华北陆块东南缘徐淮推覆-褶皱带构造变形特征研究

摘要

文章对华北陆块东南缘徐淮推覆-褶皱带构造变形特征进行了研究.研究表明,该带经历了两期构造变形,早期为推覆-褶皱变形,伴有局部的左旋走滑,晚期为早白垩世的区域伸展与闪长玢岩、二长闪长玢岩的侵入,岩浆结晶年龄为131～135 Ma.根据80多口煤田钻井和有关地球物理资料,将徐淮推覆-褶皱带划分为三个亚带,分别是:(1)变质结晶基底:靠近郯庐断裂带,大部分被第四系覆盖,零星出露古元古代片岩、片麻岩等;(2)推覆-褶皱带:由中、新元古代—奥陶纪期间沉积的碳酸盐岩夹碎屑岩和石炭纪—二叠纪期间沉积的含煤碎屑岩组成,厚2600 m;(3)推覆前缘带:为晚古生代大型煤田的赋存区.推覆-褶皱带底部以一个大型拆离断层与古元古代结晶基底接触.所有前中生代岩层都被卷入了一个上地壳层次的强烈推覆和褶皱变形,属于脆性变形构造,构成徐淮推覆-褶皱带;其上被近水平展布的早白垩世陆源碎屑岩、火山岩和晚白垩世—古近纪红层不整合覆盖.本文鉴定出了6～7个不同规模的拆离断层,它们具有犁式形态,向下产状渐趋平缓,归并到一个主拆离断层上.几何学研究表明,主拆离断层的根部在东侧,紧邻郯庐断裂带.还鉴定出了6套非能干岩层,由泥岩、粉砂岩、石膏层组成,其厚度与推覆规模呈正相关关系.断裂上盘底部为能干岩层,下盘顶部为非能干岩层.区内存在数个千米规模的飞来峰构造,其底部为低角度逆断层,断层之上为一到数个构造岩片,各岩片之间以次级断裂为界.台阶状逆断层(由断坡、断坪复合而成)、叠瓦状逆断层和断层相关褶皱(断层转折褶皱和断层传播褶皱)构成了徐淮推覆-褶皱带构造变形的最显著特征.其缩短量从20.6 km到29.6 km,缩短率为43.6％～46.4％.结合区域地质的综合分析,认为这种强烈的缩短-增厚变形是早中生代华北陆块东南缘的上部地壳朝SE朝苏鲁造山带和大别造山带下冲的产物.%The Xu-Huai thrust and fold belt in the SE margin of the North China Block mainly consists of thrust slices bordered by imbricated faults and fault-related folds with ramp and fiat-structures.Geologic evidence suggests that this belt was subjected to two phases of structural deformation,namely,an early-phase of thrust-fold deformation with partly sinistral strike-slip component and a second phase of extension followed by thc intrusion of dioritic and monzodioritic magma dated at 131-135 Ma.From 80 bore holes and geophysical data,three units can be distinguished:(1)the crystalline basement in the eastern segment,(2)the nappe unit or the thrust and fold belt in the central segment,which is composed of Neoproterozoic to Ordovician carbonate rocks and Carboniferous-Permian coal-bearing strata,about 2600 m thick,and(3)the frontal basin unit in the western segment that the large-scale coal-fields were mainly formed in this unit.A major detachment fault occurs in the bottom of the napple unit;all pre-Mesozoic depositional sequences were involved into this thrust and fold unit and were then overlain unconformably by the sub-horizontal Cretaceous-Paleogene cover consisting of coarse-grained terrigenous clastic rocks,volcanic rocks,and red beds.The km-scale thrust body is composed of allochthonous slices,autochthonous strata,and detachment fault.Six uncompetent-rock layers with biostratigraphic ages have been recognized and each uncompetent-rock layer mainly occurred in the top of the footwall,playing an important role in the development of the Xu-Huai thrust and fold belt.Geometry of the major detachment fault suggests that this thrust and fold belt was rooted in its eastern side,near the Tan-Lu Fault Zone.Plenty of ramp-flat composite faults and fault related folds were identified,from which a shortening of 20.6～29.6 km was obtained,corresponding to the shortening rate of 43.6％ ～46.4％.The formation of the Xu-Huai thrust and fold belt was likely derived from a SE-ward continental-crustal underthrust of the North China Block beneath the Su-Lu and Dabie belts during the early Mesozoic.