Trishear Fault-propagation Research Articles

Late Pleistocene Fault Slip Rate within the Bole Basin: Insights into deformation kinematics in the Central Tian Shan

Tian Shan is a vast and highly seismically active intracontinental mountain range. GPS measurements and field studies have shown that E-W and NW-SE trending thrusts and fault-related folds are distributed across central Tian Shan. However, few studies have determined the fault and fault-related-fold activity in the Bole Basin, the northernmost part of central Tian Shan. In this study, we focus on the Alashak Fold, which is situated along the southern margin of the Bole Basin. Using high-resolution uncrewed aerial vehicle-Digital Elevation Model data, field observations, and detailed mapping, we determined that the Alashak Fold is characterized by a trishear fault-propagation fold. Along the Alashak River, we identified five levels of fluvial surfaces, the T3 terrace was continuously preserved across the fold. Radiocarbon and surficial 10Be dating were used to determine the exposure age of the deformed T3 terrace. The folding characteristics of T3, trishear inversed modeling, and abandonment age indicate that the Alashak Fault has an uplift rate of 0.8 ± 0.3 mm/yr, a dip-slip rate of 4.2 ± 1.2 mm/yr, and a crustal shortening rate of 4.0 ± 1.2 mm/yr since the late Pleistocene, respectively. The Alashak and Latgan Faults intersect at a depth of 1.2 km, enclosing a wedge-shaped block. Our study emphasizes that tectonic activity along the southern margin of the Bole Basin has played a significant role in north-south shortening within central Tian Shan, as measured by geological measurements since the late Pleistocene. The Alashak Fault can potentially generate moderate–strong (Mw 5.0–7.0) earthquakes, while the Alashak and Latgan Faults can generate major (Mw 7.2) earthquakes.

A new superimposed model of the Tongnanba anticline in northeastern Sichuan and its exploration implications

Understanding the structural style, kinematic process, and timing of superimposed structures worldwide is often difficult due to complex structure deformation process. Fortunately, the newly acquired high-quality seismic reflection data and geological observations covering the Tongnanba anticline provide an excellent chance to characterize such structures. Here, we used geological and seismic data from the Tongnanba region to evaluate the structural style and deformation sequence of Tongnanba anticline. In this regard, we propose a new model of the northeastern Sichuan Basin, which are different from the model that deep structures formed earlier than shallow structures demonstrated by previous studies, and we also discussed the implications of this new model for the deep oil and gas exploration. Compressed by Micangshan and Dabashan thrust belts and controlled by three detachment layers, the Tongnanba anticline shows a complex multi-stage, multi-directional, and multi-level superimposed structure. There were three deformation layers vertically, leading to the multi-level detachment thrust structure style. Specifically, the upper and middle deformation layers were mainly controlled by South Dabashan thrust belt in the early stage, forming long-distance detachment thrust structure extended in the NW-SE direction. A series of pop-up structures propagated toward the upper and middle detachment layers. On the other hand, the lower deformation layer was primarily controlled by the Micangshan thrust belt in the late stage, forming complex basement faults extended in the NE-SW direction, which was consistent with Trishear fault-propagation fold. Along the basement detachment developed multiple branch slopes spread from northeast to southwest. The middle and upper deformation layers was transformed by the basement faults, thus forming the complex superimposed structure of north-south zonation and east-west segmentation at present. It was such complex superimposed structure that control the process of hydrocarbon accumulation and adjustment in each deformation layer, and the deep-ultra-deep ancient oil and gas reservoirs may be worth of exploring.

Characterization of the Soapaga Fault deformation in the central part of the Eastern Cordillera, Colombia

