**2. The geological setting**

#### **2.1 The triangular block in northwestern Sichuan**

The triangular block is bordered by the Longmen Mountain Range fractures in the south, the Maqin-Lueyang fracture in the north and the Xianshuihe fracture in the west (SCNWTB

(1) Maqin-Lueyang fracture; (2) Longmen Mountain Range fractures; (3) Xianshuihe Fracture, (4) Jinshajiang fracture; (5) Huya fracture; (6) Mounigou Valley fracture; SCNWTB: triangle block in the northwest of Sichuan; GZB: Ganzi Block; MSB: Minshan Block; FTLB: Motianling Block; CZB: central Sichuan block; CDRB: Chuan(Sichuan) -Dian(Yunnan) rhombic block; SJB: Sanjiang block

Fig. 1. Tectonic framework in the northwestern Sichuan

Peng(zhou)-Guan(xian) complex has been thrusted over the lower Pleistocene, especially since the Quaternary. During this overthrusting, nappes in Longmenshan structural belt have been formed. The landform of the block shows a great contrast in elevation and relief. As a result of the uplifting of the Minshan block, strong erosion caused the deposition of a thick Quaternary accumulation in the Chengdu plain and formation of the Longquan mountain. Due to the repeated Minshan block lock-up→earthquake→geo-stress release (stick-slip) mechanism, the middle segment of the Longmen Mountain Range structural belt is more active than the southwestern and northeastern segments. The earthquake history of northwestern Sichuan reveals that the migration of seismicity in northwest Sichuan is around the Minshan block, and the seismic activities on the northern margin of the Minshan

The aim of the Chapter is to present a new idea through the tectonic background analysis of Wenchuan earthquake, so that people pay more attention to the important role of the

The triangular block is bordered by the Longmen Mountain Range fractures in the south, the Maqin-Lueyang fracture in the north and the Xianshuihe fracture in the west (SCNWTB

(1) Maqin-Lueyang fracture; (2) Longmen Mountain Range fractures; (3) Xianshuihe Fracture,

Sichuan block; CDRB: Chuan(Sichuan) -Dian(Yunnan) rhombic block; SJB: Sanjiang block

Fig. 1. Tectonic framework in the northwestern Sichuan

(4) Jinshajiang fracture; (5) Huya fracture; (6) Mounigou Valley fracture; SCNWTB: triangle block in the northwest of Sichuan; GZB: Ganzi Block; MSB: Minshan Block; FTLB: Motianling Block; CZB: central

block created the conditions for the Wenchuan earthquake.

**2.1 The triangular block in northwestern Sichuan** 

**2. The geological setting** 

Minshan Block activities in the northwestern seismic tectonics.

in Fig.1). The block used to be an ocean (geosyncline) and was folded and uplifted in the late Triassic. Pre-Cambrian rocks are outcropping in Pingwu county and Maowen county, they are composed of metamorphic intermediate-basic and intermediate-acid volcanic rocks, volcanic clastic rock, intercalated with little siliceous rock. Lower Proterozoic is outcropping along the boundary of the Block. Slightly metamorphic Devonian and Carboniferous clastic rocks and carbonates are exposed in the Pingwu-Maowen region; mainly composed of carbonate, intercalated with little volcanic rock in the south part of the Block. The Permian exposed near the northern boundary of the block (Animaqin) consists of flysh, intermediatebasic volcanic rocks and carbonates. The Triassic system is widely exposed in the block, and is mainly composed of flysh. The triangular block can be divided into three sub-blocks, they are Ganzi block (GZB), Minshan block (MSB) and Motianling block (FTLB) (Fig.1). According to geophysical survey (Wang Xuben, 2000), the lithosphere of the block can be divided into three layers vertically: Upper crust, low-velocity layer in the crust, and the lower lithosphere (Fig.2).

Fig. 2. Inversion result of the MT profile in northwestern Sichuan

#### **2.2 The boundary fractures of the triangular block**

The Xianshuihe Fracture is the most active fault with large magnitude (Ms6-7) earthquakes and 30-40 years recurrence period, while the Longmen Mountain Range fracture and the Maqin-Lueyang fracture earthquakes occur with super magnitude(Ms8)and low frequency( more than 2000 years).

