内蒙古大青山侏罗纪生长地层对燕山运动“绪动”的制约

New time constraint on the initiation of Yanshan movement from Jurassic growth strata in the Daqingshan area, Inner Mongolia

在大青山石拐盆地南部中下侏罗统煤系地层变形前锋,可见中侏罗统长汉沟组与倒转褶皱相伴生,并呈现出生长地层特有的同构造沉积地层楔.该特征暗示了煤系地层褶皱变形与其底部近水平拆离断层的发育密切相关.生长几何形态由下部的上超和超覆过渡为上部的退覆,刻画了变形初期构造抬升速率逐渐加快的过程.长汉沟组上部火山灰夹层的锆石U-Pb定年(163.7±1.0 Ma)和区域地层对比限定了大青山陆内挤压变形起始于中侏罗世(~170 Ma).因此,这应该是制约燕山运动"绪动"最有力的证据.

In recent years, based on the accumulation of new data of plate tectonics and isotopic ages, Yanshan movement has been reinterpreted as regional intraplate orogenesis responding to multi-directional convergent tectonic system that extended across the East Asia, which is a much larger region than originally thought. However, the timing of Yanshanian onset remains debated, mainly including the Middle Jurassic and Late Jurassic-Early Cretaceous. The correct answer to this key scientific problem will contribute to better understand the coupling relationship between Jurassic plate tectonics in East Asia and intraplate deformation. The Daqingshan range is located in the eastern segment of the Yinshan belt, and mainly consists of the southern Daqingshan fold-and-thrust system（DFTS） and the northern Shiguai basin. In this article, the authors examined the western part of DFTS that is characterized by a series of E-trending, N-directed thrusts with a displacement no more than 10 km. The complete Jurassic sequences are well preserved in the Shiguai basin, including upwards the Lower Jurassic Wudanggou Formation, the Middle Jurassic Zhaogou and Changhangou formations, and the Upper Jurassic Daqingshan Formation. Note the contact between the Wudanggou and Zhaogou Formations is conformable and both of them are coal-bearing. ^（40）Ar-^（39）Ar dating of synkinematic minerals with two ages of 193.7±3.9 and 121.6±1.6 Ma indicate the Daqingshan range possibly underwent two phases of compressional events. Unfortunately, this conclusion is challenged by the discovery of syndepositional normal faults cutting the Early Jurassic-Archean unconformity and by the occurrence of Hohhot metamorphic core complex. Based on cross-cutting relationship, a Late Jurassic-Early Cretaceous age has been assigned to the thrust belt. However, growth strata preserved in the deformation front drew a very different conclusion. Growth strata record much syndepositional tectonic information and have been widely used in the study of orogenic belt. In the study area, the Early-Middle Jurassic synrift coal-bearing sequences are generally involved into shortening deformation in the southern Shiguai basin, resulting in the depocenter of Middle Jurassic Changhangou Formation（J2c） migrating markedly northwards. Along the deformation front, J2c commonly crops out ahead of overturned anticlines and exhibits typical syntectonic depositional wedge with bedding dips gradually shallowing upwards and each single horizon thinning or tapering towards fold crest. These features indicate J2c is typical growth strata, and the contact between it and synrift sediments preserved in the overturned forelimbs is called progressive unconformity, demonstrating the synchronicity of folding and sedimentation. This kind of growth geometry implies limb rotation is the main folding mechanism. Because contractional fault-related folding theories invoking limb rotation only include certain classes of detachment folds and the Lower Jurassic Wudanggou Formation mainly consist of incompetent materials, folding deformation is considered to be controlled by the movement of basal bedding-parallel detachment fault. Moreover, an increasing uplift rate can be delineated by the transition of growth geometries from onlap and overlap to offlap upsections. Fortunately, several 10-cm-thick volcanic ash layers have been found in the upper J2c, and the uppermost one has been sampled（SG0436-5） for LA-ICP-MS zircon U-Pb dating. The dating experiment that was conducted in the LA-ICPMS Laboratory of Nanjing Normal University, obtained 43 concordia ages with a weighted average age of 163.72±1.0 Ma. Given that the latest reported zircon U-Pb age of the upper Xiahuayuan coal-bearing sediments（be well correlated with the Zhaogou Formation） is 171±1.0 Ma, the initial activity of DFTS should occur in the Middle Jurassic（~170 Ma）. Therefore, these discoveries provide convincing constraint on the initiation of Yanshanian deformation and have significant implications for understanding its coupling relationship with the Jurassic plate tectonics in East Asia.