Mechanics of low-relief detachment folding in the Bajiaochang field, Sichuan Basin, China

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1 GEOHORIZONS Mechanics of low-relief detachment folding in the Bajiaochang field, Sichuan Basin, China Andreas Plesch, John H. Shaw, and David Kronman ABSTRACT Using three-dimensional (3-D) seismic reflection data, we apply new methods of two-dimensional (2-D) and 3-D structural restoration based on mechanical constraints to gain insights into the development of the Bajiaochang anticline, Sichuan Basin, China. This structure forms the trap for the Bajiaochang field, which is reported to contain substantial gas reserves within thick, Upper Triassic deltafront and lakeshore facies siliciclastics. The basal detachment lies in a Middle Triassic evaporite unit, above which layers are folded and faulted to various intensities based on their mechanical strengths. The new restoration methods enable us to test a variety of faultdisplacement directions and to resolve that the Bajiaochang structure grew primarily by dip slip and contractional folding. Moreover, we quantify the styles and amounts of shortening within various tectonic levels of the structure. Large amounts of shortening and ductile thickening at the basal detachment level are consistent with general detachment fold models. However, we demonstrate that slip on the basal detachment is much larger than would be predicted by these models, implying that substantial slip on the basal detachment extends beyond the structure into the foreland. Moreover, structural thickening in the core of the fold is localized by a series of small ramps in the basal detachment that produce structural imbrication. This implies that the Bajiaochang detachment fold grew by a mechanism akin to duplexes, which has important implications for the manner in which shortening and strain are partitioned within different mechanical layers of the fold. Copyright #2007. The American Association of Petroleum Geologists. All rights reserved. Manuscript received June 21, 2006; provisional acceptance September 12, 2006; revised manuscript received May 19, 2007; final acceptance June 20, DOI: / AUTHORS Andreas Plesch Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138; andreas_plesch@harvard.edu Andreas Plesch is a research associate in the Structural Geology and Earth Resources Group in the Earth and Planetary Science Department at Harvard University. He received an M.S. degree in geology from State University of New York, Albany (1994), and his Ph.D. from the Free University Berlin, Germany, (1999). His research interests involve three-dimensional modeling and analysis of contractional structures on the reservoir to mountain belt scale using a variety of methods with a focus on quantitative aspects and interpretation of seismic reflection data. He has been able to work on structures worldwide, commonly in cooperation with industry partners. John H. Shaw Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts John H. Shaw is the Harry C. Dudley Professor of Structural and Economic Geology at Harvard University and leads an active research program in structural geology and geophysics, with emphasis on petroleum exploration and production methods. He received a Ph.D. from Princeton University in structural geology and applied geophysics and was employed as a senior research geoscientist at Texaco s Exploration and Production Technology Department in Houston, Texas. Shaw s research interests include complex trap and reservoir characterization in fold and thrust belts and deep-water passive margins. He heads the Structural Geology and Earth Resources Program at Harvard, an industry-academic partnership that supports student research in petroleum systems. David Kronman Burlington Resources, ConocoPhillips, 600 North Dairy Ashford, Houston, Texas David Kronman is a principal geologist with ConocoPhillips Global New Ventures Exploration in Houston, Texas. He earned his B.S. degree in geology from the State University of New York at Cortland (1980) and his M.S. degree in geology from Vanderbilt University (1982). AAPG Bulletin, v. 91, no. 11 (November 2007), pp

2 David has more than 25 years domestic and international experience with Shell Oil Company, EOG Resources, and Burlington Resources (prior to its merger with ConocoPhillips). For the past 17 years, his primary focus has been on integrated exploitation analysis, synthesis of basins in compressional regimes, unconventional resources, and stratigraphic modeling. ACKNOWLEDGEMENTS The authors thank the management of Burlington Resources, Inc., and PetroChina for permission to publish this article. Many thanks also go to Warren S. Duncan (Burlington Resources geophysicist) for preparing seismic interpretations used in the study. For seismic interpretation, we used software provided by Landmark Graphics Corporation through its University Grant Program. The software tool used for model building and interpretation was Gocad TM, developed by Earth Decision Sciences and the Gocad Research Consortium. P. Muron was instrumental in implementing the restoration plug-in for this tool. Reviews by Sandro Serra, Catherine L. Hanks, and R. H. Groshong are gratefully acknowledged. INTRODUCTION The Sichuan Basin lies at the eastern margin of the Tibetan Plateau in central China and is surrounded on all sides by fold and thrust belts. These mountain belts include the Longmen Shan to the northwest and the Micang Shan, which is part of the Qinling Shan, to the northeast (Figure 1) (Burchfiel et al., 1995; Meng et al., 2005). Contractional deformation and shortening are most intense at these range fronts. However, deformation extends far into the basin along a series of detachments or decollements. Most of the gas and oil reserves in the basin occur within contractional folds that formed above these detachments. The purpose of this study is to investigate the mechanism of deformation of one such detachment fold, which forms the structural trap for the Bajiaochang field. The Bajiaochang structure was originally identified by seismic studies in 1953, and the first exploration well was spudded in 1974 by PetroChina Southwest Oil & Gasfield Company. In 2002, Burlington Resources acquired 100% working interest in the Chuanzhong Block Production Sharing Contract and drilled several wells to further appraise economic potential of the structure. Burlington submitted an Overall Development Program for the development of the Upper Triassic Xiangxi Formation gas reserves, which was approved by PetroChina in February To date, 30 exploration and development wells have been drilled in the field through the Triassic section. The Bajiaochang anticline is located in the northwest corner of the basin, about 100 km (62 mi) from the Longmen Shan range front and 150 km (93 mi) from the Micang Shan foothills. Based on its location, it seems reasonable to consider that the Bajiaochang anticline formed as a basinward extension of deformation that formed the Longmen Shan. However, the fold trends east-west at an angle of about 45j relative to the Longmen Shan range front (Figure 1). Thus, if Bajiaochang is associated with the deformation of the Longmen Shan, it likely manifests a large component of oblique strain and slip. Alternatively, the fold may represent a southern extension of deformation associated with the formation of the Micang Shan and Qinling Shan (Figure 1). In this case, the structure would likely have formed primarily by north-south shortening perpendicular to its fold axis. Our structural investigation of the Bajiaochang anticline focused on two objectives. First, to resolve this ambiguity about the affinity of the anticline, we apply new horizon-based (two-dimensional [2-D]) and volumetric (three-dimensional [3-D]) structural restoration approaches to quantify the directions of shortening and fault slip in this structure. Second, we employ the 3-D mechanical restoration approach and balancing concepts to gain insights into the mechanism of detachment fold growth. Specifically, we address how strain and styles of secondary faulting and folding vary between the ductile interval that contains the basal detachment and the overlying, gently folded strata that include the reservoir sections. The restoration methods we apply are based on area or volume conservation and global strain minimization criteria, with the 3-D 1560 Geohorizons

