| The far-field effect of the India-Eurasia collision in the Cenozoic is the main mechanism for understanding the changes in the tectonic pattern of Central Asia.In principle,paleomagnetism can be used to determine the net rotation experienced by a structural package of rocks since the time that they were magnetized,and such information can provide powerful constraints with which to test conceptual tectonic models.This makes the Junggar Basin in the hinterland of Central Asia relatively unexplored.In this paper,rock magnetic and paleomagnetism studies were carried out on the Cenozoic sedimentary rocks of the Burqin section(BS)and the Tieersihabahe section(TS)in the northern area of the Junggar Basin.506 hand block samples were collected from BS and TS at sampling intervals of 50 cm.The results of rock magnetism show that the main magneticcarrying minerals in the sedimentary rock samples from the Burqin section are magnetite or a combination of magnetite and hematite.The main magnetic-carrying minerals in the sedimentary rock samples of the Tieersihabahe section are magnetite and hematite.After the system thermal demagnetization,the principal component analysis and the remagnetization great circle are mainly used to fit the directions of the primary magnetization.Finally,135 characteristic remanence directions were obtained from 162 paleomagnetic samples contained in BS and 329 characteristic remanence directions were obtained from 344 samples contained in TS.According to the dispersion of remanence direction,the filtering method of 45 ° cutting angle is selected to eliminate the direction data away from the average value of 45 °.Finally,82 samples of BS and 157 samples of TS are tested by understanding the reversal of bootstrap method at the 95% confidence level.To assess if this region has suffered vertical axis rotations with respect to stable Eurasia,we compare the observed paleomagnetic directions with reference directions from cratonic Europe.Given the time interval studied here and the inherent quality of the magnetic data,we elected a temporal averaging of 5 Myrs.Thus,we divide our data in the following independent bins: 45-40 Ma(BS),40-35 Ma(BS),35-30 Ma(BS)and 25-20 Ma(TS).However,to enable a finer temporal resolution,we can also construct an alternative set of bins offset from those above: 42.5-37.5 Ma(BS),37.5-32.5 Ma(BS),22.5 Ma-17.5 Ma(TS);although clearly these two sets of bins are not independent.The above 7 time average windows show magnetic declination deviation values of-17.5 ± 8.7 °,-18.4 ± 11.4 °,-12.6± 9.1 °,-5.8 ± 11.2 °,-3.9 ± 15 °,3.4 ± 5.6 °,-3.3 ± 9.0 °,respectively.Considering that the rotation of young strata may lead to the rotation of old strata,and combined with the regional geological tectonic background,it is finally obtained that after the India-Eurasia collision,there is a counterclockwise tectonic rotation of about-18.4 ± 11.4 ° in the north of Junggar Basin,which occurred at 40±2.5 Ma ～ 35±2.5 Ma.This is mainly due to the compressive stress generated by the India-Eurasia collision passing through the relatively hard Junggar Basin,reviving its basement fault,the uplift of the northern fold mountain system,and the strong thrust nappe accompanied by strike slip activity in the deep basement fault zone. |
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