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Structural model of the central Longmen Shan thrusts using seismic reflectionprofiles: Implications for the sediments and deformations since the Mesozoic

Renqi Lu, Dengfa He, Suppe John, Jonny E. Wu, Bo Liu, Yuegau Chen

PII: S0040-1951(14)00240-6DOI: doi: 10.1016/j.tecto.2014.05.003Reference: TECTO 126304

To appear in: Tectonophysics

Received date: 11 June 2013Revised date: 30 April 2014Accepted date: 5 May 2014

Please cite this article as: Lu, Renqi, He, Dengfa, John, Suppe, Wu, Jonny E., Liu,Bo, Chen, Yuegau, Structural model of the central Longmen Shan thrusts using seismicreflection profiles: Implications for the sediments and deformations since the Mesozoic,Tectonophysics (2014), doi: 10.1016/j.tecto.2014.05.003

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Structural model of the central Longmen Shan thrusts using seismic reflection

profiles: Implications for the sediments and deformations since the Mesozoic

Renqi Lua,c, *

, Dengfa Heb, Suppe John

d, Jonny E. Wu

d, Bo Liu

c, Yuegau Chen

d

a. Institute of Geology, China Earthquake Administration, Beijing 100029, China.

b. College of Energy Resources, China University of Geosciences, Beijing 100083, China.

c. School of Earth and Space Science, Peking University, Beijing 100871, China

d. The Department of Geosciences, National Taiwan University. Taipei 10617, China.

Abstract: The structural setting and deformation history are very important aspects of

understanding the frequent earthquakes and assessing the hazards. In this paper, we interpret

two new seismic profiles that collected after the Wenchuan Mw7.9 earthquakes occurred on

May, 2008. Our interpretation has integrated four wells, three seismic profiles and the surface

geology to reveal structural characteristics of the central Longmen Shan thrust. The drilled

wells reveal there are two detachments within the upper Triassic and the lower Triassic at the

Longmen Shan thrusts. The shallow detachment fault transferred its slip to the basin and

made another deformation inner the Sichuan basin. Between the shallow detachments, many

faults developed and the major structures at the thrust front have formed as structural wedges.

Both the drilled wells and seismic interpretation show the upper Triassic repeated in the depth.

The shortening at the central Longmen Shan thrust is approximately 30% -37%. According to

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geological survey and the seismic interpretation, the foreland basin was developed during the

late Triassic and the angular unconformity between the lower Jurassic and the upper Triassic

at the Longmen Shan front. There are several different tectonic events can be identified since

the Mesozoic, which indicated the Longmen Shan have a complex evolution history. In our

model, the major Beichuan fault (F2) and the Pengguan fault (F3) at the central Longmen

Shan dip steeply near the surface but are more gentle-dipping at depth. Both of the shallow

thrusting and the basement shortening made contribution to the rapid uplift of the Longmen

Shan during the Cenozoic. The co-seismic ruptured fault is like a branchy fault that

developed on the major fault system. The Wenchuan Mw7.9 earthquake and the Lushan

Mw6.7 earthquake may be related to the reactive of the basement structures.

Keywords: Beichuan fault; Wenchuan earthquake; Multiple detachment; Structural

wedge; basement structure; Longmen Shan thrust belt.

1. Introduction

The Longmen Shan thrust belt, located at the eastern of the Qinghai-Tibet Plateau (Fig.

1), has a long, complicated tectonic evolution history that continues to the present day (Luo et

al., 1994; Deng et al., 1994; Liu et al., 1996; Tapponnier et al., 2001; Zhang et al., 2013). The

Wenchuan Mw7.9 earthquake occurred on May 12, 2008 and produced two large surface

ruptures along the Beichuan-Yingxiu fault (F2) and Pengguan fault (F3) that caused extensive

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damage and loss of life (Xu et al., 2008, 2009). Numerous aftershocks last occurred in recent

years at the southeastern of the Qinghai-Tibet Plateau. The April 20, 2013 Lushan Mw6.7

earthquake happened at the southern segment of Longmen Shan (Xu et al., 2013; Wang et al.,

2013a), indicating the faults of the southern segment is active.

