Abstract Numerous seismic studies have reported the absence of a resolvable subducted slab beneath parts of northern Kamchatka, though the mechanism for this anomaly remains debated. We utilize 1,696 P‐to‐S receiver functions to image the mantle transition zone (MTZ) beneath the Kamchatka Peninsula and surrounding areas. Beneath southwestern Kamchatka, the thickened MTZ and the pronounced 520‐km discontinuity support cold Cretaceous Pacific slab‐related material with possible hydration within the MTZ. In contrast, significant MTZ thinning is observed beneath northern Kamchatka, where previous tomography studies also show slab absence in the shallow upper mantle. This argues against the presence of a large cold detached slab remnant in the sampled MTZ. Combined with previous seismic anisotropy, tomography, and geodynamic constraints, our results support a model that thermally buoyant mantle material flows around the slab edge and ascends into the upper mantle, forming a semi‐toroidal mantle flow. The inferred slab‐edge mantle flow may extend to at least MTZ depths and provide a plausible setting for progressive modification and weakening of the slab margin beneath northern Kamchatka, while allowing slab penetration farther southwest. These observations highlight the first‐order role of slab–mantle interaction in controlling slab fate at convergent margins.
Abstract Dynamic topography changes as mantle convection evolves, making its past surface expression difficult to reconstruct. Widespread Neogene‐Quaternary intraplate basaltic magmatism across the Western Mediterranean Region allows us to test whether dated basalts preserve the mantle thermal structure responsible for regional dynamic support. We combine major, trace, and rare earth element (REE) analyses and modeling from six volcanic provinces with calibrated earthquake tomographic models and residual topographic and bathymetric measurements. The geochemical data set includes both newly acquired and published analyses. Rare earth element inverse modeling of samples least affected by lithospheric‐mantle influence indicates that melting occurred beneath lithosphere typically 60–70 km thick and at asthenospheric potential temperatures ranging from approximately ambient to 50°C above ambient mantle. These estimates agree with independent tomographic constraints. Dynamic support calculated from the inferred thermal structure is also consistent with measured residual anomalies. Lithospheric thinning accounts for most of the largest positive anomalies of approximately 0.5 km without requiring large asthenospheric temperature anomalies. The agreement demonstrates that Western Mediterranean intraplate basaltic magmatism records upper‐mantle thermal structure capable of generating regional surface support and provides a basis for reconstructing past mantle conditions and transient dynamic topography from dated basalts.
Abstract Ophiolitic mélanges and associated intrusions provide key archives for reconstructing the evolution of ancient ocean basins and subduction‐related mantle modification. The Hongliuhe–Niujuanzi–Xichangjing ophiolitic mélange in the central Beishan Orogen preserves important records of the Paleo‐Asian Ocean, but the timing of oceanic subduction, terminal closure, and subsequent postcollisional reworking in the Niujuanzi remain debated. Here we integrate field observations, zircon U–Pb geochronology, whole‐rock and mineral geochemistry, and Sr–Nd–Hf–Pb isotopic data from the newly dated and geochemically characterized ophiolitic rocks and associated dioritic to granodioritic intrusions. Zircon U–Pb ages define three Paleozoic magmatic episodes: Early Silurian diorites at 435–427 Ma, Late Silurian ophiolitic gabbros at 423–418 Ma, and Early Devonian granodiorites at ca. 403 Ma. The diorites display continental arc‐like geochemical signatures and were derived from a lithospheric mantle source modified by slab‐derived fluids. The gabbros show N‐MORB‐like affinities with weak subduction‐related enrichment, consistent with formation in an SSZ setting. The granodiorites were generated mainly by partial melting of the ancient lower crust with minor mantle input, recording a transition to postcollisional extension. Silicate in situ Pb isotopes exhibit a temporal shift toward Indian MORB‐like signatures from the Early Silurian to Early Devonian, indicating progressive mantle source modification by recycled subduction components. These results demonstrate that the Niujuanzi basin remained active until the Late Silurian, closed by the end‐Silurian, closed during the latest Silurian to the earliest Devonian (ca. 418 Ma), and transitioned to postcollisional extension in the Early Devonian, providing a refined geodynamic model for the central Beishan Orogen.
Abstract Large‐offset detachment faults frequently occur at slow‐ and ultraslow‐spreading mid‐ocean ridges, especially where magma supply is limited. Observations show that detachments at ultraslow ridges often alternate in polarity (“flip‐flop” regime), but the mechanical controls governing their formation remain poorly understood. Here we use two‐dimensional thermo‐mechanical models to systematically investigate how frictional and cohesive strength contrasts between fault zones and surrounding lithosphere influence faulting styles in nearly amagmatic ultraslow ridge environments. Our models incorporate a self‐evolving thermal structure and Mohr‐Coulomb elasto‐plasticity with friction and cohesion weakening during deformation, along with parameterizations of fluid circulation. We identify three primary deformation regimes: distributed minor faults, sequential flip‐flop detachments, and persistent runaway detachments. Flip‐flop detachments emerge under moderate frictional and cohesive weakening, where faults alternate polarity as antithetic faults periodically replace dominant detachments. Persistent runaway detachments, as produced by models, show excessive cross‐axis migration that is not typically observed in nature. A strong, viscous lower lithosphere promotes distributed deformation and hinders the onset of persistent detachments. Analytical force balance modeling further shows that flip‐flop faulting occurs when fault strength loss balances the defocusing effect of lithospheric flexure, which necessitates moderate frictional weakening (∼0.2–0.3) achievable via serpentinization of the fault zone. In contrast, runaway detachments require further weakening. These findings clarify the mechanical conditions driving flip‐flop detachment formation and highlight how relatively modest rheological contrasts between intact and deformed lithosphere enable large‐offset faulting at ultraslow mid‐ocean ridges.
Abstract Precambrian supercontinent reconstructions largely rely on robust geological constraints. Among these, the basin affinities recorded by Precambrian sedimentary fragments preserved in orogenic belts provide critical evidence for resolving the paleogeographic configurations of microcontinents and continental blocks. In this study, we integrate detailed field investigations with detrital zircon U–Pb geochronology, zircon Hf isotope analysis, Monte Carlo unmixing modeling, and multidimensional scaling to constrain the depositional age, provenance and paleogeographic significance of the Sayanggou metasedimentary fragment in the southern East Kunlun accretionary belt, southern Qaidam Block. The detrital zircon age spectra of the Sayanggou metasedimentary fragment feature prominent Ediacaran and older Neoproterozoic multimodal age populations. The absence of late‐stage detrital zircons, multiple calculating maximum depositional age approaches and stratigraphic evidence further indicates deposition in a comparatively stable Ediacaran depositional environment, possibly a passive‐margin setting. Corresponding zircon ε Hf ( t ) values are predominantly negative, indicating sediment derivation primarily from recycled ancient crust. Statistical evaluation of these data sets against global detrital zircon archives reveals a diagnostic Gondwanan signature. Monte Carlo unmixing analyses and multidimensional scaling models firmly tether the Sayanggou metasedimentary fragment to the contemporary basins in the South Qiangtang Block and related Ediacaran peri‐Gondwanan margins. Zircon U–Pb ages of crosscutting granitoid and dioritoid intrusions indicate that this metasedimentary fragment was probably accreted to the southern East Kunlun accretionary belt during the early Paleozoic in response to Proto‐Tethyan subduction. Our findings further advance the understanding that the Qaidam and South Qiangtang blocks were located in relatively close paleogeographic positions within the peri‐Gondwanan domain during the Ediacaran.
