褶皱的地震波反射响应及其大陆地壳构造地质意义

褶皱是挤压造山带中最普遍的地质构造,但是为什么在地震反射剖面上却极少呈现褶皱呢?本文从一个构造地质学者的角度出发,数值模拟了几种常见褶皱构造的地震波反射响应。结果表明,传统的反射地震技术并不能正确成像闭合褶皱、斜歪或倒转褶皱及二级以上的多级复合褶皱。褶皱陡倾翼的反射消失,而褶皱转折端的反射形成一系列断断续续分布的假水平层理,这些假水平层理在下地壳中极易被错误地解释成由地壳拉张形成的水平构造层理或由底侵作用形成的水平基性岩席。笔者认为,老、中、新褶皱造山带的下地壳中大多数不连续水平反射可能是由褶皱造成的。

Seismic Reflection Response of Folds and Implications for Structural Geology of Continental Crust

In order to understand seismic response of folded structures, reflection modeling has been performed on various types of fold trains including single-order open folds, close folds and asymmetric inclined folds as well as multi-order folds. The synthetic sections show that, unlike large scale single-order open folds, multi-order folds and inclined folds cannot be correctly imaged by conventional reflection techniques. The reflections from fold limbs tend to be muted, leaving the crests and troughs that merge laterally into pseudo-horizontal reflections. These reflections bear no resemblance to folds but mislead, instead, the presence of subhorizontal reflections. The latter is often interpreted as subhorizontal layering or compositional lamination formed by crustal extension or underplating. The modeling results also suggest that the compositional lamination is not necessarily the sole source of subhorizontal reflections. Complex folded structures due to intensive shortening may also produce such subhorizontal reflections, but the latter are generally more discontinuous laterally and less dense than those caused by simple lamination. Interestingly, the reflections observed in cratons and fold belts are generally laterally shorter and less dense (except in ductile shear zones) than those observed in the extensional regions. This indicates that seismic lamination may correspond to laminated lower crust in extensional regions such as the Basin and Range (USA), but most likely to complex folded structures beneath both ancient and modern fold belts.