SCIENCE CHINA Earth Sciences • RESEARCH PAPER • June 2011 Vol.54 No.6: 798–822 doi: 10.1007/s11430-011-4180-7 Mesozoic contraction deformation in the Yanshan and northern Taihang mountains and its implications to the destruction of the North China Craton ZHANG ChangHou*, LI ChengMing, DENG HongLing, LIU Yang, LIU Lei, WEI Bo, LI HanBin & LIU Zi State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Received August 5, 2010; accepted January 30, 2011 Mesozoic contraction deformation in the Yanshan and Taihang mountains is characterized by basement-involved thrust tec- tonics, basement-cored buckling anticlines and ductile thrust and nappe tectonics. Most of these deformations are orientated west-east, west-northwest and northeast to north-northeast. The contraction deformations began in the Permian, continued through the Triassic and Jurassic and terminated in the Early Cretaceous, and constitute an important part of the destruction of the North China Craton. It is estimated, from balanced cross-section reconstructions, that the north-south shortening of the central part of the Yanshan belt before 135 Ma was around 38%. The initial crust thickness, pre-dating the major contraction deformation in late Paleozoic and early Mesozoic, was estimated to be around 35 km based on paleogeographic characteristics. Assuming that the inferred depth of ductile thrusting deformation, 20–25 km, was the crust thickness involved in the contrac- tion deformation, and also assuming that the N-S contraction deformation was accommodated by vertical crust thickening, the thickness of the crust after the contraction deformation was expected to be around 47–50 km. This was the approximate crust thickness required for the eclogitization of the lower crust for delamination. The gravity potential accumulated by the isostatic uplift of the thickened crust, together with the decrease in crustal strength caused by the coeval magmatisms associated with the contraction deformation, led to the subsequent extensional collapse of the middle and upper crust although the regional stress regime associated with the plate interactions remained constant. It is inferred that the Mesozoic contraction deformations in the Yanshan and Taihang mountains were not only a significant tectonic process contributing to the destruction of the craton in middle and upper crust but also stimulated delamination at a deep level and the extension of the shallow crust. In other words, both the suspected delamination of the lower crust and upper mantle and the well constrained extension deformations of the shallow crust in the eastern North China Craton during the late Mesozoic are a consequence of crust thickening due to pre- vious contractions. Extensional deformations could be expected to occur independently in the shallow crust, and are not nec- essarily associated with or responding to delamination at a deep level. North China Craton, Yanshan belt, Taihang Mountain, contraction deformation, gravitational collapse Citation: Zhang C H, Li C M, Deng H L, et al. Mesozoic contraction deformation in the Yanshan and northern Taihang mountains and its implications to the destruction of the North China Craton. Sci China Earth Sci, 2011, 54: 798–822, doi: 10.1007/s11430-011-4180-7 The destruction of the North China Craton (NCC) involves gether with significant changes in the lithospheric composi- the voluminous thinning (>120 km) of the lithosphere to- tion and thermal state (gradient) and extensive tectonic de- formations [1–8]. A series of progressions and achieve- ments regarding the temporal and spatial aspects of the *Corresponding author (email: [email protected]) geological processes of lithospheric thinning have been © Science China Press and Springer-Verlag Berlin Heidelberg 2011 earth.scichina.com www.springerlink.com Zhang C H, et al. Sci China Earth Sci June (2011) Vol.54 No.6 799 made and have mainly been based on investigations into lized basements and overlying supracrustal sequences. The igneous activities during the destruction of the NCC. Most contraction deformations differ profoundly from the fore- researchers agree that the peak lithosphere thinning oc- land fold-and-thrust belt associated with the subduction or curred around 130–110 Ma [9–13]. The start of the litho- collisional orogens that developed mainly within the sedi- spheric thinning, however, remains controversial, and in- mentary prism along the former continental passive margins. cludes the end of Paleozoic [13], the Late Carbonifer- Even though a competence contrast exists between the su- ous-Late Triassic [14], Late Triassic [11] (225–205 Ma), pracrustal sequences and the underlined rigid basement, it Early Jurassic [9, 10, 12] (190–180 Ma), Late Jurassic [1, seems