Magnetotelluric signatures of Neoproterozoic subduction, and subsequent lithospheric reactivation and thinning beneath central South China
Magnetotelluric signatures of Neoproterozoic subduction, and subsequent lithospheric reactivation and thinning beneath central South China
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DOI：
10.1016/j.tecto.2022.229365
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发表时间：
2022-04
期刊：
影响因子：
2.9
通讯作者：
Xin Li;Denghai Bai;Yun Chen;Yangfan Deng;Suqing Zhang;Shuai Xue
中科院分区：
文献类型：
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作者：
Xin Li;Denghai Bai;Yun Chen;Yangfan Deng;Suqing Zhang;Shuai Xue
为探讨华南地块中部古构造作用的可能残留，在横跨江南造山带西南段的600 km东西向剖面上采集了沿着宽频带和长周期大地电磁测深资料。在JOB之下，电阻率模型揭示了一个向西倾斜的跨岩石圈导体（<20 Ω·cm），嵌入相对厚（80-120 km）和高电阻率（>1000 Ω·cm）的岩石圈中，可能与扬子和华夏地块之间新元古代俯冲带的遗迹有关。该导体的地壳部分被解释为固体导电材料（例如，石墨、硫化物），它们在俯冲和造山过程中侵位到地壳中，而其岩石圈地幔部分更有可能是由俯冲相关交代过程形成的含挥发性矿物。JOB之下的软流层折射率显著降低（1~10 Ω·m），这是由于早期形成的交代地幔的低程度熔融所致。与此相反，华夏地块的岩石圈明显变薄（60-100 km），并被几个近垂直的岩石圈尺度导体（1 - 30 Ω m）分割，这些导体可能与沿着活化的剪切/断裂带的化石流体通道有关。值得注意的是，最薄的岩石圈位于软流圈地幔中显著的低电阻率（10 ~ 30 Ω·cm）异常之上，并且在空间上与晚侏罗世玄武岩露头的位置一致。我们解释这些功能的岩石圈伸展和软流圈上涌的古太平洋板块在晚中生代回滚驱动的迹象。这些结果为新元古代SCB的组装以及随后的中生代岩石圈演化提供了重要的制约因素。
Broadband and long-period magnetotelluric (MT) soundings were collected along a 600-km-long EW-trending profile across the southwestern segment of the Jiangnan Orogenic Belt (JOB) to explore the possible remnants of ancient tectonic processes beneath the central South China Block (SCB). Beneath the JOB, the resistivity model reveals a west-dipping, trans-lithosphere conductor (<20 Ω ∙ m) embedded within the relatively thick (80–120 km) and highly resistive (>1000 Ω ∙ m) lithosphere, possibly associated with relics of the Neoproterozoic subduction zone between the Yangtze and Cathaysia blocks. The crustal portion of this conductor is interpreted as solid conducting materials (e.g., graphite, sulfide) that were emplaced into the crust during subduction and orogenesis, whereas its lithospheric mantle portion is more likely to be volatile-bearing minerals formed by subduction-related metasomatic processes. We attribute the greatly reduced asthenospheric resistivities (1~10 Ω ∙ m) below the JOB to low-degree melting of the early-formed metasomatized mantle. In contrast, the lithosphere of the Cathaysia Block is evidently thinned (60–100 km) and segmented by several subvertical, lithospheric-scale conductors (1~30 Ω ∙ m) that are likely associated with fossil fluid pathways along reactivated shear/fault zones. Notably, the thinnest lithosphere sits above a prominent low-resistivity (10 ~ 30 Ω ∙ m) anomaly in the asthenospheric mantle and is spatially coincident with the location of Late Jurassic basaltic outcrops. We interpret these features as signs of lithospheric extension and asthenospheric upwelling driven by roll-back of the Paleo-Pacific Plate during the Late Mesozoic. The results provide key constraints on subduction–accretion tectonics associated with the Neoproterozoic assembly of the SCB and the subsequent lithospheric evolution during the Phanerozoic.