地球脉动——表现、级别及与古地磁的联系

THE EARTH'S PULSATIONS, THEIR MANIFESTATION, HIERARCHY AND CONNECTION WITH PALAEOMAGNETISM

地球脉动的概念是指地球在其历史中曾发生膨胀和收缩的周期性变化。其根据是构造变动、岩浆活动、地磁极倒转以及海平面升降等方面，在地球的膨胀和收缩期，均有各种表现。构造海平面升降在地球收缩期形成海退，在膨胀期形成海侵。在一个地球脉动旋回的不同时期，地表、海平面和洋底之间的相互关系均有变化，因而形成“层序”的沉积记录。地磁场的强度和地磁极倒转频率在显生宙有明显变化，有高峰期和平静期。这种频率变化与构造变动和岩浆活动都有对应关系，特别是４Ｍａ为准周期的地磁极倒转频率与海底扩张、洋壳形成速率之间具有良好的对应关系。地球脉动旋回可以分为不同的级次，构成级别体系：超级旋回约１Ｇａ，巨旋回２５０～３００Ｍａ，一级旋回５０～１５０Ｍａ，二级旋回５～５０Ｍａ，三级旋回１～５Ｍａ，均可与构造旋回相对应，更高的级次对构造不形成影响。三级及更长的脉动旋回可能受深部过程的控制。中、新生代以来，地球脉动是在地球适度膨胀的背景下进行的。

The idea of geopulsation denotes that the earth has undergone pulsation cycles of repeated expansion and contraction in its history. It is based on the various manifestations of tectonic movement, magmatic activities, geomagnetic polarity reversal, and eustatic changes in the earth's expansional and contractional stages. The tectono eustatic changes show a regression in the contractional, and transgression in the expansional stages. Within a geopulsation cycle, the relationship between the earth's surface,the sea level and the sea floor underwent different changes, and produced the sedimentary record of stratigraphic “sequences”. At least in the Phanerozoic, the geomagnetic intensity and polarity reversal frequency show evident changes and contain alternating stages of activeness and of quiescence. These frequency changes bear generally a correlation with tectonic movements and magmatic activities. The geomagnetic reversal frequency at 4 Ma interval shows a specially good correlation with the rates of sea floor spreading and of oceanic crust generation. The geopulsation cycles are of different ranks which may form a hierarchy system. The cycles in the order of their length of time intervals are: Supercycle about 1 Ga, Megacycle 250—300 Ma, first order cycle 50—150 Ma, second order 5—50 Ma, and third order 1—5 Ma. All of them may be correlated with the different ranks of tectonic cycles, but higher order cycles above the third are without tectonic effect. The third and longer geopulsation cycles are probably controlled by deep processes of the earth. In the Meso and Cenozoic the geopulsation cycles proceeded under a background mainly of earth expansion.