| Late Mesozoic Basin-Range evolution in the Middle Yangtze region is of great significance to understanding of Mesozoic tectonic transformation and the formation of the North-South gravity gradient zone in South China continent.Systematic analysis of sedimentary facies and sandstone clastic components of Jurassic strata in Zigui Basin,combined with the low-temperature thermochronological data,discussing the sedimentary filling of Zigui Basin,Basin-and-Range Tectonic Evolution in the Peripheral Orogenic Belt,and characteristics of regional tectonic and geomorphological transformation.1.Based on the field geological observation of several sections in Zigui Basin,the sedimentary facies of Zigui Basin are analyzed according to the rock association and sedimentary structure.The early Jurassic is characterized by fluvial sedimentary environment,and a set of stable lacustrine clastic rocks were deposited in the middle and late stages of the early Jurassic.During the Middle Jurassic,the warm and humid lake environment was transformed into the dry river environment,and a set of meandering river facies deposits with huge thickness were deposited.Late Jurassic inherited the sedimentary characteristics of Middle Jurassic and mainly deposited a set of meandering fluvial deposits.2.Based on the analysis of sandstone clastic composition,paleocurrent evolution,uplift of Huangling anticline and evolution history of surrounding orogenic belt,the provenance area of Zigui basin experienced two obvious changes in Jurassic.In the early Jurassic,the Lower Jurassic Tongzhuyuan Formation in Zigui Basin is with NNW to SSE Paleocurrent direction,and sandstone composition maturity is high,quartz content is high,and metamorphic rock clasts are the main clastic components,indicating provenance area is South Qinling region,which to the NNW of the Zigui basin.In the Middle Jurassic,the lake basin sedimentary center of foreland basin gradually migrates westward,and the direction of paleo-current changed to south-west.The content of feldspar and lithic debris in sandstone of Middle Jurassic increased obviously,and the characteristics of volcanic and metamorphic debris were obvious.Combined with the U-Pb age spectrum of some clastic zircons,it is indicated that the Qinling-Dabie orogenic belt located in the NE direction of the basin is the main provenance area of the basin.Vertical variation of sandstone clastic components in the Middle Jurassic recorded the denudation process from shallow to deep in the Qinling-Dabie orogenic belt located in the NE direction of the Zigui Basin.In the late Jurassic,sediments inherit some provenances of Middle Jurassic.The clastic material entered the Zigui Basin along the paleochannel from NE to SW,but the carbonate rock debris content of the sandstones in the Upper Jurassic increased significantly,suggested that the large-scale uplift of the Middle Yangtze area,which to the east of the Zigui Basin,provides part of the material source for the Zigui Basin.3.According to the analysis of the thermal history evolution data from the low temperature thermochronology,there is a differential uplift of Huangling anticline,which on the east side of Zigui basin,from Late Jurassic to Early Cretaceous.The southern part of Huangling anticline uplifted in Late Jurassic,which provided part of the sediment source to the Zigui Basin.Up to the early Cretaceous,the Huangling anticline uplifted as a whole,and the uplift rate in the northern part of the Huangling anticline was significantly higher than that in the southern part.4.The source of sediments in Zigui Basin and the uplift process of Huangling anticline reflect the transformation of regional stress field from nearly north-south compression in early Jurassic to nearly East-West compression in early Cretaceous.In the early Jurassic,the Yangtze block continued to converge towards the North China blockthe in north-south direction,but the tectonic activity gradually weakened.During the Middle Jurassic,the Yangtze block rotated clockwise and continued to subduct laterally.Strong NE-SW tectonic compression occurred in the Qinling-Dabie orogenic belt.In the late Jurassic,thrusting nappe took place along the Xiangfan-Guangji fault zone along the southern margin of Qinling-Dabie orogenic belt in the northeastern part of Zigui Basin towards the Middle Yangtze Block.In the early Cretaceous,the regional stress was transformed into nearly EW-trending compression,forming the EW-trending geological pattern of Zigui Basin-Huangling anticline-Dangyang Basin.In the Late Cretaceous,the stress in the study area changed to nearly EW-trending extension.Because the Zigui Basin-Huangling anticline region is situated on the gravity gradient belt of Taihang Mountain-Wuling Mountain,which geological structure quite different between East side and west side.5.Based on the analysis of regional thermal evolution history indicated by low temperature thermochronology,it is concluded that: Late Jurassic morphostructure shows NWW-SEE trending distribution.Up to the early Cretaceous,the morphostructure changed into East-West differentiation along the present Taihang Mountain-Wuling Mountain gravity gradient belt,indicating that the regional tectonic stress is near the E-W direction.The early Early Cretaceous(140-120Ma)may be the initial stage of the formation of the present Taihang-Wuling Mountains gravity gradient belt. |
