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滇中中元古代软沉积物变形构造及其地质意义
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摘要
滇中易门地区昆阳群大龙口组是一套浅海碳酸盐岩沉积,其中发育了大量的软沉积物变形构造和molar-tooth构造(臼齿构造)。软沉积物变形构造主要包括水塑性褶皱、液化沙脉、液化沙侵构造、液化泄水构造、球枕构造、流化底辟构造、同沉积断裂等,均是与地震诱发的变形相适配的变形构造。这些变形构造常沿限定的岩层发育,上、下均为未变形的岩层,显示事件变形特征。
    研究区内共发现三个地震事件层,均分布于大龙口组下部的三元宫段。追索事件层,对比不同剖面同一事件层中软沉积物变形构造特征,发现其发育与分布呈规律性变化。远离可能震中区的软沉积物变形类型单一,以水塑性变形为主,臼齿构造脉体小,排列方式简单。在可能的震中区,事件层大量发育液化沙侵构造、流化底辟等高能量软沉积物变形构造。液化脉体类型多样、规模更大;臼齿构造脉体密度变大、类型增多,脉体排列更加无序。臼齿构造发育特征与地震驱动的变形构造在时空分布上具有一定的相似性,表明地震振动可能是它们共同的触发机制。
    地震事件所在的大龙口组之下发育一套厚近百米的安山质凝灰岩、层凝灰岩;属富良棚段。这套凝灰岩是昆阳群最早出现的中性火山岩系,与昆阳群沉积由陆源碎屑岩转化为碳酸盐岩,以及震积岩开始大量发育同步。为标定大龙口地震事件沉积发育的时间,在这套凝灰岩下部、顶部分别取样,精选出锆石,测得SHRIMP U-Pb年龄为1032±9Ma,指示这套火山岩发育于中元古代晚期,证实地震发育时间在中元古代末期。富良棚凝灰岩SHRIMP U-Pb年龄也表明康滇南部发育的中元古代晚期造山活动,可能与全球Grenville期造山运动相关。
Soft-sediment deformational structures and molar-tooth structures developed well inDalongkou Formation, Kunyang Group, which is a succession of marine carbonate rocks,outcropping mainly in Central Yunnan. The types of soft-sediment deformationalstructures include hydroplastic folds, sand dikes, mushroom-like intrusions of sand or silt,dish-like water-escape structures, ball-and-pillow structures, diapers and syn-depositionalfaults. They formed in temporally and stratigraphically constrained horizons bounded byundeformed strata, indicating that they are syn-depositional.
    Three event beds of soft-sediment deformational structures have been recognized inDalongkou Formation, and all of them are in Sanyuangong Member. The Characteristicsof soft-sediment deformational structures are not same at different sections, even indifferent parts of the same event bed. In the distal area to possible epicenter, thesoft-sediment deformation structures are simple and mainly hydroplastic folds, and themolar-tooth structures are small and simple too. While in the proximal area, thesoft-sediment deformational structures are larger and more complicated, and new types ofstructures appear, such as liquefied sand dikes and mushroom-like intrusions of sand orsilt, indicating that these structures were related to higher energy than that far away fromthe possible epicenter. The characteristics of molar-tooth structures, developed in theproximal area to possible epicenter, change analogously. The above facts lead to theconclusion that soft-sediment deformational structures and the molar-tooth structuresmight be triggered by the same agent—paleoearthquakes.
     A set of ash tuff and bedded tuff lies right beneath Dalongkou Formation. This set ofvolcanic rock, about 100m thick, belongs to Fuliangpeng Member, and is the first set ofAndesitic volcanic rock of Kunyang Group. This set of ash tuff is coeval with both thechange of the sedimentary facies from terrigenous detritus to carbonate and the beginningof much seismite to develop. In order to date seismites, two tuff samples were collectedrespectively from the bed's lower and uppermost parts. A weighed mean U-Pb age of1032±9Ma were obtained using zircons selected from the samples, which indicate thevolcanic rock is late Mesoproterozoic in age, and the earthquakes developed in lateMesoproterozoic. The result also support that the Mesoproterozoic orogen in KangdianArea may be related to Grenville orogeny.
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