After the occurrence of the 2008 Wenchuan earthquake, many tectonic and geodynamic

models of the Longmen Shan area have been proposed in the literature (e.g. Burchfiel et al.,

2008; Royden et al.,2008; Hubbard and Shaw, 2009; Xu et al., 2009; Zhang et al.,2010; Guo

et al., 2013).These published models have been constrained by seismic reflection profiles (Jia

et al., 2010), seismological and gravity data (Robert et al., 2010), magnetotelluric data (Zhao

et al., 2012), regional seismic tomography models (Pei et al., 2010), and seismic cycle

deformation modeling (Luo and Liu, 2010). Of these studies, seismic reflection profiles have

provided that clearest constraint on the detailed upper crustal structure of the Longmen Shan

(Jia et al., 2006; Hubbard et al., 2010; Li et al., 2010). However, it’s little known about the

sedimentary -tectonic background of the Longmen Shan since the Mesozoic. In particular, the

relationship between the surface co-seismic reptures and the deep structures is unclear. The

Lushan earthquake was a case and indicated previously undetected seimogenic hazards at the

Longmen Shan (Ren et al., 2012; Xu et al., 2013; Wang et al., 2013b).

In this paper, we interpret two new seismic reflection profiles that were collected to

study the Wenchuan earthquakes and specifically design to reveal the detailed structure of the

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Longmen Shan thrusts. For comparison, we also interpret a seismic profile extracted from 3D

seismic data. In comparison to 2D seismic profiles, 3D seismic data have the advantage that

seismic reflectors are generally more accurately positioned (Cartwright and Huuse, 2005).

The seismic interpretation presented here is further constrained by surface geology and

subsurface geology revealed by four well penetrations. These four wells include the

Wenchuan Fault Scientific Drilling project wells WFSD-1 and WFSD-2 that targeted the

Beichuan fault zone structure and provided important constraints from the basin to the thrust

belt (Zhang et al., 2012; Li et al., 2013; Nie et al., 2013). Fault-related folding theories and

axial surface analysis were used to constrain the seismic interpretation (Suppe, 1983; Shaw et

al., 2004). These data and constraints are intergrated to present a new structural model for the

central segment of Longmen Shan thrusts.

2. Geological setting

The Longmen Shan thrust belt experienced at least two major periods of contractional

deformation in the Late Triassic and Cenozoic (Burchfiel et al., 1995; Jia et al., 2006; Godard

et al., 2009; Yin 2010). During the Late Triassic, the tectonic setting changed from a rifted

passive margin to a foreland setting in response to the final closure of the Paleo-Tethys and

the continent-continent collisions of the North China, South China and Qiangtang blocks

(Chen et al., 1995, Liu et al., 2009). During the Cenozoic, the tectonism in the Longmen Shan

was reactivated by the India-Asia collision (Royden et al., 2008), and caused thrusting,

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dextral shear and dramatic uplift (Chen and Wilson, 1996; Densmore et al., 2007). The

Longmen Shan thrust belt is at present composed of a series of NE striking thrust sheets

bounded by several NW-dipping thrusts (Fig. 1).

Fig. 1 Digital elevation (DEM) map showing major thrust faults in the Longmen Shan and

location of the 2008 Wenchuan earthquake and 2013 Lushan earthquake (modified from Xu

et al., 2013). Inset map show position of the study area relative to Tibet.

F1: Qingchuan-Maowen fault；F2: Beichuan-Yingxiu fault；F3: Pengguan fault；

F4: Guankou blind fault；F5: Pengxian blind fault ( the same to below legends).

The Longmen Shan thrust belts are to the north of Guangyuan (Fig. 1). It crosses

Jiangyou, An'xian and Guan'xian to Tianquan in the south, and has a total length of about

400km (Lin et al., 1991; Zhao et al., 1994). Generally, the Longmen Shan thrusts were

distinguished by the Qingchuan-Maowen fault (F1), the Beichuan-Yingxiu fault (F2), the

Pengguan fault (F3). The Guankou blind fault (F4) and the Pengxian blind fault (F5) look like

the blind faults at the frontal mountain and inner basin (Luo et al., 1994). The longmen Shan

thrusts are divided into three segments along the strike: the north segment, the central

segment and the south segment (Bai et al., 2010; Li et al., 2012).