**The Xianshuihe fracture** starts in Donggu, Ganzi, runs southeastward, via Luhuo, Daofu, Qianning, Kangding, Moxi, and ends in Tianwan. The strike of the fracture is NW in the north of Kangding, it is NNW in the south of Kangding, showing an arc shape. The dipping direction is SW, with medium- steep angles (45-80°). According to the satellite image and geological data, the fault is large in scale and clearly linear. Valleys such as of the Xianshuihe River are developed along this fracture, the total displacement along the fracture can reach 5000m. Since its formation in the Jurassic, it showed strong activity during the uplift of the Tibet plateau. The seismic activity along the fracture is high. Since 1725, 36 earthquakes with magnitude Ms≥5.0 have been recorded, of which 13 had a Ms≥6.0 and 6 had a Ms≥7.0. The strongest recorded earthquake with Ms7.9 of Luhuo happened on 6th of Feb., 1973, with a focal source depth of about 10-20km.

**The Maqin-Lueyang fracture:** starts at the Tuosuohu lake, and leads via Maqin, Maqu, Nanping, Kangxian, to Lueyang, with a length of over 2000km. The strike ranges from 290º, 270º, 70º, with a southward arc. 6 earthquakes with a magnitude larger than Ms7.0 happened along this fracture(see references: Gu Gongshu (1983); Liu Guangshun (1996); Seismic bureau of Qinghai Province (1999); Yi Guixi (2002); Xu Xiwei et al. (2005)). In Yangbuliang, Jiuzhai, Carboniferous-Permian metamorphic carbonate rocks have thrusted onto Triassic clastic rocks, forming nappes. There is basic rock and super-basic rock outcrop along the fracture in Langmusi.

**The Longmen Mountain Range fracture:** Longmen Mountain Range fracture belts are composed of three deep fractures. From the southeast to the northwest: the front range fracture, the central fracture and the back range fracture. The strike of the three fractures is NE-SW, with dipping direction towards the northwest. Since the Cenozoic, these fractures have experienced thrusting from the northwest to the southeast with some right-slip component. The front range fracture is composed of the Dachuan-Shuangshi fault, the Guanxian-Anxian fault, and the Jiangyou fault. The central fracture is composed of the Yanjing-Wulong fault, the Yingxiu -Beichuan fault and the Beichuan-Linyansi fault. The back range fracture is composed of the Longdong-Gengda fault, the Wenchuan-Maoxian fault and the Pingwu-Qinchuan fault(Fig.3).

#### **2.3 The Minshan Block**

The Minshan Block is an important tectonic element in the northwest region of Sichuan. It not only includes the Minshan uplift in a narrow sense, but also the middle segment of Longmen Mountain Range structural belt (Fig.2, 3).

Although the west and east boundaries are still controversial, their existence and new activities are widely accepted. Analysis of seismic geological setting and the deformation data shows the Minshan uplift to be activated by the Mounigou Valley fracture in the west, Huya fracture and Leidong fracture in the east, back range fracture in the south and Maqin-Lueyang fracture in the north. Also the whole middle segment of the Longmen Mountain Range structures are especially strong active along south boundary since the Mesozoic, especially since Quaternary, namely, the Paleo-Peng(zhou)-Guan(xian) complex has been thrusted onto lower Pleistocene, nappes have been formed. There is great contrast in elevation and drop on the landform in the block. In addition strong erosion caused a thick Quaternary accumulation along the Chengdu plain and Longquan mountain was formed as a result of the uplifting of the Minshan block.

The northern part of the Minshan block (MSB in Fig. 3) is narrow (50 km wide) and it widens ( 90km ) towards the south. The SN length is about 200km (Fig.3); the three dimensional shape is narrow in the lower, deeper part and wider near the surface (Fig.4-9). The rigidity of the Block is stronger than that of the adjacent areas because the Vs (≥3.2) of the Minshan Block is larger than that of the adjacent areas (Fig.5).

