ALBERT

Description: Three main rivers—the Ganges, Brahmaputra, and Meghna—coalesce in the Bengal basin to form the world’s largest delta system, which serves as filter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fluvial systems. Such mobility developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Description: The magmatic, tectonic, and hydrothermal processes forming the oceanic crust along mid-ocean spreading centers are strongly modified at sedimented spreading centers but poorly understood owing to the lack of good crustal images. Here we present high-resolution deep seismic reflection images across the sedimented slow-spreading Andaman Sea spreading center. Several sub-horizontal sills are injected within the sedimentary strata, and no surface eruption is observed. On-axis reversed-phase reflections within the igneous crust correspond to axial magma lenses at different depths. The faults within the axial valley are steeply dipping (65°–75°) in a staircase pattern forming the axial graben. Their base coincides with a shallow-dipping (30°) reflection, defining the zone of extension and magmatism. As the sill-sediment sequences are rafted away from the axis, they are rotated and buried due to subsidence and faulting, forming the upper oceanic crust. The gabbroic lower oceanic crust is separated from the mantle by a complex Moho transition zone probably containing dunite lenses.

Description: The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis.

Description: The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis.

Description: Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28'E, we report the presence of a large low-velocity anomaly (LVA) ~4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.