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Fig. 2 Local geological map (see Data and Resources section) showing the position of

seismic reflection profiles and drilled wells used in this study.

1- Cenozoic; 2- Cretaceous; 3- Jurassic; 4- Upper Triassic; 5- Lower Triassic; 6-Permian; 7-

Carboniferous; 8-Devonian; 9- Sinian; 10- Pengguan Complex; 11- Thrust faults; 12-

Blind faults; 13- Wells; 14- Seismic Reflection profiles.

In our research area (Fig. 1 and Fig. 2), the outcrops include Sinian to middle Triassic

shallow marine clastics and carbonate rocks and Late Triassic to Cretaceous non-marine

clastics (Liu et al., 1996; Ma et al., 2001). The WFSD-2 well is located on top of the

Pengguan Complex. Many klippes have formed on top of the upper Triassic. The

Beichuan-Yingxiu fault (F2) and the Pengguan fault (F3) ruptured co-seismically during the

Wenchuan earthquake. The foreland ahead of the mountain front displays two blind faults

according to previous studies (Li et al., 2012). The Guankou blind fault (F4), which might be

occurred the Lushan Mw6.7 earthquake on April 20, 2013 (Xu et al., 2013). The Pengxian

blind fault (F5), which was thought as a brittle steep thrust (Jin et al., 2010).

3. The interpretation of seismic reflection profile

3.1 Seismic reflection profile A-B

The seismic reflection profile A-B across the Beichuan-Yingixu fault (F2) and the

Pengguan fault (F3), and extends to the basin (Fig. 3). The Wenchuan earthquake Fault

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Scientific Drilling project (WFSD) was initiated in response to the 2008 Mw 7.9 Wenchuan

earthquake (Li et al., 2013a) and its data have been integrated into the structural models

presented in this paper. Both the WFSD-1 and WFSD-2 well were drilled to penetrate the

Beichuan fault (F2) (Fig. 2 and Fig. 4). The seismic reflection profile A-B was acquired to

study the Wenchuan Mw7.9 earthquakes at the central segment of Longmen Shan (Fig. 2).

Fig. 3 Seismic reflection profile A-B showing the surface geology and DEM elevations.

The surface geological information are obtained from both the 1: 20 0000 and 1:50 000

regional geological maps (see Data and Resources section). Black boxes indicated the

locations of detailed sections shown in Figure 5.

Fig. 4 Structural interpretation of the WFSD-1 and WFSD-2 well results modified from

(Zhang et al., 2012 and Li et al., 2013a). The F2-2 fault was interpreted as the co-seismic

fault during the Wenchuan earthquake (Zhang et al., 2012).

According the results and constraints of the drilled wells (Zhang et al., 2012; Li et al.,

2013a; Nie et al., 2013), we give a structural model of this section (Fig. 4). The model shows

there are many previously-formed thrusts at the Beichuan-Yingxiu thrust belt. The F2-2

might be a new branchy thrust at the shallow strata and made co-seismic surface ruptures

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during the Wenchuan Mw7.9 earthquake. The F2-1 cut the Pengguan Complex underground

and formed earlier than F2-2. The slip of the F2-1 is about 0.5km in this section we measured.

Both the F2-1 and F2-2 have limited displacements and were likely developed during the

Late Cenozoic. They should converge in a major thrust fault at depth. If the results of the

drilled wells are reliable (Zhang et al., 2012; Li et al., 2013a; Nie et al., 2013), the two faults

are differentiated from the old major Beichuan-Yingxiu faults (F2), which was thrusted and

formed the Pengguan Complex before the Cenozoic.

Fig. 5 Interpretation of detailed seismic sections from seismic reflection profile A-B.

T3m: The upper Triassic Maantang formation; P1: The lower Permian. The red lines are the

major faults in this section and the violet line is the co-seismic fault during the Wenchuan

Mw7.9 earthquake. The Pengguan Complex was indicated by the yellow fill. The dark dash

lines are the axial surfaces. Red arrows show the motion direction of fault slipping. The

layers are the bottom of each stratum.