3 Figure 1. Location map for the central Sichuan Basin and for the study area (rectangle outlined in red) identifying main tectonic units (FTB = fold and thrust belt). The white line within the study area locates approximately the axis of the Bajiaochang anticline. National Aeronautics and Space Administration Blue Marble (Modis sensor) remote sensing data. technique implemented using finite-element methods (FEM). Therefore, unlike most current restoration techniques (i.e., inclined shear, flexural slip), these methods do not require the investigator to specify fold kinematics. Thus, we consider these approaches to be a valid and objective means of defining fold and fault mechanisms and kinematics and apply them here to investigate these processes in the Bajiaochang anticline. THE BAJIAOCHANG STRUCTURE The Bajiaochang structure is imaged by a high-quality, wide-azimuth 3-D seismic survey acquired in the spring of The survey has a full fold coverage area of approximately 300 km2 (115 mi2). Data were acquired down to 5 s two-way traveltime (TWT), corresponding to approximately 10 km (6 mi) depth and provides excellent resolution of the upper Paleozoic and Mesozoic sections. The Bajiaochang structure is a gentle, doubly plunging fold with a broad, flat crest separating fold limbs (Figures 2, 3). In the Triassic section, the limbs dip to the north at about 7j and to the south at about 4j. At this level, the northern limb of the structure is cut by a series of low-angle faults that exhibit a maximum of m ( ft) of upward-diminishing thrust separation. At Jurassic levels, the fold assumes a more symmetric shape with limb dips of about 4j. The fold involves Lower Triassic through Upper Jurassic units, with the Tertiary section being mostly Plesch et al. 1561

4 Figure 2. Seismic depth section without (A) and with (B) interpretation defining stratigraphy and the main tectonic layers. The purple horizon in layer 4 corresponds approximately to the Xiangxi 4 horizon. Red lines are faults. Not all normal faults in the Jurassic sections are interpreted. Vertical exaggeration is 2:1. sd = seismic datum Geohorizons

5 Figure 3. Contour map of the Xiangxi 4 horizon. Depth is in meters below sea level, whereas depth in Figures 2 and 8 is below the seismic datum. AA 0 is the location of section shown in Figure 2; BB 0 is the location of section shown in Figure 8. Coordinates were omitted at the request of the data provider. absent in the region. The Jurassic section is composed primarily of fluvial and lacustrine deposits. In the reflection data, the Middle and Upper Jurassic sequences are characterized by laterally discontinuous reflections with moderate- to low-impedance contrasts (Figure 2). In contrast, the Lower Jurassic section, which includes the Daanzhai and Maanshan formations, forms a strong, laterally continuous set of reflections that are broadly folded in the Bajiaochang anticline (Figure 2). The underlying Upper Triassic sequence comprises the Xiangxi Formation, which contains a series of continental floodplain, fan-delta, and lacustrine deposits that include the field s main gas reservoirs. The underlying Middle and Lower Triassic Leikoupo and Jianlingilang formations are shallow-marine sequences that are both known regionally to contain evaporites (Burchfiel et al., 1995). The lower parts of the Leikoupo and the Jialingilang Formation are more tightly folded and faulted than the overlying sections, whereas the lower Jialingilang appears to not be involved in the broad anticlinal folding. The fold clearly dies out into a basal detachment at about 4500 m (14,763 ft) depth (2.0 s TWT) near the base of the Leikoupo Formation in the upper Jialingilang Formation. The flat-lying detachment at the base of the fold and the overall symmetric shape classify the structure as a detachment fold (Jamison, 1987; Dahlstrom, 1990; Scanlin and Engelder, 2003). In a regional context, this fold trend may relate to folding in the interior basin with similar trends as shown but not investigated in detail by Burchfiel et al. (1995). The closest regional cross section by those authors is across the external Longmen Shan, subparallel to the fold axis and terminating about 30 km (18 mi) to the north of the Bajiaochang field. It suggests a detachment at about the same depth level as exists here, but does not continue the detachment into the basin. Given the high quality of the seismic data and the low relief of the structure, Bajiaochang offers a rather unique opportunity to study how styles and magnitudes of deformation vary within a detachment anticline. Based on structural character, this detachment fold can be divided into five major tectonic layers (Figure 2). Layer 1 represents flat-lying Paleozoic and perhaps lowermost Triassic rocks that lie below the basal detachment. This layer is unaffected by the broad anticlinal folding and is intersected by high-angle faults with minor normal offsets. Layer 2 is a narrow, highly deformed zone at and above the basal detachment that is about m ( ft) thick. This layer is structurally thickened beneath the anticline and is thought to represent a Triassic evaporite sequence. The overlying section of the Leikoupo Formation defines tectonic layer 3, which is characterized by numerous short-wavelength contractional folds and faults. Layer 4 consists of the Plesch et al. 1563

