摘要:
收缩裂缝在前寒武纪到显生宙各个地质时代都有发现,其成因一直存在争议。作者在灵山岛莱阳群泥岩和砂岩中都发现了大量的非构造裂缝,除了收缩裂缝外还有应力脆性裂缝和混合裂缝。这些裂缝通常在单层内发育,极少量延伸到邻层,具有多形态、多尺度、多充填方式、多期次和多级别的特征,与干燥泥裂和后期构造裂缝区别明显,结合地层中无暴露标志,认为是在水下环境形成。总结水下裂缝的关键特征并对比Pratt提出的地震收缩裂缝特征,推测灵山岛裂缝亦是地震触发而形成。灵山岛已经识别出了大量地震成因的软沉积变形构造,证明当时本区域地震活动频繁,具有裂缝地震成因的要素。前人总结收缩裂缝特点发现裂陷盆地明显多于被动大陆边缘挤压盆地,作者认为收缩裂缝在裂陷盆地拉张环境下容易形成和保存。灵山岛裂缝解释为水下环境地震成因具有更多的地质意义:可以解决沉积时水体深度问题;记录了古地震能量与频次,有效反映古地震的节律;是灵山岛莱阳群属于裂陷盆地沉积的有利证据。
关键词:
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灵山岛
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莱阳群
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收缩裂缝
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地震
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液化
Abstract:
Shrinkage cracks exist in Precambrian-Phanerozoic strata, but their origins remain controversial. Lots of cracks were found in the sandstone and mudstone of Laiyang Group in Lingshan Island, along with some brittle cracks and liquefied sandstone veins. These shrinkage cracks are usually developed in single layers and rarely extend to their adjacent layers. They possess the characteristics of multi-formation, multi-scale, multi-stage, multi-order and multiple filling ways and are distinct from desiccation cracks and post-structural fractures. Since there is no evidence of exposure, they are considered to have been formed under water. A comparison with the characteristics of seismic cracks proposed by Pratt shows that they might have been triggered by earthquakes. Many soft-sediment deformation structures were found in Lingshan Island, implying that seismic activities were frequent at that time and these seismic activities provided the necessary formation conditions for cracks. Different from the previous opinions, the viewpoint held by the authors argues that shrinkage cracks are more likely to be formed and get preserved in rift basins than in compressional basins in the passive continental margin. The interpretation of the cracks found in Lingshan Island as being caused by earthquakes under water bears much geological significance in that such an interpretation can indicate depth of water, record paleoseismic energy and frequencies, reflect paleoseismic rhythm and provide evidence that the Laiyang Group in Lingshan Island was developed in a rift basin.
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