Study On The Evolution And Fluids Migration And Aggregation In The Foreland Thrust Belt, Central-southern Longmen Mountains

The Longmen Foreland Thrust Tectonic Belt lies on the boundary line between the east border of Tibetan Plateau and Sichuan basin. Because of critical structure position and the model foreland thrust structure, it always receives the domestic and foreign scholar's high attention. Nevertheless,few works have focused on the chronological constraints for the uplifting of each thrust sheet, and also few study have been done with the coupling relationship between the structural evolution and fluids migration and aggregation in this foreland thrust belt.According to the tectonic deformation characteristics, the system of Longmen Mountains and West Sichuan Foreland Basin can be divided into five subbelts. They are(1)Maowen-Longdong ductile shear belt(,2)Pengguan-Baoxing complex nappe tectonic belt,(3)foreland thrust tectonic belt,(4)foreland fault-fold tectonic belt,(5)foreland uplift tectonic belt. The research indicates that the thrust uplifting direction of nappes is mainly southeastward. The tectonic uplifting took place in the foreland thrust tectonic belt since late Triassic epoch(207.3Ma), then in the foreland thrust tectonic belt and foreland fault-fold tectonic belt since late Cretaceous epoch(95.5Ma). Each tectonic subbelt was raised with differential uplifting velocities in the whole fast uplifting stage since Miocene Epoch(24.1Ma), and the uplifting rate in the foreland thrust tectonic belt was faster than which is in the foreland basin.Along the Longmen thrust belt, from orogenic belt to foreland basin, the structural deformation becomes less extensive while the fluid activity attenuates. Multi-phase thrust uplifting movements result in episodic fluid expulsion. The multi- episodic fluid infilling characteristics in foreland thrust belt are obvious,but which are simple in foreland basin. By geological and geochemical traces of fluid source for each tectonic subbelt's fluid, it has determined that fluids infilling in different thrust sheets came from the same or similar sources, and suggesting fluids'allochthonous origin, probably came from marine carbonate rocks with redissolution.In this paper, the thrust uplifting movements in foreland thrust belt and their coupling relationship to fluids migration and aggregation are constructed. Frist of all, with Songpan-Ganzi ocean basin transformation to orogenic belt, fluids migrated from orogenic belt into Pengguan-Baoxing complex nappe tectonic belt. And then, with Pengguan-Baoxing complex nappe tectonic belt uplifting, fluids infilled into foreland thrust tectonic belt. Finally, fluids removed into foreland fault-fold tectonic belt with foreland thrust tectonic belt uplifting. During the process of fluids migration, structure controled the distribution of hydrothermal dolomitization, which attenuated from orogen to foreland basin.An ancient oil and gas reservoir can be identified the frist time in southern Longmen Mountains and the process of development and destruction of ancient oil and gas reservoir is reconstructed. The hydrocarbon from Songpan-Ganzi ocean basin migrated from west to east and infilled into Dengying Formation,Guanwushan Formation and Permian located in the foreland thrust belt to form oil reservoir before late Triassic. With Songpan-Ganzi ocean basin transformation to orogenic belt and forland basin development in late Triassic, ancient oil reservoir was deep buried and cracked to form ancient gas reservoir, which was destroyed further during progression and migration of foreland thrust belt from orogenic belt to foreland basin.