17:15 〜 18:45
[SCG48-P22] Structural and compositional analyses of pelagic sediments beneath the Nicobar fan, Indian Ocean: insights on compaction, diagenesis, and past intraplate deformation
キーワード：スマトラ・スンダ海溝、Wharton海盆、遠洋堆積物、国際深海科学掘削計画
Along the Sumatra-Sunda trench, the Indo-Australian plate subducts beneath the Sunda and Burma plates, forming an active seismogenic zone that have generated large, destructive earthquakes such as the 2004 Mw 9.2 Sumatran-Andaman earthquake (Chlieh et al. 2007). Seaward of the Sunda trench, the incoming section composes the Wharton Basin, which is shaped by N-S oriented fracture zones that formed during Cretaceous to Eocene spreading along the fossil Wharton Ridge (Jacob et al. 2014). These fractures zones are currently reactivated as left-lateral strike-slip faults in the present-day stress field, hosting frequent earthquakes represented by the 2012 Mw8.6 Wharton Basin earthquake sequence ~120 km southwest of the Sunda trench (Wei et al. 2013). The stress states are inferred to interact between the oceanic plate and subduction system in this region (Delescluse et al. 2012), but the mechanisms of the interaction, together with the evolution of past intraplate stresses on the Indo-Australian plate, are not well understood.
The full sedimentary succession in the Wharton Basin from the seafloor to 1415.4 meters was recovered by International Ocean Discovery Program (IODP) Expedition 362, east of the Ninety-East Ridge, ~225 km distance from the trench offshore Northern Sumatra (McNeill et al. 2017). They consist of predominantly siliciclastic sediments interpreted as part of the Bengal-Nicobar Fan (Lithologic Units I–II), underlain by mixed tuffaceous and pelagic sediment (Unit III) and thin intervals of intercalated pelagic and igneous material (Units IV and V) overlying oceanic crust (Unit VI). Major transitions in lithology, age, structure, and physical properties mark the boundary between the unlithified Nicobar fan sediments (late Miocene-recent) and lithified pre-fan pelagic sediments (late Paleocene-late Miocene), which is a horizon traceable throughout the Wharton Basin, and is one of the potential pre-décollement horizons at the subduction zone offshore Sumatra.
A distinct concentration of normal faults is observed in the pelagic units (Units III-V), characterized by thin anastomosing faults which randomly cross-cut each other. Sand injections, sediment-filled veinlets, syn-sedimentary recumbent folds, and diagenetic spots also occur in the sediments. The flattened ellipsoidal geometry of the diagenetic spots which may serve as indication of strain, overprint the normal faults and incur no offsets. The high conjugate angle and wavy geometry of the faults may imply that the normal faulting occurred before significant compaction. In one of the shear zones, red sediments that are darker than the surrounding sediments are distributed, similar to diagenetic spots, suggesting that the shear zones may have served as pathways of focused fluids. The concentrated deformation in the pelagic section may have occurred in an early burial history during active ridge environment at that time, followed by rapid sedimentation of the Nicobar Fan, associated with sediment diagenesis and fluid flow.
The motivation of our research is to investigate the internal architecture of the late Miocene-Paleocene pelagic section in the northern Wharton Basin, and to reconstruct the intraplate conditions and interplay of deformation, consolidation, and diagenesis, prior to and/or during the Nicobar Fan sedimentation. Using core samples from IODP Exp. 362, we conduct 1) three-dimensional microstructural observations using a micro-focus X-ray CT scanner and optical microscopy on thin sections, 2) textural and compositional analyses using field emission scanning electron microscope, X-ray diffraction and fluorescence analyses to examine the mineral occurrences, and 3) paleo-stress analysis utilizing structural data obtained during the expedition and from microstructural observation. In this presentation, we report on preliminary findings from these analyses, to discuss possible mechanisms of observed deformation and syn-sedimentary processes.
The full sedimentary succession in the Wharton Basin from the seafloor to 1415.4 meters was recovered by International Ocean Discovery Program (IODP) Expedition 362, east of the Ninety-East Ridge, ~225 km distance from the trench offshore Northern Sumatra (McNeill et al. 2017). They consist of predominantly siliciclastic sediments interpreted as part of the Bengal-Nicobar Fan (Lithologic Units I–II), underlain by mixed tuffaceous and pelagic sediment (Unit III) and thin intervals of intercalated pelagic and igneous material (Units IV and V) overlying oceanic crust (Unit VI). Major transitions in lithology, age, structure, and physical properties mark the boundary between the unlithified Nicobar fan sediments (late Miocene-recent) and lithified pre-fan pelagic sediments (late Paleocene-late Miocene), which is a horizon traceable throughout the Wharton Basin, and is one of the potential pre-décollement horizons at the subduction zone offshore Sumatra.
A distinct concentration of normal faults is observed in the pelagic units (Units III-V), characterized by thin anastomosing faults which randomly cross-cut each other. Sand injections, sediment-filled veinlets, syn-sedimentary recumbent folds, and diagenetic spots also occur in the sediments. The flattened ellipsoidal geometry of the diagenetic spots which may serve as indication of strain, overprint the normal faults and incur no offsets. The high conjugate angle and wavy geometry of the faults may imply that the normal faulting occurred before significant compaction. In one of the shear zones, red sediments that are darker than the surrounding sediments are distributed, similar to diagenetic spots, suggesting that the shear zones may have served as pathways of focused fluids. The concentrated deformation in the pelagic section may have occurred in an early burial history during active ridge environment at that time, followed by rapid sedimentation of the Nicobar Fan, associated with sediment diagenesis and fluid flow.
The motivation of our research is to investigate the internal architecture of the late Miocene-Paleocene pelagic section in the northern Wharton Basin, and to reconstruct the intraplate conditions and interplay of deformation, consolidation, and diagenesis, prior to and/or during the Nicobar Fan sedimentation. Using core samples from IODP Exp. 362, we conduct 1) three-dimensional microstructural observations using a micro-focus X-ray CT scanner and optical microscopy on thin sections, 2) textural and compositional analyses using field emission scanning electron microscope, X-ray diffraction and fluorescence analyses to examine the mineral occurrences, and 3) paleo-stress analysis utilizing structural data obtained during the expedition and from microstructural observation. In this presentation, we report on preliminary findings from these analyses, to discuss possible mechanisms of observed deformation and syn-sedimentary processes.