3D geometry of range front blind ramp and its effects on structural segmentation of the southern Longmen Shan front, eastern Tibet

Journal of Asian Earth Sciences 181 (2019) 103911

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3D geometry of range front blind ramp and its effects on structural segmentation of the southern Longmen Shan front, eastern Tibet ⁎

T

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Chuang Suna,b, Zhigang Lia,b, , Wenjun Zhenga,b, , Dong Jiac, Dongli Zhanga,b, Xiaogen Fanc, Peizhen Zhanga,b a

Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou, China Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China c School of Earth Sciences and Engineering, Nanjing University, Nanjing, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Fault geometry Longmen Shan Sandbox modeling Wenchuan earthquake Lushan earthquake Seismic gap

A 3D fault morphology model has been constructed from seismic profiles covering the piedmont of the southern Longmen Shan (LMS), eastern margin of the Tibetan Plateau. It reveals a spatial correlation between an ∼3.5 km-high geometrical bump on the range front blind ramp (RFBR) and the gap between the 2008 Mw 7.9 Wenchuan and 2013 Mw 6.6 Lushan earthquakes. Sandbox modeling is then used to probe the effects of the geometrical irregularity of the RFBR on fault behaviors and the LMS range front deformation. Sand wedges are designed to thrust over artificial blind ramps, analogous to the thrusting of the LMS orogenic wedge toward the Sichuan Basin through the RFBR. The experiments reproduce the first-order deformation features presented in the southern LMS range front, confirming that along-strike geometrical variations of the RFBR have induced the segmentation of both fault motion and the range front deformation. Their results also suggest that the gap between the Wenchuan and Lushan earthquakes is located directly within the structural transition zone originating from the geometrical bump on the RFBR. Additionally, there is a duplex structure occurring in the hanging wall of the geometrical bump, which localizes significant shear strain with progressive shortening and represents a fracture zone corresponding to the observed geophysical anomaly (low seismic velocity) underneath the seismic gap. It is likely that this fractured body protected the gap from the ruptures of the Wenchuan and Lushan earthquakes.

1. Introduction Two devastating earthquakes, the 2008 Mw 7.9 Wenchuan and 2013 Mw 6.6 Lushan earthquakes, occurred on the east margin of the Tibetan Plateau, producing nearly 400-km-long ruptures in the ∼500-km-long Longmen Shan (LMS) thrust-fold belt, which has been shortened by the expansion of the growing Plateau (e.g., Shen et al., 2009; Wang et al., 2011; Fig. 1). However, an ∼45-km-long seismic gap has been left between the ruptures of the two earthquakes, with this zone devoid of any notable co-seismic ruptures or associated aftershocks (e.g., Hao et al., 2013, Li et al., 2013b; Fig. 2). Simulations of Coulomb stress change indicate that this gap was loaded by the two earthquakes and now represents a high seismic hazard (e.g., Parsons et al., 2008; Shan et al., 2009; Luo and Liu, 2010; Wang et al., 2010; Nalbant and McCloskey, 2011; Li et al., 2014a; Liu et al., 2014). Geophysicists and geologists hold different opinions about the

nature of this gap, which leads to contrasting inferences about whether large earthquakes (M > 7) can occur in the southern LMS range front. Geophysical evidence suggests that underneath the gap, there is a ductile crustal body characterized by a lower velocity (Vp, Vs), a high conductivity and a high Poisson’s ratio (Zhan et al., 2013; Pei et al., 2014; Wang et al., 2015b). The seismogenic faults, even across this gap, cannot accumulate enough crustal stresses to generate large earthquakes. This point of view has been challenged by recent trench studies, which revealed unambiguous historical ruptures in the gap (Dong et al., 2017; Wang et al., 2015a). Background microearthquakes in the gap are not weaker than in adjacent areas (Wang et al., 2018), and post-seismic GPS data obtained after the Wenchuan earthquake argue against ductile deformation beneath the gap as well (Diao et al., 2018). Therefore, many geological studies favor fault segmentation for the southern LMS and consider this gap to be a structural transition zone (e.g., Wang et al., 2010; Chen et al., 2013; Wang et al., 2014a; Shao et al., 2018).

⁎ Corresponding authors at: Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou, China. E-mail addresses: [email protected] (Z. Li), [email protected] (W. Zheng).

https://doi.org/10.1016/j.jseaes.2019.103911 Received 16 April 2019; Received in revised form 27 June 2019; Accepted 29 June 2019 Available online 02 July 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (A) Topographical map of Tibet and its adjacent area with our study area highlighted by a while rectangular. Convergence rate of India and Tibet is from Zhang et al. (2004). (B) Landsat image of the southern Longmen Shan (LMS) and the Sichuan Basin, showing the epicenters and focal mechanisms of the 1970 Ms6.2 Dayi earthquake, the 2008 Mw7.8 Wenchuan earthquake, the 2013 Mw6.6 Lushan earthquake, and major faults. The mainshock hypocenters and focal mechanisms are modified from Li et al. (2017). The yellow dashed line marks the surface projection of the range front blind ramp (RFBR). Co-seismic ruptures of Wenchuan earthquake from Liu-Zeng et al. (2009) are presented as red lines against white background. The locations of typical cross sections are also indicated by pink lines. (C) Geological cross section after Hubbard et al. (2010) with the ramp-flat structure underneath the range front represented by yellow bold line. WLF: Wulong fault; BXF: Baoxing fault; WMF: Wenxian-Maoxian fault; YBF: Yingxiu-Beichuan fault; PGF: Pengguan fault; RFBR: Rang front blind ramp; LQF: Longquan fault; XSHF: Xianshuihe fault. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

from the Plateau toward the adjacent Sichuan Basin (Fig. 2; see Section 3 for further details). A geometrical bump is present beneath the gap, with a positive relief of up to ∼3.5 km related to the master fault plane of the RFBR, extending ∼40 km along the strike and ∼12 km downwards (Fig. 3). Its southern boundary has limited and forced the aftershocks of the Lushan earthquake to a half-bowl-shaped distribution (Li et al., 2014a; Zhang et al., 2013). The aftershocks and co-seismic ruptures of the Wenchuan earthquake have also terminated when approaching this geometrical bump (Fig. 3A). Consequently, a reasonable speculation can be made that the fault geometry of the RFBR, especially the presence of the geometrical bump, may be the primary factor controlling segmentation in the southern LMS range front. The sandbox modeling method has been widely used to investigate both two- and 3D, upper crust deformation, especially the faulting processes (e.g., Zhou et al., 2007; Yan et al., 2016; Sun et al., 2016; Wang et al., 2016; Deng et al., 2014, 2016; Deng et al., 2017; Caer et al., 2018; Ritter et al., 2018; Zhang et al., 2019). Thrust motion over ramps of variable dip angles is also one of the well-investigated issues

Additionally, the combination of fault slip data and an analysis of the Gutenberg–Richter relationships of historic earthquake catalogs suggests that the potential maximum moment magnitude of earthquakes in the southern LMS could reach Mw 7.7 if the entire length of the seismogenic faults were to rupture (Li et al., 2017). Consequently, the factors controlling the segmentation of fault motion in the southern LMS and their contributions to the occurrence of the gap are of great importance in further assessments of regional seismic hazards. Fault geometry impacts not only long-term deformation, such as folding, secondary faulting, and fracturing but also short-term seismogenic behavior, such as rupture initiation and termination (Aki, 1979; Lay and Kanamori, 1981; King and Nábělek, 1985; Kilsdonk and Fletcher, 1989; Zhang et al., 1991, 1999; Zhu and Zhang, 2010, 2013; Hubbard et al., 2016; Qiu et al., 2016). As illustrated by the 3D fault morphology model built from seismic profiles, the gap between the ruptures of the Wenchuan and Lushan earthquakes roughly corresponds to the geometric irregularity occurring on the range front blind ramp (RFBR), which is the major structure transferring active displacement 2

