovered from drill cores. In this study, we use these two acoustic datasets to build a facies model of sedimentation in the northeast Lau Basin, an actively opening back-arc basin in the southwest Pacific Ocean. Using 830 km of Parasound and 730 km of seismic lines along 4 transects of the Lau Basin, we constructed one of the most detailed models of sedimentation in a back-arc basin to date. Parasound data show distinct echoes with sub-bottom reflections indicative of a high proportion of hemipelagic sediment, whereas the indistinct echoes with few to no sub-bottom reflections indicate a higher proportion of coarse, bedded, volcaniclastic turbidites. Hyperbolic echoes are associated with regions of rugged or uneven terrain characterized by exposed, rough basement or deposits formed by contour currents, turbidity currents, slumps or slides. These relationships form the basis of an echo-facies legend developed for typical back-arc basin sediments. The echo-facies observed in the Parasound, and confirmed by deeper-penetrating seismic reflection data, provide important insights into the sedimentary processes involved in back-arc sedimentation. We observed mass transport deposits (MTDs) in all of the sub-basins and slope deposits within and on the flanks of active rifts (e.g., the Fonualei Rift and Spreading Centre, FRSC), suggesting a direct correlation between MTDs and zones of active rifting. We observed an overall increase in sediment thickness toward the Tofua Arc which suggests it is the main sediment source, but local variations in sediment thickness suggest significant input from local intrabasinal seamounts. The uppermost echo-facies in over 60% of the sub-basins in the study area is dominated by hemipelagic material, which suggests an abrupt transition in the dominant sediment source from volcaniclastic to hemipelagic at around 0.3 Ma, when a period of volcanic quiescence from the Tofua Arc began. The study shows that a near complete record of basin evolution can be constructed using geophysical and acoustic methods and that this work may help to locate future drill sites where in situ data can be collected.