The Early Paleozoic Tectono-sedimentary Characteristics And The Basin-Orogen Process In South Tarim Basin

The orogenic belts and basins are unified dynamic system,which are related to each other in space time development and formation mechanism.They are mutually dependent in space,and they mutually transform in matter,with mutually adjusting and balancing in surface morphology.The formation of the basin is controlled by the orogeny of the peripheral orogenic belts.The subsidence of the basins and the uplift of the orogenic belts are temporally synchronized and spatially adjacent.So,the deposition and deformation records of the basin reflect the tectonic process of the basin-margin orogenic belts.As one of the largest basins in China,the Tarim Basin has recorded multiple orogenic events around the basin during the long evolution of basin.During the Early Paleozoic,strong tectonic deformation and transition of sedimentary characteristics developed in the southern part of the Tarim Basin,and the nature of the basin underwent fundamental changes..This paper focuses on the sedimentary and deformation records of the Early Paleozoic in the South Tarim.Based on the development characteristics and spatial migration of paleo-uplifts in the southwestern Tarim Basin,the tectonic development characteristics and evolution process of the thrust belts in the late Early Paleozoic in southeastern Tarim Basin,and the transition of early Paleozoic depositional environment and sedimentary sources,this paper wants to establish the evolution law of tectonic deformation and sedimentary environment in the early Paleozoic in the southern Tarim Basin.Based on the basin-orogen coupling theory,the basin-orogen process between the Tarim Basin and the West Kunlun and Altun orogens in the Early Paleozoic was discussed from the perspective of basin tectonic deformation and sedimentary environment change.There are the main achievements of this paper:(1)This paper identifiedthe Southwestern Tarim Basin Paleo-Uplift developed from the Middle Ordovician,revealed that the paleo-uplift has a migration pattern from west to east,and proposed that the paleo-uplift is the front uplift of the Early Paleozoic foreland basin in the southwest of Tarim Basin.Through the structural interpretation of the regional seismic profiles in the southwest of the Tarim Basin,it is identified that there was a regional paleo-uplift in the early Paleozoic in the southwest of the Tarim Basin,and it is called Southwestern Tarim Basin Paleo-Uplift.The Southwestern Tarim Basin Paleo-uplift formed during the Middle Ordovician,and the scale of the paleo-uplift increased during the Late Ordovician,mainly distributedin the Southwestern Tarim Basin.During the Silurian,the paleo-uplift was uplifted steadily with its range expanding rapidly to the east,and the Pishan and Hetian(Pishanbei 1 Well、Pishanbei 2 Well、Shenghe 2 Well、Luosi 2 Well)became parts of the paleo-uplift with the tectonic height of the paleo-uplift mainly locating in the Pishan and Hetian Area.During the Devonian,the range of the Southwestern Tarim Basin Paleo-uplift began to shrink gradually,and the range was limited to the Pishan-Hetian Area.At the end of Middle Devonian,the paleo-uplift uplifted in a small range.Since the Late Devonian,the Southwestern Tarim Basin Paleo-Uplift gradually disappeared,and the Upper Devonian surpassed the paleo-uplift,and the Lower Carboniferous covered the whole early paleo-uplift.During the Late Ordovician-Silurian period,the Southwestern Tarim Basin Paleo-upliftwas widely distributed in the entire southwestern part of the Tarim Basin,and the paleo-uplift migrated to Pishan-Hotian direction at the beginning of the Devonian,and the range also began to gradually decrease,with the characteristics of eastward migration and shrinkage.Combined with the previous research results on the West Kunlun orogenic belt,it is considered that the Southwestern Tarim Basin Paleo-uplift was the front uplift of the Southwestern Tarim Basin Early Paleozoic Foreland Basin relating to the West Kunlun Early Paleozoic Orogeny.The migration of the Southwestern Tarim Basin Paleo-uplift reflects that the orogenic process of the West Kunlun orogenic belt may have an orogenic process of gradually colliding and closing from west to east.(2)This paper determines the tectonic development characteristics and evolution process of the thrust belt in the late Early Paleozoic in Southeastern Tarim Basin,and proposes that the thrust belt is a foreland fold-thrust belt related to the Early Paleozoic orogeny of the Altyn orogenic belt.According to the structural interpretation of the seismic profiles in the Southeastern Tarim Basin,several thrust faults developed from south to north in the Tangnan Area,while in the Madong Area,there are five rows of thrust faults with NE-SW trending developed from north to south.In the thrust belt,the bottoms of the thrust faults in both directions converged on the regional detachment of the Middle Cambrian,and the hedging structure formed where the faults in the two directions intersect.According to the analysis of unconformity and growth strata,the early deformation in the Madong Area occurred between the end of the Late Ordovician and the Early Silurian,and there was still weak tectonic deformation before the deposition of the Upper Devonian.But the deformation in the Tangnan Area began at the end of the Late Ordovician(slightly earlier than the Madong Area),and a strong tectonic deformation occurred before the deposition of the Upper Devonian.But there are two angular unconformities at the Aqike section in the South Tarim Basin,the lower unconformity is the large angle unconformity between the Upper Devonian and the Ordovician,and the upper unconformity is the small angle unconformity between the Carboniferous and the Upper Devonian,indicating that the thrust deformation in the southern part ended between the Late Devonian-Carboniferous.Based on previous studies about the Altyn Orogenic Belt,it is considered thatthe thrust belts in the Southeastern Tarim Basin during the Early Paleozoic are the foreland fold and thrust belts relating to the orogeny of Altyn Early Paleozoic Orogenic Belt.