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Fig. 2. Different views of 3D geometric configuration of the range front fault system, consisting of RFBR, the basal detachment underneath the LMS and the shallow detachment with the Sichuan Basin. (A-B) Morphological models colored according to depth. (C) Morphological model colored to dip angles. USGS focal mechanisms of the Wenchuan and Lushan earthquakes are also presented. The yellow and white balls are aftershocks of the Lushan earthquake from Fang et al. (2015), and that of Wenchuan earthquake from Wang et al. (2011), separately. Black dotted lines outline the area of geometric bump (GB). The pink arrow line represents the distance between epicenters of the Wenchuan and Lushan earthquakes. While the blue arrow line marks the gap between ruptures of these two earthquakes. The green one indicates the extension of GB. The red lines are coseismic surface ruptures of Wenchuan earthquake from Liu-Zeng et al. (2009). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2. Geological setting The LMS range overthrusts the adjacent Sichuan Basin and has produced a 500-km-wide thrust-fold belt in response to the outgrowth of the Tibetan Plateau, which is caused by the India-Eurasia collision since ∼50 Ma ago (e.g., Burchfiel et al., 1995; Chen and Wilson, 1996; Tian et al., 2013; Yan et al., 2018, Fig. 1A). The LMS range itself consists of two distinct deforming belts: a thrust stack belt in the hinterland and a fold belt in the western Sichuan Basin (Fig. 1C). These two belts display distinct structural features controlled by the deformed materials and their respective detachments (Jia et al., 2006, 2010; Hubbard et al., 2009, 2010; Li et al., 2010; Wang et al., 2014a; Zhang et al., 2018, 2019b). In the southern LMS, the hinterland thrust belt contains four major east-vergent thrusts (from NW to SE): the Wenchuan-Maoxian Fault (WMF), the Wulong Fault (WLF), the Baoxing Fault (BXF) and the RFBR. These faults sole to a basal detachment located at a depth of ∼18 km, forming a typical orogenic wedge showing a great topographic relief of ∼4500 m over a horizontal distance of ∼50 km (Hubbard et al., 2010; DeDontney and Hubbard, 2012). The foreland fold belt, extending for ∼100 km forward, is characterized by isolated fault-related fold structures, such as the Dayi and Qiongxi anticlines, with the Longquan anticline as its current deformation front. Almost all these anticlines nucleate from the Triassic salt detachment at a depth of ∼5 km within the Sichuan Basin (Fig. 1C). The 2008 Wenchuan earthquake, 2013 Lushan earthquake, and Quaternary fault activity recorded by growth strata (e.g., Wang et al., 2014a; Li et al., 2016) all suggest that compression strain is currently located near the LMS range front, where a transition zone between the hinterland thrust belt and the foreland fold belt occurs. It is also the place where the RFBR connects the basal detachment beneath the LMS range with the midlevel detachment in the adjacent Sichuan Basin, forming a crustal flat-ramp-flat structure that feeds the active displacement from the Tibetan Plateau into the adjacent Sichuan Basin (e.g., Jia et al., 2006, 2010; Li et al., 2010; Li et al., 2013a, 2014a, 2016; Wang et al., 2014a, Fig. 1C). As the key connection, this blind ramp poses a significant earthquake threat to the adjacent densely populated Chengdu Plain and may have decisive effects on deformation partitioning within the LMS fold-and-thrust belt, for example, determining whether active slip would be transferred to the ramp as in the way that

Fig. 3. Simplified geometric parameters of RFBR along the strike of southern LMS range. Orange and pink lines represent synclinal-bend and planar types of crustal ramp structure in and out of the gap between the Wenchuan and Lushan earthquakes. The geometric bump (GB) is highlighted by yellow color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(e.g., Bonini et al., 1999, 2000; Mulugeta and Sokoutis, 2003; Maillot and Koyi, 2006; Koyi and Maillot, 2007; Rosas et al., 2017). These studies mainly focus on planar fault geometry. The kind of along-dip and along-strike geometry variations that the RFBR presents beneath the southern LMS range front remain less investigated. In this paper, we present sandbox experiments simulating the thrusting of the LMS orogenic wedge toward the adjacent Sichuan Basin through the RFBR. A total of four sandbox experiments are constructed according to the true geometric features of the RFBR. Both the alongstrike and along-dip fault geometries have been investigated. The experimental results are compared with the structural features observed in and out of the gap between the ruptures of the Wenchuan and Lushan earthquakes to evaluate the effects of the RFBR geometry on the segmentation of fault activities. We further propose a plausible explanation for the geophysical observations underneath the gap; this explanation is also consistent with geological studies.

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the Lushan earthquake ruptured the RFBR, or just be accommodated by the breaking of the splay faults in its hanging wall, as with the Wenchuan earthquake (Wang et al., 2014a; Li et al., 2017; Zhang et al., 2018). 3. 3D geometry of the RFBR High-resolution seismic reflection profiles that cover the LMS range front have long been employed to analyze the subsurface structure of the region, providing important constraints on regional seismogenic faults (Jia et al., 2006, 2010; Hubbard et al., 2009, 2010; Li et al., 2010, 2014; Wang et al., 2013a, 2013b, 2014; Li et al., 2016, 2017; Lu et al., 2014, 2017). Here, we focused on the detailed geometry of the RFBR, as follows: We first recognized the direct fault-plane reflections and fault cutoffs in the hanging wall and footwall. However, a range of possible fault planes came out, due to indistinction and segmentation of the referred features. Therefore, we furtherly calculated preferred dip angle of the RFBR based on the limited information about fault geometry and geometric parameters of its associated fold, through the quantitative fault-related folding relationship (Suppe, 1983; Shaw et al., 2005). We determined sedimentary layers according to borehole data and tracked their corresponding reflections along entire seismic profiles, to delineate the shallower fold geometry. The folded stratum was then divided into reflection regions of equivalent dipping, termed as dip domains (Shaw et al., 2005). The surfaces separating these dipping domains were axial surfaces, which could be used to infer the locations of where fault plane bends (Medwedeff and Suppe, 1997). On basis of both fold geometry and axial surfaces, we calculated and chose the preferred fault traces. Additionally, the moment tensor solutions for the Dayi and Lushan earthquakes were taken as additional constraints on the deeper segment of RFBR geometry (see Li et al. (2017) for more details). Next, we present two representative seismic reflection profiles that are oriented perpendicular to the strike of the LMS to illustrate the along-strike geometric variations of the RFBR (Fig. 4). The first seismic reflection profile, A, passes through the epicentral zone of the 1970 Ms 6.2 Dayi earthquake (Fig. 1B). The geometric relationship between the fault and the overlying fold suggests that the shallower section (from ∼5 to 12 km depth) of the RFBR dips 52° to the northwest, while its deeper section dips 35° (Fig. 4A), coinciding with the moment tensor solution of the Dayi earthquake (Zhang et al., 1990; Li et al., 2017). The RFBR is therefore a typical synclinal-bend fault exhibiting concave or listric geometry (Shaw et al., 2005), with a middle-level bend at ∼12 km depth. The shallow section of the RFBR merges with a Triassic detachment level in the Sichuan Basin at ∼5 km depth, while its deeper section soles into the detachment beneath the LMS range at 17–20 km depth (Hubbard et al., 2010; Li et al., 2010). This basal detachment is out of the scope of our seismic profile, but has been imaged by the SinoProbe-02 deep seismic reflection profile directly (Guo et al., 2013). Moreover, its location is compatible with the overlapping zone between the aseismic horizon beneath the Longmen Shan, below which very limited earthquakes occurred (Li et al., 2010), and the low-seismic-velocity zone observed by crustal tomography of Huang et al. (2009). Therefore, this deep detachment not others, is the major active structure transferring deformation toward the Sichuan Basin. The second seismic profile, B, is near the epicentral zone of the 2013 Lushan earthquake, ∼60 km to the south of profile A (Fig. 1B). This profile images a simple linear RFBR that dips 32° to the northwest and is comparable to its counterpart near the epicenter of the Wenchuan earthquake, which is illustrated in Fig. 1C. A total of 40 seismic reflection profiles, oriented perpendicular to the structural strike of the RFBR, were interpreted and rebuilt in Gocad™ (Mallet, 1992; Fig. A1 for the locations of the used profiles). The resultant 3D fault model reveals an ∼40-km-wide geometrical bump on the RFBR that is located between the hypocenters of the