6 Upper Triassic Xiangxi to Lower Jurassic Daanzhai sections, which are broadly folded in the Bajiaochang anticline. This layer is locally cut by a family of lowangle faults with small thrust separations. Layer 5 is composed of the thick Middle and Upper Jurassic sequences, which are folded concordantly with layer 4. However, the faults with thrust separation apparent in layer 4 generally do not extend upward into layer 5. Instead, layer 5 is cut by a series of normal faults that dominantly trend north to northeast across the fold. In the sections below, we investigate how the growth of the Bajiaochang structure was governed by this mechanical stratigraphy. Based on a 3-D structural model developed from interpretations of the 3-D seismic survey, we apply various methods of structural analysis to quantify the magnitudes and styles of deformation in each tectonic layer. Our 3-D restorations, in particular, consider how the mechanical properties of the reservoir intervals influence the styles of deformation manifest within it. RESTORATION METHODS Since the advent of structural balancing concepts in the 1960s, restorations of folded and faulted structures have become a major research theme in petroleum geoscience, both in academics and industry. Structural restorations serve many purposes, helping to validate structural interpretations, define fault and fold kinematics, and assess timing and magnitudes of deformations. Moreover, structural restorations are increasingly used as a means to estimate strain distributions within structures that may help predict patterns of natural fractures or other small-scale deformations. These strain estimates, in turn, are commonly used to infer the distributions of porosity and permeability in hydrocarbon reservoirs. Applications of the restoration method therefore directly benefit the petroleum geoscientist on a variety of levels, from better understanding trap formation and reservoir evolution to guidance in well and production planning. This study focused on achieving these benefits by deciphering the reservoir-scale mechanics of low-relief detachment folding in the Bajiaochang field with the help of new structural restoration methods. Whereas the many kinematic restoration approaches (e.g., Dahlstrom, 1969; Suppe, 1983; Suppe and Medwedeff, 1990; Allmendinger, 1998) employed today (e.g., Dahlstrom, 1970; Woodward et al., 1985; Novoa et al., 2000) have proven very successful in many aspects of structural analysis, they generally fail to address inherently 3-D patterns of deformation (i.e., strike-slip or oblique displacements) and situations where major contrasts in the mechanical strength of rock units impart a prominent influence on styles of folding and faulting. The contrasts in deformation styles from tightly folded and thrusted to broadly extended within the tectonic layers of the Bajiaochang field clearly highlight that mechanical strength contrasts are indeed an important factor here, and the orientation of the fold relative to the Longmen Shan range front leaves open the option that the structure may have been formed by some component of oblique deformation. Thus, we seek to apply a different approach to restoration that is based on mechanical constraints and from which we can objectively infer the sense of fault displacements. We use a combination of 2-D and 3-D restoration techniques (Muron, 2005), implemented in a geologic CAD-based modeling software tool (Mallet, 1992). The horizon-based (2-D) restoration method employs a parametric method (Muron et al., 2005), which restores deformed surfaces to a horizontal datum with a global minimum of area change and internal strain, calculated in a least-square sense. In contrast, the volumetric (3-D) restoration technique (Müller et al., 2005; Muron et al., 2005; Guzofski, 2007) uses mechanical properties to guide volume conservation and strain minimization constraints. The method employs a standard finite-element approach to minimizing total strain energy imposed on a model by restoration of a datum horizon, within the confines of pin line and pin wall boundary conditions. Pin lines and pin walls are fixed objects during a restoration. The restoration field is calculated using a dynamic relaxation technique (de Santi et al., 2003; Muron, 2005), coupled with the FEM. The equilibrium calculation is currently governed by either linear (Hookean) or nonlinear (neo-hookean) elastic constitutive laws. The user specifies rock properties (as Lame s constants), which are allowed to vary among different regions (stratigraphic layers) in the model. These parameters, in effect, govern the balance between strain minimization and volume conservation during the restorations. Varying parameters between different regions of the models allows stiffer layers to restore competently, whereas strain is localized in weaker units. Clearly, these simple constitutive laws are being used in the course of restorations to investigate large, permanent, inelastic strains. We seek to address these limitations in our study by restoring only modest amounts of strain and, in some cases, summing these small strains 1564 Geohorizons

