Mesozoic-Cenozoic Superimposed Orogeny And Foreland Deformation In The Northeastern Sichuan Basin

A superimposed belt of several continental orogens and their associated foreland is anintensively deformed zone. Its deforming and coupling relationship is a front topic in continentaldynamics. Additionally, its special tectonic position and hydrocarbon potential arouse muchattention. Although geometric and kinematic models of single fold-thrust belt and associatedforeland basin have been put forward by many geologists, more complex superimposed deformationof multi-boundaries fold-thrust belts and foreland with multi-stages and multi-directions has notbeen studied in detail. So the theories and models about superimposed deformation of multi-orogensand foreland need to be established based on deep research.The Northeastern Sichuan basin(NSB)is located at the north-west edge of the Yangtz plate.Regoinally, it is bounded on the north by the west-east trending Micang Shan fold-thrust belt(MSB),on the east by the north-west trending Daba Shan arcuate fold-thrust belt(DSB), and onthe northwest by the north-east trending Longmen Shan fold-thrust belt(LSB). This region behavedas a passive margin of the Yangtz plate from the Paleozoic to middle Triassic times and then evolvedinto a peripheral foreland basin in response to the continental collision of Qinling belt since the latemiddle Triassic. With the uplifting mountains advancing toward the foreland, multi-stages andmulti-boundaries deformation with east-west, north-east and north-west superimposed structureswere formed. So the NSB provides an ideal area to study superimposed deformation of amulti-boundaries fold-thrust belt and a foreland with multi-boundaries.Based on previous research of the NSB, this dissertation synthesizes and analyze the updatedsurface geology, borehole and seismic data, especially more than two thousand 2D seismicexploration profiles, and studies superimposed deformation of the Tongnanba(TNB) tectonic zoneas a breakthrough. This dissertation conducts detail analysis of surface and sub-surface structures ofMSB, DSB and its foreland, and mainly discusses superimposed structural relationship anddeformation mechanism of MSB, southern DSB and TNB.Main contents are(:1)To establish a NSB regional Mesozoic stratigraphic framework, restoresedimental facies and paleo-geographical environment, and unravel coeval basin-margin tectonics.(2)To analyse the zoning structure of MSB and its foreland on the basis of surface and sub-surfacegeological data, discuss early west-east structure superimposed by north-east structure, and mainlydiscuss deformed relationship between surface and sub-surface structures at the front of MSB.(3)To analyse the zoning structure of DSB and its foreland on the basis of surface and sub-surfacegeological data, discuss early north-east structure superimposed by north-west structure, and mainlydiscuss deformed relationship between surface and sub-surface at the front of DSB.(4)To analysethe deformation style of upper and lower deformation strata and 3D geometric and kinematiccharacters of north-east and north-west faults in TNB on the basis of detailed interpretation of 2Dseismic profiles, and further discuss the structural relationship between TNB and peripheralmountains such as MSB and DSB.Research methodologies are:(1)To use the sub-surface structural information disclosed by2D seismic profiles on the basis of field geological survey and investigation, strengthen the researchof surface and sub-surface structure at margins of basins and mountains(.2)To use borehole data, theupdated processing and interpretation methods of seismic data as well as kinematic models offold-thrust to improve the credibility of seismic structural interpretation.(3)Because there are nomiddle Cretaceous-Neocene strata or the strata were eroded, the main structural deformationhappened from late Yenshan Orogeny to early Himalaya Orogeny. So using the traditional structuralanalysis method to infer deformational history by coeval sediment is difficult. Fortunately, thesuperimposed deformation information provides a key record about tectonic evolution of this region.In a word, the most important method is to deduce the periphery mountains tectonic evolvement byanalyzing the surface and sub-surface superimposed structures recorded by foreland deformation atTNB.Main conclusions: This dissertation has discussed superimposed structures of multi-stages,multi-directions, surface and sub-surface on the basis of 2D seismic data interpretation. The surfaceand sub-surface superimposed structures of MSB, DSB and TNB are analyzed. The 3D structure ofTNB which is the foreland of MSB and DSB is put forward. It also discusses the tectonicevolvement of TNB and periphery mountains. It's the first time to discuss the deformation ofperipheral mountains by the sub-surface superimposed structure and evolution of foreland. The mainconclusions are as follows:(1)The foreland of NSB is characterized by thick accumulation(3000-5000m)of lateTriassic-early Cretaceous continental sedimentary deposits including alluvial plain, lacustrine andfan delta depositional systems. The strata from late Triassic Xujiahe Formation to the middleJurassic Qianfuya Formation are composed of 8 sequences according to sequence interfaces ofuplifting erosion, ablating erosion and lithological surface of discontinuities. The analysis results ofold current direction show that the sediment supply is from the north-east, and the mountain uplift ismore active on the north and east sides. From late Triassic to early Jurassic times, the NSB behavedas a typical foreland basin in response to continental collision and southward thrust of the Qinlingmountain belt. After Jurassic time, the lacustrine depositional system dominated this area.