Fig. 4. Two typical seismic reflection profiles showing subsurface structures underneath the range front of the southern LMS from Li et al. (2017). (A) Profile across the epicenter of the 1970 Dayi earthquake, showing synclinal-bend ramp geometry. (B) Profile near the epicenter of the 2013 Lushan earthquake, displaying planar ramp geometry. Their detail locations are indicated in Figs. 1B and 2C. Traces of the RFBR are represented by the bold yellow lines. According to the fault-related folding theory (Suppe, 1983), geometrics of the shallow fold are extracted to determine detail geometry of the upper section of RFBR. While the geometry of lower section is mainly constrained by focal mechanism, for details see Li et al. (2017). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Wenchuan and Lushan earthquakes (Fig. 2). This bump represents the steeper segment of the shallow RFBR, with dip angles of 55 ± 5°, while the lateral and deeper sections possess shallower dips of 30 ± 5° (Fig. 2B). According to this 3D fault morphology model, we then conducted scaled sandbox experiments. 4. Sandbox modeling 4.1. Experimental setup Footwall blocks are commonly used experimental design to explore the fault geometry effect on the associated thrust deformation in sandbox experiments (Bonini et al., 1999, 2000; Mulugeta and Sokoutis, 2003; Maillot and Koyi, 2006; Koyi and Maillot, 2007; Rosas et al., 2017) as well as in numerical modeling (e.g., Erickson et al., 2001; Hughes et al., 2014). Building the fault planes of variable geometry by rigid blocks, such design only focuses on the structural evolution of the hanging wall, such as the development of shear zones and folding processes. In the LMS range front, shortening induced by the advancing LMS orogenic wedge has primarily been accommodated by thrusting above the RFBR, accompanied by superficial structures forming above the shallower detachment within the adjacent Sichuan Basin (Fig. 1C and 4). Accordingly, we adopt a fixed footwall that is buried by a horizontal sand layer representing the shallower deforming strata in the Sichuan Basin. (Fig. 5). Such a design highlights both the deformation occurring in the hinterland thrust wedge and the shallow 4

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Fig. 5. Schematic diagram of the used artificial ramps and experimental setup. (A and B) The planar and synclinal-bend ramps used in the 2D experiments M1 and M2, separately. (C) Artificial ramps used in the 3D experiments M3 and M4, involving alongstrike changes analogue to that of RFBR in the southern LMS. The sharp edges of the steep upper ramp are polished, forming smooth transition along the strike. (D) Experimental setup. Critically-tapered proto wedges are adopted to represented the advancing LMS orogenic wedge in all our experiments. The Plexiglas plates are fixed and buried by 1 cm thick sand layer in the foreland, acting as the footwall of RFBR. The black arrows indicate the direction of shortening.

4.2. Modeling apparatus, materials and scaling law

fold belt in the adjacent foreland, analogous to what has occurred in the LMS range front. The deformation of experiments would be induced by pushing the deforming materials from the rear toward the artificial ramps (Fig. 5D). This process allows the deformation to migrate forward progressively and interact with the foreland ramp, consistent with the processes involved during the thrusting of the LMS orogenic wedge over the RFBR. We set the initial boundary condition, i.e., the geometry of the deformable materials, based on the critical taper theory (Davis et al., 1983; Dahlen, 1990). This theory describes the relationship between the geometry and the mechanics of fold and thrust belts, predicting a mechanically stable wedge geometry that is dependent on the strengths of both the deforming materials and the basal detachment. It states that when Coulomb-type wedges achieve this stable geometry, the whole thrust wedge could slip along its base with deformation only concentrated in the wedge front (Davis et al., 1983; Dahlen, 1990). Previous studies have already verified that both the long-term tectonics (Hubbard et al., 2010; Sun et al., 2016) and the special rupture pattern presented during the Wenchuan earthquake (DeDontney and Hubbard, 2012) could be well explained using the critical taper theory. The LMS orogenic wedge is consist of not only the related weak sedimentary rocks near the range frontal zone, but also high strength rocks in the hinterland, like the Pengguan and Baoxing massifs. However, we did not take this type of rock contrast into our sandbox experiments. Instead, we used homogenous sand assemblages. That is both the hinterland wedges and foreland sedimentary horizons were built by identical simulation materials. According to the prediction based on the critical taper theory, when the RFBR nucleated in the range front of the LMS, the hinterland orogenic wedge must have achieved its critically tapered geometry (Hubbard et al., 2010; DeDontney and Hubbard, 2012). Since then, the displacement from the hinterland begun to be propagated into the range frontal zone directly, with major active deformation concentrated near the RFBR, rather than the hinterland massifs. In other word, the behaviors of RFBR and its associated range front deformation would not be affected by hinterland massifs significantly. Therefore, even some mismatch between the real Longmen Shan and our modeling, mainly associated with the topographic relief, would be introduced. We still could build critically tapered protowedges using the same materials as that simulating foreland sediments, to achieve an analogue mechanical state of the LMS orogenic wedge when the RFBR began to nucleate, and not disturb the range frontal deformation pattern.

Our modeling was performed in a rectangular sandbox with two glass sidewalls, a fixed wall and a mobile backstop, situated on a horizontal Plexiglas base plate. The initial dimensions of the apparatus were 60 × 40 × 25 cm in length, width and depth (height), respectively. Six 1-cm-thick rectangular plastic blocks with one edge inclined at 30° or 60° were used to build ramps of different geometries. Three of these were 40 × 40 cm in length and width; the remaining three were 40 cm long and 8 or 16 cm wide. These blocks could be stacked to form a kinked ramp. The base plate of our sandbox and the upper surface of the plastic blocks and their inclined edges form the flat-ramp-flat structure that serves as an analogue to the blind structure underlying the LMS range front. The shortening of our experiments was induced by the mobile backwall, which was connected to a motor-driven piston. Quartz sand and glass microbeads are used to simulate brittle rocks in the upper crust, as all these materials obey the Coulomb failure criterion (e.g., Lohrmann et al., 2003; Panien et al., 2006). The quartz sand used is well sorted and 200–400 µm in diameter with a density of ∼1400 kg/m3. Its internal coefficient of friction of 0.6 is similar to that of upper-crustal rocks (μ = 0.60–0.85, Byerlee, 1978). We employ glass microbeads for the weak frictional detachment because they have a lower frictional strength of ∼0.4 (e.g., Colletta et al., 1991). A 3-mmthick layer of glass microbeads was introduced on the lower flat to act as the basal detachment beneath the LMS range. Sand was sieved onto the basal glass microbeads to a total thickness of 2.7 cm. The footwall ramps were also covered by 1 cm-thick sand layers to represent sedimentary rocks in the west Sichuan Basin. Protowedges (PW) of quartz sand, dipping at 13°, were added atop the 3-cm-thick sand body against the mobile back wall. The widths of these sand protowedges were 20 cm in all experiments. The geometry of the PW was obtained from our test experiments starting with 3-cm-thick horizons, which revealed that the first critical taper attained for our modeling materials was 13° and was associated with a 20-cm-long deformation zone (Fig. A2). All sandbox experiments should be properly scaled to their prototype (Table 1). The involved dynamics for brittle failure and associated frictional slip could be expressed using the Smoluchowsky number (Sm), defined by the ratio of the gravitational force (σ = ρgh) to the frictional force (τ = μσ + C), where C is the cohesion (Ramberg, 1981). Dynamic similarity is achieved when this ratio is of the same order in both nature and the sandbox. In nature, the density of brittle upper crustal rocks is ∼2400 kg/m3, and their cohesive stresses are 10–20 MPa (Handin, 5