7 to describe larger deformations. Moreover, structures can be separated into different regions, or stratigraphic intervals, bounded by faults, detachments, at flexuralslip surfaces. Thus, deformation within specific regions may be governed by elastic constitutive relations, but the structure as a whole can include inelastic components of deformation. Ultimately, by examining the geometries of restored structures and their deformation paths, we seek to evaluate how well these simple constitutive laws can describe the deformation. One of the powerful aspects of this approach is that it can restore folding and faulting simultaneously (Muron, 2005). In contrast, most kinematic restoration approaches (e.g., inclined shear) restore folding and then simply recover fault displacement by static displacement of unfolded beds to match fault cutoffs. The mechanical restoration approach performs the unfolding by minimizing the total strain energy and then removes fault gaps and overlaps by mapping the hanging-wall fault surface to the footwall, or vice versa, at all steps adhering to the boundary conditions and strain minimization constraints. This process ensures complete fault compliance and facilitates a dependency between fold and fault displacements, which is well documented in a wide variety of fault-related folds (e.g., Suppe, 1983; Suppe and Medwedeff, 1990; Erslev, 1991; Suppe et al., 1992; Shaw and Suppe, 1994; Poblet et al., 1997; Shaw et al., 2005). To perform these restorations, we develop topologically defined, volumetric (tetrahedral) meshes of the Bajiaochang structure that are based on geometric models derived from interpretations of the 3-D seismic volume. Both the 2-D and 3-D restorations yield a set of restoration vectors that connect particles before and after unfolding and unfaulting. Deformation vectors are equal to restoration vectors but with an opposite sign, and gradients in the restoration vector field describe restoration strains. Thus, the approach defines the full displacement field of the structure as well as all components of the strain tensor. MODEL BUILDING Stratigraphic horizons and faults were precisely mapped in the Bajiaochang structure from the 3-D seismic survey and well control. The mapped points were then converted from traveltime to depth using a 3-D velocity model, which had been calibrated by well picks of horizon tops, sonic logs, and check-shot surveys. These picks were used to generate evenly meshed triangular surface meshes (tsurfs) by discrete smooth interpolation (Mallet, 1992). We used a structural modeling work flow that allows for iterative improvements of the surfaces through a variety of methods. The generated tsurfs were modified by adjusting mesh triangle size to the existing data density (30-m [98-ft] grid spacing) by excluding individual data mispicks and by assuring mutual consistency of fault and mapped horizon cutoff relations. To ensure consistency of fault geometry with horizon cutoffs, it was necessary to smooth horizons near faults in places where offsets along the faults were less well imaged in the seismic data. We assumed that the sense and amount of fault-slip changes only gradually along the fault. This approach resulted then in mildly undulating cutoff lines with a consistent sense of separation along faults. The final model consists of eight horizon and five fault tsurfs, which offset horizons. Triangle sizes average 90 m (295 ft) for the horizons and 150 m (492 ft) for the faults. From the Triassic horizon tsurfs, we created a single-layer volumetric mesh with the help of a meshing module (Lepage, 2003) for use with the 3-D restoration method. The resultant 3-D model of the Bajiaochang structure represents the different tectonic layers, including the basal detachment and the overlying highly strained and thickened layers 2 and 3 (Figure 4). Layers 4 and 5 are broadly folded, with the former cut by faults with thrust separation and the latter by high-angle faults with normal displacement. This structural model was used as the basis for our 2-D and 3-D structural restorations. RESTORATIONS OF THE BAJIAOCHANG STRUCTURE We restored various surfaces from the 3-D model using both the horizon-based (2-D) and volumetric (3-D) restoration methods to investigate the deformation of the detachment anticline. We focused our analysis on the Xangxi Formation, which contains the main reservoir units. Two-Dimensional Restoration The 2-D restoration method we use (Muron et al., 2005) requires that we specify a fault-slip direction, and then the restoration method calculates the displacements and strains required to restore the fault and fold. The 2-D restoration is divided into two steps. First, a strain Plesch et al. 1565

8 Figure 4. Three-dimensional perspective views of the doubly plunging fold. The image in (A) shows the volumetric mesh of the restored horizon with well coverage. Red corresponds to 3300 m (10,826 ft) depth below sea level, and white corresponds to 2950 m (9678 ft). The image in (B) shows a complete view of most of the mapped horizons and faults. The basal detachment is not shown. The shown horizons do not correspond to the horizons interpreted in Figures 2 and 8. The high-frequency undulations of the horizon surfaces are artifacts only at the boundaries. The interior high-frequency structure is a result from careful mapping. Faults above the intermediate layers (transparent) have normal fault displacement, whereas faults below these layers have largely thrust displacement. Vertical exaggeration is 3: Geohorizons

