ALBERT

Description: Highlights • We present three geophysical profiles along the southern Chatham Rise margin. • Structural variations relate to Hikurangi Plateau collision and Zealandia rifting. • SE Chatham Terrace represents a broad COT (thin and modified continental crust). • The southern Chatham Rise margin is a unique hybrid-rifted margin. • Initial rifting was passive, but upwelling mantle affected the Chatham Rise margin. Abstract Passive continental margins are commonly classified as magma-poor and magma-rich types. Related breakup processes are often associated with far-field tectonic stresses or upwelling mantle plumes. The Chatham Rise east off New Zealand records a sequence of Late Cretaceous tectonic events, which include subduction and collision of the oceanic Hikurangi Plateau to subsequent continental rifting and breakup. The mechanisms triggering the change in tectonic forces are poorly understood but address open questions regarding the formation of passive margins. We acquired wide-angle seismic reflection/refraction, multi-channel seismic and potential field data along three profiles crossing the southern Chatham Rise margin and SE Chatham Terrace to the oceanic crust in order to image and understand the crustal structure and breakup mechanisms. Variations in crustal thickness along the highly faulted Chatham Rise are most likely related to the collision with the Hikurangi Plateau. Our data indicate that the SE Chatham Terrace represents a broad continent-ocean transition zone (COTZ), which we interpret to consist of very thin continental crust affected by magmatic activity. Along the southern Chatham Rise margin, features of both, magma-poor and magma-rich rifted margins are present. We suggest that passive rifting initiated at 105–100 Ma related to slab dynamics after the Hikurangi Plateau collision. We revise the onset of seafloor spreading south of the eastern Chatham Rise to ~88 Ma from the extent of our inferred COTZ. Geographically extensive, but low-volume intraplate magmatism affected the margin at 85–79 Ma. We suggest that this magmatism and the onset of seafloor spreading are a response to upwelling mantle through a slab window after 90 Ma. After 85 Ma, spreading segments became connected leading to the final separation of Zealandia from Antarctica. We interpret the southern Chatham Rise margin as a unique hybrid margin whose tectonic history was influenced by passive continental rifting and mantle upwelling.

Description: Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.