地球科学中时间段的定年问题——一种不同于地球化学的地球物理定年方法

Duration measurement in geological sciences——A geophysical approach

地质事件时间结构的度量有两种方法,一是测算事件发生时间点的距今时间,另一是测算事件过程的时间段的持续时间.作为时间点的测量方法,地球化学的同位素地质年代学经过近百年的发展,在国际上已很成熟(其误差精度一般为百万年量级),但这一方法不能测算地质事件持续时间.测定地质事件始末的时间间隔,则需要地球物理的构造地质年代学研究,这是目前国际前沿研究的一个新领域.经过努力,现今探索了一种测定地质事件时限年龄的方法(一般误差精度为万年量级),其工作程序主要分为四个阶段:构造地质工作——观测构造改造序次强度;构造物理工作——测算差应力和应变速率,这是地球物理定年的基本环节;构造化学工作——求算构造变形地质体的体积因子;构造数学工作——求算构造形成时限.我们已将该方法应用于中国东部和西南韧性剪切带的研究,获得了一定的成果.但此方法还不成熟,若要使之成为尚在创建中的构造地质年代学的有效理论,还需要进一步完善,本文探讨了构造地质年代学的发展前景,指出了当前地质定年工作存在的局限性.

There are two ways of measuring the time structure of geological events： the time-point at which an event occurred and its duration. Although the isotopic geochronological method, after nearly a century of development, is now a mature approach to the time-point measurement of a geological event （,generally with a precision of a millions years＇ error）, it is incapable of measuring its duration. The measurement of a geological event from beginning to end requires an insight into structural geochronology, a new field at the forefront of international research. A geophysical approach was thus proposed （generally with a precision of 104 years＇ error ）. The approach can be divided into four steps, tectonic-geological work -- observation of order and intensity of tectonic reworking; tectonic-physical work -- measurement of differential stress and strain rates, a basic component in geological dating; tectonic-chemical work -- calculating volumetric factors of deformed geological bodies; and tectonic-mathematical work -- calculating the duration of structure formation. The approach has been applied to the study of the ductile zone in East and Southwest China and some results have been achieved. The approach is not yet mature and further improvement is needed for it to qualify as an effective theory of structural geochronology. The prospects of structural geochronology were given and its application limitations were pointed out.