| Southern Anhui is located in the eastern part of the Jiangnan orogenic belt.During the late Mesozoic,this area experienced a transition from compressional environment to extensional environment.Many predecessors have done a lot of work on this.However,the time,mode and geodynamic background of the transition are still not unified.In this paper,we try to understand the transition from compressional structure to extensional structure in South China in Late Mesozoic and the approximate time of the transition from compressional structure to extensional structure,based on the growth strata found in Tunxi-Xiuning basin and Jixi-Huayang basin in southern Anhui.Through the combination of field investigation and laboratory work,it is firstly found that growth strata are developed in the lower layer of Early Cretaceous Huizhou Formation in Tunxi-Xiuning Basin,Huangshan Sity and Huayang basin,Jixi County.In Yuting area of Tunxi-Xiuning basin,asphalt plant-T-junction-on the opposite side of highway intersection,near the northeast,the occurrence of Huizhou formation changes from 49° to 22°;In the area of new highway in Jixi-Huayang basin,intersection beside burying and descendingnorthern Xima village-under S01 Lihuang expressway bridge,near the northeast,the occurrence of Huizhou formation changes from 50° to 11°;In the Hukou-Shangyan-liqukou route of Tunxi-Xiuning basin,near the northeast,the occurrence of Huizhou formation changes from 44°to 26°;In Tunxi-Xiuning basin,northeast of Shangcun-northeast gas station of Donglinxi townnext to Wuliting South G56 expressway,NNE direction,the occurrence of Huizhou formation is from 23° change to 8°;In the northwest of Songkou village in Tunxi-Xiuning basin,near the northwest,the occurrence of Huizhou formation ranges from 35° change to 17°,The occurrence change of these strata shows a slow transition in space,not a sudden change,and the thickness of single layer strata has the form of downward thickening.There are angular unconformity between Huizhou formation and underlying old strata,synsedimentary fault,normal fault,left strike slip fault and lithologic change.Combined with the previous measurements in 2019,the zircon U-Pb age of rhyolite near the top of Shiling formation under Huizhou formation in Tunxi area is 139.0± 5 Ma.It is suggested that the early Cretaceous extensional growth strata in Tunxi-Xiuning basin and Jixi-Huayang basin began at about 140 Ma,which may be related to the subduction and rollback of the Paleo-Pacific plate. |