In section a, the cut-offs (shows with yellow arrows) indicate the F2 and F3 faults very

clearly (Fig3. and Fig. 5a). The WFSD-2 well encountered the major fault (F2-2) at the same

position we interpreted. The fault (F2-2) made surface ruptures during the Wenchuan Mw7.9

earthquake. There is no evidence to confirm the co-seismic fault (F2-1) extending to the

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basement with high angle in this seismic reflection profile.

The geology map and the co-seismic surface ruptures show both the Beichuan fault (F2)

and Pengguan fault (F3) have steep dips at the surface (Xu et al., 2008). But the 3-D model of

seismic interpretation show the Pengguan fault (F3) with gentle dip at depth (Li et al., 2010;

Lu et al., 2012). The Beichuan fault (F2) was generally considered as the oblique, high-angle,

listric-reverse faulting (Zhang et al., 2010). The seismic reflectors are continuous with gentle

dips at depth in this section (Fig. 5a). We interpreted the old major Beichuan fault (F2) to

have a steep-dip near the surface and a lower dip-angle at depth.

In section b, the strata between the T3m and P1 seismic reflectors are thickened and the

strata above are conspicuously deformed into an anticline (Fig3 and Fig. 5b). Here we

interpret a thrust fault has shortened and deformed the T3m and P1 section and uplifted the

overlying strata. At the same time, several axial surfaces have terminated on the lowermost

fault. Based on these observations we interpret the structural wedges in the section b.

The drilling cores show three sections of the Pengguan Complex and three sections of

the Xujiahe Formation that occur in alternate repetition in the WFSD-2 well (Zhang et al.,

2012), indicating that the Longmen Shan thrust belt is composed of a series of thrust sheets

(Fig. 5 and Fig. 6). Most of the structures we interpreted were coinciding with the surface

geology. We get the strata from the western Sichuan depression, where have many oil drilled

wells. At the mountain front is a monocline belt (Fig. 2).

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According to the seismic geology interpretation (Fig. 6), at the mountain front, the lower

Jurassic displays a parallel unconformity with the fifth member of upper Triassic Xujiahe

Formation. Beneath the monocline belt, the fifth member of upper Triassic Xujiahe

Formation has pinched-out. The lower Jurassic was overlapping on the fourth member of

upper Triassic Xujiahe Formation with an angular unconformity on the surface (Fig. 2 and

Fig. 6). It indicates that the central segment of Longmen Shan uplifted during the late Triassic,

and formed a foreland basin (Liu et al., 1996; Li et al., 2013b).

Fig. 6 The interpretation of the seismic reflection profile A-B.

There is an important detachment between the middle-lower Triassic that has formed

within gypsum beds (Tang et al., 2008). The structural wedges have developed on this

detachment deformed the upper strata to form a monocline belt. Fault slips toward the

foreland basin formed the Longquan Shan structure (Jia, et al., 2009). The Sui'ning Ms5.0

earthquake that occurred at the inner Sichuan basin on January, 2010 might be related to the

slip of basement detachment from the Longmen Shan to the central Sichuan basin (Lu et al.,

2010a, 2011).

Above the structural wedges, the strata are relatively flat-lying in contrast to the more

steeply-dipping surface geology (Fig. 6). Geometric constraints require more than a single

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thrust form this monocline. In this seismic profile, the Guankou blind fault (F4) is not a steep

thrust, but is a gentle reverse fault with back-thrust and may be a structural wedge. At the

basement, we also find deformed strata that indicate some pre-existing structures (Lu et al.,

2010b). According to the structural style of the sedimentary cover and the deformation

characteristics of the basement, we infer that a structural wedge model is reasonable.

Here we use the "Area Error Prop" software to calculate the shortening of different strata

(Judge and Allmendinger, 2011). In the profile A-B, the lower Triassic and the Paleozoic have

likely remained buried during their history and were not denudated. We hypothesize they

keep the same thickness and use area balancing. However, the upper Triassic strata have been

exposed to the surface and eroded. Therefore, it need take into account the impact of erosion,

as well as the deposition thickness variation (Liu et al., 2009). The results show the upper

Triassic was uplifted and shortened approximately 13.8 km. The lower Triassic and the

Permian have been shortened about 9.6 km (Table. 1). The lower Paleozoic has less

shortening because the profile A-B at the mountain belt. The lower Paleozoic may be more

highly shortened at the hinterland.

