EUROBRIDGE: new insight into the geodynamic evolution of the East European Craton
Svetlana BogdanovaRoland GorbatschevM. GradTomasz JanikA. GuterchElena KozlovskayaGediminas MotuzaGražina SkridlaitėV. I. StarostenkoL. Taran
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Abstract The Palaeoproterozoic crust and upper mantle in the region between the Ukrainian and Baltic shields of the East European Craton were built up finally during collision of the previously independent Fennoscandian and Sarmatian crustal segments at c. 1.8-1.7 Ga. EUROBRIDGE seismic profiling and geophysical modelling across the southwestern part of the Craton suggest that the Central Belarus Suture Zone is the junction between the two colliding segments. This junction is marked by strong deformation of the crust and the presence of a metamorphic core complex. At 1.80-1.74 Ga, major late to post-collisional extension and magmatism affected the part of Sarmatia adjoining the Central Belarus Zone and generated a high-velocity layer at the base of the crust. Other sutures separating terranes of different ages are found within Sarmatia and in the Polish-Lithuanian part of Fennoscandia. While Fennoscandia and Sarmatia were still a long distance apart, orogeny was dominantly accretionary. The accreted Palaeoproterozoic terranes in the Baltic-Belarus region of Fennoscandia are all younger than 2.0 Ga (2.0-1.9, 1.90-1.85 and 1.84-1.82 Ga), whereas those in Sarmatia have ages of c. 2.2-2.1 and 2.0-1.95 Ga. Lithospheric deformation and magmatism at c. 1.50-1.45 Ga, and Devonian rifting, are also defined by the EUROBRIDGE seismic and gravity models.Abstract While the western part of northern Tarim Craton has long been considered as a Paleozoic passive margin, a pronounced Silurian‐Devonian magmatism developed on eastern part of this margin may indicate different but active margin setting. In this contribution, detailed structural mapping, petro‐structural analysis, and geochronological investigations were conducted in the Korla area, eastern part of northern Tarim Craton. Three main generations of fabrics were recognized. The earliest pervasive fabric is an originally sub‐horizontal metamorphic S1 foliation that is in part associated with migmatization characterized by high temperature/low pressure metamorphic mineral assemblages, interpreted as reflecting crustal extension. S1 foliation was affected by D2 contraction forming regional‐scale F2 upright folds associated with sub‐vertical axial planar foliation S2. D3 is marked by development of NW‐SE oriented dextral fault, asymmetric mega‐folding of S2 and spaced NW‐SE‐striking S3 foliation, likely in response to dextral transpression. Geochronological data indicate that D1 extension occurred from ca. 420 to 410 Ma, D2 contraction started around 410 Ma and lasted till 400 Ma or later, and D3 transpression was ongoing around ∼370 Ma. Integrated with regional data, an updated geodynamic model is proposed by interpreting the Central Tianshan, South Tianshan and NE Tarim Craton as an early Paleozoic supra‐subduction system. We suggest that the Silurian‐Devonian event reflects thermal softening and horizontal stretching of the supra‐subduction crust, resulting in drifting of the Central Tianshan continental arc from the proto Tarim Craton in association with opening of the South Tianshan back‐arc basin in‐between.
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华北克拉通（华北）南缘中元古代早期熊耳群火山活动之后，在渑池-确山地区发育了一套中-新元古代沉积（汝阳群、洛峪群、黄连垛组、董家组以及罗圈组和东坡组），记录了该时期沉积-构造演化过程。通过该区碎屑锆石和洛峪群顶部凝灰岩年龄的约束，将汝阳群-洛峪群的沉积时代基本限定于约1750~1600Ma，导致洛峪群由中元古界上部或新元古界下划到中元古界长城系。因此，在现有的地层年代格架下，需要重新对该区中-新元古代沉积演化进行讨论。本文通过对洛峪口组上覆黄连垛组和董家组沉积环境和物源分析，同时借助古生界辛集组源区分析的约束，揭示华北南缘中-新元古代沉积-构造演化。沉积环境分析显示，黄连垛组沉积初期发育了河口湾沉积环境。伴随海侵扩大，在下汤地区沉积了潮上带长石石英砂岩与泥质粉砂岩，而叶县地区发育了潮间带泥晶白云岩。晚期下汤和叶县地区发育潮下带泥晶白云岩与含硅质条带白云岩。董家组与下伏黄连垛组为平行不整合接触，董家组沉积初期为陆源碎屑物质供给充分的滨海相，在下汤和叶县地区沉积底部细砾岩及长石石英砂岩。随后，两个地区发育潮坪相长石石英砂岩与泥质粉砂岩，在顶部沉积了局限台地钙质泥岩。黄连垛组在豫西下汤和叶县地区沉积于河口湾-潮坪沉积环境，整体表现为自南向北的海进序列，而董家组总体上沉积于局限海盆的滨浅海-潮坪环境。由于下伏汝阳群-洛峪群分别沉积在河流-滨海-潮坪和浅海-滨海-潮坪环境，黄连垛组和董家组指示其沉积时期盆地产生收缩。碎屑锆石定年结果显示，黄连垛组和董家组碎屑锆石年龄主要峰值为1800Ma、2250Ma、2350Ma、2650Ma，两者物源均为华北克拉通。但下汤地区早古生界辛集组碎屑锆石显示主要峰值年龄为1850Ma、2500Ma、2200Ma、2700Ma，其次为1200Ma。结合华北克拉通发育大量的中元古代末期到新元古代碎屑锆石以及南缘中元古界官道口群和新元古界栾川群沉积特征，黄连垛组和董家组代表的局限盆地（海盆）沉积可能构成了该时期盆地的边缘相，期间伴随的抬升和盆地中心迁移可能与同期大地构造演化密切相关。
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Continental Margin
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Mylonite
