Study On Extensional Structure In The Eastern Slope Of Northern Da Hinggan Mountains

Da Hinggan Mountains crosses Inner Mongolia, Heilongjiang, Jilin, and Hebei Provinces with the NNE trend, which southern part connects to the Taihang Mountains. Da Hinggan Mountains crosses Central Asia - Mongolian orogenic belt, northern margin of the North China Plate suture zone and other major geological units, and constitutes the natural geographic boundaries between the western highlands and the eastern plains in China.The tectonic cause of Da Hinggan Mountains formation has been a hot topic and a concerned issue for many researchers. Li Siguang (1939, 1973), Huang Jiqing (1981), Zhang Zhengfa (1990, 1993, 1994, 1995, 1997) et al. all considered that Da Hinggan Mountains was formed by horizontal compression. In the 1980s, many researchers directly linked the uplift of Da Hinggan Mountains to subduction of the Pacific plate (Deng Jinfu et al., 1996; Zhao Dapeng, 2004; Niu Yaoling, 2005). The other researchers considered that it was related to the reactivation of the Paleo-Asian Ocean subduction zone (Zhou Xinhua et al., 2001) or mantle plume (Ge Wenchun et al., 1999) by studies on the Da Hinggan Mountains Mesozoic magmatic evolution and the distinguish geochemical characteristics. With the development of the intraplate orogeny theory, based on the researches of extensional structures and metamorphic core complex, and combining with new seismic tomography study, Shao Ji'an et al. (1993, 1994, 1999, 2005, 2007) believed that the tectonic evolution model for Da Hinggan Mountains may be extensional orogeny.At present, most studies of extensional structures focus on the middle and southern parts of Da Hinggan Mountains, the northern part is lack of detail study. Based on the studies of the field deformation structures, micro-fabrics and petrology in the Shanhe Farm of Keluo town and Galashan Forest-farm of Huma town, together with the previous research of the extensional structure in the southern and middle part of Da Hinggan Mountains, we have discussed the forming mechanism and age of the extensional structures in the two study areas, and the their roles in the uplifting of the Da Hinggan Mountains.1. The extensional structure in the Shanhe Farm of the Keluo townThe study region is located in the both sides of Keluo River in Shanhe Farm. It is exposed Keluo gneiss of Mesoproterozoic and the Neopaleozoic gneissic metasomatic granites of the Keluo and Shuanghe, also known as the Keluo Complex. On the structure section, we can see a series of low angle normal faults, decollement folds, Sheath folds, interlayered asymmetrical folds, A-type lineations consists of elongated and oriented minerals, B- type lineations, drag structures and augen structures and so on, all these types of structures indicate that the main direction of extensional dellement is SE. In the granitic gneisses collected from structural section shows that biotites and muscovites are oriented and discontinuous permutation, parts of the biotite had suffered alteration such as chloritization. The muscovite grains in the strongly deformed granitic gneiss had been curved. Some feldspar and quartz grains were elongated and oriented. Moreover some quartz grains show band extinction.2. The extensional structure in the Galashan Forest-farmGalashan Forest-farm is located in the southwest of Huma town, the study area is mainly exposed Tiemaoshan Formation and Galashan Formation which belong to the Luomahu Complex of the early Late Proterozoic, and the main rock types are granitic gneiss and mica schist with banded marble and pure marble. The overall trend of the Luomahu Complex is NE, in which there developed a number of large-scale gentle folds, a series of low angle normal faults, sheath folds, mullion structures, interlayered folds and asymmetrical folds. On the detachment planes the A-lineations and B-lineations were well developed. Quartz strip, drag structure and augen structure and so on occurred in the deformed rocks. All these structures indicate a southeast-directed extensional decollement structure. The observation of the samples under microscope shows that the biotite and muscovite are oriented, feldspar and quartz is elongated and oriented with the same direction as the micas. In the mica schist, muscovite is generally curved, and even kinked, some grains formed mica fish in the deformation process. In addition, the twin striation of calcite in banded marble is also curved; these phenomenons indicate that the rock in study area had suffered shallow level deformation. As a result of shear strain, plagioclase in the granitic gneiss and mica schist formed domino fabric and indicates dextral shearing.In order to determine the strain of rocks, this paper used the Fry method to carry out finite strain measurements for the samples from two research sections, and type of the strain was determined by Flinn diagram. It is suggested that both the results reflect extensional strain with simple shear characteristics. Meanwhile, the structural element projections indicate that two research areas were mainly suffered SE-directed stress.Combined with the previous age data, the metamorphic age of the Keluo Complex is 216±3Ma; the related granitoid rock intrusion age is 165Ma. In the Middle and Late Jurassic, the crust-mantle interaction resulted in the wide asthenosphere uplifting and the crust thinning in the Da Hinggan region. At the same time the emplacement event of granitoid took place, therefore, it is considered that the extensional structures in Shanhe Farm began in the late-middle Jurassic or later. Zheng changqing et al. (2008) have tested the granite gneiss samples of the Luomahu Complex in the Galashan Forest-farm and got the biotite 40Ar/39Ar age of 130Ma ~ 120Ma. The age should represents the cooling age of the Luomahu Complex. According to the background of asthenosphere's uplift in the middle and late Jurassic, it is believed that Galashan Forest-farm extensional structure were formed in the late Jurassic or later.Based on the discussion of the extensional structure formation ages in the two areas, It is indicated that the underplating in the middle-late Jurassic resulted in the mantle-crust interaction in the Da Hinggan Mountains, including Shanhe Farm of the Keluo town and Galashan Forest-farm, and the event might continue to the Cretaceous (Shao Ji'an, 1999). Due to the strong and continued underplating, the Da Hinggan Mountains started to uplift in the beginning of the Early Cretaceous. From the underplating started in the middle-late Jurassic to the overall uplift in the early Cretaceous in the Da Hinggan Mountains, the Keluo Complex in the Shanhe Farm and Luomahu Complex in the Gelashan Forest-farm were subjected to extension and detachment deformation under crustal extensional setting. In space the uplifted Da Hinggan Mountains is spreading with north-east trend, and the extensional structures in the Keluo and Galashan Forest-farm are southeast directed, they correspond to each other. Therefore, the author considers that the Da Hinggan Mountains uplift directly resulted in the formation of the extensional structures in Shanhe Farm of the Keluo town and the Galashan Forest-farm.On the basis of the above, combined with the studies of the metamorphic core complex in middle-south part of the Da Hinggan Mountains, the author believes that the large-scale extensional detachment structures in Shanhe Farm of the Keluo town and the Galashan Forest-farm are both a product of a large-scale regional extensional structures, their origin should be related to the Da Hinggan Mountains uplifting. They are the concrete manifestations in different locations of a unified regional decollement structure (Metamorphic core complex) caused by the Da Hinggan Mountains uplifting. The features of extensional structure and formation mechanism of the two study regions support the point of view that the Da Hinggan Mountains is an intracontinental extensional orogenic belt.