## 2 Geological Setting  
The Helanshan Mountain tectonic belt (HTB), an important tectonic boundary between East and West China with an abrupt geophysical field, trends NNE and forms the northwestern margin of the Ordos Block. The eastern Yinchuan Block is a Cenozoic rift basin with over 7000 m of Cenozoic deposits.  
The Neoarchean-Paleoproterozoic metamorphic basement rocks and the Meso-Neoproterozoic strata consist of sandstone and dolostone and are mainly exposed in the northern HTB. The Cambrian rocks include phosphorus sandstone, limestone with  
Figure 2: Geological map of HTB modified from [PERSON] et al. (2018) [3]. F1—East Piedmont Fault, F2—West Piedmont Fault, F3—Xiaosongshan thrust fault, F4—Huanghe fault, F5—Sanguankou fault [5; 20].  
mudstone, and dolomitic limestone. The Ordovician rocks consist of dolomite, sandstone, limestone, and conglomerate. They mainly occur in the southern HTB and the central HTB and have experienced multiple tectonic deformations from the Paleozoic to the Cenozoic [21; 22]. The Permian rocks mainly involve quartz sandstones, shales, and siltstone interbedded with coal layers in the central and the northern HTB [23].  
The Triassic succession is characterized by quartz sandstones interlayered with shales in the central HTB, and angular unconformity is found between the Late Triassic shale rocks and the Middle Jurassic sandstone and conglomerate. The Jurassic rocks are fluvial-lacturing deposits, which comprise predominantly sandstones, siltstones, and mudstones with coal beds. The Late Jurassic Fengfang River Formation consists of conglomerate and sandstone, which is scarcely distributed in the north of the HTB. The Early Cretaceous coarse-grained conglomerates and sandstones are unconformably overlying the previous strata in the central HTB and southern HTB.  
The Cenozoic sedimentary section has three primary groups: the Qingshuiying Group, the Hongliugou Group, and the Ganhegou Group [23]. The Oligocene Qingshuiying Group is composed of mudstone and sandstone and belongs to fluvial-lacturing deposits. The Miocene Hongliugou Group comprises mudstone and clay. Moreover, the Pliocene Ganhegou Group consists of sandstone, siltstone, and mudstone. In addition, the Miocene strata developed several parallel unconformity sections with the lower Oligocene in Dingjia from Tongxin Country and in Hongliugou from Zhongning Country.  
The East Piedmont Fault of the HTB is located on the east edge of the range belt and is a significant boundary between the uplifted HTB and the rifted Yinchuan Block. In the Late Jurassic and Early Cretaceous, a WNW-ESE horizontal shortening [6] developed, causing west- or east-directed folds and thrusts [1]. During the Late Jurassic, the Upper Jurassic comprising sandstone and mudstone was overlapped by the Early Paleozoic formation due to the compression and movement of the Xiaosongshan thrust nappe. The West Piedmont Fault of the HTB dipped to WNW and the East Piedmont Fault of the HTB dipped to SE, both of which are normal boundary faults from the NNE-trending HTB. Meanwhile, the West Piedmont Fault and the East Piedmont Fault controlled the Cretaceous sedimentation in the Bayinhaote Basin and Yinchuan Basin (Figure 2), respectively [3]. Moreover, the East Piedmont Fault of the HTB has developed different lithologic strata in the corresponding wall since Cenozoic. While the upper (east) wall has fluvial and lacustrine facies deposits, a piedmont deposit exists in the lower (west) wall. During the Cenozoic, the NE-SW extension along the HTB formed the arc-shaped Hetao graben and the NNE-SSW trending Yinchuan Block [2]. The above geology and structure suggest that the HTB underwent intense extensional deformation and exhumann during the Cenozoic.  
Since the Phanerozoic, the HTB has exhibited little magmatic activity. In the Ruijigou region, only a few Late Triassic-Early Jurassic basaltic beds developed [3; 24; 25].