9 fieldiscalculatedusingthe described minimization method. Second, from a reference pin point, restoration vectors are calculated by integration over the strain field. We applied the 2-D restoration method to folded and faulted horizons in the Xiangxi interval, investigating a range of possible fault-slip directions. Here, these tests are illustrated using two end-member displacements, one with highly oblique slip and the other with pure dip slip. In all cases, the restorations fully restored hanging-wall and footwall cutoffs across the fault. In the first case (Figure 5A), vectors linking footwall and hanging-wall cutoffs are defined to represent oblique, left-lateral thrust slip. In this model, the strike-slip component ( m; ft) of the slip vector is about three to eight times the dip-slip component (30 70 m; ft), which results in a rake angle of 7 19j. Whereas the rake angle is approximately maintained as far as the position of nodes along the cutoffs allows, the total slip amount decreases toward the lateral fault terminations where it necessarily becomes zero. The variation and not the magnitude of the slip vectors is important because it directly determines the calculated strain field at the fault. In the second case (Figure 5B), we represent pure dip slip by defining slip vectors, which link the nodes on the hanging wall and footwall, which are closest to each other. This linking results in a slip magnitude of m ( ft) and generally in rake angles of about 90j or pure dip slip. In this case, as in the strike-slip case, the rake is generally kept constant along the fault at the terminations where fault slip decreases to zero. All other modeling parameters such as the location of a seed point for the wave-front type algorithm employed by the restoration method or possible weighting of strain minimization versus area conservation are chosen to be identical for the two cases. We tested a variety of fault-slip directions (dip and oblique) on the Xiangxi surfaces and found that essentially pure dip-slip motions yielded the lowest strain gradients in all horizons. The difference in predicted strain is particularly apparent at the fault terminations. Large components of oblique displacement localize artificially high strains along the fault, particularly at its terminations. These strain patterns do not correspond to areas of intense folding in the mapped horizon. Presumably, if a large component of strike slip occurred on this fault, this displacement would have to be consumed by folding near the fault terminations to consume the displacements. This folding is not apparent, and thus, to restore the surface in a manner consistent with strike-slip motions requires large amounts of strain (area change) to occur within the layers. This is probably unreasonable given that layer thickness in the Xiangxi interval is largely maintained across the fold, and secondary faults are not localized near the major fault terminations at the imaging resolution of the seismic data. Subseismic-scale strain accommodation mechanisms may be invoked to explain the predicted strain pattern, although we observe no manifestation of this. In contrast, in the dip-slip case, regions exhibiting higher amounts of predicted strain correspond to areas of intense small-scale folding and close proximity to faults. In essence, these observations imply that the patterns of fault cutoffs and the fold shape are most consistent with dip-slip displacement. Thus, essentially pure dip-slip displacements recover the fault offset and folding with minimum internal strain within the horizons. Dip slip on these faults is also consistent with their gentle dips (17 30j), as well as the soft, en echelon linkage patterns observed in the seismic data, with both features common in thrust belts. The 2-D restorations of all of the Xiangxi intervals, within layer 4, produced only modest amounts of area changes (dilatations) in the surfaces (3%). This implies that strains imparted on these horizons during deformation were likely very low, which is consistent with the low amplitude of folding. Three-Dimensional Restoration We further explored the magnitudes and styles of restoration strains imparted on these surfaces through a series of volumetric (3-D) restorations of the Xiangxi intervals and a Jurassic interval. We used the FEM-based 3-D method to restore an approximately 100-m (330-ft)- thick, single layer volume defined by two horizons in the Xiangxi interval. The horizons included cutoffs along five thrust faults with well-defined displacement profiles. We used a linear elastic constitutive relation, and our preferred models employed Lame s constants of l = 40 GPa and m = 25 GPa (corresponding to a Poisson s ratio u = 0.31 and a Young s modulus E =65GPa).We based our choice of elastic parameters on a limited exploration of the parameter space within physically reasonable limits and on experiments reported by Müller et al. (2005). The boundary conditions were a pin point fixed at the center of the northern model boundary (footwall) and a datum depth at the depth of the pin point. The restoration method predicates that the choice of these boundary conditions has only a minor effect on the strain calculations but a large effect on the calculations of the restoration vectors. These conditions Plesch et al. 1567

10 Figure 5. Two-dimensional restoration of horizon Xiangxi 4 contrasting the influence of imposed oblique-slip (A) and dip-slip (B) effects on calculated strain distribution. The black arrows show the direction of imposed left-lateral slip. A negative dilatation is area decrease after deformation, a positive dilatation is area increase. We suggest that the smaller strains associated with imposed dip slip favor this sense of fault displacement. therefore employ constraints from the dip-slip 2-D restoration. All other mesh nodes were unconstrained. The restorations we generated yielded a set of restoration vectors that connect particles before and after unfolding. Restoration vectors are equal to deformation vectors but with an opposite sign, and gradients in the restoration vector field describe restoration strains. The 3-D restoration of the Xiangxi intervals yielded 1568 Geohorizons

11 restoration vector fields that are dominated by northsouth shortening and vertical thickening in the core of the anticline (Figure 6), consistent with the 2-D restoration results that implied primarily dip-slip motions of the major faults. This displacement pattern is consistent with kinematic models of detachment folding (Hardy and Poblet, 1994; Hardy et al., 1996; Poblet et al., 1997), with large components of vertical displacements centered in the fold core and larger amounts of horizontal motions in the synclines, which bound the fold limbs. In contrast, most other types of fault-related folds (i.e., fault-bend and fault-propagation folds) exhibit larger proportions of horizontal displacement across the entire fold. The restoration contraction values (Figure 7A) were also very low (4%), similar to the area changes (dilatation) that resulted from the 2-D restorations. Notably, these dilatation values are very low relative to similar restorations performed on other detachment anticlines with greater fold amplitudes (Müller et al., 2005). This implies that structural growth imparted only modest strains on the Xiangxi intervals. The highest strains are localized along the thrust faults, as expected, with only modest variations in strain intensity across the crest of the fold. The strain analysis of a similar, thin-layer 3-D restoration of an unfaulted shallower horizon within layer 5 resulted in similarly low levels of background strain and localized lineaments of enhanced elongation (Figure 7B). These lineaments are clearly associated with normal faulting in layer 5. They show a network of small normal faults on the fold limbs trending oblique to the fold axis, as well as larger north-south trending normal faults cutting across the crest of the fold. SHORTENING AND THE ROLE OF MECHANICAL STRATIGRAPHY Our 2-D and 3-D structural restorations of the units in tectonic layers 4 and 5 indicate that contraction was oriented generally north-south, perpendicular to the main fold axis. The total magnitude of shortening in these layers was on the order of m ( ft), which includes displacements on thrust faults as well as the folding (Figure 8). This modest amount of shortening reflects the fold s broad, gentle nature, with limb dips that are only about 4 7j. Shortening at deeper levels is much greater (Figure 8). In layer 2, the seismic imaging is generally sufficient to map a prominent horizon that lies at or immediately above the basal detachment zone in several seismic lines. Using a standard palinspastic restoration approach, which employs line-length measurements, we estimate that minimum shortening of this horizon ranges between about 1 and 1.4 km (0.62 and 0.86 mi). However, most of this shortening reflects displacement on small ramps that connect various splays of the basal detachment. Shortening caused by folding on this horizon is also modest (<100 m; <330 ft). Shortening is difficult to measure palinspastically within layer 3, but seems to be about m ( ft), intermediate between the shortening determined in layers 2 and 4. The transition between layers 3 and 4 is abrupt and is localized within the Leikoupo Formation. This contact may represent an unconformity and likely acts as a detachment to compensate the disharmonic folding above and beneath it. The large contrast between total shortening in layer 2 and total shortening in layers 3 and 4 reflect different functions these units have in the process of detachment folding (Epard and Groshong, 1993, 1995; Homza and Wallace, 1997) and provide insights into this important folding mechanism. Two competing models may explain this large contrast in total shortening (Figure 8). In the first case of classic detachment folding (Figure 9A), the fold grows by the addition of material to its incompetent core in front of a pinned foreland causing the fold crest to be lifted up relative to the fold limbs. This style of folding implies minor total slip of the upper, competent layer and major slip of the lower, incompetent layer. The added core material can be measured as excess area beneath the fold and above the regional level of the thickened horizon (e.g., Chamberlain, 1910). An excess area-based depth-to-detachment calculation considering the top of layer 2 predicts a detachment level 36 km (22 mi) below the top of layer 2 if shortening is measured as line-length difference. Because we identify the detachment level clearly at a much higher level about 500 m (1640 ft) below the top of layer 2 we are led to investigate a competing model of such folding (Figure 9B). In this model, the upper layer is transported with little internal deformation by the roof thrust at the top of imbricate bodies, similar to a duplex body (Boyer and Elliott 1982; Mitra, 1986; Tanner, 1992; Butler, 2004). This transport folds the upper layer in such a way that its total shortening is minor, but its total displacement is at least equal to or larger than the slip on the underlying detachment. These two competing models are readily distinguished by examining how shortening and displacement vary in the structure as a function of depth. The Plesch et al. 1569

