The Lithosphere Effective Elastic Thickness And Its Tectonic Significance In Central-northern Segment Of North-South Tectonic Belt And Adjacent Area

North-South tectonic Belt, the key study area of Continental Dynamics, is the East and West boundaries of China’s continental tectonics which has notable differences between bilateral blocks on its surface system, deep structure and lithospheric deformation characteristics. Researches on the strength of the lithosphere are critical component on resolving differences of surface system evolution and deep dynamic process. Lithosphere effective elastic thickness (Te) is an important physical parameter that dedicated on lithospheric flexural strength and dynamic response. Lithospheric dynamic response of resistance to deformation, which quantitatively restrict large scale of the lithosphere dynamics and usually as a parameter to characterize the mechanical strength of the lithosphere, is through flexural rigidity of the lithosphere effective elastic thickness. Therefore, the selected study area is adjacent to the North-South tectonic belt and is of Northeastern margin of the Qinghai-Tibet plateau and the Ordos block confluence area. Effective lithospheric elastic thickness in the study area and its geological significance are researched.Slepian multitaper spectral method was prepared by using the Bouguer gravity anomaly and new terrain elevation data to procedures for calculation of the lithosphere effective elastic thickness and the Te values in the study area was calculated. Combined with Te plane distribution, North-South tectonic belt and its adjacent regions’ deep structure characteristics and its significance in lithosphere Dynamics is discussed below. The primary results are as follows:1. Inversion calculation and comparison through different data based on the effective elastic thickness provided reliable Te values in the study areas.(1) Calculated the Te vaules with different Moho loading by using coherence of Moho relief and topography, and compared with the results of coherence of gravity and topography, showing that the former is more accurate than latter.(2) Calculated results on effective lithospheric elastic thickness disclose that the Ordos block has higher Te values, is surrounded by lower Te values of Qilian block (up to 20-50Km) and the Qinling orogenic belt (up to 20-40Km) on its southwest margin has characteristics of gradient lower Te values. Bayanhar terren has the lowest Te value (up to 15-35Km).2. In order to understand the relation between Te values and other geophysical parameters of the lithosphere, correlation analysis is based on Te distribution in the study area with terrestrial heat flow density, earthquake distribution, equilibrium gravity anomaly, crustal thickness distribution, thermal lithosphere thickness and depth of magnetic structure layer, and the relationship between them has been studied.(1) Comparative analysis on Te and terrestrial heat flow density and magnetic structure layer reveals that no obvious correspondence between Te and surface thermal flux distribution in Bayanhar, Qilian, Qingling and Ordos. However, magnetic structure layer and Te proved to be correlation. From regional comparison, Ordos features with low heat flow and high Te values and correspond with the characters of "cold" and "hard" cratonic blocks, however heat flow is higher and depth of magnetic layer is deeper compared with other blocks, suggesting that thermal contribution may come from the surrounding rift zone or due to increased mantle convection. Bayanhar block character with low heat flow and Te values, while the bottom surface of the magnetic structural layer is deep, due to the very thick crust make time lag block heating under, has not yet been transmitted to the surface; High inverse correlation between heat flow and low Te values presents in Qilian orogenic belt suggesting that the Cenozoic tectono-thermal event of great impact.(2) Te and the earthquake distribution illustrate that effective lithospheric elastic thicknes and distribution of focal depth, earthquake has a certain relationships under some extent. The northeastern margin of the Qinghai-Tibet block represents the most intensive crustal movement, the highest frequency and strength of earthquake activity in Chinese continent that corresponding to a relatively low (smaller) Te value. High frequency of earthquake distribution between the northeastern margin of the Tibetan Plateau and Ordos block correspond with the transition zone of crustal deformation and the broken structure. North-south tectonic belt and west of the Qilian are dense areas of highly possibility occurring strong earthquakes of which Te value is merely 20-30 km. Ordos block is weak in seismic activity internally, but with the Te value up to 80-90 km.(3) Comparative analysis on Te and crust, lithosphere thickness prove that Te distribution in Ordos region is much larger than the average crustal thickness, but less than the thermal lithosphere thickness which indicates the high intensity of the lithosphere may come from a higher intensity in the upper mantle lithosphere. For the periphery of the active region, Te value is equivalent with the crustal thickness, but the hot lithosphere is thicker which reveals that upper mantle contributes fewer to Te. Crust and thermal lithosphere are both thicker in Bayan block with low Te value indicates that the mechanical strength of the block is mainly borne by (partial) upper crust, the upper mantle contributes smaller to the Te values.3. Researches on the relationships of deep structure, lithospheric composition, density and velocity characteristics with Te in the study area and combining with the rheological characteristics of the lithosphere is discussed in this paper, and comprehensively discussed dynamic mechanism of formation of North-South tectonic belt.(1) Comparative analysis of Te, crustal thickness and the geoeletrical structure in the study area expose that the increasing thickness of the low velocity crust, low-resistivity layers development and other factors could lead to lower lithosphere comprehensive strength; comparative analyzing of Te and the basement composition shows that the earlier formation of the crystalline basement, the sooner consolidation, the higher degree of cratonization, and the bigger of its material composition and density corresponding, greater mechanical strength of the lithosphere; Comparative analysis of Te, density and velocity characteristics shows that terranes with bigger density and speed is relatively compact and rigid in constituting the rock, deformation is less unlikely at a given stress, on the contrary, ductile deformation is likely to occur, the integrated intensity of the lithosphere small.(2) Combining with the researching results of northeastern margin of the Tibetan plateau characterized by weak mantle rheology suggests the decoupling relations exist in the mechanics area, weak mantle rheology that may crust lithospheric mantleand low overall strength andbrittle crust part is foreign the main bearer of stress. On top of weaker crustal strength, the eastern edge of the plateau lower crust may block the flow of western and north western Yangtze block on the top crust which presumably causes the formation to the north and south with the gravity anomaly gradient. Ordos block has a typical Continental craton lithosphere structure characteristic with large lithospheric total intensity especially high intensity upper mantle that could be a major contribution to the high mechanical strength. No large-scale of High velocity area is found in lower crust which is dominated by felsic components, therefore, material flows East of the Tibetan plateau has resulted in lower crustal thickening.