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RFBR if no rigid blocks are present in the foreland (Fig. A2). Therefore, before the applied shortening reaches 100 mm, the modeling design of the rigid footwall blocks is mechanically reasonable. On the other hand, the time-independent Mohr-Coulomb behavior is dominant in both our sandbox experiments and the brittle upper crust. The shortening rate thus, does not impact the experimental results; therefore, we applied the same shortening rate (2.4 mm/min) to all the four sandbox experiments. All of the modeling materials were sieved into the sandbox from a height of 30 cm to reduce variations in bulk mechanics. Before sieving the deforming materials, the interior of the sandbox was carefully cleaned with an alcohol solution and dried thoroughly to minimize sidewall friction. During the experiments, deformation was monitored by a Canon 60D SLR, and photographs were taken from the side and top of the sandbox at 1-min intervals. To reveal the displacement and strain fields during each shortening increment, particle image velocimetry (PIV) analysis was performed using these photographs in the similar way of Adam et al. (2005, 2013). The resultant experiments were saturated with water at the end of shortening and then sectioned to obtain their internal structural configurations. Each of the presented sandbox experiments was performed at least twice to affirm the reproducibility of their first-order features.

Table 1 Material properties and scaling parameters between model and nature. Quantity Density (ρ) Initial layer thickness (h) Gravity acceleration (g) Lithostatic pressure (σ) Cohesion (Co) Internal friction coefficient (μ) Velocity (v) Smoluchowsky number (Sm)d a b c d

Nature (n)

Model (m) 3

Scaling ratio (m/n) 3

2400 kg/m 15 km 9.81 m/s2 360 MPa 10–20 MPaa 0.6–0.85b

1400 kg/m 3 cm 9.81 m/s2 411 Pa 80 Pa ∼0.6

0.6 2 × 10−6 1 ∼1.1 × 10−6 1–3 × 10−6 ∼1

< 3 mm/yrc 1.1–1.59

2.4 mm/min ∼1.26

∼3 × 105 ∼1

Handin (1966). Byerlee (1978). Zhang et al. (2004). Ramberg (1981).

1966). In our sandbox experiments, the internal coefficient of friction and cohesion of used sand is ∼0.6 and ∼80 Pa, respectively (measured by a Hubbert-type shear test apparatus similar to Krantz (1991) and Lohrmann et al. (2003)). Gravity acceleration (g) is 9.81 m/s2 in both nature and sandbox. When we take three centimeters in our experiments to represent fifteen kilometers in nature, that is a length scale ratio of 2 × 10−6. The Smoluchowsky numbers of our sandbox and the nature could be calculated, giving values of ∼1.26 and 1.1–1.59, separately. They meet the requirement of dynamic similarity very well. The object of this study, the RFBR is the ramp fault between the ∼16-km-deep, basal detachment of the LMS and the shallower detachment within the adjacent Sichuan Basin, at a depth of 5 km (Fig. 1C and 4). Thus, based on above length scale ratio, it could be represented by a 2-cm-high artificial ramp in our sandbox experiments with the dynamic similarity guaranteed. Balanced restoration of the interpreted seismic profiles estimated the Cenozoic shortening in the southern LMS to be ∼30 km (Chen et al., 2005; Hubbard et al., 2010; Li et al., 2014b). This estimation indicates that 60 mm of shortening in our modeling is required to mimic the current state of the southern LMS. While the test experiment revealed that the next foreland fault would nucleate after ∼100 mm of shortening following the development of the analogue

4.3. Experimental results Four sandbox experiments are presented here to illustrate the effect of fault geometry on fault behaviors and associated wedge front deformation. Experiment M1 tested the case of a simple planar ramp with a 30° dip (Fig. 5A), analogous to sections of the RFBR out of the gap. Experiment M2 investigated a synclinal-bend ramp case, which consisted of a gentle deeper section with a 30° dip and a steeper shallow section with a 60° dip (Fig. 5B). This geometry is comparable to that of the RFBR beneath the gap. Both M1 and M2, taking no account of lateral influence, are quasi plane deformation experiments. In contrast, experiments M3 and M4 explored cases involving lateral changes in the ramp geometry that were designed to mimic the RFBR along the entire southern LMS (Fig. 5C). Experiments M1, M2, and M4 underwent 60 mm of shortening, which corresponded to the ∼30 km Cenozoic

Fig. 6. Sequential stages of experiments M1 and M2, separately. (A–C) Thrust motion over a 30°-dipping, planar ramp. (D–E) Thrust motion over the synclinal-bend ramp with a 30°-dipping lower section and 60°-dipping upper section. The applied shortening amounted to 6 cm in each experiment. The dashed red lines represent traces of major thrusts. B: back thrust; T: initial ramp thrust nucleated along and from the artificial ramps; D: detachment fault; F: shallow thrust in the foreland. R: ramp thrust. These faults are labeled in Arabic numbers according to their sequence. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 6

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Fig. 7. Displacement field of our experiments during the first 6 mm shortening (A) Experiment M1-planar ramp model. (B) Experiment M2-synclinal-bend ramp model. The displacement fields were analyzed by the PIV software developed by Vision Asia Pte. Ltd. Of China. And double spatial resolution of displacement field of experiment M1 was obtained in experiment M2.

Fig. 8. Incremental displacement fields and their associated shear strain localization during the evolution of experiment M1 and M2. PIV analysis was performed in each incremental shortening of 6 mm. (A–E) The first 30 mm shortening of experiment M1. (F–J) The first 30 mm shortening of experiment M2. The pink arrows mark the termination of localized shear zones along the artificial ramps. F: shear zones in the shallow foreland. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

deactivated them (Fig. 6B and C). A detachment and small shallow thrust F nucleated along the upper flat (Fig. 6C) as the rising sand block, bounded by T and B1, stepped over the upper bend. A flat-ramp-flat thrust system consisting of D1, F, and R1 then became established as a major active deformation pathway from the moving backstop to the shallow foreland.

shortening recorded in the southern LMS range front, while experiment M3 had more shortening, up to 90 mm. In all experiments, the initial deformation took place at the toe of the PW (Figs. 6–8), but the nucleated structures were quite different in each case, especially at depth. 4.3.1. Experiment M1 (Planar ramp model) In this experiment, active slip induced by the moving backwall was transported throughout the basal detachment fault D1, reaching the artificial ramp directly. Two conjugate thrust faults, R1 and B1, nucleated to accommodate the shortening near the rigid block (Fig. 6A). While ramp thrust R1 and the basal detachment D1 formed the major active displacement delivery system T, breaking through to the surface and emerging at the toe of the protowedge. Furthermore, R1 maintained its linear geometry even as it extended away from the artificial ramp. Three additional backthrusts formed sequentially near the lower bend of T as the shortening progressed. Continuous slip along T resulted in the progressive upward migration of these backthrusts and finally

4.3.2. Experiment M2 (Synclinal-bend ramp model) Compared with M1, experiment M2 developed a complex fault system after the onset of deformation. The major thrust T in this experiment consisted of the basal detachment D1, ramp fault R1 along the artificial ramp and ramp fault R2 through the shallow layer. Thrust T also propagated along the predefined multi-bend ramp, with backthrust B1 emerging from the basal tip of the artificial ramp (Fig. 6D). Another ramp fault, R3, nucleated beneath the sand wedge in the hanging wall of T. This R3 extended to the surface and merged with R2 as shortening progressed, forming a small splay fault system in the frontal part of the protowedge (Fig. 6E). After 60 mm of shortening, active deformation had propagated into the foreland, and shallow fault F nucleated along 7

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the upper flat. F also merged with R3, forming one of the wedge-front splay faults. Simultaneously, a lenticular duplex developed between the foreland rigid ramp and R3. This lenticular duplex underwent significant shearing, which induced thrust stacking and thickening (Fig. 6E and F). Backthrusts B2 and B3 nucleated near the base of R3, suggesting considerable slip along this fault.

