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首页> 外文期刊>Global and planetary change >Quaternary tectonics in the central Interandean Valley, Ecuador: Fault-propagation folds, transfer faults and the Cotopaxi Volcano
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Quaternary tectonics in the central Interandean Valley, Ecuador: Fault-propagation folds, transfer faults and the Cotopaxi Volcano

机译：厄瓜多尔Interandean山谷中部的第四纪构造学：断层传播褶皱，转移断层和科托帕希火山

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摘要

We studied the Quaternary tectonics of the central Interandean Valley (IV, Ecuador), around the active Cotopaxi volcano, by field geological-structural survey, analysis of seismicity, precise levelling of river terraces and numerical modelling. North of the volcano, there are main Quaternary west-dipping reverse faults located along the western side of the valley. At the Cotopaxi foothills, we found NNE-SSW-striking, vertical, right-lateral oblique strike-slip faults and E-W-striking normal faults, which offset Pleistocene deposits. South of the volcano, there are several Quaternary folds that show minor normal faults at the hinge zone linked to local extension at the extrados. The core of the folds sometimes shows reverse faults. Most of the folds have flexure geometry whereas the Guambalo fold shows a double flexure resembling a huge box fold. By numerical modelling we computed the best solution that fits the observed geometry as fault-propagation folds. The fold system shows a double vergence linked to reverse faults dipping to the west along the western side of the IV and to the east along the eastern side. Computed stress tensors by striated fault inversion indicate a Quaternary E-W-trending horizontal greatest principal stress (σ1) and a vertical least principal stress (σ3) south of Cotopaxi, whereas at the volcano σ1 is ENE-WSW and σ3 is also horizontal. At the Yanayacu flexure we measured an increment of deformation along late Quaternary river deposits, suggesting a potentially very recent activity. These results are consistent with the distribution of crustal seismicity. We consider a larger Quaternary tectonic shortening south of the volcano than to the north of it, with the transcurrent faults acting as transfer structures. Magma emplacement at Cotopaxi in this compressional setting has been favoured by the horizontal σ3 linked to the transfer fault zone.
机译：我们通过现场地质构造调查，地震活动性分析，河阶的精确平整和数值模拟研究了活跃的科托帕希火山周围的中部安第斯河谷（厄瓜多尔四世）的第四纪构造。在火山以北，沿山谷的西侧有主要的第四纪向西倾斜的逆断层。在科托帕克西山丘上，我们发现了NNE-SSW走向，垂直的，右斜斜走滑断裂和E-W走向的正常断裂，它们抵消了更新世的沉积。在火山以南，有几个第四纪褶皱，在铰链区显示出较小的正断层，与断层处的局部延伸相联系。褶皱的核心有时显示出反向断层。大多数折痕具有挠曲几何形状，而Guambalo折痕显示出类似于巨大盒子折痕的双重挠曲。通过数值建模，我们计算出了与断层传播褶皱相吻合的最佳解决方案。褶皱系统显示出与逆断层相关的双汇聚点，该断层沿IV的西侧向西倾斜，而沿东侧的东侧向东倾斜。通过条纹状断层反演计算出的应力张量表明，第四纪E-W向水平的最大主应力（σ1）和科托帕西以南的垂直最小主应力（σ3），而火山的σ1为ENE-WSW，σ3也为水平。在Yanayacu弯曲处，我们测量了第四纪晚期河床沉积物的变形增量，表明这可能是近期的活动。这些结果与地壳地震活动性的分布是一致的。我们认为，火山南侧的第四纪构造收缩比北侧的更大，而逆流断层则作为传递构造。在这种压缩环境中，科托帕希的岩浆位置受到了与转移断层带相关的水平σ3的青睐。

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