| The Eastern Sichuan-Xuefeng fold-thrust belt in the interior of the South China Plate has the structural phenomenon that the middle-upper crust structural vergence is opposited,that is,the middle crust structural vergence is opposite to the upper crust and the NW to the regional structural compression force.Structural vergence reversal in the interior of fold-thrust belt is rare in nature,difficult to explain with the classical Coulomb thrust wedge theory,and difficult to reproduce in the laboratory.At present,the mechanism of the the middle-upper crust characteristic structural vergence reversal in the Eastern Sichuan-Xuefeng fold-thrust belt is not very clear.The author uses the physical analogue means,which is effective to research this structural deformation process and mechanism,to develop a systematic study for this problem.Based on the experimental results of 9 sets of conceptual models and 7 sets of similar models,this paper draws the following main insights:?1?Under the condition of single detachment,the effect of lateral friction plays an important role in the formation of the back-thrust non-Coulomb thrust wedge.When the ratio of initial lateral friction to initial base friction is extremely large?initial?s/?d?1.19?,the back-thrust non-Coulomb thrust wedge is formed;when the ratio of initial lateral friction to initial base friction is compared large?initial 0.15??s/?d<1.19?,the double-vergence thrust wedge is formed;when the ratio of initial lateral friction to initial base friction is extremely small?initial 0<?s/?d<0.15?,the fore-thrust wedge is formed.?2?Under the condition of two detachments,the layered slippage restricts the deformation characteristics of the upper and lower layers.The deformation characteristics and the pattern of the upper and lower layers of the thrust wedge generally follow the deformation law under the single detachment,especially the non-Coulomb back-thrust deformation of the lower layer of the two detachments completely follows the conceptual model:that is,when the initial?s/?d?1.19,the structural vergence inside the thrust wedge is all directed to the hinterland,the so-called non-Coulomb fold wedge.The deformation anomaly occurs in the deformation process of the upper layer.The initial?s/?dratio of the corresponding deformation feature of the upper layer does not fall completely within the interval defined by the initial?s/?dratio of the corresponding deformation feature in the conceptual model.The critical value of the initial?s/?dof the upper non-Coulomb backthrust and double-vergence deformation is no longer 1.19 but between 0.81 and 0.98.That is,under the experimental conditions of two detachments,the condition to formate the non-Coulomb back-thrust fault in the upper layer is 0.81<initial?s/?d;the condition to formate the double-vergence thrust wedge in the upper layer is 0.34<initial?s/?d?0.81;the condition to formate the fore-thrust wedge in the upper layer is 0initial?s/?d?0.34.Compared with insights from the conceptual model{when initial?s/?d?1.19,the structures in the thrust wedge all point to the hinterland,the so-called back-thrust non-Coulomb thrust wedge;when0.15?initial?s/?d<1.19,a double-vergence thrust wedge is formed;when initial?s/?d<0.15,a fore-thrust wedge is formed},the upper layer defined intervals of the initial?s/?dof the thrust wedge under the condition of two detachments to produce a non-Coulomb thrust wedge and a fore-thrust wedge becomes larger,but the defined interval of the initial?s/?dof a double-vergence thrust wedge is smaller.?3?Under two-dimensional conditions,the reversal of the direction of the maximum principal stress?1 caused by lateral friction is the mechanical mechanism of the formation of the back-thrust non-Coulomb thrust wedge.This has certain implications for three-dimensional research.?4?The mechanism of middle-upper crust structural vergence reversal in the Eastern Sichuan-Xuefeng fold-thrust belt is most likely due to the difference of initial?s/?d ratio between the middle and upper crust.That is,the middle crust has a very weak base?initial base friction?d=7.9×105Pa?and a extremely large ratio of?s/?d,so the tectonic deformation is a back-thrust non-Coulomb thrust wedge;the initial?s/?d ratio of the upper crust is small?initial?s/?d=0.07?,and the thrust wedge deformation mode is a double-vergence groove-type thrust wedge. |
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