Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth’s surface may provide direct information about the upper mantle beneath the volcanic region. This paper presents the study on mantle xenoliths col...Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth’s surface may provide direct information about the upper mantle beneath the volcanic region. This paper presents the study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the thermal structure and rheology of the upper mantle beneath the region. The results show that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52～75km), and garnet lherzolite at greater depth (greater than 75km), instead of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of North China, and is somewhat closer to the Oceanic geotherm. The crust-mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel lherzolite. If we take 1280℃ as the temperature of the top of asthenosphere, then the lithosphere-asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometry, while the strain rate and equivalent viscosity are determined by using the high temperature flow law of peridotite. The differential stress, strain rate and viscosity profiles constructed on the basis of the obtained values indicate that asthenospheric diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, the scale and intensity of the diapir was significantly less than that occurring in the North China region. Moreover, numerous small-scale shear zones with localized deformation might occur in the lithospheric mantle, as evidenced by the extensive occurrence of xenoliths with tabular equigranular texture.展开更多
Numerical experiments are used in this study to systematically investigate the effects of convergence rate,crustal rheological strength,and lithospheric thermal structure on the dynamics of continental collision.The s...Numerical experiments are used in this study to systematically investigate the effects of convergence rate,crustal rheological strength,and lithospheric thermal structure on the dynamics of continental collision.The study focuses on the types,conditions and processes of unstable continental subduction.Modelling results suggest that the development of unstable continental subduction can be promoted by conditions that tend to decrease rheological strength of the lithosphere,such as low crustal rheological strength,"hot"thermal structure of the lithosphere,or low convergence rate.Unstable subduction mode can be further categorized into three types:(1)multi-stage slab breakoff,(2)continuously"flowing"of fluid-like slab into the upper mantle,and(3)large-scale detachment of the thickened orogenic root.These three types of unstable continental subduction are respectively associated with(1)a low convergence rate,(2)"hot"thermal structure of the lithosphere with a high convergence rate,and(3)moderate-high crustal rheological strength with a low convergence rate.It is also revealed that the evolution of crustal melting is dominated by the deformation pattern of continental collision,which is mainly controlled by crustal rheological strength.The modelling results have important implications for understanding of continental subduction mode selection under specific geodynamic conditions.展开更多
文摘Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth’s surface may provide direct information about the upper mantle beneath the volcanic region. This paper presents the study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the thermal structure and rheology of the upper mantle beneath the region. The results show that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52～75km), and garnet lherzolite at greater depth (greater than 75km), instead of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of North China, and is somewhat closer to the Oceanic geotherm. The crust-mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel lherzolite. If we take 1280℃ as the temperature of the top of asthenosphere, then the lithosphere-asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometry, while the strain rate and equivalent viscosity are determined by using the high temperature flow law of peridotite. The differential stress, strain rate and viscosity profiles constructed on the basis of the obtained values indicate that asthenospheric diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, the scale and intensity of the diapir was significantly less than that occurring in the North China region. Moreover, numerous small-scale shear zones with localized deformation might occur in the lithospheric mantle, as evidenced by the extensive occurrence of xenoliths with tabular equigranular texture.
基金supported by National Basic Research Program of China(Grant Nos.2014CB440901,2015CB856106 and 2016 YFC0600303)National Natural Science Foundation of China(Grant Nos. 41190073 and 41372198)the National "Qian-Ren " Program to Z.H.Li, the Fundamental Research Funds for the Central Universities to Sun Yat-senUniversity,and the PetroChina Project(Grant No.2016B-0501)
文摘Numerical experiments are used in this study to systematically investigate the effects of convergence rate,crustal rheological strength,and lithospheric thermal structure on the dynamics of continental collision.The study focuses on the types,conditions and processes of unstable continental subduction.Modelling results suggest that the development of unstable continental subduction can be promoted by conditions that tend to decrease rheological strength of the lithosphere,such as low crustal rheological strength,"hot"thermal structure of the lithosphere,or low convergence rate.Unstable subduction mode can be further categorized into three types:(1)multi-stage slab breakoff,(2)continuously"flowing"of fluid-like slab into the upper mantle,and(3)large-scale detachment of the thickened orogenic root.These three types of unstable continental subduction are respectively associated with(1)a low convergence rate,(2)"hot"thermal structure of the lithosphere with a high convergence rate,and(3)moderate-high crustal rheological strength with a low convergence rate.It is also revealed that the evolution of crustal melting is dominated by the deformation pattern of continental collision,which is mainly controlled by crustal rheological strength.The modelling results have important implications for understanding of continental subduction mode selection under specific geodynamic conditions.