(2)MSB extending west-east direction is composed of a fold-thrust belt and foreland fromnorth to south. From Cenozoic time, with the Qinghai-Tibet plateau uplifting and LSB growing, thetectonic compressive stress from northwest had a great effect on the Sichuan basin and MSB. As aresult, a set of north-east trending structures including folds and faults which cut or interfere thewest-east structure were formed. The deformed strata are obviously controlled by regional evaporatedecollement strata at the Leikoupo and Jialingjiang Formation. The upper continental strata withweak deformation make up of the MSB frontal monocline. On the other hand, the lower sea facialstrata with lots of folds and thrust faults make up of triangle zones buried in the front of MSB.(3)The northwest-southeast trending Daba Shang arcuate fold-thrust belt is convex towardsouthwest with different deformation styles of which the folds and faults become less intensivetoward the foreland. With the foreland-verging compression from the southern DSB hinterland in thelate Himalaya orogeny, a set of north-west trending folds and faults were formed at the late Triassicto early Cretaceous continental strata which were detached by regional evaporate decollementstratum. In the front of DSB, the Paleozoic rocks are overlapped and uplifted by duplex in which theregional decollement stratum behaves as roof faults and Sinian shale as sole thrust faults.(4)Separated by regional evaporate decollement stratum at the Leikoupo and JialingjiangFormation, TNB can be divided into upper and lower deformation strata. The north-west trendingfolds and thrust faults dominate the upper deformation stratum while the north-east trending faultsand main fold dominate the lower deformation stratum. The two deformation strata are composed ofthe inconsonant superimposed structure from surface to sub-surface. The research results ofdisplacement-distance show that north-west trending faults cut cross the early north-east trendingmain fold with SW dip and back-thrust which cut up the section of the Shaximiao Formation andmerge down to regional decollement stratum. The change of displacement of north-west trendingfaults from north-west to south-east and from two limbs and core of the main fold is as follows: thedisplacement at the fold core is little and becomes larger toward the two limbs, and larger at thesouth-west deep limb than that at the north-west gentle limb;On one seismic profile, thedisplacement at the Xujiahe Formation is the largest and decreases toward the two ends at thesouth-west weakly deformed segment of TNB, while the displacement at the Qianfuya Formation isthe smallest and increases toward the two ends at the north-east intensively deformed segment ofTNB. The analysis results imply that deformation of north-west trending faults in the north-east ismore intensive than that in the south-west TNB. The north-east trending faults dominate the lowerdeformation stratum and mainly develop at the two limbs of main north-east trending fold while thefaults at the south-east deep limb are larger than those at the north-west gentle limb, anddisplacement of the faults at the two limbs is negatively correlated along the axis of the main foldfrom south-west to north-east which adjusts the shortening amount of compressive deformation. Theformation of TNB is mainly consist of three stages since middle Cretaceous time: The initial stage ofnorth-east trending main fold, the formation of north-east trending main fold with faults, and thenorth-west trending structure superimposed stage.(5)Surface geological survey results show that the eastern segment of east-west trendingMSB is superimposed by north-west trending structure of southern DSB with typical largesuperimposed buckling fold. TNB is located at the superimposed deformation zone of MSB andDSB. Its structural frame is controlled not only by the base of Sichuan basin but also by compressionfrom MSB and DSB.The main structure of TNB is north-east trending main fold and faults which are the firstly andextensively deformed after the formation of Mesozoic foreland. The north-east TNB main foldexperienced two stages: the initial stage of main fold and the formation of main fold and thrust faultswhich are related with the formation and evolvement of MSB. The north-west superimposedstructure dominating the upper deformation stratum mainly consist of thrust faults or back-thrustfaults and thrust-related folds which are related with the DSB arcuate fold and thrust system.. Thesurface and sub-surface thrust or back-thrust faults with north-west trend consist of the front thrustsystem buried at the front of DSB freland.(6)According to the initial orogeny of peripheral mountains from late Triassic to earlyJurassic times, three tectonic evolution stages can be determined: pre-orogeny( HercynianOrogeny),orogeny(Indian Orogeny, Yanshan Orogeny) and re-orogeny and foreland deformation(Early and Late Himalaya Orogeny).Especially, the Himalaya Orogeny is the import deformationstage of TNB.