Table 1.

Shortening at stratigraphic intervals predicted by the interpretation of seismic profile A-B.

3.1 Seismic reflection profile C-D

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The seismic reflection profile C-D was extracted from a 3-D seismic data. The cut-offs

show the Pengguan fault (F3). In this profile, the Pengguan fault (F3) is gentle and with the

dip about 25°~30°, similar to the profile A-B from this study. The LS1 well encountered

several repeat sections of the upper Triassic Xujiahe Formations and about ten thrusts or

fracture zones (Fig. 2 and Fig. 8). The drilling data had revealed several coal layer

detachments, a number of fractures and some small scale duplexing (Lu et al., 2010a).

Fig. 7 Seismic reflection profile C-D that was extracted from a 3D seismic volume showing

the surface geology and DEM elevations. The surface geological information are obtained

from both the 1: 20 0000 and 1:50 000 regional geological maps (see Data and Resources

section). Black boxes indicated the locations of detailed sections shown in Figure 9. Fault

cut-offs are indicated by yellow arrows.

Fig. 8 The LS1 well shows the drilled strata and faults

HLF- The Carboniferous Huanglong Formation; XJHF- The upperTriassic Xujiahe

Formation; XTZF- The upper Triassic Xiaotangzi Formation; MATF- The upper Triassic

Maantang Formation; TJSF- The middle Triassic Tianjingshan Formation; TJSF- The middle

Triassic Tianjingshan Formation; LKPF- The middle Triassic Leikoupo Formation; LKPF-

The lower Triassic Jialingjiang Formation;

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Fig. 9 Interpretation of detailed seismic sections from seismic reflection profile C-D.

The red lines are the major faults and the dash lines are the axial surfaces. Red arrows show

the motion direction of interpreted fault slip.

In section c, the CY92 well at the frontal mountain allows us to distinguish the strata

accurately (Fig, 9c). The sedimentary characteristics are similar to profile A-B (Fig. 6 and Fig.

9c). The surface geology and drilled well provide important constraints on the model. The

seismic reflection display poor continuity in the shallow section. Here we interpreted a

structural wedge base on the surface geology and drilling data.

In section d, beneath the Pengxian blind fault (F5) area (Fig. 7 and Fig. 9d), the seismic

data show the Pengxian blind fault (F5) is not a steep thrust, but is a kink band developed on

the Triassic detachment. The paired axial surfaces were terminated on the detachment, which

is in the middle-lower Triassic. The detachment also transfers the slip to the Longquan Shan

several kilometers (Lu et al., 2010a).

The seismic reflection profile C-D reveals the detail of structures clearly for it is

extracted from a 3D seismic data (Fig. 10). The interpretation results are the same to the

seismic reflection profile A-B. There are some structural wedges at the mountain front. The

LS1 drilled well reveal alternating repeat sections of the upper Triassic Xujiahe Formations

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(Fig. 8). According to the axial surfaces analysis, the Pengguan fault (F3) cut the pre-existing

structures and ruptured to the surface. The slip of the major Pengguan fault (F3) is about 2~3

km.

Fig. 10 The interpretation of the seismic reflection profile C-D.

In the seismic profile C-D (Fig. 10), we can find deformation of the Paleozoic strata based

on changes in strata' thickness between the lower Permian and the Sinian.

Deep-seated basement uplift has uplifted and deformed the sedimentary cover. Therefore,

deformation within the central Longmen Shan thrust belts are closely related to the basement

structures (Lu et al., 2010b).

The shortening of strata in the profile C-D was calculated with the same methods (Table.

2). The statistics show the shortening of the upper Triassic is 18.8km, which is more than the

profile A-B. The lower Triassic and the Permian have shortened approximately 12km. The

data indicated that tectonic activities were more intense in the profile C-D area. The surface

geology show a similar relationship, where the profile C-D area has more faults outcropping

than the profile A-B area. In addition, the surface ruptures occurred in the profile C-D area

had more vertical slips than the profile A-B area during the Wenchuan Mw7.9 earthquake (Xu

et al., 2008).

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Table 2.

Shortening at stratigraphic intervals predicted by the interpretation of seismic profile C-D.

