The Mesozoic-Cenozoic Tectonics of North China as Revealed in the Shanxi Rift

Abstract

The Shanxi Rift is a ~1000 km long active rift zone in North China defined by en-echelon NE-SW–trending basins and uplifts. The Taihangshan and Luliangshan (‘shan’ = mountains) constitute the eastern and west-central rift shoulders of the Shanxi Rift. The southern and northern margins of the Shanxi Rift are delimited by the E-SE–striking Qinling fault and the E-trending Yanshan, respectively. The rift zone is thought to have initially formed during the Late Miocene to Pliocene based on the age of alluvial-fluvial-lacustrine and loess deposits that infill the grabens and half-grabens. The age and thickness of these synrift sediments are understood from sparse well control and 2D seismic data, but much of this subsurface data is inaccessible and poorly documented in the international literature; therefore, addressing the timing and magnitude of uplift for the rift-flank uplifts remains challenging. The geology of the footwall blocks is defined by Neoarchean–Paleoproterozoic basements rocks unconformably overlain by Proterozoic metasedimentary rocks and Paleozoic–Mesozoic strata. These rocks are folded and faulted as part of the NE-SW trending Taihang-Luliangshan fold belt, formed during the Middle-Late Jurassic–Early Cretaceous (ca. 160-135 Ma) Yanshanian orogenic event. Shortening at this time is recognized throughout much of North China and was likely a consequence of flat-slab subduction of the Izanagi (i.e., paleo-Pacific) plate under the eastern margin of Asia, although far-field deformation associated with terrane amalgamation in Eurasia may have also played an important role. Shortening transitioned to a phase of extension in North China at ca. 135-130 Ma, which led to the development of high-magnitude extensional core complexes in North China from ca. 130-100 Ma, widespread magmatism, and the formation of Early–Late Cretaceous backarc basins near the trench margin, such as in the East China Sea and Korean Peninsula. This phase of Early–Late Cretaceous extension is associated with a period of rollback of the Izanagi plate following the earlier episode of shallow-slab subduction. By ca. 60-50 Ma, the mid ocean ridge between the Izanagi and Pacific plates had subducted under the eastern margin of Asia and convergence rates between the Pacific and Eurasian plates decreased concomitantly with an increase in convergence obliquity. This resulted in a renewed phase of extension and slip reactivation of pre-existing fault systems (e.g., Qinling and Tanlu fault), which produced several large continental rift basins, most notably the Bohaiwan (Bohai) basin. The western margin of the Bohai Basin is bordered by the NNE-striking Eastern Taihangshan fault, which is characterized by a thick (>6-8 km) section of late Paleocene–Oligocene synrift sediment in its hanging wall. Eocene-Oligocene sediments are also known from the Weihe Graben in the southernmost rift, and this basin formed as a response to Eocene–Oligocene offset along the ESE-striking Qinling and Huashan faults. Low-temperature thermochronologic data from the Shanxi Rift help to constrain the age of these Mesozoic-Cenozoic tectonic events, with cooling episodes at ca. 160-135 Ma, ca. 110-70 Ma, and ca. 50-30 Ma. A clear low-temperature thermochronometric record of latest Neogene rifting related to the formation of the Shanxi Rift is generally absent. Instead, it appears that exhumation associated with this latest phase of rifting occurred from temperatures of <~70-60 ¬°C, and the timing for the onset of rifting remains loosely constrained. Regardless of the precise age for the onset of latest rifting, the low-temperature thermochronologic record suggests that the bulk of exhumation in the Shanxi Rift occurred by the Eocene–Oligocene and that the magnitude of footwall uplift associated with the range-bounding faults is <~2-2.5 km.