that this contact in this area could not serve as an 15] (160 Ma), Late Jurassic-Early Cretaceous (160–140 Ma) effective decollement. The contraction deformations in the [5], and the end of the Early Cretaceous (100 Ma) [16]. Yanshan belt and Taihang Mountain are characterized by Three different geodynamic models, the lithosphere de- basement-involved deformations. lamination model, the thermal-mechanical and chemical erosion model, and the mantle alteration and replacement 1.1 Large-scale basement-involved thrust tectonics model, have been proposed to account for the lithospheric thinning. Large-scale basement-involved thrust tectonics are one of Most researchers who support the delamination model the important structural styles in the Mesozoic contraction maintain that there was a profound crust thickening prior to deformations around the Yanshan belt and Taihang Moun- the delamination although they disagree with each other on tain. The Shangyi-Chicheng-Fengning-Longhua-Lingyuan- the timing, extent, tectonic level and pattern of the delami- Beipiao fault which bounds the north side of the “Yan-Liao nation. Lines of evidence including the petrological and Depression” (Figure 1) is one of the most important and geochemical characteristics of the igneous rocks [4, 17–19], largest basement-involved thrust tectonics [28–32]. The the distribution of adactic rocks [19–26], and mantle xeno- high grade metamorphic rocks within the “Inner Mongolia liths from volcanic rocks [27] have been used to support the Uplift” were thrust southward above the Middle Proterozoic existence of a thickened crust. The processes and mecha- sedimentary sequence and Mesozoic terrestrial sedimentary nisms for crust thickening, however, remain unclear. rocks along this fault zone. In the last decade, the strongly The Mesozoic contraction deformations were widespread deformed ductile shear zones along the fault system have around the eastern NCC, and some deformations have been been recognized as being south-vergent thrusting. The syn- studied for more than a century. They developed during the deformation metamorphic mineral assemblages and the in- time period between the stable craton (~457–460 Ma) and ferred P-T conditions of the mylonites developed within the the mobilized or destroyed craton (130–110 Ma) in the ductile shear zones suggest that they formed under upper eastern NCC and spatially coincide with the extensive de- amphibolite facies, equivalent to 20–25 km below the crust struction of the NCC. However, the roles these contraction surface [33–35]. 40Ar/39Ar dating of the syndeformational deformations played in the crust thickening and destruction biotites and hornblends from the thrust ductile shear zone of the NCC have not been determined. Our research aims to identified the late Permian (ca. 263 Ma) [36] and Middle evaluate the contribution of the contraction deformations to the extensional deformation in the shallow crust and the Jurassic (163 Ma) [35]. The synkinematic sedimentation suspected lower crust delamination, and the implications of features and deformations of the foredeep sequences in front this destruction process to the NCC, involving investiga- of the thrust system [37–41] imply that they underwent tions and reviews into the structural style, and the temporal south-directed brittle thrusting during and after the sedi- and spatial evolution of the contraction deformations and mentation of the Tuchengzi Formation in the Late Jurassic. the resulting shortening and thickening of the Yanshan belt The thrust faults which have developed around Xiahua- and the northern Taihang Mountain. In the present study, yuan and the Jiucaigou-Xiaosuangou area in northwestern the Taihang Mountain belt refers to the area lying between Hebei Province and Yanqing County in Beijing, share the the eastern boundary of the Ordos Basin and the western same characteristics of basement-involved deformations boundary of the Bohai Bay-North China Basin rather than [32]. The Xinglong thrust tectonics in the central Yanshan the exact geographical region of the Taihang Mountain. belt [42–45], and Anjiang-Mengjiazhuang thrust in western Chengde County (Figures 2 and 3(a)) have characteristics of both basement and cover sequences being involved in the 1 Structural styles of the contraction deforma- contraction deformations. tions In the eastern part of the Yanshan belt, in addition to the previously mentioned Lingyuan-Beipiao thrust [30], the The Mesozoic contraction deformations within the Yanshan Xifengkou-Yuerya-Baoshenmiao thrust (XBT) exposed in belt and northern Taihang Mountain developed from typical Qianxi and Kuancheng counties in Hebei Province and platform crust structures with strong and rigid crystal-