| Recent observations show distinct positive and negative topography in back-arc rift areas and mid-ocean ridge rift zones,corresponding to the areas of rift basin uplift and subsidence,respectively.Studies indicate that these topographic differences are controlled by deep-seated magma and properties,particularly the influences of viscosity.While previous research has established the influence of deep-seated magma on rift basin evolution,the direct correlation between basin evolution and variations in magma properties,with quantitative analysis,has only recently been completed.Numerical simulations suggest that high-viscosity deep-seated magma causes topographic subsidence,resulting in sedimentation in rift,whereas low-viscosity magma results in rift uplift,weakening or reversing sedimentation.The spatial variability of modern rift topography controlled by the magma suggests that if translated into temporal changes in topography for a specific rift area,it would reflect the subsidence patterns of rift activity over time,with evolution pattern controlled by variations in the nature of deepseated magma.Therefore,the discovery of deep-seated magma plumbing system in the rift basin and the establishment of a direct link between basin subsidence patterns and changes in magma properties(volcanic activity)could reveal the dynamic mechanisms controlling rift basin evolution,combining the above principles with other geophysical and geochemical research evidence.In this paper,we focus on the Yitong Basin located in the Yilan-Yitong graben(rift)in the Northeast China(NEC)as the research subject to conduct the above-mentioned studies.This article employs the three-dimensional forward and inversion technology of magnetotelluric(MT)method as the main research method,supplemented by the hightemperature and high-pressure rock physics experiments,and volcanic petrology as constraints to analyzes the properties of the magma plumbing system in the Yitong Basin.The geochemistry of volcanic rocks,volcanic eruption history,burial history of the basin stratum in the Yitong Basin,and the subduction history of the Pacific plates are encompassed in the research scope,to explore and discuss the formation mechanism of the magma plumbing system of Yitong Basin and the dynamic mechanism of Yitong Basin evolution influenced by the subduction of the Pacific Plates.Firstly,we organized,summarized,and analyzed the deep electrical structures of the volcanoes in various tectonic environments and their respective observation systems.We established theoretical models of magma plumbing systems and conducted a comprehensive evaluation of the effectiveness of large-scale MT array based on the observation array for detecting magma plumbing system in the middle-lower crust and lithospheric mantle,considering 3D inversion results,sensitivity analysis and point spread functions.Based on this,we proposed an observation design for large-scale MT sites covering extensive areas and locally dense MT sites.Subsequently,we carried out MT data collection in the NEC and Yitong area.Secondly,the collected MT data were processed,analyzed,and conducted to 3D inversion.After processing the raw data,13 long period and 38 broadband MT data were selected,with effective periods of 10 s-13000 s and 0.003125 s-2000 s,respectively,for 3D inversion.In order to obtain stable and reliable 3D inversion results,the paper also conducted trial inversions on parameters such as grid spacing,initial model resistivity,regularization factor,and data errors that could affect the inversion results.The 3D resistivity model of Yitong Basin exhibit the following resistivity characteristics: Within the crustal range,the electrical structures of the southern and northern sides of the Yitong Basin are notably different.In the southern side(Yitong volcanoes),the deep parts are mainly characterized by low resistivity,with the low resistivity anomaly tilting and extending towards the Moho.In contrast,the northern side exhibits high resistivity anomalies extending to the lower crust;In the deep lithospheric mantle of the Yitong Basin,there exists a large-scale low resistivity anomaly,which forms an "upwelling" pattern and connects with the low resistivity within the crust of the Yitong volcanoes;There are also near-vertical low resistivity anomalies in the upper mantle of the Yitong Basin,rooted in the mantle transition zone.These anomalies consist of multi-level of low resistivity anomalies located at the deep upper mantle,asthenosphere,and lithospheric mantle,forming a near-vertical "mushroom" shaped structure.Then,the 3D resistivity structure,combined with high-temperature and highpressure rock physics,as well as the petrology,geochemistry,and other geophysical evidence of the study area,elucidated the multi-level trans-upper mantle magma plumbing system.This plumbing system is composed of melts with the following properties: The low-resistivity anomaly in the deep upper mantle is formed by hydrous carbonate melts with a volume faction of approximately 0.5 vol.%;The low-resistivity anomaly in the asthenosphere is formed by carbonated silicate melts,which is generated by metasomatic reaction between the hydrous carbonate melts and eclogites in the asthenosphere,with melt fraction of at least 0.13 vol.%;The low-resistivity anomaly in the lithospheric mantle is interpreted as being due to basaltic melts with a melt fraction of 4 vol.%.Alkaline basaltic melts are generated by reactions between carbonatesilicate melts derived from the upper mantle and background peridotites;Integrating the effects of different types melts by basaltic melt crystallization,it is speculated that the anomaly in the middle crust may be composed of intrusive basaltic melt dike swarms.Furthermore,the study delves into the formation mechanism of the deep-seated trans-upper mantle magma plumbing system in the Yitong Basin and the rapid response of surface volcanic activity to changes in the subduction rate of the Pacific Plate.Combining results from seismic tomography,geomagnetic sounding,and volcanic petrology,it is proposed that the release of "water" and "carbon" from the subducted western Pacific slab stagnated in the mantle transition zone will lead to the formation of magma plumbing system.Based on reconstructed subduction histories of the Pacific Plate,volcanic eruption records in the Yitong and adjacent areas,as well as statistically revealed lag times,it is suggested that the western Pacific subduction state and subduction rate may control and influence surface volcanic activity in the Yitong and adjacent areas tens of thousands to millions of years later.Finally,further integrating the evolution history of Yitong Basin,the burial history of the basin stratum,and the volcanic activity within the basin,it shows that changes in the subduction rates of the Pacific Plate establish a coupled relationship between volcanic activity and the evolution of the Yitong Basin.Based on the discovery,a mechanism for the evolution of the Yitong Basin is proposed,wherein rhythmic changes in the subduction rates of the Pacific Plate influence the dehydration and decarbonation processes of slabs stagnated in the mantle transition zone.This,in turn,leads to rhythmic changes in viscosity within the deep-seated magma plumbing system of the Yitong Basin,affecting the coupling capacity between the lithospheric mantle magma reservoir and the ductile lower crust.Consequently,it regulates the evolutionary process of the Basin.The findings of this research indicate that the evolution of rift basins in the NEC is not only influenced by the lithospheric-scale force caused by the subduction of the Pacific Plate,but also regulated by variations in the subduction rates of the Pacific Plate.These variations affect the fluxes of water and recycled materials released by the plate,thereby influencing the viscosity of the magma plumbing system and regulating the basin evolution. |
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