Multistage structural deformations of a superimposed basin system and its tectonic response to regional geological evolution: A case study from the Late Jurassic-Early Cretaceous Tanan depression, Hailar-Tamtsag basin

The spatial-temporal extent and influence of the Mongol-Okhotsk and Paleo-Pacific tectonic regimes in northeast (NE) China and adjacent areas during late Mesozoic times have long been controversial. As a superimposed rift system which formed due to extensive intracontinental extension during the Late Jurassic to Early Cretaceous periods, the Hailar-Tamtsag basin is a key area to study the relationship between the Mongol-Okhotsk and Paleo-Pacific tectonic regimes and their superposition. With the aim of constraining the evolution of those two regimes, in this paper, the Tanan depression, which is a second-order structural unit in the western part of the Hailar-Tamtsag basin, has been chosen as the research object. On the basis of structural analysis of three-dimensional (3D) seismic data in the Tanan depression, and taking into account the controlling effect and influence of the pre-existing facies on the basin evolution, the deformation events in the Early Cretaceous can be subdivided into four periods: 1) A NW-SE trending extension during the deposition of the Tongbomiao Formation to the upper part of the second Member of the Nantun Formation; 2) A NW-SE trending compression during the deposition of the upper part of the second Member of the Nantun Formation to the first Member of the Yimin Formation; 3) A near EW trending extension during the deposition of the second Member to the lower part of third Member of the Yimin Formation; 4) A near EW trending compression during the deposition of the upper part of the third Member of the Yimin Formation. Additionally, we identify compressional structures in the basement in the southeast part of the depression, which is a key study that resulted from the Mongol-Okhotsk collisional orogeny during the Late Jurassic. The above five major deformational events make up a relatively complete tectonic deformation sequence of the Hailar-Tamtsag basin. Moreover, the deformation sequence, together with other available information from the literature in NE China and adjacent areas, including petrology, geochemistry and geochronology data, also record the whole process from the orogenic stage after the closure of the Mongol-Okhotsk Ocean, over the post-orogenic collapse of the Mongol-Okhotsk orogen, to the weakening of the influence of the Mongol–Okhotsk regime, and eventually to be replaced by the Paleo-Pacific tectonic regime. Considering that the superposition and transformation of the two tectonic regimes involves almost all the geological process in NE China and adjacent area during the late Mesozoic, the complete geological record in the Tanan depression during this period provides new constraints on the research of regional tectonic evolution, and the model of multiphase deformation is also expected to have useful implications for future studies of complex basin systems.