Filling The Lanping Basin: Response To The Neotethyan Tectonics In The Sanjiang Orogenic Belt

The Sanjiang orogenic belt is located in the east of the Eastern Syntax. It may have a different Neotethyan tectonic evolution in contrast to its counterpart in central Tibetan due to their different tectonic locations. However, we know very little about Neotethyan tectonics of the Sanjiang belt because of the lack in Late Mesozoic magmatic rocks. Being the largest Mesozoic to Cenozic one in the Sanjiang orogenic belt, the Lanping Basin may have recorded a complete Neotethyan tectonic history in its basin filling. Besides, the Lanping basin is enriched in mineral resources as well. Detail study upon the geological evolution of the Lanping Basin can provide positive constraints on the kinetic correlation between the collision orogenisis and mineralization. However, the current understanding of the basin is still fuzzy, major problems may include:(1) Previously published studies rarely took the Neotethyan tectonics into account during modeling the basin evolution, although it is obviously a Mesozoic to Cenozoic one. Therefore, no any dynamic relationship has been built between the Sanjiang orogenic belt and Tibetan Plateau.(2) Avaliable basin-evolution models were constructed on the basis of 1/200,000 geological mapping results, which were completed before 70s-80s, last century. A large amount of new data has been published since then. Obviously, these new data should be merged into any tectonic model. So far in fact, neither the basement nor the fillings of the Lanping Basin has been clearly constrained.In order to solve these problems and to reveal its tectonic implications, this thesis, selecting the northern part of the Lanping basin (north of Yunlong County and south of Weixi County) as the study area, tries to conduct detail field study along five transactions across the basin; our purpose is to establish the basin sedimentation framework and discuss the tectonic evolution history of the basin.Detailed study along the five profiles revealed:1. The outcropped basement of the Lanping Basin consists of two parts; between them is an angular unconformity:(1) The lower part consists of the Late Permian to Middle Triassic continental margin arc volcanic rocks and coeval sedimentary rocks. These rocks belong to the Jamda-Weix-Yunxian arc-like volcanic belt that had resulted from the north-, northeast-, and eastward subduction of the Paleotethyan Ocean under the Qiangtang and Yangtz plates.(2) The upper part of the basin-basement belongs to the Transitional Unit defined by Burchfiel & Chen(2014). This part is composed of the Upper Triassic Maichuqing Formation in lower and the Sanhedong Formation in upper. The Maichuqing Formation consists of sandstone and mudstone with coal-lines, while the Sanhedong Formation is made of marine carbonate rocks, sandstone, and mudstones. The Maichuqing Formation forms a complete deltaic sedimentary facies assemblage. The rocks of the Sanhedong Formation deposited in carbonate platform and slope environment. Syn-sedimentary deformation structure widespread developed within the Sanhedong and Maichuqing formations. They have a similar rock aseemblage, sedimentary texture, and syn-sedimentation structures as foreland basin.2. The Lanping Basin started to fill since the Early Cretaceous and the filling continued to the Eocene (-35 Ma). After the 35 Ma volcanism, a thick conglomerate sequence (the Baoxiangsi Formation) marked the demission of the Lanping Basin. The basin fillings can be divided into four segments in upwards according to the sedimentary features and the nature of sedimentary boundaries:(1) The Lower Cretaceous Jingxing Formation and upper Lower to Upper Cretaceous Nanxin Formation are made rhythmically of several upward-fining clastic sequences; each one composes of sandstone with local gravel-layers in low, muddy siltstone in middle, and mudstone in upper. A mud-layer enriched by calcite nodules or calceous crusts developed in top of most rhythmic sequences, each of which indicates a hatius. Such sedimentary pattern suggests that the Lanping Basin was a foreland one during the Cretaceous.A brief analysis in clastics and U/Pb results of detrital zircons indicate that the Cretaceous sediments were derived from the coeval arc-belt locating in west side of the basin. The arc-belt was thought as resulting from easternward subduction of Neotethys. Therefore, the Lanping Basin was a Cretaceous back-arc foreland one.(2) The Upper Cretaceous to Lower Paleogene Hutoushi Formation consists of river-facies light-gray, gray-green, massive to thick, coarse to fine sandstones, which is sandwiched by muddy siltstone, mudstone and conglomerate. In contrast to its underlain Lower Cretaceous, a huge change in sedimentary texture can be identified that likely correspondes to the tectonic transition from subduction to continent-continent collision.(3) The Paleocene to Eocene Yunlong and Guolang Formations are a package of lacustrine and lacustrine delta sediments mainly composing of brick-red calceous siltstone and mudstone. They represent basin-fillings during a relatively quiescent tectonic period.(4) The Upper Eocene Baoxiangsi Formation (<35Ma) consists of large pebbles with diameter up to 30cm. Development of the huge conglomerate suite suggests that the Sanjiang orogen start to uplift after 35Ma.3. Our data presented in this thesis demonstrate that the Sanjiang orogen seems to have a similar early Neotethyan subduction tectonics as that of central Tibetan Plateau although their exhibition is variable. On the other hand, after closure of the Neotethys, their collision tectonics is quite distinctive and widespread magmatism, intensive deformation, and rapid uplifting took place in central Tibetan Plateau but the Sanjiang Orogen kept relatively quiet.In summary, the Lanping Basin is a Late Mesozoic to Early Cenozoic continental basin which developed on the basement of the Paleotethyan tectonostratigraphic units. Its earlier stage likely formed in a back-arc foreland environment; while in the later stage, it changed to an intermountain basin under relatively constant tectonic settings.