Crustal Shortening And Uplitfing Of The Longquan Anticline: Evidence From Terrace Deformation

Nowadays, continental dynamics is a frontier subject in the earth sciencefield. Continental orogenic belts and adjacent basins are two important tectonicunits, so the new continent tectonic scientific theory must be based on the finestudy of orogenic belts and sedimentary basins. Because of tectonic setting ofthe Longmen Shan thrust belt and its adjacent basin, it becomes a key area inthe solution of Tibetan Plateau uplift and peripheral orogenic belts of orogenicprocess. The Longmen Shan, the western Sichuan foreland basin and Longquananticline constitute a trinity of geology whole. The Longquan anticline islocated on the eastern margin of the western Sichuan foreland basin. To clarifythe relationship between the Longmen Shan and Longquanshan is of greatsignificance to reveal the basic characteristics of tectonic deformation anddistribution of movements in the eastern Tibetan Plateau.The Longmen Shan is a focused subject in the study of intracontinentalorogenic belts on which, Chinese and foreign geologists have made a lot ofdeep studies. However the Longquan anticline as a part of the whole geologywas studied little, especially in the quantitative study of crustal shortening anduplift since the late Quaternary. The study of Longquan anticline kinematicsmay help understand the deformation model of the eastern edge of the TibetanPlateau.In addition, it have a little confusion in understanding of geometry theLongquan fault zone. Based on petroleum seismic reflection profiles data, thiswork investigates the deformation characteristics of the Longquan Fault zoneand attempts to improve the understanding of the Longquan fault.Located at east of the Longmen Shan thrust, the Longquan anticline is a foreland uplift of the Sichuan foreland basin. Across the Longquan anticline，the Kai river grow three terraces. According to optically stimulatedluminescence（OSL） and¹⁴C samples, the ages of the terrace T1and T2are4.6ka、（11.7±0.4） ka, respectively. With growth of the anticline, terracesurfaces deformed obviously, which are characterized by folding scarps. Thelongitudinal profiles of the terraces at the Kai River have been measured by theTrimble5700RTK GPS system. The subsurface fault and fold geometries areinterpreted using petroleum seismic reflection profiles, which suggest that theLongquan anticline grows laterally through kink-band migration and depth todetachment is about6kilometers, and yielding a total cumulative shortening of0.8km. In this thesis, the T1, T2and T3of the excess area are calculated to be42000m²,110000m²and224000m², respectively. Taking depth of thedetachment layer as6km, the area of conservation obtains the crustalshortening7.0m,18.3m and37.3m. In addition, combined with the terracesages, the crustal shortening and uplift rates are1.36mm/a and0.7mm/a,respectively. Using function relation （l=u/sinΦ） between uplifting andshortening of the fault-bend fold model, I calculate the uplift rate of0.57mm/aand obtain the average of the two uplift rate of0.64mm/a as the uplift rate inthe north segment of the Longquan anticline since the late Pleistocene.Errors in shortening and uplifting rate are estimated as follows. Frequenthuman activities the have changed the terraces. The measurement error is about10%. The error of dating of terraces is about10%. The error of calculatingexcess area and the depth of the detachment layer is about10%. The total erroris estimated to be30%. The crustal shortening and the uplift rate in latePleistocene are （1.36±0.41） mm/a and （0.64±0.19） mm/a, respectively.Collision of India with Eurasia resulted in E-SE extrusion of the Bayan Harblock which is strongly resisted by the South China block，so most of theenergy is released in the Longmen Shan，and part of by decollement transferred to the formation of the Longquan anticline. The12May2008Wenchuanearthquake （Mw=7.9） of China occurred in the Longmen Shan thrust belt as aconsequence of the ongoing uplift process of the eastern margin of the Tibetanplateau. After the earthquake, stress was reduced in the northern section of theLongquan fault and increased in the southern section. This clearly illustratesthat the Longquan anticline and the Longmen Shan thrust belt are an integralwhole. The mechanism uplift of the Longquan anticline provides someevidence for brittle crustal thickening orogenic theory in the eastern TibetPlateau.Through above study this work obtains some knowledge, but has also someproblems. First, if the terraces were eroded, the calculated result may beeffected. If a sign-layer in the terrace profile can be found, the result would bemore reliable. Second, the terrace age is also an important parameter, and theaccurate age depends on the dating samples and measure means. The¹⁴C datingis more accurate. If charcoal is available, the data will be more reliable. Third,only the data in the northern segment of the Longquan anticline for shorter timeis present. For deep research, combined with drilling operations for identifyinggrowth strata to obtain different segments on long time scale data, the segmentsactivity for the Longmen Shan fault can be revealed and thus further discussionof scientific issues can be made.