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We used a newly developed Pn tomography method to obtain high-resolution uppermost mantle velocity and anisotropy structures beneath the Northwest Pacific region. The observed Pn velocities are consistent with the local tectonic background, where high Pn velocities are observed beneath the Japan Trench area and Songliao Basin, and low Pn velocities beneath the Kuril Islands, Japan Archipelago-Izu Islands, Kyushu Island, Changbaishan-Jingpohu volcanoes, Korea Peninsula, and Japan Basin. The new Pn velocity image outlines the subducting slabs along the trenches and the young seafloor within the Japan Basin. Our results also support the existence of hot upwelling feeding the Changbaishan, Jingpohu, and Chuga-Ryong volcanoes, where small-scale mantle convection may exist below the Northeast China region. Further east, both trench-parallel anisotropy below arcs and trench-perpendicular anisotropy within the back-arc region suggest subduction-dominant mantle flow, where anisotropy may be attributable to the lattice-preferred orientation of olivine induced by flow-related strain. The highly accurate uppermost mantle velocity and anisotropy structures provide crucial information outlining the complex dynamic processes near convergent plate boundaries.

The CL images, LA-ICP-MS in situ trace elements analysis, and U-Pb dating for zircons indicate that the metamorphic ages of the sillimanite-garnet-biotite gneiss and the garnet-amphibole gneiss from eastern Taxkorgan of the Western Kunlun Mountains are 220±2 and 220±3 Ma respectively, and their protolith ages are younger than 253±2 and 480±8 Ma respectively. Two samples were collected at the same outcrops with HP mafic granulite and HP pelitic granulite. Mineral assemblage of the sillimanite-garnet-biotite gneiss (Grt+Sill+Per+Q) is consistent with that of HP pelitic granulite at early high amphibolite-granulite facies stage. Mineral assemblage of the garnet-amphibole gneiss (Grt+Amp+Pl+Q) is consistent with retro-metamorphic assemblage of HP mafic granulite at amphibolite facies stage. The dating results suggest that these HP granulites underwent peak metamorphism at 220±2 to 253±2 Ma. Thus, the Kangxiwar tectonic zone was probably formed by subduction and collision of the Paleo-Tethys Ocean during Indosinian. Protolith ages of the two samples, together with previously published U-Pb zircon dating age, suggest that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit, not a part of the Paleoproterozoic Bulunkuole Group. Western Kunlun Mountains, LA-ICP-MS zircon dating, Bulunkuole Group, Indosinian, Kangxiwar tectonic zone Citation:Yang W Q, Liu L, Cao Y T, et al. Geochronological evidence of Indosinian (high-pressure) metamorphic event and its tectonic significance in

The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S.The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the 2 existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.

Although the density structure of the cratonic lithospheric mantle (CLM) is critical for understanding the evolution of continents, there is little consensus on this important geodynamic property. The traditional model of strong isopycnicity (Jordan, 1978) assumes that the compositional buoyancy balances the thermal effect at any given depth within the mantle lithosphere. This proposition is supported by the xenolith data showing ancient subcontinental lithosphere could be more depleted than recent lithosphere (e.g., Griffin et al., 2009). However, the isopycnicity hypothesis faces challenges from recent observations, such as the stratified lithospheric anisotropy implying different ages of lithospheric layers (Yuan & Romanowicz, 2010), variations in mantle xenolith composition suggesting lithospheric alteration (Lee et al., 2011), large vertical motions of cratons due to lithospheric delamination (DeLucia et al., 2018;Hu et al., 2018), cratonic lithosphere destruction (Zhu et al., 2011), and cratonic basin subsidence resulting from thermal-chemical alteration within the cratonic lithosphere (Liu et al., 2019). These debates necessitate an updated view on the density structure of the CLM.Geodynamics studies provided contrasting views on the density of the CLM. While early geophysical calculations based on seismic data argued for a potentially dense CLM (

Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190 Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200 km to <60 km. This change was associated with a peak time (approximately 120 Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two‐stage evolutionary process. First, approximately 100 km of cratonic “keel” underlying a weak midlithospheric discontinuity layer (approximately 80–100 km) was rapidly removed in <10–20 Ma. This keel delamination stage was followed by a protracted (approximately 50–100 Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on numerical exploration of the well‐recorded second stage of the eastern North China Craton's lithospheric evolution. We find that (1) lithospheric mantle capped by thick crust can be locally replaced by deeper mantle material in 100 Ma due to small‐scale convective erosion; (2) asthenospheric upwelling and related extension can replace lithospheric mantle over horizontal length scales of ~50–150 km, and account for observed “mushroom‐shaped” low‐velocity structures; (3) modeling shows conditions that could lead to the multiple eastern North China Craton magmatic pulses between 190 and 115 Ma that are associated with temporal and spatial changes in magma source petrology and a magmatic hiatus; and (4) a “wet” midlithospheric discontinuity layer provides a potential source material for on‐craton magmatism.