12 Figure 6. Top and perspective view of the 3-D deformation field of the Xiangxi 4 horizon resulting from 3-D restoration. The top view shows the deformation vectors and contours of their inclination. Note that the horizontal components of displacement vectors are largely north-south, consistent with a contractional origin for this fold. In both the top and perspective views, the deformation vectors have been scaled to enhance observation. shortening profiles are similar in the two models, with no observed shortening below the detachment, and large amounts of shortening in the lower layer that decreases upward into the upper layers. In contrast, the displacement of the various layers relative to the footwall showed marked contrast between the models. Note that this measure of displacement cannot account for motion on a planar detachment that does not contribute to shortening in the structure. Instead, it considers displacement produced by slip on detachments that is 1570 Geohorizons

13 Figure 7. Distribution of elongation (maximum principal strain) in a Jurassic horizon at about 0.7 s two-way traveltime (A) and of contraction (minimum principal strain) in the Triassic Xiangxi 4 horizon (B) resulting from 3-D restoration. In addition, the image in (B) shows the direction of the contractional strain component as short black lines. Plesch et al. 1571

14 Figure 8. Seismic section without (A) and with (B) interpretation of horizons and faults (red), with vertical shortening profiles derived from line-length measurements. No vertical exaggeration is present. Abbreviation: sd = seismic datum. required to cause the observed structural thickening and shortening in the lower layer. In the classic detachment fold model, the vertical total displacement profile is largest in the lower layer and decreases upward, whereas displacement is maintained or increases upward in the duplex model. Notably, the upward-decreasing displacement profile of the pinned detachment fold model implies that in the Bajiaochang structure, layer 2 would need to have been intruded into the structure. However, we do not see evidence for such intrusion, which would require substantial motion on an upper detachment that is in the opposite direction to that on the basal detachment. Moreover, the intrusion model would require that the displaced area is balanced elsewhere above the upper detachment, and/or that intruded layers are thinned at some other location to balance the thickening manifest 1572 Geohorizons in the core of the fold. In a regionally extensive seismic reflection data set, we see no excess shortening in layers 4 or 5 or substantial thinning of layer 3; thus, we disfavor the intrusion model. This, coupled with the erroneous depth-to-detachment prediction, suggests that the classic detachment fold model is not valid for the Bajiaochang structure. We therefore prefer the duplex model, but acknowledge that these two alternative solutions are end-member models and mixed models are possible. The preferred duplex solution implies that most of the slip entering the structure on the basal detachment exits to the foreland, and that only a modest amount of this slip is consumed in detachment folding. Moreover, based on the observed local duplication of the horizon that lies at and above the basal detachment, we argue that the detachment ramps to a shallower level within the fold. This shallower detachment may only be 100 m (330 ft)

