摘要
从渤海湾和辽宁省开始 ,燕山山脉沿北纬 40°左右一直向西延伸到河北和内蒙古的边界。继续向西 ,以太古宙为基底的燕山山脉被上第三系所覆盖 ;因此 ,虽然内蒙古的阴山很可能就是燕山向西的延伸 ,但还缺乏有力的证据。中生代燕山褶皱冲断带以陆源沉积、岩浆作用和构造变形 (包括多期的褶皱作用 ,挤压、伸展及走滑断层作用 )为特征。对北京、冀北与辽西地区侵入岩及火山岩的野外工作及测年 (U Pb,40 Ar 39Ar方法 )研究揭示出该区中生代复杂的构造变形特征 ;而这种构造变形特征被归因于 :(1)中生代以前 (二叠纪到中生代早期 )复杂的早期变形序列 ,以及 (2 )中生代时期交替发生的主要构造形迹向南和向北的构造汇聚作用。燕山在中侏罗世之前 (早于 180 Ma)向南运动的低角度的逆冲断层发育在一条二叠纪三叠纪岩浆弧的南部 ,太古宙基底及元古宙和显生宙盖层均卷入其中。该次推覆作用可能是 (1)古生代时期蒙古弧与安第斯型的大陆弧沿华北板块的北部边缘发生碰撞的后果 ,或 (2 ) (北部陆块 )向南俯冲于华北太古宙“克拉通”之下时形成的 ,类似于美国科迪勒拉型的弧后、前陆褶皱冲断带。此次褶皱冲断发生之后 ,该推覆断层的上盘经历了广泛的剥蚀 ,随后中侏罗世火山沉积地层同时沉积在上、下盘之上。
The Yanshan(Yan Mountains)of northern China extend westward at about lat 40°N from Bohai Bay and Liaoning province to the border between Hebei provincc and Inner Mongolia. It is likely,but not unequivocally demonstrated,that the Archean-floored Yanshan continue farther westward under a cover of Neogene strata to emerge as the yinshan belt of Inner Mongolia.Mesozoic terrestrial sedimentation,magmatism and deformation-including multiple phases of folding and contractional,extensional,and strike-slip faulting-characterize the Yanshan fold and thrust belt. Field studies and radiometric dating(U-Pb, 40Ar- 39Ar)of plutonic and volcanic rocs units in Beijing Municipality and northern Hebei and western Liaoning provinces have revealed that the complexity of Mesozoic deformation in these areas is in large part(1)a consequence of profound earlier deformation of Permian(?)to carly Mesozoic age,and(2)an unusual younger Mesozoic history of alternating northward and sorthward tectonic vergence of major structures. Major south-directed low-angle thrust faulting of pre-Middle Jurassic age (>180Ma)in the Yanshan involved Archean basement rocks and their Proterozoic and Phanerozoic cover,and developed south of a Permian-Triassic magmatic arc.Thrusting could have been(1)a consequence of the collisional suturing of Paleozoic Mongolian arcs against an Andean-style continental arc along the northern margin of the North China plate,or(2)an expression of a backarc, foreland fold and thrust belt of U.S.Cordilleran Type formed during southward subduction beneath the North China Archean'craton.'This episode of thrusting and folding was followed by widespread erosion of upper plate rocks and subsequent terrestrial deposition of Middle Jurassic volcanic and sedimentary strata across both plates. Our studies indicate that Late Jurassic and Early Cretaceous contractional deformation in the Yanshan was also much more intense than generally believed,and that it followed a heretofore unrecognized phase of Middle Jurassic or early Late Jurassic east-west extension in northern Hebei province.Prior to our studies, popular views of east-to east-northeast-trending Yanshan contractional deformation proposed that(1)involvement of Archean basement rocks in faulting indicates a thickskinned tectonic style analogous to the U.S.Laramide Rocky Mountains;(2)thrust faults steepen downward into the basement;(3)vertical movements predominated in the development of the belt;and(4)Mesozoic contraction across belt was only a few tens of percent;some workers have believed that the Yanshan developed independently of plate interactions.We question all of these assumptions.We have identified a major,synformally folded,thin-skinned thrust plate just south of Chengde,Hebei province,that is of Late Jurassic age and had a minimum northward displace-ment of～40km.In earlier studies in the Yunmeng Shan area of the Yanshan north of Beijing we defined major south-vergent Late Jurassic-Early Cretaceous ductile structures involving Archean basement rocks and their cover.These structures include a uecumbent,basement-cored anticlinal nappe and a lower limb ductile thrust fault within a 6-km-thick gneissic shear zone. The Jurassic-Cretaceous Yanshan belt, and its probable western continuation,the Yinshan belt of Inner Mongolia,appear to be reflect regional north-south intraplate shortening. However,some Yanshan patterns of deformation,e.g.,ductile nappe formation in the Yunmeng Shan and northeast structural trends of the belt in Liaoning province,were influenced by thermal regimes related to magmatism(≤180～190Ma)accompanying westward or northwestward Pacific pacific plate subduction beneath eastern Asia.We thus believe it likely that two contrasting modes of plate interaction occurred synchronously in the Yanshan segment of the Yinshan-Yanshan belt during Middle Jurassic through Early Cretaceous time. Jurassic-Cretaceous collision of an amalgamated North China-Mongolian plate with the Siberian plate is widely believed to have accompanied closure of a Mongolo-Okhotsk sea m
出处
《北京地质》
2002年第4期1-40,共40页
Beijing Geology
