specially, regarding its period of formation and its structural geometry. In view of the complexity of arc structures, it is generally believe that the most effective means of studying arc structures are structural analysis and paleomagnetic combination(Van der Voo&Chanell .,1980；Schwartz&Van der Voo,1983;Borradaile et al.,1997; Weil &Sussman，2010;Daniel et al.,2011; Weil et al , 2013). Here, we have discussed the basic structural features of the Dabashan foreland arc tectonic belt including the distributional geometrical feature of the arc structure, the basin-mountain system style, the paleo tectonic stress field characteristics and so on. On this basis, the main controlling factors of the formation of the Dabashan arc are discussed. And The formation mechanism of Dabashan arc is different from the classic collision models.
1 Geologic setting
The endpoints of the East and West end of the Dabashan arc structure belt are the Fangxian and Xixiang County. The external arc distributional length of the whole Dabashan arc structure is about 310km. The internal length of this arc structure is about 270Km (Zhang Y Q, 2010). The arc length is about 230km and the amplitude is about 60Km as shown in figure 1.
The Dabashan arc structure belt is divided into the western, middle and eastern by the Maliu-Guanba-Sanxi and the Dongan-Tianba. The North border and the South border of the arc structure are the Chenkou-Fangxian fault and the Tiexi-Wuxi buried fault just shown as figure 2. The west width of this zone is about 43km. The width in the middle of this zone changes large and the width is between 36km to 45km.The east this zone is widest and the width is up to 55km. The east wing of this structure is in the direction of the east-west (Zeng J, 2012). The west wing of this structure is in the direction of the north-south. The two wings of this structure is in the vertical direction and the included angle between the two wings is changing by 90 degree.
Fig. 1 The Geometric features of the Dabashan arc structure belt
Fig.2 The main faults and folds of the Dabashan and the front of Dabashan（Draw according to 1:200000 and 16 geological maps including the Zhenba county, Chengkou cunty and so on.）
The fault and fold development between the Chengkou-Fangxian fault belt and the Zhenba-Jichang fault belt. The fault between the Zhenba-Jichang fault belt and the Tiexi-Wuxi buried fault is much more developed than the fold of this zone. The superposed folds in the western part of the arc structure of Dabashan are developed and especially across the intersection of the Micangshan and Dabashan are much more (Zhiwu L I, 2006).
According to the geometry classification of arcuate structure by Macdonald&Marshak (Macdonald&Marshak, 1999), the Dabashan arc structure belongs to the asymmetrical arc structure with parallel strikes.
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Conclusion
This paper discusses the basic structural features of the Dabashan foreland arc tectonic belt including the distributional geometrical feature of the arc structure, the basin-mountain system style, the paleo tectonic stress field characteristics and so on. On this basis, the main controlling factors of the formation of the Dabashan arc are discussed. The formation mechanism of Dabashan arc is different from the classic collision models. This arc tectonic zone experienced the Paleozoic continental margin extensional rifting stage, the initial Triassic collision orogeny in late three embryonic period and the late Jurassic intracontinental orogenic period. The extension of the paleo continental margin before the orogeny and the irregular boundary shape played a decisive role i