(3)This paper proposes that the sedimentary environment changed from the passive continental margin during the Cambrian-Early Ordovician into the foreland basin during Middle Ordovician-Middle Devonian in the South Tarim Basin.And the region of Southwestern Tarim Basin and the Southeastern Tarim Basin belong to different foreland basin systems during late of Early Paleozoic.By analyzing the Early Paleozoic sedimentary facies in the southern Tarim Basin,it is found that the sedimentary facies of southern Tarim Basin changed from the open platform facies during the Cambrian to Middle Ordovician to the shelf facies during the Late Ordovician,and then to the emergence of the uplift and denudation area in the southern part of the Tarim Basin during the Silurian to Devonian,and this suggests that the Southern part of Tarim Basin has undergone a complex transition of sedimentological characteristics.Detrital zircon dating shows that the Silurian-Lower Devonian on the Sangzhu section and the Ordovician on the Aqike section have similar peaks in the age of detrital zircon,both recording regional tectonic events in the Neoproterozoic(around800 Ma),whose provenances all came from the provenance of the Tarim Basin itself.However,the two sections of the Upper Devonian show obvious differences.Although they both record the peak age of regional tectonic events in the Neoproterozoic(about 800 Ma),the Upper Devonian on the Sangzhu section has no age of about 430-450 Ma.records,while the Devonian on the Aqike section has a peak age around 430-450 Ma.This shows that the Upper Devonian sediment sources of the two sections are different,the Upper Devonian sediment sources of the Sangzhu section still came from the Tarim Basin itself,while the Upper Devonian sediment sources of the Aqike section came from the Altyn Orogenic Belts and the Tarim Basin itself.Based on the analysis of the evolution and deformation characteristics of the paleo-uplift,it is concluded that both two sections were located near the front uplift side of the foreland basin before the Late Devonian,but the Aqike profile was involved in the late Early Paleozoic Southeastern Tarim Basin Thrust Belts during the Late Devonian,and may have received the wedge-top deposits of the thrust belt which its provenance came from both the denudation of the thrust block(the Tarim provenance recycling)and the denudation of the Altyn orogenic belt on the southeast side,thus there is a peak age around 430-450 Ma.Combining with the evolution of the West Kunlun Orogenic Belt and the Altyn Orogenic Belt,it is considered that there were two foreland basins in the South Tarim Basin during the Early Paleozoic,one is controlled by the orogenic process of the West Kunlun during the Early Paleozoic,the other one is controlled by the orogenic process of the Altyn during the Early Paleozoic.(4)This paper proposes that the orogeny of Western Kunlun and Altyn Early Paleozoic orogen belts jointly controlled the tectonic deformation and migration process during the early Paleozoic in the southern Tarim Basin,as well as the evolution of the depositional environment,and also preliminarily establishes different basin-orogen coupling evolution models between the Tarim Basin and the two orogenic belts.The basin-orogen interaction between the Tarim Basin and the West Kunlun Orogenic Belt lasted from the Middle Ordovician to the Late Devonian.During the Middle Ordovician,the Tarim Plate and the South West Kunlun Block collided first in the western segment,and a peripheral foreland basin developed in the southwest of the Tarim Basin,and the Southwestern Tarim Basin Paleo-uplift,which was a frontal uplift,began to develop.From the Silurian to the Early Devonian,the continuation of the collision led to the large-scale development of the Southwestern Tarim Basin Paleo-uplift as a frontal uplift.Due to the scissor closure of the West Kunlun Ocean from west to east,the collisional orogenic process gradually migrated eastward,and the strong collision between the two occurred in the eastern part of the West Kunlun orogenic belt during the Early Devonian to the Middle Devonian,and the foreland basin also moved eastward.And the Southwestern Tarim Basin Paleo-uplift,which was the front uplift,moved to the Pishan-Hotian area accordingly.In the Late Devonian-Carboniferous,due to the weakening of orogeny,the Southwestern Tarim Basin Paleo-uplift as the front uplift gradually disappeared.The basin-orogen interaction between the Tarim Basin and the Altyn Orogenic Belt lasted from the Late Ordovician to the end of the Devonian.In the Late Ordovician,the collision between the southeastern part of the Tarim Plate and the North Altyn Block resulted in the development of the Early Paleozoic Foreland Basin(including the foreland fold-thrust belt)and the Late Ordovician shelf facies in the southeastern Tarim region.In particular,the existence of the region detachment layer in the Middle Cambrian made the deformation of the thrust belt propagate rapidly to the interior of the Tarim plate,forming a Tangnan Thrust System with thrusting direction to the northwest and a Madong Back Thrust System which constituted a hedging structure.During the Silurian-Middle Devonian,with the continuation of the collision,the orogenic wedge at the root of the thrust belt continued to propgate in the northwest direction,resulting in the continuous uplift and denudation of the strata in the southeast of the Tarim Basin.And the uplift and denudation area was formed and continued to advance into the southeast of Tarim Basin,and the Lower Silurian-Upper Devonian was missing in most areas,while the Upper Devonian were deposited unconformably above the formations of the Ordovician in some areas.This collision continued until the end of the Devonian,resulting in the deformation of the Upper Devonian deposits in the southern part of the thrust belt,and the Carboniferous unconformably overlaid the underlying deformed Upper Devonian.