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Magnetotelluric images of Paleoproterozoic accretion and Mesoproterozoic to Neoproterozoic reworking processes in the northern Sao Francisco Craton, central-eastern Brazil

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摘要

Broadband and long period magnetotelluric (MT) data were collected along an east-west oriented, 580-km-long profile across the northern Sao Francisco Craton where extensive Proterozoic and Phanerozoic sedimentary cover and lack of deep-probing geophysical surveys have prevented to establish unequivocally the cratonic character of the Archean-Paleoproterozoic lithosphere. Following dimensionality analyses, the MT dataset was interpreted using both 2-D and 3-D inversion procedures. The near-surface structure is better resolved in the 2-D model due to its finer resolution. A huge upper crustal conductor is found all along the shallow early Neoproterozoic Irece Basin in the central domain of the craton, extending laterally for approximately 150 km and restricting signal propagation below the basin. Its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package. Another upper crustal conductor is observed on the west side of the profile, interpreted as fractured metasedimentary rocks of the Rio Preto belt thrusted on top of the craton basement during Neoproterozoic marginal collision. The 3-D model explains significantly better the measured data related to deep structure. Contrary to what is expected for a stable cratonic block, the geoelectric model shows pronounced electrical complexity and heterogeneity, an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events. Different lithospheric resistive blocks bounded by major conductive zones are identified. Constrained by geochemical and isotopic data, these vertical conductive interfaces are interpreted as cryptic suture zones due to large-scale amalgamation of continents and microcontinents leading to the assembly of the Sao Francisco Craton in the Paleoproterozoic. The conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism. At upper mantle depths, high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the Paleoproterozoic subducting slabs and Mesoproterozoic to Neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle.
机译:宽带和长期磁音(MT)数据沿着东西部化,580公里长的围绕圣弗朗西斯科·克拉顿,其中广泛的正古古和山峰沉积覆盖,缺乏深入的地球物理调查,防止了明确建立Archean-Paleoprotroioicoic岩石圈的克拉廷特征。在维度分析之后,使用2-D和3-D反转过程来解释MT数据集。由于其更精细的分辨率,在2-D模型中,近表面结构更好地解决。所有巨大的上层地壳导体都沿着浅层早期的新核zoice Ireece盆地在Craton的中心领域中发现,横向延伸约150公里并限制盆地下方的信号传播。它的高电导是通过沉积包装中的高孔隙率和高液体盐度的组合来解释。在概况的西侧观察到另一个上层导体,被解释为在新蛋白古代边际碰撞期间在Craton地下室的顶部推动的RIOPRIOPER带上的裂缝后的岩石。 3-D模型明显更好地解释了与深结构相关的测量数据。与预期的稳定克拉廷块相反,地电气模型显示出明显的电气复杂性和异质性,这表明克拉特罗斯岩石圈在过去乘以构造进一步的事件。鉴定由主要导电区界定的不同的岩石层电阻块。由于地球化学和同位素数据受到限制,这些垂直导电界面被解释为阴道缝合区,由于大陆和微加密的大规模融合,导致古普罗佐的圣弗朗西奇克拉顿的组装。导电性增强更有可能通过沿着先前的缝合区域在迈克斯·岩浆中施加硫化物来解释。在上部地幔深度下,大部分轮廓下观察到的高导电性表明了古普罗佐奇岩体底板和Mesproterozoio对深流体的NeoProoterozoic升高的不相容元素,并且熔化的碳酸根系和熔化的偏差过程是粗大的碎屑垫。

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