ON TECTONOCLIMATIC CYCLE OF QUASI PERIOD OF 1.2Ma IN LATE CENOZOIC

The tectonoclimatic cycle which states that the climate is determined by tectonic movement is different from Milankovich's glacioclimatic cycle.However,both of them are controlled by the Earth's orbit.On the basis of the late Cenozoic geological records in the Qinghai-Xizang (Tibet) Plateau and the Loess Plateau,with the magnetostratigraphy as timescale and the maximal amplitude position of ecliptic tilt (ε) as model age,the major cyclothems of tectonoclimatic cycle with a quasiperiod of 1.2Ma in the last 8Ma are divided as MTC₇、MTC₆、MTC₅、MTC₄、MTC₃、MTC₂、MTC₁、MTC₀,whose horizon lines in the red clay loess sequence and boundary ages in the ε curve are RCL₄/Br 7.3Ma,RCL₃/RCS₄ 6.1Ma,RCL₂/RCS₃ 4.9Ma,RCL₁/RCS₂ 3.7Ma,WL₃/RCS₁ 2.5Ma,L₁₅/WS₁ 1.3Ma and L₂/S₂ 0.2Ma B P,respectively.And an analysis of the evolution of the eolian deposits of Loess Plateau in periods of about 1.2Ma and the tectonic uplift of Qinghai-Xizang Plateau shows that there existsa causality between them that the climate depends on the uplifting.
The uplift model of the Qinghai-Xizang Plateau constructed by the related information indicates that it has undergone different tectonic stages.First,the pre plateau stage,in which no integrated Qinghai-Xizang block had been formed,existed before 7.3Ma B P in the late Miocene.Second,the stage of isostatic adjustment of the newly- formed inner plateau block ensued during a time from 7.3 to 2.5 Ma B P,in which the uplift of the West Kunlun Mts.has began since 3.7Ma B P.Third,the stage of forming plateau surface followed from 2.5 to 1.3Ma B P,in which the Qilian-Qaidam block coalseced with the major part of the plateau,to form an initial Qinghai-Xizang Plateau similar to its present cnfiguration,and the aggradational plateau surface reached a mean altitude of 2400m.An en bloc uplifting stage has commenced since 1.3Ma B P in which the plateau surface has uplifted from a mean altitude of ca.2400m,successively to ca.2900m(0.9Ma B P),ca.3600m(0.6Ma B P),ca.4200m(0.13Ma B P),ca.4560m(0.09 Ma B P) and today's mean altitude of ca.4800m.
It is evident that the differences of climatic and environmental effects on the eastern China monsoon area,including the Loess Plateau,have been produced around the time of formation and uplift of the Qinghai-Xizang Plateau,in which the change of seasonality resulting from the stage-strengthening winter monsoon is most important.At first the winter monsoon was not conspicuous for a time span of 7.3-3.7Ma B P and strengthened somewhat after 3.7Ma B P.Afterwards,the East Asian monsoon being typical of a much stronger winter monsoon than summer monsoon have been formed.Then,in the process of winter monsoon strengthening,it abruptly changed before 2.5Ma B P,transformed before 1.3Ma B P and altered progressively before 0.2Ma B P,in which the different features of strong-weak associations of the winter and summer monsoon have been shown in every variation,enhanced seasonality step by step and resulted in the differentiation of ecological environment between south ant north,east and west.