新元古代沂沭海峡地震引发的软沉积物变形及其微量元素信息

Earthquake-induced soft sediment deformations and their trace-element information in the Neoproterozoic Yishu Strait

新元古代沂沭海峡位于郯庐断裂带昌邑—大店断层西侧宽60~90km、长300km的区域内,其分布范围基本与郯庐断裂带中段一致。新元古代,该海峡盆地先后沉积了青白口系、南华系及震旦系的滨—浅海相陆源碎屑、泥质及碳酸盐沉积物,其岩石地层称之为土门群。本次研究从该海峡盆地土门群佟家庄组(由石英砂岩、泥灰岩和页岩组成,属南华系)、浮来山组(由细—粉砂岩夹页岩组成,属震旦系)及石旺庄组(主要由碳酸盐岩组成,下部夹页岩,属震旦系)中鉴别出了一系列地震触动软沉积物形成的变形构造,包括液化砂墙和砂脉、负载构造、火焰构造、球枕构造、软布丁构造、底辟构造、滑塌褶曲和同沉积断层等。它们是地震事件的记录——震积岩。新元古代中晚期(约800—600Ma)沂沭海峡盆地拉张裂陷是它们形成的动力机制。根据石旺庄组中的微亮晶方解石脉与地震成因的变形构造(如负载、火焰、软布丁及同沉积断层等)共(伴)生的事实,并结合模拟地震试验结果分析,认为微亮晶方解石脉很可能是地震引发的饱和灰泥(方解石微粒)液化脉。微量化学元素分析结果表明,这些震积岩中的w(La)/w(Sc)和w(La)/w(Th)值高于上地壳平均值和正常沉积层,而w(Th)/w(Sc)值则较低,深源微量元素Ni和Cr的含量也高于上地壳平均值及正常沉积层。这些微量元素特征是当时的盆地基底不稳定、快速沉积且伴有深部物质元素混入的反映,强地震很可能是快速沉积和促进深源物质混入的主要动力。土门群中发育的一系列地震引起的软沉积物变形(震积岩),再次证明了古郯庐断裂带的存在,由它们记录的地震事件也是晋宁运动乃至Rodinia超大陆的裂解的响应。

The Yishu Strait is a region of 60～90,km wide and 300,km long that is located at the western side of the Changyi-Dadian Fault of the Tanlu Fault Zone,which extent is essentially as same as that of the middle segment of the Tanlu Fault Zone. During the Neoproterozoic,littoral neritic sediments composed of terrigenous clastics,clays and carbonates were deposited in the basin. These sediments belong stratigraphically to the Tumen Group from the Qingbaikou to Sinian. A variety of soft-sediment deformation structures（SSDS）were identified from the Tumen Group,including dikes or veins of liquefied sand,load cast structures,flame structures,ball-and-pillow structures,soft boudinages,diapirs,slumps and synsedimentary faults. They occur in the Nanhua Tongjiazhuang Formation（which consists of quartz sandstones,marls and shales）,the Sinian Fulaishan Formation（which consists of fine sandstones and silty sandstones interbedded with shales）and the Shiwangzhuang Formation（with mainly carbonate rocks,interbedded in the lower part with sandy shales）of the Tumen Group. They are records of seismic events,i.e. seismites,which were related to tectonic tension and seismic activity in the Paleo-Tanlu Fault Zone. Strong earthquakes caused by the rifting of the strait basin was the dynamic trigger mechanism responsible for the formation of the SSDS during the mid-late stage of the Neoproterozoic（approx. 800-600,Ma）. Because of the joint occurrence of microspar calcite veins（molar tooth structures）and seismogenic soft-sediment deformation structures（such as load casts,flames,boudinage features and synsedimentay faults）,incombination with the results of earthquake-simulation experiments, it must be deduced that the micro-spar calcite veins represent most probably veins of fluidized saturated lime-mud（a water-saturated sediment consisting of fine calcite particles）in the Shiwangzhuang Formation. The results of trace-element analyses show that w（La）/w（Sc） and the w（La）/w（Th） ratios in the seismites are higher than the average ratios in the upper continental crust and in normal sedimentary layers,whereas the w（Th）/w（Sc） ratio is lower. The Ni and Cr content in seismites is also higher than in the upper continental crust and in normal sedimentary layers. These characteristics of the trace-elements reflect the instability of the basin basement,rapid deposition,and the mixing with elements from deep sources. A series of seismite with soft-sediment deformation structures in the Tumen Group once more prove the existence and activity of the paleo-Tanlu Fault Zone. The seismic events recorded by these seismites might also be considered as a response to the Tsinning tectonics and even to the breakup of the Rodinia supercontinent.