Late Cenozoic magnetostratigraphy of the Yuncheng Basin, central North China Craton and its tectonic implications

Abstract

Yuncheng Basin, the southernmost basin in the Shanxi Graben System, is considered to be one of the earliest formed in this region. However, due to the insufficient study on the strata and the lack of drilling data, the initial time of the Yuncheng Basin has not been accurately constrained, which heavily restricts our understanding about the formation and evolution of the Shanxi Graben System. In this study, borehole SG-1, which penetrated through the entire Late Cenozoic sediments and reached the basement bedrock, was obtained in the northern part of the Yuncheng Basin, and a detailed magnetostratigraphic study has been carried out on the samples selected from the borehole SG-1. Results show that the borehole SG-1 records 17 normal (N1-N17) and 16 reversed (R1-R16) polarity zones which can correspond to the chron C1n-C4An.1n of the global geomagnetic polarity timescale (GPTS). The bottom of the borehole SG-1 is within chron C4An, and the age of the bottom can be extrapolated to 9.1 Ma according to the sediment accumulation rates calculated by two lowermost major polarity transition boundaries. This age provides precise limitation for the initiation of the Yuncheng Basin and the formation of the Shanxi Graben System. The sediment accumulation rates of the Borehole SG-1 was calculated by major polarity boundaries of chrons and subchrons in the borehole SG-1 and showed significant changes at 9.1, 3.6, and 1.2 Ma. Unlike other basins around the Ordos Block which formed much earlier under the influence of western subduction of the Pacific Ocean, the initiation and evolution of the Shanxi Graben system were mainly controlled by the uplift and outward expansion of the Tibetan Plateau after the Late Miocene. The initiation of the Shanxi Graben System indicates that the far-field effect of the uplift and outward expansion of the Tibetan Plateau reached the central North China Craton by at the latest 9.1 Ma. The effect posed a persistent impact on the evolution of the Yuncheng Basin since 9.1 Ma and led to significant changes of the sediment accumulation rates and sedimentary facies at 9.1, 3.6, and 1.2 Ma in the Yuncheng Basin which correspond well to the prologue of the Qingzang Movement, phase A of the Qingzang Movement, and Kunhuang Movement.

Dynamic Divide Migration as a Response to Asymmetric Uplift: An Example from the Zhongtiao Shan, North China

“…The hangingwall block is a tilted block, dipping toward south and producing a depocenter close to the NZTF. This is highlighted by the thickening of basin fill deposits moving from north (few hundred meters) to south (more than 5000 m), as testified by drilling and geophysical data [25,40] ( Figure 2B).…”

“…Previous results from geology, seismicity, and geodesy investigations indicated that the graben systems were developed under a regional extensional stress field and characterized by the fault-uplift side of ranges and the subsidence of basins [29][30][31][32]. In addition, the ranges and basins are controlled by active normal faults and show the mirror image symmetry [25,28,32,33].…”

“…The highest peak, Xuehua Shan, with an elevation of 1994 m a.s.l, is located at the western part of the range (see Figure 2A). It has been suggested that the uplift of the Zhongtiao Shan is controlled by its range-front fault, the north Zhongtiao Shan normal fault (NZTF) [25,33,34]. The fault has a total length of 130 km and trends from ENE to NNE (Figure 2A).…”

“…In this study, we choose to investigate the landscape characteristics and possible divide migration history of the Zhongtiao Shan, a mountainous range which is located at the southernmost tip of the Shanxi Graben System. First, the tectonic background of the Zhongtiao Shan is distinct and its uplift was argued to be only controlled by the north Zhongtiaoshan normal fault [25,26]. Second, the Zhongtiao Shan is crossed by the Yellow River and both northern and southern sides of the range share a common base level (i.e., the Yellow River) (Figure 1), thus the geomorphic condition of the Zhongtiao Shan is simple.…”

“…(B) Simplified cross-section of the Yuncheng Basin and the Zhongtiao Shan along the a-a' profile shown in Figure 2A. The faults and the strata are adopted from [25].…”