The main part of the Minshan block is composed of series of peaks such as Gonggaling, Hongxingyan, Xuebaoding, Xueguzhai, Maoheshan and Jiudingshan, whose summits are at about 4000-5000m a.s.l. The landform is intensively dissected and deep gorges are well developed. The Songpan plateau is situated to the west of the Minshan block and is slightly dissected while a peneplain(planation surfacce) with an elevation of about 4000m is preserved.

**The Maqin-Lueyang fracture:** starts at the Tuosuohu lake, and leads via Maqin, Maqu, Nanping, Kangxian, to Lueyang, with a length of over 2000km. The strike ranges from 290º, 270º, 70º, with a southward arc. 6 earthquakes with a magnitude larger than Ms7.0 happened along this fracture(see references: Gu Gongshu (1983); Liu Guangshun (1996); Seismic bureau of Qinghai Province (1999); Yi Guixi (2002); Xu Xiwei et al. (2005)). In Yangbuliang, Jiuzhai, Carboniferous-Permian metamorphic carbonate rocks have thrusted onto Triassic clastic rocks, forming nappes. There is basic rock and super-basic rock outcrop

**The Longmen Mountain Range fracture:** Longmen Mountain Range fracture belts are composed of three deep fractures. From the southeast to the northwest: the front range fracture, the central fracture and the back range fracture. The strike of the three fractures is NE-SW, with dipping direction towards the northwest. Since the Cenozoic, these fractures have experienced thrusting from the northwest to the southeast with some right-slip component. The front range fracture is composed of the Dachuan-Shuangshi fault, the Guanxian-Anxian fault, and the Jiangyou fault. The central fracture is composed of the Yanjing-Wulong fault, the Yingxiu -Beichuan fault and the Beichuan-Linyansi fault. The back range fracture is composed of the Longdong-Gengda fault, the Wenchuan-Maoxian

The Minshan Block is an important tectonic element in the northwest region of Sichuan. It not only includes the Minshan uplift in a narrow sense, but also the middle segment of

Although the west and east boundaries are still controversial, their existence and new activities are widely accepted. Analysis of seismic geological setting and the deformation data shows the Minshan uplift to be activated by the Mounigou Valley fracture in the west, Huya fracture and Leidong fracture in the east, back range fracture in the south and Maqin-Lueyang fracture in the north. Also the whole middle segment of the Longmen Mountain Range structures are especially strong active along south boundary since the Mesozoic, especially since Quaternary, namely, the Paleo-Peng(zhou)-Guan(xian) complex has been thrusted onto lower Pleistocene, nappes have been formed. There is great contrast in elevation and drop on the landform in the block. In addition strong erosion caused a thick Quaternary accumulation along the Chengdu plain and Longquan mountain was formed as

The northern part of the Minshan block (MSB in Fig. 3) is narrow (50 km wide) and it widens ( 90km ) towards the south. The SN length is about 200km (Fig.3); the three dimensional shape is narrow in the lower, deeper part and wider near the surface (Fig.4-9). The rigidity of the Block is stronger than that of the adjacent areas because the Vs (≥3.2) of

The main part of the Minshan block is composed of series of peaks such as Gonggaling, Hongxingyan, Xuebaoding, Xueguzhai, Maoheshan and Jiudingshan, whose summits are at about 4000-5000m a.s.l. The landform is intensively dissected and deep gorges are well developed. The Songpan plateau is situated to the west of the Minshan block and is slightly dissected while a peneplain(planation surfacce) with an elevation of about 4000m is

along the fracture in Langmusi.

**2.3 The Minshan Block** 

preserved.

fault and the Pingwu-Qinchuan fault(Fig.3).

Longmen Mountain Range structural belt (Fig.2, 3).

a result of the uplifting of the Minshan block.

the Minshan Block is larger than that of the adjacent areas (Fig.5).

