Early cretaceous topographic evolution associated with the collapse of the Mongol-Okhotsk orogen in Western Transbaikalia: an integrated analysis

The Early Cretaceous topographic evolution of Transbaikalia was largely governed by the tectonic evolution of the Mongol-Okhotsk orogen. The collapse of the Mongol-Okhotsk orogen triggered the formation of metamorphic core complexes and associated extensional basins, widespread throughout Transbaikalia, North Mongolia, and North China. Numerous lithofacies and biostratigraphic studies have been carried out from the sedimentary deposits of the Transbaikalia basins. However, the absence of absolute ages for the sedimentary series, as well as sediment source-to-sink analysis do not allow to accurately characterize the regional topographic evolution. We focused our study on the Gusinoozersk Basin of Western Transbaikalia, where extensive sedimentary sections of Lower Cretaceous deposits have been preserved. We provide new U/Pb (LA-ICP-MS) data on detrital zircons from sedimentary series and ⁴⁰Ar/³⁹Ar data on intruding rocks. We review the paleontological data to clarify the age of the paleogeographic events associated with the collapse of the Mongol-Okhotsk orogen, as well as to correct the age of faunal complexes in Western Transbaikalia. Our geochronological results show that the formation of the Cretaceous basins of Transbaikalia began around 136–130 Ma, accompanying the main episode of extension associated with the exhumation of the metamorphic core complexes. The lowest coarse-clastic formation characterizes the rapid subsidence and the predominance of proximal sediment sources. Distal provinces also made a contribution to sedimentation indicating the rise of a positive topography characterizing the exhumation of the metamorphic core complexes. Overlying fine-grained formations indicate a significant smoothing of the topography, suggesting that from middle Aptian, Western Transbaikalia developed in a relatively calm tectonic regime. We also show that the basins of Transbaikalia were formed both in conjunction with the exhumation of metamorphic cores complexes and reactivated structural sutures. Revised data on dinosaur fauna and palynology, together with the dating of host deposits, provide insights on the Early Cretaceous paleoenvironmental evolution.

外贝加尔地区（Transbaikalia）的早白垩世地形演化主要受控于蒙古-鄂霍茨克造山带（Mongol-Okhotsk orogen）的构造演化。蒙古-鄂霍茨克造山带的垮塌，触发了遍布外贝加尔、蒙古北部及华北地区的变质核杂岩（metamorphic core complexes）与伴生伸展盆地的形成。此前学界已对外贝加尔盆地的沉积地层开展了大量岩相学与生物地层学研究，但由于沉积序列缺乏绝对年代数据，且物源-汇系统（sediment source-to-sink）分析不足，难以精准刻画区域地形演化过程。本次研究聚焦于西外贝加尔地区的古西诺奥泽尔斯克盆地（Gusinoozersk Basin），该区域保存有大面积的下白垩统沉积剖面。我们获取了沉积序列中碎屑锆石的铀-铅（U/Pb，LA-ICP-MS）测年数据，以及侵入岩的氩-40/氩-39（⁴⁰Ar/³⁹Ar）测年数据。我们梳理了古生物学数据，以明确与蒙古-鄂霍茨克造山带垮塌相关的古地理事件年代，并校正西外贝加尔地区的动物群组合年代。地质年代学结果显示，外贝加尔白垩纪盆地的形成始于约136~130 Ma，这一过程伴随变质核杂岩抬升剥露的主要伸展幕。最底部的粗碎屑岩系代表快速沉降与近端物源占主导的沉积过程；远端物源区同样为沉积作用提供物源，这指示了伴随变质核杂岩抬升剥露的正地形发育。上覆的细粒岩系则表明地形已显著趋于平缓，暗示自阿普第期中期开始，西外贝加尔地区处于相对稳定的构造环境。我们还证实，外贝加尔的盆地既形成于变质核杂岩的抬升剥露过程，也与复活的构造缝合带相关。经校正的恐龙动物群与孢粉学数据，结合宿主沉积层的年代测定结果，为揭示早白垩世古环境演化提供了新的认识。