Young rifts are shaped by combined tectonic and surface processes and climate, yet few records exist to evaluate the interplay of these processes over an extended period of early rift-basin development. Here, we present the longest and highest resolution record of sediment flux and paleoenvironmental changes when a young rift connects to the global oceans. New results from International Ocean Discovery Program (IODP) Expedition 381 in the Corinth Rift show 10s–100s of kyr cyclic variations in basin paleoenvironment as eustatic sea level fluctuated with respect to sills bounding this semi-isolated basin, and reveal substantial corresponding changes in the volume and character of sediment delivered into the rift. During interglacials, when the basin was marine, sedimentation rates were lower (excepting the Holocene), and bioturbation and organic carbon concentration higher. During glacials, the basin was isolated from the ocean, and sedimentation rates were higher (~2–7 times those in interglacials). We infer that reduced vegetation cover during glacials drove higher sediment flux from the rift flanks. These orbital-timescale changes in rate and type of basin infill will likely influence early rift sedimentary and faulting processes, potentially including syn-rift stratigraphy, sediment burial rates, and organic carbon flux and preservation on deep continental margins worldwide.

This work is distributed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Contents 1 Introduction and operations 5 Lithostratigraphy 9 Structural geology 13 Micropaleontology 16 Geochemistry 18 Physical properties 25 Paleomagnetism 26 Downhole measurements 30 Core-log-seismic integration 33 References

We are able to decipher the tectonic evolution of the Bay of Bengal, a puzzle which has not been satisfactorily solved in the past, and we are also able to shed new light on origin of the buried 85°E Ridge. We do so by incorporating a number of disparate items into a unified solution. These items are the marine magnetic anomalies in the Western Basin of the Bay of Bengal, the Rajmahal and Sylhet traps, and Deep Seismic Sounding lines in India, a prominent magnetic anomaly doublet and seismic Seaward Dipping Reflectors in Bangladesh, and a new precise gravity map of the Bay of Bengal. We identify seafloor‐spreading magnetic anomalies ranging in age from 132 Ma (M12n) to 120 Ma (M0) in the Western Basin. These anomalies are “one sided”; the conjugate anomalies lie in the Western Enderby Basin, off East Antarctica. The direction of spreading was approximately NW‐SE, and the half‐spreading rates varied from 2.5 to 4.0 cm/yr. With the arrival of the Kerguelen plume around M0 time, seafloor spreading was reorganized and a new spreading axis opened at or close to the line joining the Rajmahal and Sylhet traps. The prominent magnetic anomaly doublet connecting the Rajmahal and Sylhet traps indicates that these traps are not individual eruptions at about 118 Ma, but rather, together, define the new line of opening. Spreading started at this line, and subsequently, India changed direction from west to north. The new oceanic crust, thus generated, underlies Bangladesh and the Eastern Basin of the Bay of Bengal and is younger than 118 Ma. The western boundary of the new ocean floor is a transform fault, which was generated by the spreading axis jump. This transform fault appears as the 85°E Ridge, and further north, on land, as a negative free‐air gravity anomaly strip. A unique feature of the northern boundary of the new oceanic crust is that due to the later deposition of enormous sediments derived from the Himalayan orogeny, it lies onshore Bangladesh, in contrast to most continent‐ocean boundaries in the world, which lie offshore.

The sediment succession in the Bay of Bengal (BoB) records the signatures corresponding to India-Asia collision, regional climate change, and erosional processes of both the Himalayan orogen and Indian subcontinent. The Bengal Fan-the world's largest submarine fan-has long been studied to understand the link between the Himalayan tectonics and Asian monsoon. But, lack of detailed information on corresponding signals hampered the understanding of related processes of tectonics, climate and erosion. The present study of long-streamer seismic reflection profile data and information from deep drill well logs in the western BoB has revealed two different phases of sediment deposition. In the first phase, until Oligocene-Miocene (~23 Ma), Indian peninsular rivers discharged sediments to the BoB which accumulated at a rate ~20 m/m.y. with an aberration of two fairly enhanced sediment pulses during the periods from 65 to 54 Ma and 34 to 23 Ma. In the second phase, since 23 Ma, the Ganges and Brahmaputra rivers added huge volumes of sediments to the bay at variable rates ranging from 40 to >1000 m/m.y. A distinct increase in sediment discharge (~140 m/m.y.) during the Oligocene-Miocene (~23 Ma) together with the development of regional onlap unconformity and the start of turbidity system provide an important age marker corresponding to rapid exhumation of the Himalaya, which intensified the erosional process and commencement of Bengal Fan sedimentation. Further rise in the rate of sedimentation during the period 6.8-0.8 Ma is coincident with the change in monsoon intensity, but surprisingly not in agreement with the decrease in sediment rate reported at ODP Leg 116 sites in the distal Bengal Fan. Here we provide wellconstrained ages for the commencement and growth of the Bengal Fan, which can serve as benchmark information for understanding the interaction between the Himalayan exhumation and Asian climate.