Tancheng-Lujiang Fault Research Articles

Widespread late Mesozoic to Recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from South China

Paleomagnetic results are described from eighteen formations sampled in the Nanjing (Jiangsu Province) and Yichang (Hubei Province) areas of South China. The marine platform carbonates and elastics range in age from early Cambrian to early Triassic. Progressive alternating field and/or thermal demagnetization analyses reveal predominantly single component magnetizations, sometimes of both normal and reverse polarity. These magnetizations, isolated in 457 of the 549 samples studied, typically have directions in geographic coordinates coincident with late Mesozoic to Recent fields. A secondary origin for the magnetizations is supported by negative fold tests in several units. A lower Carboniferous and possibly a lower Triassic limestone may have escaped the pervasive remagnetization; if the remanences isolated are indeed prefolding, a large counterclockwise tectonic rotation of the Nanjing area is implied by comparison with available Carboniferous and Triassic paleopoles reported from elsewhere in South China. This rotation may have been associated with Mesozoic left-lateral motion on the Tancheng-Lujiang fault zone.

Formation and evolution of the Tancheng-Lujiang wrench fault system: a major shear system to the northwest of the Pacific Ocean

In the northwestern peri-Pacific tectonic belt on the continental margin of East Asia, there is a series of major faults trending NE-NNE, i.e. the Tancheng-Lujiang wrench fault system. This fault system extends more than 5000 km, with a width of 800–1000 km in the continent and a general strike of N30°E. On the basis of the results of special research on the major faults and a lot of new information about the fault system, the authors discuss early Mesozoic (Triassic-Jurassic period) sinistral transcurrent faulting in the different regions. It is suggested that in the eastern continent of East Asia, the E-W trending tectonic pattern of the Paleozoic and beginning of the Mesozoic appear in the nuclei of the Sino-Korean Continent Massif and the Yangtze Continent Massif. These ancient continent massifs accreted both southwards and northwards, which finally led to their collision and suturing to the Angura Continent Massif. Later, this tectonic framework was changed greatly by the Tancheng-Lujiang wrench fault system, along which left-lateral horizontal displacement of 1000–1500 km took place. Since the Mesozoic, the dislocation and deformation of horizontal displacement of the Tancheng-Lujiang fault system produced a series of structures trending NE-NNE in the continental margin, which have greatly transformed the ancient structures of the preceding Mesozoic. This is an important diktyogenese in the continent of East Asia. During the horizontal displacement motion of these faults, the folds, nappes and decollement structures, pull-apart basins, and subductions within the continent occurred in the continent at the same time. Some large sinistral ductile shear zones of horizontal displacement appeared. The depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin. Whilst the ductile shear deformation occurred, the fault dynamo-thermal metamorphism and the fault remelting took place, and magma activity appeared within the continent. In the Mesozoic, the fault zones used to be high energy zones and recently became the seismic belts within the continent. Formation of the Tan-Lu fault system was due to the collision between the southern and northern blocks. Its large sinistral horizontal displacement in the Mesozoic was closely related to the Kula Plate moving northwards. Therefore, the continent disintegrated by shearing, which produced some new terranes. The shear edge belt was formed between the continent and ocean. The change of relative motion between the continent and Pacific Ocean plate transformed the Tan-Lu fault system into first extension and then compression. Clarifying the characteristics of the Tan-Lu fault system is an important key in understanding the evolution of the East Asia Continent since the Mesozoic.

Ductile deformation and regional strain field in the southern segment of the Tancheng-Lujiang fault zone, eastern China

Studies in a segment of the Tancheng-Lujiang fault zone show that it is a major transcurrent ductile shear zone with a considerable sinistral displacement. The formations (Pt→T1+2) at depths of 5 to 15 km may have cropped out in this area during subsequent erosion. Many ductilely deformed structures are in it. The deformed zone was formed during the Indo-Sinian orogeny. On the basis of measurements and analyses of many deformed pebbles within the shear zone, it is suggested that the deformation in the zone studied may be catagorized as a variation of simple shear and plane strain with a constant volume. The intensely deformed belt is generally 40 to 50 km wide, with the average strain ratio 27.68 and the maximum greater than 87.37. From analysis of elongation strain, we estimate that the deformed belt has been elongated by 164.23 km and narrowed by 222.5 km. The internal belt was formed by a high shear strain γ, calculated to be more than 11.34. The deformed belt is associated with syntectonic dynamothermal metamorphism, represented by greenschist facies and retrogressive metamorphism on previous amphibolite facies.