On the other hand, active shear zones could easily spread to the surface in experiment M1, even when they extended beyond the artificial ramp (Figs. 8A–E and 9A–E). Although the localized shear strain was significantly reduced relative to that of the basal detachment when going up and propagating along the artificial ramp. Because significant strain had already been released by fault slip, secondary faulting and fracturing occurred before propagating into a steeper fault section. Similar features could also be identified in experiment M2. For example, the incremental shear zones along the deeper, gentle ramp sections terminated near the area where the artificial ramp geometry steepened (Fig. 8F and H). In addition, the pattern of fault activation was episodic where the fault plane became steeper. The active shearing could happen along the steeper segment in one incremental shortening (Fig. 8G), while during its next or former increment, the shearing failed to activate the same fault segment (Fig. 8F and H). This phenomenon was not unique; it also appeared after R3 developed in experiment M2. Active deformation along the basal detachment sometimes failed to enter the shear zone that had already formed along the artificial ramp (Fig. 8I and J). Therefore, it can be inferred that a certain threshold characterizing fault activity should exist, which determines whether active deformation could propagate into a steeper fault section. Only when this threshold was satisfied or exceeded could incremental shear strain enter the steepened fault section and the activity of this steeper fault section persist. If significant incremental strain was partitioned in other structures or fractures, the steep upper fault section may fail to be activated, as illustrated in experiment M2.

4.3.3. Deformation localization controlled by along-dip fault geometry The structural evolution of experiments M1 and M2 differed greatly due to the different ramp geometries (Fig. 6). The most striking difference was the development of a duplex structure adjacent to the synclinal-bend ramp in experiment M2 (Fig. 6D and F). To characterize the associated differences in deformation localization, we analyzed and compared the displacement and strain fields of experiments M1 and M2 in stages using the PIV software package (Vision Asia Technology Pte. Ltd., China). We found that both displacement fields of M1 and M2 displayed gradual changes during the first incremental shortening of 6 mm, with the displacement vectors rotating from a nearly horizontal orientation near the moving backstop to a parallel orientation with the underlying ramps toward the toe of the protowedge (Fig. 7). This change in displacement direction produced small-offset backthrusts (Fig. 6), because of the concentration of local differential stresses in response to active slip across the sharp bend of the underlying flat-ramp structure (Erickson et al., 2001; Rosas et al., 2017). Approaching the boundaries between the sand assemblage and the rigid ramp or base, the magnitude of these displacement vectors decreased quickly, and intense deformation was localized in active shear zones (Fig. 8A and F). As suggested by the distribution of localized shear strain, both the planar and synclinal-bend ramps were initially activated synchronously with active slip on the basal detachment (Fig. 8A and F). However, the linear artificial ramp in experiment M1 had most of its active slip confined within an ∼3 mm-thick shear zone along the boundary. Limited deformation occurred in the hanging wall, as indicated by a rather uniform displacement field (Fig. 8A). In contrast, experiment M2 displayed a much wider transition zone where the displacement vectors decreased toward the artificial ramp and rotated according to the dips of the adjacent artificial ramp, thus located significant but distributed shearing within (Fig. 7B and 8F). The progressive shortening in experiment M2 led to the transformation from diffuse to localized deformation near the protowedge front (Fig. 8F and G), with the occurrence of a shear zone R2 that gradually propagated downward to the basal detachment (Fig. 8G and H). At the same time, the shallow section of the artificial ramp was activated as well (Fig. 8G). However, after the evolving shear zone R2 merged with the basal detachment, the incremental shear that was localized on the artificial ramp diminished (Fig. 8H–J). The activation of the frontal part of the basal detachment beneath R3 also decreased significantly. The active deformation switched from the boundary between the rigid blocks and the sand to the interior of the sand wedge, mainly localized along shear zone R3 (Fig. 8J). The duplex zone interbedded between R3 and the artificial ramp was not completely deactivated. It still accommodated incremental shortening from the hinterland, though no preferred fault planes or zones dominated (Fig. 9F–J). Either steep-dipping shear zones (Fig. 9G) or gentle-dipping shear zones (Fig. 9I) could occur and be activated, producing a fracture network that was far more complex than macroscopic faults within the duplex zone (Fig. 6E and F). In contrast to the complex deformation localization presented in experiment M2, the shearing localization in experiment M1 was extremely simple. Throughout the evolution of this experiment, active slip was mainly concentrated on the linear artificial ramps and the basal detachment (Figs. 8A–E and 9A–E). The accompanying displacement field within the hanging wall of the rigid ramp was quasi-uniform in magnitude, with zones of a minor displacement gradient corresponding to macroscopic backthrusts (Figs. 8A–E and 9A–E).

4.3.4. Experiments M3 and M4 (planar ramp versus synclinal ramp model) In experiments M3 and M4, the effects of along-strike variation of the ramp geometry that was analogous to the RFBR extending along the entire southern LMS range front were investigated (Figs. 2 and 5). A geometrical bump was built in the center of the blind ramp (Fig. 5C). The experimental results revealed that the geometrical bump, which consisted of the steeper section of the synclinal-bend ramp, segmented the resultant structures as the hinterland wedge thrusted over it. Shortly after the experiment was initiated, motion over the synclinal-bend ramp induced rapid uplift and accumulated more hanging wall thickening in the center of the experiment than in the planar ramp on either side of it (Fig. 10A). The emerged foreland thrust T developed a recess zone in response to the deep geometric irregularity because more shortening was accommodated by hanging wall thickening and thus less by slip along the shallower T section. When shortening reached 48 mm, deformation propagated into the shallow foreland, with nucleation of segmented foreland faults (Fig. 10B). These foreland shallow faults possessed a curved map-view geometry that was similar to T. More foreland thrusts nucleated with progressive shortening, and a clear structural transition zone developed in response to the underlying geometrical bump. The faults on either side of the transition zone could extend into the center (Fig. 10C and D). Therefore, more intense deformation occurred in the transition zone, with approximately twice as many structures observed within this zone compared to either side. 4.3.5. Evolution and reorganization of active fault planes We detected the evolving surface displacement field of experiment M4 using PIV technology. As the most pronounced feature, a deficit zone of horizontal displacement occurred immediately above the synclinal-bend ramp (Fig. 11). This deficit zone initiated before T reached the surface, with the horizontal displacement contours curving backward (Fig. 11A). The backward retreat of contour lines became closed with increasing shortening (Fig. 11B), and more contour lines were involved until the experimental shortening increased to 12 mm (Fig. 11C). It indicated that the along-strike difference accumulated in these stages. Moreover, as suggested by experiments M1 and M2, active slip should occur along the basal detachment and artificial ramps during these initial shortening stages (Fig. 8A, B, F, and G). The 8