4. The structural model for the central segment of Longmen Shan thrusts

There are several important detachments in the Longmen Shan and the Western Sichuan

Depression (Yan et al., 2010). The deformations in the Longmen Shan thrusts are controlled

by the multi-level differential detachments (Tang et al., 2008). Our research reveals that the

shallow-level detachment is in the upper Triassic Xujiahe Formation and develops imbricate

thrusts (Fig. 8). The medium-level detachment is in the middle-lower Triassic and develops

structural wedges and fault-related folds. This detachment slipped forward and propagated to

the Longquan Shan. The deep-level detachment is in the basement and develops the duplex or

structural wedges (Fig. 11). Both the shallow and the deep detachments have transferred slip

to the foreland basin and resulted in deformation at inner Sichuan basin.

Seismic interpretation show there are many thrust splays at the shallow surface at the

Pengguan thrust belt. The major Pengguan fault (F3) cut the pre-existing structures (Fig.10

and Fig. 11). Beneath the front monocline belt are some structural wedges. The Guankou

blind fault (F4) is not a steep thrust but is the tip of a wedge at the central segment of

Longmen Shan. The Pengxian blind fault (F5) is a kink band which developed on the

medium-level detachment.

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Fig. 11 Comparison of interpreted structure of the Central Longmen Shan from section A-B

and C-D that separated along-strike by approximately 20km. Both sections show the

development of a structural wedge formed by blind thrusts under the frontal monocline. In

contrast, the hinterland is deformed by forethrust imbricates that have cut pre-existing

structures and reach the surface.

According to the seismic interpretation, there were four tectonic events at least identified

at the central segment of Longmen Shan thrusts. The first tectonic event was occurred during

the Late Triassic (Liu et al., 1996; Yan et al., 2011; Li et al., 2012), for the angular

unconformity between the lower Jurassic and the upper Triassic (Fig. 11). The second

tectonic event was identified by the fontal monocline belt. The structural wedges made the

upper Triassic, the Jurassic and the Cretaceous deformed, which indicated that this tectonic

event occurred after the Late Cretaceous. The third tectonic event was recognized by the

basement was deformed and made the upper sedimentary cover uplifted, which may be

related to the early Cenozoic time (Wang et al., 2012). The fourth tectonic event was the

neo-tectonics. The Beichuan fault (F2) and the Pengguan fault (F3) were reactivated. They

cut the pre-existing structures and ruptured to the surface. The multi-superimposed

deformation played an important role in the evolution of the Longmen Shan.

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5. Discussions and Conclusions

5.1 Discussions

1. Both the LS1 well and the WFSD-2 well revealed there are many pre-existing faults

(Fig. 4 and Fig. 8). These indicate the Longmen Shan thrust belt have experienced quite a lot

of large paleo-earthquakes or historical earthquakes (Li et al., 2013a; Nie et al., 2013). On the

basis of the seismic profiles interpreted (Fig. 11), the upper Triassic repeated several times

and has thickened the strata more obvious. The shortening at the central Longmen Shan front

is nearly 30% -37% (Table 1 and Table 2). It shows clearly that the crustal shortening was

typical of the central Longmen Shan thrust belts, which may not support to the lower crust

beneath the Longmen Shan (Royden et al., 2008; Burchfile et al., 2008). In our model, the

Beichuan fault (F2) and the Pengguan fault (F3) are high-angle dipping near the shallow, but

gently dipping at depth (Fig. 12a). Structural wedges beneath the monocline belt indicate that

the Guankou fault (F4) is not like a simple "listric" fault (Jin et al.,2010).The Pengxian fault

(F5) is only a kink band, which is consistent with the previous interpretation (Jia et al., 2010).

2.The sedimentary research show there were developed the foreland basin at the

Longmen Shan thrust belt during the Late Triassic (Liu et al., 1996; Li et al., 2013b), which

are not agree with previous opinions, that thought there were absence of a foreland basin in

Longmen Shan (Burchfiel et al., 1995; Densmore et al., 2007). Since the Late Cretaceous

period, the Sichuan Basin has been in an overall uplifting stage with the exception of the

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Western Sichuan Depression (Liu et al., 2009). During the Cenozoic, many structures in the

Longmen Shan were developed and have deformed both the basement and sedimentary cover

(Lu et al., 2012). Seismic reflection profiles crossing the central segment of Longmen thrust

belt, show the lower Paleozoic and basement were deformed (Jia et al., 2010; Li et al., 2010).

