Tectonic Evolution And Formation Mechanism Of Leshan-Longnüsi Paleo-uplift, Sichuan Basin

With abundant oil and gas resources, Leshan-Longnüsi paleo-uplift is a significant tectonic unit to be studied at the area of its tectonic evolution as well as genetic mechanism in Sichuan Basin. Meanwhile, analysing that paleo-uplift is advantageous to the analysis of basin intracontinental deformation characteristics, at the same time of great importance to hydrocarbon exploration. In this paper, we aim to analyze the structure evolution and genesis mechanism of Leshan-Longnüsi paleo-uplift. Based on abundant seismic data and well data in Sichuan Basin, combined with data of outcrops as well, the theory of fault related fold is used in order to discuss the structural interpretation. Besides, by tracking the fine seismic profile, recognising the relationship of seismic wave groups, we focus on the evolution of the Sinian and early Paleozoic in the study area. Also, the balanced section technique and the apatite fission track method are used to conclude the meso-cenozoic tectonic evolution characteristics. Therefore, according to the comprehensive analysis above, the conclusion of the ancient uplift formation mechanism is draw.In front of Longmenshan, Leshan-Longnüsi palaeohigh showed the most intense tectonic deformation. The southern segment of Longmen Mountains, imbricate structures developed in the piedmont region, while reverse thrust anticlines and synclines appeared at the upper part of the formation. The middle Longmen Mountains mainly formed structure wedge base. There are multiple anticlines appared in Southwest Sichuan. In Longquan Mountain anticline, the tectonic style varied from the southern to the north. Sichuan uplift tectonic performed weak deformation relatively, showing a large wide slow anticline, and the palaeohigh axis located in the line of Laolongba-Weiyuan-Longnüsi.The research shows that Leshan-Longnüsi paleo-uplift has multi-detachment layers with different structural deformation among them in seismic sections. Leshan-Longnüsi paleo-uplift, which is controlled by basement with a certain inheritance in geologic history, has experienced multiple synsedimentary uplifting and erosion, and formed before the Permian, then experienced several movements, and formed the current structural features.The tectonic evolution of the paleo-uplift is divided into six stages. Analyzing the formation mechanism of the Tectonic Evolution of the paleo-uplift, it is suggested that it is influenced by basement, the layered detachment system and the dynamic evolution setting.Weiyuan Anticline is controlled by basement uplift. Its overlying strata deformed with the same amplitude. Based on apatite fission track data, the time of Weiyuan Anticline uplift was migration in northeast-southwest of anticline. Forward modelings were carried out to further verify that Weiyuan Anticline is controlled by imbricate structure in basement, and the procession of uplift was piggyback type propagation. Combined with the structural geometric and kinematic features, we argue that the deformation of Weiyuan Anticline is influenced by the uplift of Longmen Shan,the transmission of tectonic stress was largely blocked from the structures in east Sichuan, and lead to the rapid uplift of Weiyuan Anticline.Moxi-Gaoshiti Structure is an anticline from the bottom up, with northeast-southwest tectonic trending axis. The anticline amplitude in formation of sinian and palaeozoic strata is the highist, compared with that of the upper Triassic and Jurassic which taked second place and that of Permian to the middle Triassic strata is lowest. The formation of sinian and palaeozoic still developed anticline, whose tectonic trend axis is south- north. The formation mechanism of Moxi-Gaoshiti Structure is the result of multi period structural activity. The Lower detachments below the Pre-Cambrian basements and Lower Triassic Jialingjiang Formation with gypsolith interval played an important role in the coordination of structural layers.Analyzed using normal fault propagation fold theory, Weiyuan-Anyue intracratonic rift was affected by Nanhua Period-sinian period extension, developing multi-stages rifting as well as the combination of half-grabens and depressions. Weiyuan-Anyue rift trough is formed in the lower substrate by the effect of normal fault propagation.The evolution of Paleouplift was controled by basement tectonic, with inherited processing, which can be divided into six stages. At Sinian prototype development stage, palaeohigh underwent two tectonic lifting movements. Cambrian-Silurian Period was the development phase, and the early Cambrian superimposed nearly north-south rift trough deposition, then evolving the end of the Early Cambrian, the late Cambrian and Ordovician, three phases of construction lifting movement, palaeohigh range increased.The late Silurian-Carboniferous stage was the uplift and erosion. In Caledonian cycle, because of the movement of the Yangtze region of Guangxi massive uplifted and subjected to intense erosion, Leshan-Longnüsi uplift nuclear unit has been seriously eroded. In Permian-Triassic burial stable phase, palaeohigh accepted stable deposition. In the late Triassic, tectonic inversion movement occurred in Sichuan Basin, showing uplift in southeast and subsidence in northwest. At Late Triassic-Early Jurassic of western Sichuan foreland basin developmental stage, inherited the structural pattern in the late Triassic, a foreland basin developed in the western area.In the adjusting phase, Jurassic, mainly due to the impact Himalayan movement, palaeohigh axis continued migrated to the southeast, at the same time, Weiyuan anticline uplifted and obliqued rapidly. The Palaeohigh shaped up.We believes that paleouplift formed at different times, a variety of properties, and the results of multi-phase superposition. The formation process is mainly affected by three aspects etc., which are the base, the layered slip deformation and the kinetic evolution factors controlled by the background.Combined reservoir characteristics and tectonic evolution of Paleo-uplift, we found that the favorable exploration zone in Sinian formation located on both sides of the rift trough, which are the high position of Paleo-uplift. Furthermore, the favorable exploration zone of Cambrian located on Gaoshiti-Moxi-Suining area.