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Fig. 9. Incremental displacement fields and their associated shear strain localization during the evolution of experiment M1 and M2. PIV analysis was performed in each incremental shortening of 6 mm. (A–E) The second 30 mm shortening of experiment M1. (F–J) The second 30 mm shortening of experiment M2. The black and pink arrows mark the termination of localized shear zones. The hatched areas delineate some missed data, which form some fake boundary and shear zones. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

have become greatly reduced as it migrated away from the lower bend. Therefore, this backthrust cannot explain the gradual extinction of the deficit zone, which continued from 12 mm of shortening to 60 mm. In fact, another important change occurred during this stage. As indicated by experiment M2, a new shear zone labeled R3 developed and connected with emerging thrust R2 near the toe of the protowedge (Fig. 8G and H). R3 replaced the synclinal-bend rigid ramp, forming a new major active structure T at ∼12 mm of shortening (Fig. 8F–H). This T is of a simple linear geometry. Therefore, the decreased alongstrike variation in horizontal displacement was actually associated with active slip switching from the synclinal-bend ramp to the newly generated linear ramp. It was a structural reorganization of the active

observed surface deficit zone was therefore a direct reflection of the differential kinematics induced by the underlying synclinal bend versus the linear ramps. When backthrust B1 nucleated in the hinterland boundary of the deficit zone, the percentage of along-strike deficit and its magnitude began to decrease, with observed reductions of ∼20% to ∼10% and 1.6–1.3 mm, respectively (Fig. 11C and D). The deficit zone became stretched along the strike as well, which also reflected a reduced alongstrike kinematical variation (Fig. 11B–F). The weakening of the deficit zone was accompanied by the nucleation of B1. It seemed that the activation of this backthrust had reduced the along-strike variations of horizontal displacement. However, the backthrust activation should

Fig. 10. Sequential stages of experiment M3 involving along-strike geometric variations. Thrust motion occurred over the blind ramp analogue to the RFBR in the LMS range front. This experiment emphasizes on foreland response to the underlying geometric bump on RFBR with a total shortening of 90 mm. The black arrows show the emergence of shallow thrusts in the foreland. Recess of these shallow faults formed a structural transition zone in the front of geometric bump.

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Fig. 11. Horizontal displacement fields of experiment M4 which had the same initial boundary condition as experiment M3. The total applied shortening was 60 mm. (A–C) Formation of a deficit zone above the geometric bump. In these stages, thrust motion mainly occurs on the artificial ramps, which induced the kinematical difference on the surface. (D–E) Dissipation of the deficit zone, after a simple and smooth slip plane forms underneath, for details see text. Surface view photos of M4 are inserted as well. Isolines of horizontal displacement are indicated. The traces of underlying artificial ramps are represented as pink and dashed line in each Figure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5. Discussion

thrust system to a simple fault plane (Ben-Zion and Sammis, 2003). As revealed by the inner cross-sections of experiment M4, the final, large-displacement thrust T in the center was parallel to the 30°-dipping rigid ramp (Fig. 12A and B), having similar geometry to the sections on either side (Fig. 12C and D). While along the strike, the tapering edge of a duplex formed a gradual geometric transition of the fault plane (Fig. 12E and F). Therefore, the emplacement of the hanging wall during this stage took place on a rather uniformly dipping thrust plane that consisted of the roof thrust of the presented duplex and the planar artificial ramps on either side. This simple fault plane had quasi-uniform kinematics along the strike. The accumulated slip on this fault would thus reduce the overall percentage of the along-strike deficit that had developed in the initial shortening stages.

5.1. Simplification and limitation We added a critically tapered protowedge against the moving backstop to mimic the initial state of the LMS orogenic wedge when the RFBR nucleated. Assuming that the used materials were cohesionless, we determined the critical taper angle (∼13°) of our materials through a test experiment, which also yielded the detailed dimensions of the protowedge (∼20 cm long and ∼3.6 cm high) (Fig. A2). However, this protowedge does not represent an exact, realistic topographic relief. The reasons why we added this “larger” (relative to the real LMS orogenic wedge) protowedge are as follows: (1) Stronger Precambrian metamorphic and igneous rocks are present in the LMS hinterland, west of the Yingxiu–Beichuan fault (e.g., Burchfiel et al., 1995; Chen and

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Fig. 12. Cross sections of experiment M4 showing the 3D geometry of buried duplex structure associated with the geometric bump. (A and C) Line draws of typical cross sections develop over the synclinal-bend and planar ramp, separately. They are parallel with the shortening direction. (B and D) Photos of the typical cross sections with the major slip plane T indicated by dotted yellow lines. (E–F) Oblique view of the duplex structure, marked by red dash lines, and its spatial relationship with the geometric bump on the blind ramp. The yellow part of ramp is of synclinal-bend shape. The yellow dotted lines represent the relatively smooth fault plane which formed and dominated in the late stages of evolution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

dip angles of the underlying ramps, mainly in the form of conjugated fore- and back-thrusts. Additionally, the number of backthrusts and their dips also varies with ramp geometry. In our low-dipping (30°) ramp experiment M1, four small-displacement backthrusts developed near the lower bend of the rigid ramp and became inactive when moving upwardly along the ramp. This deformation mode was identical to that of previous modeling works. Our sandbox experiments drove the deformable hanging wall toward a fixed footwall, which is opposite to the indention of a rigid footwall block into the hanging wall like some previous modeling (e.g., Bonini et al., 1999; Koyi and Maillot, 2007). Although such differences in boundary conditions induce no significant differences on deformation associated with gently dipping ramps. Total distinct hanging wall deformation styles emerge when steep (≥60°) dipping ramp sections are present. In the case of a moving footwall, a forethrust ahead of the steep-dipping moving footwall block would occur and form an effective indenter that forces subsequent deformation over the forethrust (Fig. 5 in Bonini et al., 1999). The area between this forethrust and the artificial ramp would suffer little deformation. In contrast, as in our experiment M2, the rigid ramp had a nonlinear fault geometry, and its upper section was too steep to allow stable slip. Thus, a hanging wall forethrust was also required. However, the zone between the major forethrust and preexisting steep ramp accommodated a large amount of experimental shortening by the development of duplex structure (Fig. 6D–F). The phenomena mentioned above, emphasize that boundary conditions should be properly selected according to prototype in nature. Because they do impact modeling results in certain cases. And to our knowledge, this is still a question less addressed before.

Wilson, 1996). The thrust wedges, which consist of stronger rocks, generally have smaller critical taper angles (Davis et al., 1983). Therefore, when we employ a homogeneous sand assemblage, which is a good substitution for sedimentary rocks, the hinterland sand wedge must have a higher taper angle to achieve the same stable state as that of the LMS orogenic wedge. (2) Our modeling was performed with a horizontal basement, which ignored the ∼1.5° (Hubbard et al., 2010) or 7° (Wang et al., 2011) dip of the basal detachment beneath the LMS. The critical taper theory defines the taper angle of a thrust wedge as the sum of its slope angle and the dip angle of the basal detachment (Davis et al., 1983). Therefore, the protowedge in our models should also compensate for the blind relief of the basal detachment underneath the LMS orogenic wedge, rather than simply compensating for the topographic relief. A “larger” protowedge was therefore required here as well. Noteworthy, the topographic relief and hinterland uplift cannot represent the real case any more due to this simplification of initial boundary condition. It should also be noted that no old faults or variations in the rock properties were taken into account. However, our focus is on range front deformation near the RFBR. The old faults and stronger massif rocks are mainly limited to the west of the Yingxiu-Beichuan fault (Mag in Fig. 13) and therefore have limited impact on fault behaviors of the RFBR. Similarly, the foreland Triassic evaporite detachment and the sedimentation in the western Sichuan Basin have also been ignored in our modeling experiments. 5.2. Comparison with previous modeling works of fault geometry Several previous modeling studies have systematically investigated thrust motion over linear ramp faults of different dip angles (Bonini et al., 1999, 2000; Maillot and Koyi, 2006; Koyi and Maillot, 2007). Their results suggest that hanging wall thickening increases with the 11

