Cenozoic Sedimentary Characteristics And Their Tectonic Implications Of The Hongsanhan No.1 Area In Western Qaidam Basin

The Altyn fault zone, more than 1900km long, is a large-scale sinistralstrike-slip fault zone in the middle Asia continent. The fault zone not onlycuts across the western Kunlun range to the south and the Qilian Mountainto the north, but also separates Tarim and Qaidam basins. It plays a veryimportant role in the tectonic evolution of the western China and even of themiddle Asia region. The strike-slip of the Altyn fault, accompanying withthe uplift of the Altyn Mountains, should certainly control the sedimentationand structural evolution of the basins on the both side of the Mountains. Inthis paper, we carried out sedimentary characteristic study of the differentepoch sedimentary strata in Cenozoic in the front basin of the AltynMountains, western Qaidam basin. Furthermore, we also carried out detritalapatite fission track research to obtain effective chronological informationof the thermal evolution history of the provenance. Combining with coolingevents and cooling velocity based on the isotope chronological data from thebasement rocks in the Mountains, the exhumation progress of the AltynMountains has been backdated, the study provides reliable evidences for theformation and evolution of the Altyn Mountains.In the past, the division of the Cenozoic stratum in the Qaidam basinwas not consistent with the international standard in the Qinghai Oil Field.The stratum division was only based on the pollen fossils, the differentformations were not exactly demarcated. The preconditon to study thethermal history of the provenanace by detrital mineral dating in the basin isto know the sedimentary ages of the samples. Therefore, in this paper, firstlywe determined the sedimentary ages of the High No.3 section in theHongsanhan No.1 area by magnetostratigraphy. Secondly, the provenanaceis analyzed by the component analyses of the standstone samples, modelanalysis by Dicknson-Gazzi method, and the paleocurrent data in the HighNo.3 section. Thirdly, we use grain statistic analyses in the High No.3section to reflect the Altyn Mountains' influence to the basin sedimentationsince Eocene. Finally, the thermal history of the Altyn Mountains and activehistory of the Altyn slip-fault from Eocene to now is firmly proved bydetrital apatite fission track dating of the samples from the differentCenozoic sediment strata in front of the Altyn Mountains. We conclude asfollows:(1) The strata dividing age of the Shangganchagou and XiaganchangouFormations in the west Qaidam is 35.5Ma. The Shangganchagou andXiaganchangou Formations in High No.3 section are in the range of 40 to26.5Ma. The Thickness-Magnetism age curve of the Shangganchagou andXiaganchangou Formations suggests that the sediments were transported fora long distance with weak dynamical conditions, and there was no intensetopography relief in the Xiaganchangou Formation. Whereas, the sedimentprovenances became near, the transported distance shorter, and there wasintense topography relief in the Shangganchagou Formation.(2) The Altyn Mountain and the southern Qilian area are the provenancesof the Hongsanhan No.1 area. After Oligocene, the strike-slip of the Altynfault occurred obviously and displaced the western part of the northernQilian belt to the current place. In the Xiaganchaigou Formation, there is anincrease trend of the percentage of bulk of Quartz with the time, whereas,the feldspar and debris decrease. The increase of maturation degreessuggests that the sedimentary surroundings are very stable, the distancebetween the provenance and the sedimentary area was long, and there is norapid uplift in the Altyn Mountain. In the Shangganchaigou Formation, thedecrease of the maturation degrees suggests that the sedimentarysurroundings are not stable, from which we can affirm that the provenanceAltyn Mountains uplift rapidly and the detrital minerals accumulated rapidlyin the area not far away from the provenance.(3) The provenance, the Altyn Mountains, didn't uplift rapidly relative tothe Qaidam basin in Eocene, and possibly this phase was a slow or a steadyuplifting phase with a low velocity. The provenance of the ShanganchaigouFormation uplifted rapidly relative to the Qaidam Basin in Oligocene, anddeposit area became drought obviously. The most of the Hongsanhan areawas uplifted from Oligocene to Pliocene (Xiaganchaigou Formation,Shanganchaigou Formation and Shizigou Formation), but the AltynMountains probably were not much higher than the Qaidam Basin. Therewas a rapid uplift in the Altyn Mountains in the Early Pleistocene (QigequanFormation), and the Altyn Mountains were obviously higher than theQaidam Basin. The uplift continues up to now.(4) The ages of the single detrital apatite provides a wide fission track agerange, which reveals that the uplift of the Altyn Mountains are multi-stagedand unequilibrium. The Lulehe Formation in Paleocene in the HongsanhanNo.1 section, close to the Altyn Mountains, gives out a complex thermalhistory of 40Ma, and this history can be explained as strike-slip events ofthe Altyn faults since Late Eocene. The detrital apatite fission track ages offrom the High No.3 far away from the Altyn Mountain indicate that thereare two periods of uplifting and exhumation in the Altyn Mountains inCretaceous. Excepting for the sample H220-1, six of the others fromHongsanhan area record thermal events range from 10Ma to 20Ma, whichindicate a rapid uplifting of the Altyn Mountains between 10-20Ma.