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The debate on the role of vertical versus horizontal tectonism in Archean cratons is intimately linked to the initiation of plate tectonics. The dome-and-keel architecture has been considered as a consequence of vertical tectonism. Although such a structural pattern is documented in some Mesoarchean and older cratons, such as the Kaapvaal and Pilbara cratons, whether it also occurs in Neoarchean cratons is poorly constrained. Determining the kinematics, structural evolution, and the timing of these structures is crucial in understanding how the tectonic behavior operated during the evolution of the early Earth. The North China Craton, especially its eastern part, is a Neoarchean continental block and preserves typical greenstone-granite rock assemblages. Detailed structural mapping reveals that the Anziling area (east China) is characterized by a typical dome structure without significant reworking by later deformation. The dome is in tectonic contact with a supracrustal rock assemblage that is now the dip-slip Shuangshanzi ductile shear zone. In the supracrustal rocks, compositional layers are folded into upright isoclinal folds. Meanwhile, along the shear zone, foliation varies from NNW to SW with sub-vertical dip. Mineral stretching lineations indicate a sinistral shear sense with a slightly oblique-slip component in the north, but show NWW-directed and SW-directed steep dip-slip shear in the west and south, respectively. Kinematic indicators imply that the granitic dome formed through a vertically upward movement accompanied by an uneven clockwise rotation. The supracrustal rocks sank downwards to form the regional keel structure. Structural data suggests that the Anziling area is a typical dome-and-keel structure. U-Pb zircon dating on pre-, syn-, and post-tectonic dykes indicate that the dome-and-keel structure formed at 2530−2500 Ma, and was intimately related to the emplacement of tonalite-trondhjemite-granodiorite granitoids. New data from this study suggest that until the late Neoarchean, the vertical tectonism was still a dominant tectonic regime that was operating in the eastern North China Craton.
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Geochronology
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The E–W trending Yanshan belt, an intraplate fold-thrust belt located in the northern North China Craton, has experienced several episodes of Mesozoic deformation, which resulted in the widely distributed magmatism and mountain-basin tectonics that completely re-shaped the topography of the eastern North China Craton. The eastern part of the famous Chinese Great Wall was built on the high range of the southeastern Yanshan mountain belt juxta-posed to the plain, which directly relates to the Bohai Bay basin. Our study focuses on which tectonic processes created such mountain-basin couple in Mid-Late Mesozoic times. The U-Pb LA-ICP-MS dating of zircons yield ages of 114 to 201 Ma for various granites and 115 to 116 Ma for volcanic rocks from Yixian and Jiufotang Fms. The detrital zircons from the Lower Cretaceous sandstones yield four age groups of 2587 to 2460 Ma, 2222 to 1828 Ma, 297 to 190 Ma and 187 to 100 Ma, which are all sourced from the Qinglong and surrounding areas and indicating that the Qinglong area started to uplift after the Middle Jurassic. The Qinglong area underwent multiple deformation by NE–SW compression in the Middle-Late Jurassic times, WNW–ESE compression in the Late Jurassic to Early Cretaceous, ENE–WSW extension in the Early Cretaceous and NNW–SSE compression in the Late Cretaceous during the final stage of the Yanshanian orogeny. Meanwhile, widely distributed granite intrusions and emplacement of the Upper Jurassic-Lower Cretaceous volcanic rocks indicate a large amount of magma input into the area. The Qinglong area with the Great Wall along its southern margin close to adjacent plain to the south was uplifted to form the Qinglong highland and surrounding related basins by combination of the following three processes: the multiple tectonic Late Jurassic and Late Cretaceous shortening processes related to Yanshanian orogeny, Early Cretaceous regional extension triggered by slab-retreat of the Paleo-Pacific ocean and inflation of large amount magma at depth during Jurassic and Early Cretaceous.
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