15 Figure 9. Contrasting models of low-relief detachment folding with shortening and displacement values for the lower and upper layers shown. (A) The detachment fold model has a completely pinned foreland and an incompetent core. It requires a displacement of the lower layer larger than that of the upper layer. (B) In the duplex model, the detachment climbs to a higher level and allows slip to be transmitted into the foreland. Whereas the shortening of the lower and upper layer is similar to (A), here, the upper layer experiences the same displacement as the lower. The transmitted slip shown is a model minimum. or so above the basal detachment, but nonetheless, it is sufficient to localize structural thickening within layers 2 and 3 in the core of the fold. In this sense, lowrelief detachment folding in the Bajiaochang structure reflects a process of structural imbrication. The upper detachment within layer 2, as well as the folding in layer 3 and its overlying detachment, acts as the roof, which separates disharmonic folding at shallow and deep levels. The fact that there is an additional upper detachment between layers 3 and 4 where differentially shortened layers appear to be decoupled can be described as a double duplex. The lower duplex body provides the roof thrust, on top of which the upper duplex body is both shortened and also passively transported. In turn, the upper detachment of the upper duplex body accommodates slip responsible for thrusting and thrust-related folding and passive transport of the overlying hanging wall. We infer the overall transport on the detachment levels to be south directed given that the restoration analysis points to pure thrusting, which we associate with the formation of the Micang Shan and Qinling Shan to the north. Therefore, the main thrust faults, which are south dipping, are in fact hinterland directed or backthrusts and are located in the foreland of the fold and thrust belt. Such backthrusting is commonly observed as part of conjugate faults at the toe of low-taper thrust wedges, which exhibit a low-friction basal detachment (e.g., Spratt and Lawton, 1996; Cotton and Koyi, 2000; Bilotti and Shaw, 2005), and has been shown in numerical and analog models (e.g., Liu et al., 1992; Jamison, 1996). Consequently, we suggest that the hinterlanddirected thrusting in Bajiaochang is the result of the low friction provided by evaporites in layer 2 and of a horizontal attitude of the main detachment. The mechanism of detachment folding we infer based on the different amounts of shortening in the tectonic layers that comprise the Bajiaochang structure has some important similarities with, as well as differences from, classic kinematic detachment fold models (Figure 9). Consistent with these models, deformation is governed by displacement on a weak basal detachment, which, in the case of Bajiaochang, appears to be localized within a thin evaporite sequence. Moreover, folding at shallow levels is caused in part by ductile or distributedthickeninginanintervalthatoverliesthe basal detachment, which, in Bajiaochang, corresponds to layers 2 and 3. The Bajiaochang structure differs from standard detachment fold models in that it is not pinned in the foreland, but instead has been displaced toward the foreland by slip on its basal detachment. Although shortening at shallow levels is modest, slip on this basal detachment is substantial (in excess of 1 km [0.6 mi]), implying that this fold developed above a well-established detachment system that extends across large parts of the basin. The presence of Plesch et al. 1573

16 other detachment folds lying between the Bajiaochang anticline and the Micang Shan and Qinling Shan belts, as well as south of the Bajiaochang trends, supports the idea that a substantial amount of slip on the detachment extends far into the foreland. Similar detachment levels have been recognized to the west, which were activated by the deformation in the Longmen Shan belt (Burchfiel et al., 1995). It appears that these far-reaching detachments at similar levels were able to readily transfer both south- and east-directed slip, presumably in different phases of deformation originating from basin-bounding fold and thrust belts. Moreover, a series of modest ramps in the basal detachment that transfer slip to a higher detachment level seem responsible for localizing structural thickening within the core of the fold within layers 2 and 3. Given that slip likely extends across this structure on the basal detachment, it is these ramps and the local imbrications that they induce that largely drive the detachment folding process. Modest amounts of deformation in layer 4 are consistent with the competency of this unit, which comprises primarily well-lithified sandstones. The thrust faults that extend up into this interval appear to emerge from the basal detachment localized within layer 2. Their presence presumably reflects some inability to completely decouple shortening between layers 4 and 3. Finally, folding in layer 5 is concordant with deformation in layer 4. The primary difference in these units is that normal faults are common in layer 5. Based on our analysis, many of these faults seem to be part of a regional fracture set that extends across the fold. Nevertheless, normal slip on these faults is locally amplified because they trend across the crest of the fold, presumably driven by an outer arc extension in the folding process. CONCLUSIONS We apply new methods of structural restoration based on mechanical constraints to gain insights into the development of the Bajiaochang anticline in the Sichuan Basin, China. The structure is a detachment fold, consisting of a basal detachment that is localized in a Triassic evaporite layer. Overlying layers are folded to various intensities based on their mechanical strengths. The restoration methods we employed allowed us to test various fault-displacement directions and to resolve that slip and shortening in the Bajiaochang structure is primarily contractional. This suggests that the structure grew as a component of foreland deformation most likely related to the development of the Micang and Qinling fold and thrust belt, which bound the basin to the north. Moreover, we quantify the styles and amounts of shortening within various tectonic levels of the structure. Large amounts of shortening of lower units and low amounts of shortening at shallower levels are consistent with general detachment fold models. However, slip on the basal detachment is much larger than would be predicted by these models, implying that substantial slip on the basal detachment extends beyond the structure into the foreland. Moreover, structural thickening in the core of the fold is localized by a series of small ramps in the basal detachment that produce structural imbrication. This implies that the detachment fold grows by a mechanism akin to duplexes, which has important implications for the manner in which shortening and strain are partitioned within different mechanical layers of the fold. REFERENCES CITED Allmendinger, R. W., 1998, Inverse and forward numerical modeling of trishear fault-propagation folds: Tectonics, v. 17, p Bilotti, F., and J. H. Shaw, 2005, Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles: AAPG Bulletin, v. 89, p Boyer, S., and D. Elliott, 1982, Thrust systems: AAPG Bulletin, v. 66, p Burchfiel, B. C., Z. Chen, Y. Liu, and L. H. Royden, 1995, Tectonics of the Longmen Shan and adjacent regions, central China: International Geology Review, v. 37, p Butler, R. W. H., 2004, The nature of roof thrusts in the Moine thrust belt, NW Scotland: Implications for the structural evolution of thrust belts: Journal of the Geological Society (London), v. 161, p Chamberlain, R. T., 1910, The Appalachian folds of central Pennsylvania: Journal of Geology, v. 18, p Cotton, J. T., and H. A. Koyi, 2000, Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan: Geological Society of America Bulletin, v. 112, p Dahlstrom, C. D. A., 1969, Balanced cross sections: Canadian Journal of Earth Sciences, v. 6, p Dahlstrom, C. D. A., 1970, Structural geology in the eastern margin of the Canadian Rocky Mountains: Bulletin of Canadian Petroleum Geology, v. 17, p Dahlstrom, C. D. A., 1990, Geometric constraints derived from the law of conservation volume and applied to evolutionary models for detachment folding: AAPG Bulletin, v. 74, p de Santi, M. R., J. L. E. Campos, and L. F. Martha, 2003, 3-D geological restoration using a finite element approach, in Gocad Proceedings: 23th Gocad Meeting, Association Scientifique pour la Geologie et ses Applications, p. 1/12 12/12. Epard, J.-L., and R. H. Groshong Jr., 1993, Excess-area and depth to detachment: AAPG Bulletin, v. 77, p Geohorizons

