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 province 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 rock 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 early Mesozoic age, and (2) an unusual younger Mesozoic history of alternating northward and southward tectonic vergence of major structures. Major south-directed low-angle thrust faulting of pre-Middle Jurassic age (7180 Ma) 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 the 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 displacement of &40 km. 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 recumbent, 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 (≤ 180-190 Ma) accompanying westward or northwestward 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 more than 800-1100 km north of the Yanshan belt. This collision might have been responsible forYanshan (andYinshan) intraplate contractional deformation farther south, but such a hypothesis is severely complicated by reports of widespread, basin-forming Late Jurassic and Early Cretaceous extension in the terranes between the Mongolo-Okhotsk suture and the Yinshan-Yanshan belt. Following Early Cretaceous contraction in both the Yinshan and Yanshan belts, the northern margin of the Archean-floored plate was also the site of major northwest-southeast regional extension beginning soon after 120 Ma. Subducting plate rollback or postorogenic collapse are only two of several possible explanations for the development of extensional metamorphic core complexes in northern InnerMongolia and theYanshan belt farther east.
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