The rapid uplifting of the Longmen Shan may be associated with both the shallow thrusting

and the basement shortening (Fig. 12).

Fig. 12 The long seismic profiles L1 was interpreted (modified from Li et al., 2013a). The SE

section is the same to the profile A-B (Fig. 6).

3. The new co-seismic faults at the Beichuan thrust belt during the Wenchuan Mw7.9

earthquake may be different from the early major fault system (Fig.2 and Fig. 4). Previous

studies found that the deformation of the Beichuan thrust belt and the Pengguan thrust belt

began from the Mesozoic (Liu et al., 2009; Li et al., 2012). However, it is quite difficult to

distinguish these faults developed during the Mesozoic. The co-seismic fault at the Beichuan

thrust belt and the Pengguan thrust belt have limited slip. Our interpretation of the drilling

results is that these new thrusts have cross-cut older, more gently-dipping thrusts (Fig. 4 and

Fig. 12b). We interpret the Wenchuan Mw7.9 earthquake co-seismic fault to be a branchy

fault that developing at the Beichuan thrust system, whcih were controlled by the multiple

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detachments (Tang et al., 2008; Jin et al., 2010). It is possible that uplifting of the Longmen

Shan is related to the activity along the basement faults, which had occurred the Wenchuan

Mw7.9 earthquake and Lushan Mw6.7 earthquake (Fig. 12b). With continued shortening of

the Longmen Shan, our model indicates potential for continued large earthquakes at the

Longmen Shan thrust belt. However, the relationship between the co-seismic faults and the

pre-existing major faults requires further study.

5.2 Conclusions

Multiple tectonic events and multi-superimposed deformations have occurred since the

since the Mesozoic at the central segment of Longmen Shan. The deformations of the

Longmen Shan thrusts were controlled by the multi-level differential detachments. The

shallow-level detachment in the upper Triassic Xujiahe Formation develops imbricate

structures. The medium-level detachment in the middle-lower Triassic develops wedges. The

medium-level detachment slipped forward and propagated to the Sichuan basin and formed

Longquan Shan. The deep-level detachment in the basement develops the duplex or wedges.

Both the shallow and the deep detachments transfer its slip to the basin and made another

deformation inner the Sichuan basin. The shortening at the central segment of Longmen Shan

front is nearly 30% -37%, which indicated that the crust shortening was the typical

characteristics of the Longmen Shan thrust belt. Both of the shallow thrusting and the

basement shortening made contribution to the uplifting and shortening of the Longmen Shan.

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The Wenchuan Mw7.9 earthquake and Lushan Mw6.7 earthquake were related to the

reactivated of the basement faults.

Data and Resources

The 1：200 000 geology maps are come from the first and third geological team of

Sichuan geology and mineral exploration and development bureau. 1996-1997 geology maps.

The 1：50 000 geology maps are come from the first region geology survey team of Chengdu

College of Technology. 1991-1994. The seismic reflection profiles were provided courtesy of

Chinese Academy of Geological Sciences and the SINOPEC.

7 Acknowledgements

This research received financial support from National Science and Technology Major

Project (No. 2011ZX05005-003-004), the National Natural Science Foundation of China (No.

41202142, 40739906), the China Postdoctoral Science Foundation funded project (No.

2012M520120, 2013T60027). We especially thank Haibin Li, the researcher of Chinese

Academy of Geological Sciences for their seismic data. We thank Xiwei Xu, the researcher of

China Earthquake Administration and the chief editor, Laurent Jolivet gave us significantly

suggestions to improve the manuscript.

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Fig. 1 Digital elevation (DEM) map showing major thrust faults in the Longmen Shan and

location of the 2008 Wenchuan earthquake and 2013 Lushan earthquake (modified from Xu

et al., 2013). Inset map show position of the study area relative to Tibet.

F1: Qingchuan-Maowen fault；F2: Beichuan-Yingxiu fault；F3: Pengguan fault；

F4: Guankou blind fault；F5: Pengxian blind fault (the same to below legends).