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Fig. 13. Comparison between surface structures of experiment M3 and its prototype. (A) Geological map of the gap between the Wenchuan and Lushan earthquakes in the southern LMS (location shown in blue polygon in the inset Figure). The white dotted rectangle indicates the rough scope of synclinal-bend RFBR, the same as in Fig. 13B. (B) Landsat image of the gap showing geomorphic features. (C) Modeling structural features appear in the surface view, at shortening of 50 mm. The red dotted lines highlighted by yellow show the surface projection of RFBR. The red solid lines represent shallow foreland structures. Focal mechanisms of the Wenchuan, Dayi and Lushan earthquakes are shown as well. WMF: Wenchuan-Maoxian fault; WLF: Wulong fault; BXF: Baoxing fault; YBF: Yingxiu-Beichuan fault; PGF: Pengguan fault; QXF: Qiongxi fault; DYF: Dayi fault; DYA: Dayi anticline; RFBR: Range front blind ramp; Mag: Mesozoic magmatites; (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

too large to compare to the Dayi anticline. This mismatch between the size of the Dayi and Qiongxi anticlines may result from E–W compression following the late Cenozoic stress field changes, which favors the growth of N-S striking structures (Li et al., 2014b, 2018). However, the geomorphology of the Qiongxi anticline is comparable to that of the Dayi anticline (Fig. 13B) and consistent with the modeled recess zone in the sandbox (Fig. 13C). The similarity between the surface structures in our experiments and those presented in the southern LMS range front therefore suggests that the gap between the ruptures of the Wenchuan and Lushan earthquakes is a structural transition zone that developed in response to the along-strike geometric variations of the underlying RFBR.

5.3. Comparison with the southern LMS range front The investigated ramps in experiments M3 and M4 were designed according to the 3D configuration of the RFBR in the southern LMS range front. A synclinal-bend ramp section, analogous to the section of RFBR beneath the rupture gap between the Wenchuan and Lushan earthquakes, was built in the center. Such a geometrical irregularity segmented the range front deformation with the development of a structural transition zone that was characterized by pronounced hanging wall thickening and a foreland recess zone with high-density structures (Fig. 10). These observed characteristics in our experiments are also traceable in the southern LMS range front. In the rupture gap between the Wenchuan and Lushan earthquakes, an ∼40-km-long outcropping belt of Triassic strata in the core of the Mingshan anticline develops (Fig. 13A). Toward either side of the gap, the RFBR switches from a synclinal bend to a planar ramp, where the oldest exposed strata is Jurassic in age. This change occurs over a few tens of kilometers, which excludes differential erosion as the dominant effect. Such an alongstrike contrast in exposed rocks therefore reflects additional structural thickening caused by the synclinal-bend ramp beneath the gap, similar to that presented in our experiments (Fig. 13). In addition, a recess structure also appears in the corresponding foreland of the gap, which consists of the Dayi and Qiongxi anticlines. These two emergent active structures sole into the Triassic detachment at a depth of 4–6 km (Wang et al., 2013a, 2013b; Li et al., 2014b); both are approximately parallel to the inboard RFBR (Fig. 13A and B). The Qiongxi anticline is interpreted as a north-trending, ∼50-km-long dextral en-echelon fault with a plunging southern segment (Wang et al., 2013a, 2013b), which seems

5.4. Implications for nature of the gap between the Wenchuan and Lushan earthquakes The gap between the Wenchuan and Lushan earthquakes is characterized by low seismic velocities (Vp, Vs) and a high Poisson’s ratio (e.g., Pei et al., 2014; Wang et al., 2015b). Accordingly, it was proposed that there is a weak, ductile crustal body beneath the gap, and therefore this gap cannot accumulate enough stress to generate large earthquakes (M > 7). If the ductile body exists and reflects fluids or partial melt products from the mid-lower crust of Tibet (Wang et al., 2014b, 2015), it should be a long-term feature because significant time is needed to complete the emplacement of these materials to the LMS range front. Thus, certain permanent deformation should have occurred and been recorded there, waiting for further exploration. While the other possibility is that only fluids migrate and now concentrate beneath the gap. If so, why these fluids gather in this zone 12

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beneath the gap of Wenchuan and Lushan earthquakes, could develop due to the presented fault geometry irregularities (Fig. 14). 5.5. Insights for fault activity in the southern LMS range front The occurrence of the 1970 Ms 6.2 Dayi earthquake (Gao et al., 2014; Li et al., 2017, Fig. 1B) and the paleo-seismic investigations that identified rupture events right in the seismic gap (Wang et al., 2015a; Dong et al., 2017; Shao et al., 2018) suggest that brittle deformation should be dominant in the southern LMS range front. However, the 3D geometric model of the RFBR revealed a geometrical bump with a relief of up to ∼3.5 km on the major fault plane (Figs. 2 and 3). This means that the behaviors of the seismogenic fault here are strongly impacted by this structure. However, as illustrated by experiments M3 and M4, slip on the complex fault plane of the RFBR would not persist all the time (Fig. 11). Instead, the complex ramp would evolve toward a simple and smooth fault plane (Fig. 6). The sharp bends or steep-dipping ramps could be smoothed via either hanging wall accretion of fault slices as observed in our experiments (Fig. 6) or via peeling by footwall faulting. Obviously, the former should dominate currently in the LMS range front since minimal deformation has been identified in the footwall of the RFBR from the seismic profiles (Fig. 1C and 4). This process, called structural reorganization, would enhance fault continuity and reduced the difficulty of through-going rupture (Ben-Zion and Sammis, 2003). The secondary faults accompanied by the geometrical bump on the RFBR are also important factors determining the regional fault behaviors. These secondary faults could act as fragmentation barriers, distributing the stress by fracturing and stopping rupture propagation (King and Nábělek, 1985; King, 1986). However, motion on the small faults or fractures of distinct orientations was also proposed to be capable of locking the fractured zone and then initiating future earthquakes by the coalescence of smaller faults into a larger one there (King, 1986). The NE-propagating rupture of the 2013 Mw 6.6 Lushan earthquake terminated to the south extent of the gap, possibly because the fractured zone underneath the gap had behaved as a fragmentation barrier. The 1970 Ms 6.2 Dayi earthquake occurred immediately beneath the gap, perhaps reflecting the formation or activation of a related large fault plane within the fractured duplex structure. Therefore, we cannot exclude the possibility that the rupture events along the whole southern LMS range front may happen as proposed by Li et al. (2017).

Fig. 14. Sketch showing 3D structures of the gap between the Wenchuan and Lushan earthquakes, according to seismic profile Fig. 4A, field data and sandbox experiments. The geometrical bump (GB) on RFBR has segmented range-front deformation of the south Longmen Shan and produces a recess zone which is consist of DYF and QXF. It also contributes to the occurrence of a fractured duplex zone which corresponds with the observed low-seismic-velocity body underneath the gap (e.g., Pei et al., 2014; Wang et al., 2015b). QXF: Qiongxi fault; DYF: Dayi fault; DYA: Dayi anticline; RFBR: Range front blind ramp;

remains poorly understood. Based on our sandbox experiments and a comparison with the structures in the range front of the LMS, we have confirmed that the gap between the Wenchuan and Lushan earthquakes is a structural transition zone caused by the geometrical variation of underlying fault. And therefore, the highly fractured zone associated with the investigated fault geometrical irregularity in our sandbox should also occurred where geometrical bump of the RFBR presents. Consequently, we speculate that the observed geophysical anomaly represents a region with a higher degree of fracture than its surrounding region (Fig. 14). The blind, highly fractured duplex provides a suitable “reservoir” holding high pore pressure in this area, if any. Two lines of evidence supporting this proposition can be summarized as follows: (1) Brittle rocks respond to large amounts of strain by fracturing (Scholz, 2002). We therefore, analyzed the shear strain fields of our experiments, which could constrain the distribution of structural fractures caused by an analogue fault of the RFBR. The analysis results revealed that the duplex structure nucleating adjacent to the synclinal-bend blind ramp suffered diffuse shearing throughout the entire duplex, rather than localized shearing as a thin shear zone in the case of a planar ramp (Fig. 9). So the along-strike extension of the fractured zone underneath the gap of the Wenchuan and Lushan earthquakes depends on the size of the geometrical bump on the RFBR (Fig. 12C), and is therefore comparable to the detected geophysical anomaly (Zhan et al., 2013; Pei et al., 2014; Wang et al., 2015b). (2) Structural fractures are capable of creating significant low-velocity zones (Zhao and Kanamori, 2013; Zhan et al., 2013; Liu et al., 2015). It is well known that accumulated movement over crustal faults usually produce damage zones consisting of complex fault cores and surrounding fracture belts (e.g., Chester et al., 1993; BenZion and Sammis, 2003). Seismic observations have revealed that these damage zones could induce a 40%-50% seismic velocity reduction related to the wall rock (e.g., Cochran et al., 2009; Yang et al., 2015). Additionally, a single fault damage zone may be up to several kilometers wide, such as the Calico Fault in eastern California (Cochran et al., 2009) and the L’Aquila Fault in the central Apennines (Valoroso et al., 2014). While Bistacchi et al. (2010) revealed that the damage zone nucleated around a contractional jog was five times larger than that of a planar fault. All these cases support the idea that low-velocity, fractured zones, like that

