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Abstract Magma emplacement in the top unconsolidated sediments of rift basins is poorly understood. We compare two shallow sills from the Guaymas Basin (Gulf of California) using core data and analyses from IODP Expedition 385, and high‐resolution 2D seismic data. We show that magma stalling in the top uncemented sediment layer is controlled by the transition from siliceous claystone to uncemented silica‐rich sediment, favoring flat sill formation. Space is created through a combination of viscous indentation, magma‐sediment mingling and fluidization processes. We show that sills emplace above the opal‐A/CT diagenetic barrier. Our model suggests that in low magma input regions sills emplace at constant depth from the seafloor, while high magma input leads to upward stacking of sills, culminating in a funnel‐shaped intrusions. Our petrophysical, petrographic, and textural analyses show that magma‐sediment mingling creates significant porosity (up to 20%) through thermal cracking of the assimilated sediment. Stable isotope data suggest carbonate formation at 70–90°C, consistent with background geothermal gradient at 250–325 m depth. The unconsolidated, water‐rich host sediments produce little thermogenic gas through contact metamorphism, but deep diagenetically formed gas bypasses the low‐permeability top sediments via hydrothermal fluids flowing through the magma plumbing system. This hydrothermal system provides a steady supply of hydrocarbons at temperatures amendable for microbial life, serving as an incubator that may be abundant in magma‐rich young rift basins and play a key role in sustaining subseafloor ecosystems. 

Abstract Faults on microcontinents record the dynamic evolution of plate boundaries. However, most microcontinents are submarine and difficult to study. Here, we show that the southern part of the Isla Ángel de la Guarda (IAG) microcontinent, in the northern Gulf of California rift, is densely faulted by a late Quaternary‐active normal fault zone. To characterize the onshore kinematics of this Almeja fault zone, we integrated remote fault mapping using high‐resolution satellite‐ and drone‐based topography with neotectonic field‐mapping. We produced 13 luminescence ages from sediment deposits offset or impounded by faults to constrain the timing of fault offsets. We found that north‐striking normal faults in the Almeja fault zone continue offshore to the south and likely into the nascent North Salsipuedes basin southwest of IAG. Late Pleistocene and Holocene luminescence ages indicate that the most recent onshore fault activity occurred in the last ∼50 kyr. These observations suggest that the North Salsipuedes basin is kinematically linked with and continues onshore as the active Almeja fault zone. We suggest that fragmentation of the evolving IAG microcontinent may not yet be complete and that the Pacific‐North America plate boundary is either not fully localized onto the Ballenas transform fault and Lower Delfin pull‐apart basin or is in the initial stage of a plate boundary reorganization. 

Abstract Solander Basin is characterized by subduction initiation at the Pacific‐Australia plate boundary, where high biological productivity is found at the northern edge of the Antarctic Circumpolar Current. Sedimentary architecture results from tectonic influences on accommodation space, sediment supply and ocean currents (via physiography); and climate influence on ocean currents and biological productivity. We present the first seismic‐stratigraphic analysis of Solander Basin based on high‐fold seismic‐reflection data (voyage MGL1803, SISIE). Solander Trough physiography formed by Eocene rifting, but basinal strata are mostly younger than ca. 17 Ma, when we infer Puysegur Ridge formed and sheltered Solander Basin from bottom currents, and mountain growth onshore increased sediment supply. Initial inversion on the Tauru Fault started at ca. 15 Ma, but reverse faulting from 12 to ca. 8 Ma on both the Tauru and Parara Faults was likely associated with reorganization and formation of the subduction thrust. The new seabed topography forced sediment pathways to become channelized at low points or antecedent gorges. Since 5 Ma, southern Puysegur Ridge and Fiordland mountains spread out towards the east and Solander Anticline grew in response to ongoing subduction and growth of a slab. Solander Basin had high sedimentation rates because (1) it is sheltered from bottom currents by Puysegur Ridge; and (2) it has a mountainous land area that supplies sediment to its northern end. Sedimentary architecture is asymmetric due to the Subtropical Front, which moves pelagic and hemi‐pelagic sediment, including dilute parts of gravity flows, eastward and accretes contourites to the shelf south of Stewart Island. Levees, scours, drifts and ridges of folded sediment characterize western Solander Basin, whereas hemi‐pelagic drape and secondary gravity flows are found east of the meandering axial Solander Channel. The high‐resolution record of climate and tectonics that Solander Basin contains may yield excellent sites for future scientific ocean drilling. 

Abstract Subduction initiation often takes advantage of previously weakened lithosphere and may preferentially nucleate along pre‐existing plate boundaries. To evaluate how past tectonic regimes and inherited lithospheric structure might lead to self‐sustaining subduction, we present an analysis of the Puysegur Trench, a young subduction zone with a rapidly evolving tectonic history. The Puysegur margin, south of New Zealand, has experienced a transformation from rifting to seafloor spreading to strike‐slip, and most recently to incipient subduction, all in the last ∼45 million years. Here we present deep‐penetrating multichannel reflection and ocean‐bottom seismometer tomographic images to document crustal structures along the margin. Our images reveal that the overriding Pacific Plate beneath the Solander Basin contains stretched continental crust with magmatic intrusions, which formed from Eocene‐Oligocene rifting between the Campbell and Challenger plateaus. Rifting was more advanced to the south, yet never proceeded to breakup and seafloor spreading in the Solander Basin as previously thought. Subsequent strike‐slip deformation translated continental crust northward causing an oblique collisional zone, with trailing ∼10 Myr old oceanic lithosphere. Incipient subduction transpired as oceanic lithosphere from the south forcibly underthrust the continent‐collision zone. We suggest that subduction initiation at the Puysegur Trench was assisted by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike‐slip along the plate boundary. The Puysegur margin demonstrates that forced nucleation along a strike‐slip boundary is a viable subduction initiation scenario and should be considered throughout Earth's history.