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Recent studies have debated the timing and spatial configuration of a possible intersection between the Pacific-Izanagi spreading ridge and the northeast Asian continental margin during Cretaceous or early Cenozoic times. Here we examine a newly compiled magmatic catalog of ∼900 published Cretaceous to Miocene igneous rock radioisotopic values and ages from the northeast Asian margin for ridge subduction evidence. Our synthesis reveals that a near-synchronous 56–46 Ma magmatic gap occurred across ∼1500 km of the Eurasian continental margin between Japan and Sikhote-Alin, Russian Far East. The magmatic gap separated two distinct phases of igneous activity: (1) an older, Cretaceous to Paleocene pre–56 Ma episode that had relatively lower εNd(t) (−15 to + 2), elevated (87Sr/86Sr)0 (initial ratio, 0.704–0.714), and relatively higher magmatic fluxes (∼1090 km2/m.y.); and (2) a younger, late Eocene to Miocene post–46 Ma phase that had relatively elevated εNd(t) (−2 to + 10), lower (87Sr/86Sr)0 (0.702–0.707), and a lower 390 km2/m.y. magmatic flux. The 56–46 Ma magmatic gap links other geological evidence across northeast Asia to constrain an early Cenozoic, low-angle ridge-trench intersection that had profound consequences for the Eurasian continental margin, and possibly led to the ca. 53–47 Ma Pacific plate reorganization.

The plate tectonic history of the hypothesized "proto-South China Sea" (PSCS) ocean basin and surrounding SE Asia since Cenozoic times is controversial. We implement four diverse proto-South China Sea plate reconstructions into global geodynamic models to constrain PSCS plate tectonics and possible slab locations. Our plate reconstructions consider the following: southward versus double-sided PSCS subduction models; earlier (Eocene) or later (late Oligocene) initiation of Borneo counterclockwise rotations; and larger or smaller reconstructed Philippine Sea plate sizes. We compare our modeling results against tomographic images by accounting for mineralogical effects and the finite resolution of seismic tomography. All geodynamic models reproduce the tomographically imaged Sunda slabs beneath Peninsular Malaysia, Sumatra, and Java. Southward PSCS subduction produces slabs beneath present Palawan, northern Borneo, and offshore Palawan. Double-sided PSCS subduction combined with earlier Borneo rotations uniquely reproduces subhorizontal slabs under the southern South China Sea (SCS) at 400 to 700 km depths; these models best fit seismic tomography. A smaller Philippine Sea (PS) plate with ã 1,000-km-long restored Ryukyu slab was superior to a very large PS plate. Considered together, our four end-member plate reconstructions predict that the PSCS slabs are now at <900 km depths under present-day Borneo, the SCS, the Sulu and Celebes seas, and the southern Philippines. Regardless of plate reconstruction, we predict (1) mid-Cenozoic passive return-flow upwellings under Indochina; and (2) late Cenozoic downwellings under the SCS that do not support a deep-origin "Hainan plume." Modeled Sundaland dynamic topography strongly depends on the imposed plate reconstructions, varying by almost 1 km. Plain Language Summary The past motion of tectonic plates (i.e., plate tectonic reconstructions) is the source of fundamental boundary conditions for many studies of Earth history. The South China Sea lies at a key junction between NE and SE Asia, which is one of the most tectonically complex regions in the world. A great diversity of plate reconstructions has been proposed for the South China Sea area. In this work we assimilate multiple SE Asia plate tectonic reconstructions into numerical models of mantle convection, in the form of time-dependent velocity boundary conditions at the Earth's surface. This technique leads to a prediction for the time evolution of mantle structure and its flow field that is consistent with the reconstructed plate motions. Each reconstruction produces a different prediction of where subducted, colder oceanic lithosphere (i.e., slabs) should exist within the Earth's mantle. These predictions are transformed into a "synthetic" seismic tomography using published tomographic resolution filters and compared against seismic tomographic images of the Earth's mantle with special emphasis on the present South China Sea area, and its past plate tectonic history. For each plate reconstruction we also computed t...

The configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400−600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.

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