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Fig. 2 Local geological map (see Data and Resources section) showing the position of

seismic reflection profiles and drilled wells used in this study.

2- Cenozoic; 2- Cretaceous; 3- Jurassic; 4- Upper Triassic; 5- Lower Triassic; 6-Permian; 7-

Carboniferous; 8-Devonian; 9- Sinian; 10- Pengguan Complex; 11- Thrust faults; 12-

Blind faults; 13- Wells; 14- Seismic Reflection profiles.

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Fig. 3 Seismic reflection profile A-B showing the surface geology and DEM elevations. The

surface geological information are obtained from both the 1: 20 0000 and 1:50 000 regional

geological maps (see Data and Resources section). Black boxes indicated the locations of

detailed sections shown in Figure 5.

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Fig. 4 Structural interpretation of the WFSD-1 and WFSD-2 well results modified from

(Zhang et al., 2012 and Li et al., 2013a). The F2-2 fault was interpreted as the co-seismic

fault during the Wenchuan earthquake (Zhang et al., 2012).

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Fig. 5 Interpretation of detailed seismic sections from seismic reflection profile A-B.

T3m: The upper Triassic Maantang formation; P1: The lower Permian. The red lines are the

major faults in this section and the violet line is the co-seismic fault during the Wenchuan

Mw7.9 earthquake. The Pengguan complex was indicated by the yellow fill. The dark dash

lines are the axial surfaces. Red arrows show the motion direction of fault slipping. The

layers are the bottom of each stratum.

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Fig. 6 The interpretation of the seismic reflection profile A-B.

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Fig. 7 Seismic reflection profile C-D that was extracted from a 3D seismic volume showing

the surface geology and DEM elevations. The surface geological information are obtained

from both the 1: 20 0000 and 1:50 000 regional geological maps (see Data and Resources

section). Black boxes indicated the locations of detailed sections shown in Figure 9. Fault

cut-offs are indicated by yellow arrows.

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Fig. 8 The LS1 well shows the drilled strata and faults

HLF- The Carboniferous Huanglong Formation; XJHF- The upperTriassic Xujiahe

Formation; XTZF- The upper Triassic Xiaotangzi Formation; MATF- The upper Triassic

Maantang Formation; TJSF- The middle Triassic Tianjingshan Formation; TJSF- The middle

Triassic Tianjingshan Formation; LKPF- The middle Triassic Leikoupo Formation; LKPF-

The lower Triassic Jialingjiang Formation;

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Fig. 9 Interpretation of detailed seismic sections from seismic reflection profile C-D.

The red lines are the major faults and the dash lines are the axial surfaces. Red arrows show

the motion direction of interpreted fault slip.

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Fig. 10 The interpretation of the seismic reflection profile C-D.

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Fig. 11 Comparison of interpreted structure of the Central Longmen Shan from section A-B

and C-D that separated along-strike by approximately 20km. Both sections show the

development of a structural wedge formed by blind thrusts under the frontal monocline. In

contrast, the hinterland is deformed by forethrust imbricates that have cut pre-existing

structures and reach the surface.

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Fig. 12 The long seismic profiles L1 was interpreted (modified from Li et al., 2013a). The SE

section is the same to the profile A-B (Fig. 6). The figures show there are some basement

faults existing and making the strata deformation from mountain to the basin.

3- Strata of the Cenozoic, Cretaceous and Jurassic; 2- Xujiahe Formation of Upper Triassic;

3- Upper Triassic; 4- Lower Triassic; 5-Permian; 6- Lower Paleozoic; 7-Basement rock;

8- Pengguan Complex;

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Table 1

Shortening at stratigraphic intervals predicted by the interpretation of seismic profile A-B.

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Table 2

Shortening at stratigraphic intervals predicted by the interpretation of seismic profile C-D.

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Highlights

► We conducted a detailed structural model of the central Longmen Shan thrust.

► Structural wedges are important structures developed on the different multiple

detachments at the Longmen Shan thrust belt.

►The shortening at the central Longmen Shan thrust is approximately 30% -37%.

► The shallow thrusting and the basement deforming have contributed to the uplift of the

Longmen Shan since the Cenozoic.