东秦岭柞水—镇安地区泥盆纪沉积环境和沉积盆地演化

DEVONIAN SEDIMENTARY ENVIRONMENTS AND BASIN EVOLUTION IN ZHASHUI-ZHENAN DISTRICT, EASTEPN QINLINC CHIAN

东秦岭陕南柞水—镇安地区泥盆纪沉积盆地中主要发育有冲积扇、小型辩状河、河口湾、潮坪、潮下洼地、台地边缘生物滩、生物礁、开阔台地、斜坡和盆地沉积环境。台地沉积主要由陆源碎屑和碳酸盐混合类型岩石组成,斜坡—盆地为巨厚的陆源碎屑浊积岩充填。根据沉积作用特征和大地构造差异,在构造活动期和构造稳定期的沉积模式就反映了沉积盆地演化特征。 根据沉积作用,沉积环境的发展变化,结合区域构造特征、地层层序、岩浆活动和变质作用、地球物理和化学资料,论证了当时沉积盆地为南北向展布(与秦岭造山带总体方向垂直),具向东走滑的大陆边缘裂谷—断陷盆地。盆地的发展演化阶段为:(1)岩石圈裂前拱起—陆内裂谷早期阶段(Z_2—S);(2)大陆边缘裂谷—断陷盆地的中期阶段(D_1—D_3);(3)晚期回返—陆内俯冲阶段(C—T_2)。盆地的关闭是由于强烈陆内俯冲,同时盆地被挤压进东西向展布的秦岭造山带。

The Devonian sedimentary basin of the Zhashui-Zhanhan district in southern Shaanxi Province shows well developed alluvial fan, braided stream, estuarine, tidal flat, platform margin sands, point reef, slope and basin facies. A mixed terrigeneous clastic-carbonate succession mainly comprises an intercalation of carbonate platform facies with slope-basin and basinal turbidites of very great thickness. The basin shows the following features: (1) Turbidite mineral compositions fall into the passive margin and rift basin field of Valoni & Maynard' s QLF figure. (2) The sedimentary succession shows rapid and complex facies variation. (3) From east to west, facies vary from shallow to deep marine indicating that the main trend of the basin is N-S. (4) There are major abrupt facies changes between three large faults in the basin. These show right lateral slip and indicate eastward basin migration. (5) The sedimentary association of the basin shows the characteristics of continental margin rift basins. (6) Volcanic rocks are of the sub-alkaline and alkaline series. (7) The structure of the basin comprises a series of south trending thrust or reverse faults, with the intervening blocks showing complex east- west trending tight folds. Overall, the sedimentary basin is a continental margin rift depression shpwing elongate N-S orientation, and right lateral strike slip faulting. The basin developed on the northern margin of the Yangtze plate, which underwent extensional crustal thinning. The stages of basin evolution are as follows: (1) Primary stage (Z2-S) ——pre-rift arch to intracontinental rift. Mantle upwelling caused thinning of the Yangtze plate margin crust. Alkaline volcanism occurred along major faults. In the early Palaezoic, there was great lateral variation in sedimentation, with areas of differential uplift and subsidence. (2) Intensive spreading middle stage (D1-D3) ——continental rifting to faulted basin developerrient. In the early part of this stage, spreading led to large scale block subsidence, aulacogen formation, and right lateral strike slipping. The basin was filled with a great thickness of flysch, with some intercalated pyroclastics. In the later part of the stage, the Yangtze and North China plates began to move together, with continental collision taking place locally in the middle Devonian. (3) The inversion stage (C-T2)——intracontinental subduction and uplift. In this late stage, from Carboniferous to middle Triassic, the basin showed little change in sedimentary pattern, with a suite of shallow marine limestones, mudstones and sandy mudstone facies being developed. Following the middle Triassic, basin closure occurred as a result of purely intracontinental, shallow coastal deformation along a large decollement with about 120km of horizontal transport. Intracontinental subduction resulted in compression of the sedimentary basin into the east-west trending Qinling belt.