6. Conclusions The southern segment of LMS orogenic wedge has been thrusted over the adjacent Sichuan Basin through the RFBR (e.g., Jia et al., 2006, 2010; Hubbard et al., 2010). To enhance our understanding about the role of this key fault, we have constructed its 3D morphologic mode on basis of high-resolution seismic profiles, and preformed sandbox modeling accordingly. Our main conclusions including: (1) Based on high-resolution seismic profiles, the 3D configuration of the RFBR which generated the 1960 Dayi earthquake and the 2013 Lushan earthquake, was established. It reveals a geometrical bump standing on the main plane of the RFBR, right beneath the seismic gap between the 2008 Wenchuan and 2013 Lushan earthquakes. (2) According to the 3D morphologic mode of RFBR, we have designed and performed sandbox experiments. The experimental results suggest that the geometrical bump on the RFBR has resulted in the development of a duplex structure in the hanging wall. Strain analysis through PIV technology finds that this duplex structure has accumulated shearing continually since its initiation, thus representing a highly fractured zone which may account for the observed geophysical anomaly beneath the seismic gap. (3) Our sandbox experiments also display comparable surface 13

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deformation with that of the southern LMS range front, suggesting that geometrical irregularities of the RFBR contribute greatly to the observed structural segmentation. While the seismic gap corresponds to the structural transition zone resulted from the geometrical bump on the RFBR.

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Acknowledgments This work was jointly supported by the National Natural Science Foundation of China (41772209 and 41590861), Guangdong Province Introduced Innovative R&D Team of Geological Processes and Natural Disasters around the South China Sea (2016ZT06N331), and the China Postdoctoral Science Foundation (2018M633211); We are very grateful to Dr. Li Yiquan, Wang Maomao, Yan Bin and Jiao Liqing for their valuable discussions. We also thank the editors and reviewers for their constructive reviews which have improved the manuscript greatly. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.103911. References Adam, J., Urai, J.L., Wieneke, B., Oncken, O., Pfeiffer, K., Kukowski, N., et al., 2005. Shear localisation and strain distribution during tectonic faulting—New insights from granular-flow experiments and high-resolution optical image correlation techniques. J. Struct. Geol. 27 (2), 283–301. https://doi.org/10.1016/j.jsg.2004.08.008. Adam, J., Klinkmüller, M., Schreurs, G., Wieneke, B., 2013. Quantitative 3D strain analysis in analogue experiments simulating tectonic deformation: integration of X-ray computed tomography and digital volume correlation techniques. J. Struct. Geol. 55, 127–149. https://doi.org/10.1016/j.jsg.2013.07.011. Aki, K., 1979. Characterization of barriers on an earthquake fault. J. Geophys. Res. Solid Earth 84 (B11), 6140–6148. https://doi.org/10.1029/JB084iB11p06140. Ben-Zion, Y., Sammis, C.G., 2003. Characterization of fault zones. Pure Appl. Geophys. 160 (3–4), 677–715. https://doi.org/10.1007/PL00012554. Bistacchi, A., Massironi, M., Menegon, L., 2010. 3D characterization of a crustal-scale fault zone: the Pusteria and Sprechenstein fault system (Eastern Alps). J. Struct. Geol. 32 (12), 2022–2041. https://doi.org/10.1016/j.jsg.2010.06.003. Bonini, M., Sokoutis, D., Talbot, C.J., Boccaletti, M., Milnes, A.G., 1999. Indenter growth in analogue models of Alpine-type deformation. Tectonics 18 (1), 119–128. https:// doi.org/10.1029/1998TC900008. Bonini, M., Sokoutis, D., Mulugeta, G., Katrivanos, E., 2000. Modelling hanging wall accommodation above rigid thrust ramps. J. Struct. Geol. 22 (8), 1165–1179. https:// doi.org/10.1016/S0191-8141(00)00033-X. Burchfiel, B.C., Zhiliang, C., Yupinc, L., Royden, L.H., 1995. Tectonics of the Longmen Shan and adjacent regions, central China. Int. Geol. Rev. 37 (8), 661–735. https:// doi.org/10.1080/00206819509465424. Byerlee, J., 1978. Friction of rocks. Pure Appl. Geophys. 116, 615–626. https://doi.org/ 10.1007/BF00876528. Caër, T., Souloumiac, P., Maillot, B., Leturmy, P., Nussbaum, C., 2018. Propagation of a fold-and-thrust belt over a basement graben. J. Struct. Geol. 115, 121–131. Chen, S.F., Wilson, C.J., 1996. Emplacement of the Longmen Shan Thrust—Nappe Belt along the eastern margin of the Tibetan Plateau. J. Struct. Geol. 18 (4), 413–430. https://doi.org/10.1016/0191-8141(95)00096-V. Chen, Y.T., Yang, Z.X., Zhang, Y., Liu, C., 2013. From 2008 Wenchuan earthquake to 2013 Lushan earthquake. Scientia Sinica Terrae 43, 1064–1072. https://doi.org/10. 1360/zd-2013-43-6-1064. Chen, Z.X., Jia, D., Zhang, Q., Wei, G.Q., Li, B.L., Wei, D.T., Shen, Y., 2005. Balanced cross-section analysis of the fold-thrust belt of the Longmen Mountains. Acta Geol. Sin. 79 (1), 38–45. Chester, F.M., Evans, J.P., Biegel, R.L., 1993. Internal structure and weakening mechanisms of the San Andreas fault. J. Geophys. Res. Solid Earth 98 (B1), 771–786. https://doi.org/10.1029/92JB01866. Cochran, E.S., Li, Y.G., Shearer, P.M., Barbot, S., Fialko, Y., Vidale, J.E., 2009. Seismic and geodetic evidence for extensive, long-lived fault damage zones. Geology 37 (4), 315–318. https://doi.org/10.1130/G25306A.1. Colletta, B., Letouzey, J., Pinedo, R., Ballard, J.F., Balé, P., 1991. Computerized X-ray tomography analysis of sandbox models: examples of thin-skinned thrust systems. Geology 19 (11), 1063–1067. https://doi.org/10.1130/0091-7613(1991) 019<1063:CXRTAO>2.3.CO;2. Dahlen, F.A., 1990. Critical taper model of fold-and-thrust belts and accretionary wedges. Ann. Rev. Earth Planet. Sci. 18 (1), 55–99. https://doi.org/10.1146/annurev.ea.18. 050190.000415. Davis, D., Suppe, J., Dahlen, F.A., 1983. Mechanics of fold-and-thrust belts and accretionary wedges. J. Geophys. Res. Solid Earth 88 (B2), 1153–1172. https://doi.org/10. 1029/JB088iB02p01153. DeDontney, N., Hubbard, J., 2012. Applying wedge theory to dynamic rupture modeling

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