| The South China Block(SCB)is located in the southeastern part of the Asian continent,adjacent to the Qinling-Dabie orogenic belt to the north,the Tibet Plateau to the west,and the Pacific Plate to the east.The SCB is composed of the Yangtze and the Cathaysia blocks,which collided in the Neoproterozoic and formed the Jiang-Shao fault zone.The 1300 km wide inland orogenic belt and the widespread magmatic activity in South China during the Mesozoic are affected by the Paleo-Pacific plate(PPP)flat subduction and subsequent delamination and rollback processes.However,the mechanism of the PPP flat subduction formation and the influencing factors of subsequent delamination process remain unclear and need further study.In this thesis,we use two-dimensional numerical simulation method to study the PPP flat subduction and subsequent delamination processes beneath the SCB in the Mesozoic.Based on this study case,we discuss several controlling factors of the flat subduction of an ancient oceanic lithosphere.In addition,we further discuss the influence of thickened oceanic crust and subduction velocity on the transition from flat subduction to steep subduction.The numerical simulation results combined with the observation data provide a theoretical explanation for the dynamics of the SCB in the Mesozoic,and provide a reference for other related studies.The formation of flat subduction is a result of complex multi-factor interaction,which mainly depends on the balance between resisting force and driving force of subducted plate.With “abnormal” dynamic factors,the resisting force of the subducted plate is greater than the driving force and the oceanic slab tends to evolve to flat subduction.Compared with the young oceanic plate(≤45 Ma),the older age(80 Ma)and higher average density of the PPP make the larger driving force more severe for the "abnormal" factors that can cause flat subduction.The sensitivity of the four dynamic parameters is tested,including initial subduction angle,oceanic crust thickness,continental lithosphere thickness and its thrust motion.The results show that in the range of dynamic parameters of the Mesozoic SCB and the PPP,a smaller initial subduction angle(e.g.12°)or a larger oceanic crust thickness(e.g.35 km)can induce a flat subduction of the 80 Ma oceanic plate without other favorable factors.However,the thicker continental lithosphere thickness(e.g.140 km)or the continental lithosphere trenchward motion(e.g.3 cm/yr)can only reduce the subduction angle,but not induce flat oceanic slab subduction.The numerical simulation results show that the thickened oceanic crust in the oceanic lithosphere and the coupling force between the continental lithosphere and the oceanic lithosphere are the primary controlling factors of the PPP flat subduction,while the thickness and the trenchward motion of the continental lithosphere are the secondary controlling factors.Based on the numerical simulation results and geological observation data,we conclude that the PPP subducted beneath the SCB at a moderate angle during 280-260 Ma,which triggered the Andean continental magmatic arc in the coastal area.During 260-240 Ma,the PPP under the SCB gradually formed flat subduction,which might be the result of the thickened oceanic crust,plate front break-off and the dehydration of subducted oceanic slab.At about ca.260 Ma,the thickened oceanic crust in the PPP reached the continental arc margin,which resulted in a decreased average density of the oceanic plate near the trench.The extra buoyancy provided by the thickened oceanic crust was conducive to the formation of the subsequent flat subduction.In addition,the average density of the basalt crust in the subducted part of the oceanic plate increased due to the eclogite phase change.The increased density difference between the subducted and the non-subducted oceanic plates made the break-off of subducted plate in a short time due to the thermal relaxation.The subducted front of the dense plate was separated from the PPP and sank into the deep asthenosphere mantle,which effectively reduced the driving force on the PPP.On the other hand,the dehydration of the oceanic plate during subduction and slab break-off transformed the dry lithospheric mantle at the bottom of the overlying continental lithosphere into a wet one.Its low viscosity and low density increased the contact area between the PPP and the SCB,which enhanced the coupling force between the upper and lower plates.As a result,the gradually subducted thickening oceanic crust and the strong coupling foce together increased the resisting forces.The increase of resisting force and the decrease of driving force of the subducted PPP led to the formation of the flat subduction.The magmatic activity of the SCB migrated inland from ca.250-190 Ma,reflecting the flat subduction of the PPP to the inland direction.The distribution of igneous rocks between ca.190-80 Ma showed a migration to the trench direction,which was attributed to the slab delamination caused by the eclogite phase change and the subsequent slab rollback.However,the exact factors causing the plate to sink are still unclear.At the same time,some scholars have pointed out that the late Mesozoic volcanic rocks in the southeast coastal areas of China show a northeastward younging trend,and put forward the PPP asynchronous rollback model to explain this trend.However,it is still unclear what factors caused this asynchronous rollback process.Using 2-D thermomechanical numerical models,we study the effect of the thickened oceanic crust size(including length and thickness)and subduction velocity on the transition mode of the PPP from flat subduction to steep subduction.The simulation results show that:(1)the larger thickened oceanic crust and the lower subduction velocity are more likely to cause the transition of flat-to-steep subduction as delamination;(2)Even a small(100 km)thickened oceanic crust can induce slab delamination at a sufficiently low subduction velocity;(3)Under the same conditions,the larger thickened oceanic crust requires less subduction velocity to cause slab delamination;(4)With a small size(e.g.100 km long and 20 km thick)thickened oceanic crust and a certain subduction rate,the oceanic slab can maintain a long-lasting flat subduction.Combined with the numerical simulation results and the global reconstruction model,we conclude that the eclogitization phase change of thickened oceanic crust and the motion change of the SCB from the southeast direction to the northwest direction away from the trench since ca.195±5 Ma induced the delamination of the PPP.Under similar dynamic conditions in the south and north parts of the SCB,the PPP asynchronous rollback may be caused by the north-south difference in the size of the subducted thickened oceanic crust.Future research of these dynamic processes requires the combination of 3-D high resolution thermo-mechanical numerical simulation with observation data. |
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