17 Epard, J.-L., and R. H. Groshong Jr., 1995, Kinematic model of detachment folding including limb rotation, fixed hinges and layer-parallel strain: Tectonophysics, v. 247, p Erslev, E. A., 1991, Trishear fault-propagation folding: Geology, v. 19, p Guzofski, C. A., 2007, Mechanics of fault-related folds and critical taper wedges: Ph.D. dissertation, Harvard University, Cambridge, 121 p. Hardy, S., and J. Poblet, 1994, Geometric and numerical model of progressive limb rotation in detachment folds: Geology, v. 22, p Hardy, S., J. Poblet, K. McClay, and D. Waltham, 1996, Mathematical modeling of growth strata associated with fault-related fold structures, in P. G. Buchanan and D. A. Nieuwland, eds., Modern developments in structural interpretation, validation and modeling: Geological Society (London) Special Publication 99, p Homza, T. X., and W. K. Wallace, 1997, Detachment folds with fixed hinges and variable detachment depth, northeastern Brooks Range, Alaska: Journal of Structural Geology, v. 19, p Jamison, W. R., 1987, Geometric analysis of fold development in overthrust terranes: Journal of Structural Geology, v. 9, p Jamison, W. R., 1996, Mechanical models of triangle zone evolution: Bulletin of Canadian Petroleum Geology, v. 44, p Lepage, F., 2003, Three dimensional mesh generation for the simulation of physical phenomena in geosciences: Ph.D. thesis, Institut National Polytechnique de Lorraine, France, 254 p. Liu, H., K. R. McClay, and D. Powell, 1992, Physical models of thrust wedges, in K. R. McClay, ed., Thrust tectonics: London, Chapman and Hall, p Mallet, J. L., 1992, Discrete smooth interpolation in geometric modeling: Computer Aided Design, v. 24, p Meng, Q., E. Wang, and J. Hu, 2005, Mesozoic sedimentary evolution of the northwest Sichuan Basin: Implication for continued clockwise rotation of the South China block: Geological Society of America Bulletin, v. 117, p Mitra, S., 1986, Duplex structures and imbricate thrust systems: Geometry, structural position, and hydrocarbon potential: AAPG Bulletin, v. 70, p Müller, J. P., C. Guzofski, C. Rivero, A. Plesch, J. H. Shaw, P. Muron, F. Bilotti, and D. A. Medwedeff, 2005, New approaches to 3-D structural restoration in fold-and-thrust belts using growth strata (abs.): AAPG Annual Meeting Program, v. 14, p. A97. Muron, P., 2005, 3-D numerical methods for the restoration of faulted geological structures: Ph.D. thesis, Institut National Polytechnique de Lorraine, France, 141 p. Muron, P., J. L. Mallet, and D. A. Medwedeff, 2005, 3-D sequential structural restoration: Geometry and kinematics (abs.): AAPG Annual Meeting Program, v. 14, p. A98. Novoa, E., J. Suppe, and J. H. Shaw, 2000, Inclined-shear restoration of growth folds: AAPG Bulletin, v. 84, p Poblet, J., K. R. McClay, F. Storti, and J. A. Muñoz, 1997, Geometries of syntectonic sediments associated with single layer detachment folds: Journal of Structural Geology, v. 19, p Scanlin, M. A., and T. Engelder, 2003, The basement versus the no-basement hypothesis for folding within the Appalachian Plateau detachment sheet: American Journal of Science, v. 303, p Shaw, J., and J. Suppe, 1994, Active faulting and growth folding in the eastern Santa Barbara Channel, California: Geological Society of America Bulletin, v. 106, p Shaw, J. H., C. Connors, and J. Suppe, 2005, Seismic interpretation of contractional fault-related folds: An AAPG seismic atlas: AAPG Studies in Geology 53, 156 p. Spratt, D. A., and D. C. Lawton, 1996, Variations in detachment levels, ramp angles and wedge geometries along the Alberta thrust front: Bulletin of Canadian Petroleum Geology, v. 44, p Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p Suppe, J., and D. A. Medwedeff, 1990, Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, p Suppe, J., G. T. Chou, and S. C. Hook, 1992, Rates of folding and faulting determined from growth strata, in K. R. McClay, ed., Thrust tectonics: London, Chapman and Hall, p Tanner, P. W. G., 1992, The duplex model: Implications from a study of flexural-slip duplexes, in K. R. McClay, ed., Thrust tectonics: London, Chapman and Hall, p Woodward, N. B., S. E. Boyer, and J. Suppe, 1985, An outline of balanced cross sections: University of Tennessee, Department of Geological Sciences, Studies in Geology, 2d ed., v. 11, 170 p. Plesch et al. 1575