1. regional main deep fractures; 2. common fractures; 3. synclines; 4. anticlines; 5. nappes; 6. predicted fracture; (1) Guanxian-Anxian fracture; (2) Yingxiu-Beichuan fractures; (3)Maowen fracture; (4) Pingwu-Qingchuan fracture; (5) Maqin-Lueyang fracture; (6) Huya fracture; (7) Xueshan fracture; (8) Minjiang fracture; (9) Mounigou Valley fracture; (10)Songpinggou fracture; (11) Aba-Heishui-Jiaochang arc fracture, (12) Miyaluo-Lixian fracture; (13)Maerkang-Lianghekou fracture; (14)Wudu-Chengxian fracture; (15) Longquan Mountain Range fracture; (1)-1 Dachuan-Shuangshi fracture; (1)- 2 Guanxian-Anxian fracture; (1) -3 Jianyou fracture; (2)-1 Yanjing-Wulong fracture; (2)-2 Beichuan-Yingxiu fracture; (2)-3 Beichuan-Linanshi fracture; (3)-3 Gengda-Longdong fracture; (3)-2 Maowen-Wenchuan fracture; (3)-3 Pingwu-Qingchuan fracture

Fig. 3. Skeleton of the northwest block of Sichuan

S: Silurium System; D: Devonian System; D-T: Devonian-Triassic System; C: Carboniferous System; T: Triassic System; J: Jurassic System; K: Cretaceous System; Q: Quaternary System; γ: PreCambrian;

S: Silurium System; D: Devonian System; D-T: Devonian-Triassic System; C: Carboniferous System; T: Triassic System; J: Jurassic System; K: Cretaceous System; Q: Quaternary System; γ: PreCambrian;

Fig. 4. The geological map of Minshan block

Fig. 5. Vs distribution of surface waves in the northwest of Sichuan (at depth of 8km. From Wang Xuben, personal communication ) **The coarse black line is the boundary fracture** 

According to the activities and the distribution of Quaternary and landform characteristics, Minshan Block can be divided into three segments: the Gonggaling to Zhenjiangguan, Zhenjiangguan to Maoxian and Maoxian to Guanxian segments.

The Gonggaling to Zhenjiangguan segment is composed of a series of peaks with an elevation of about 4500m and the mountain range is with SN strike, the highest peak being Hongxingyan (5010m) with outcropping Devonian-Triassic limestone, dolomite, metamorphic sandstone and slate. Its boundary fractures are active and a thick Quaternary accumulation is developed in the Zhangla basin.

The Zhenjiangguan to Maoxian segment is composed of peaks with an elevations of about 4500m and deeply eroded valleys. The summits follow a SN strike direction, and have Tertiary metamorphic sandstone, phylitte, slate and Devonian Weiguan Group schist, Silurian Maoxian Group phyllite, and slate and quartzite as its bedrock (Fig.4).

The Maoxian to Guanxian segment is located in the area with superimposed SN and NE structures. The strata are striking is NE direction and the general elevations is over 2000m. The highest peak is Jiudingshan (4989m), the next highest peak is Qianfoshan (3033m), and the area has 2000-4000m in elevation differences with respect to the Chengdu plain. The landform is deeply dissected and the tectonic deformation is intensive. The bedrock consists of Precambrian granite, granodiorite, Silurian schist and quartzite, Triassic sandstone and mudstone.

(1) front range fracture, (2) central fracture, (3) back range fracture, (4) Leidong fracture, (5) Huya fracture, (6) Mounigou fracture, MB Minshan block, SCB Sichuan basin

Fig. 6. 3D structural diagram of the Minshan block

Fig. 7. Western boundary fracture of the Minshan block, (camera facing south) The fault zone is composed of cataclastic rock and fault gouge Carbonization is obvious, fault zone width 20-30m

(1) front range fracture, (2) central fracture, (3) back range fracture, (4) Leidong fracture, (5) Huya

Fig. 7. Western boundary fracture of the Minshan block, (camera facing south) The fault zone is composed of cataclastic rock and fault gouge Carbonization is obvious, fault zone

fracture, (6) Mounigou fracture, MB Minshan block, SCB Sichuan basin

Fig. 6. 3D structural diagram of the Minshan block

width 20-30m

Fig. 8. East boundary of Minshan block, (camera facing north) The fault zone is composed of cataclastic rock and fault gouge Carbonization is obvious, fault zone width 120m.

Fig. 9. Schematic cross section of the Minshan Block and its eastern,and western boundary, (The direction of the cross section: N31º50´)

The Minshan block is a part of China's SN seismic belt. Seismic events are frequent inside the block and along its boundaries. GPS measurement data indicate that the block moves eastward and that its southern boundary moves towards the southeast.