The eastern border of the Floresta Massif is characterized by the confluence of the Soapaga Fault and the Corrales Fault, two reverse faults that are linked to each other in a simple transfer zone and whose deformation influenced the formation of the Betéitiva Syncline. The Betéitiva Syncline, located on the Soapaga Fault footwall, shows geometric features, such as upright to inverted backlimbs and thickness changes related to the fault's deformation. However, none of the previously proposed kinematic models (fault-propagation, fault-bend, and buckling–folding models) clearly explain the evolution of the Betéitiva Syncline related to the Soapaga Fault or the differentiation between this fault and the Corrales Fault. In this work, we developed a structural analysis to differentiate the Soapaga Fault from the Corrales Fault through a detailed description of the fault zones, geomorphic evidence, geometric relationships, and deformation features related to each fault. Then, we developed a structural analysis of the evolution of the aforementioned structures using five balanced cross-sections that illustrate how they are related to one another. Three kinematic models or structural processes were used to explain the evolution of the study area: the trishear fault-propagation fold model, the fault-propagation fold model, and the gravitational fold and fault model. This work presents the structural, geological, and geomorphic evidence that allowed us to differentiate the Corrales Fault from the Soapaga Fault. Additionally, our results show an increase in the Soapaga Fault displacement (4.7 km–9.5 km), the Corrales Fault displacement (0.6 km–1.3 km), and the total shortening (1.9 km–3.4 km) from north to south. In addition, we propose a structural evolution model that better explains the geometric features of the Betéitiva Syncline regarding the deformation of the Soapaga Fault, which requires 10x less shortening than previous structural models. Finally, we propose that the deformation of the Soapaga Fault is a scale-dependent variable because of the relationship that this fault has with regional structures, such as the Floresta Anticlinorium, but also with local structures, such as the Betéitiva Syncline.

Three-Dimensional Fault-Fold Growth Deciphered from Combined Seismic and Geological Data: A Case Study from the Xiongpo Anticline, Longmen Shan Piedmont

The Xiongpo fault-fold belt shows prominent NE, ENE- and ~N–S-trending relief, which resulted from multi-stage upper crustal shortening in the Longmen Shan piedmont during the eastward growth of the eastern Tibetan Plateau. Previous studies have determined its 2D structural configurations from seismic profiles and field-based geological cross-sections. Here, we extend this analysis into the entire belt to explore the 3D structural evolution of this complex fault-fold belt and have built a 3D regional fault model. The results reveal along-strike variation of subsurface structural architecture of the Xiongpo fault-fold belt, which is characterized by transformation from a complex superimposition of a deep fault-bend fold beneath a shallow structural wedge in the center segment to a simple shallow fault-bend fold on both ends of the structure, and then to a trishear fault propagation fold on the plunging edges. This structural transformation determines the contrast between the NE-striking relief of the central segment, and the ENE- and ~N-S-striking relief in the two plunging zones. We combine our results with published low-temperature thermochronology and growth strata results to propose a three-stage evolution for the Xiongpo fault-fold belt that closely relates with regional stress field changes, including a NE-striking fault under the NW–SE compression between 40–25 Ma and 15–10 Ma, lateral propagation of the NE-striking fault and initiation of ENE-striking fault by WNW–ESE compression from ~5–2 Ma, ~N–S fault under ~E–W compression until the present. This work enhances our understanding of the stress field changes of eastern Tibet since the Late Eocene. It also can serve as a typical case study deciphering 3D fault-fold growth using seismic and geological imaging, which is helpful to understand 3D structural and landscape evolutions of other complex fault-fold belts worldwide.

Effect of inherited structural highs on the structure and kinematics of the South Dezful Embayment, SW Iran

AbstractUnderstanding the structural evolution of the South Dezful Embayment (SDE) in the Zagros Fold-and-Thrust Belt (ZFTB) is significant for hydrocarbon exploration and production. The structural evolution of this area has been controlled by the reactivation of basement structures related to the oblique convergence along the ZFTB. In this work, we study the effect of the pre-contractional interaction between the basement step and overlaid salt layer superimposed by the Zagros shortening on the structural style and evolution of the sedimentary basin in the SDE. We use multidisciplinary approaches involving surface and subsurface data and analogue models. Disharmonic folds with small wavelength formed over the Kharg-Mish Palaeo-high (KMPH) where the sedimentary cover is thinner. On the other hand, faulted detachment folds with large wavelengths developed in the adjacent depocenters. Furthermore, the KMPH affected the geometry and kinematics of the frontal part of the Zagros Belt, which is characterized by the Mountain Front Flexure (MFF) topographic step. Long-lasting active deformation of the sedimentary cover over the frontal ramp of the KMPH developed the South Izeh Promontory (SIP). Localized contraction in the SIP was accommodated temporally by trishear fault-propagation folding and resulted in > 5 km elevation difference between this promontory and two adjacent local basins. Supporting scaled analogue models show that structural evolution, folding and along-strike variations in fold style of the SDE have been controlled by thickness variations of the sedimentary cover and the geometry of the inherited pre-salt structure. Based on the results of this study, the effect of inherited structural highs on the structure and kinematics of the SDE provide important insights for hydrocarbon entrapment, migration and exploration.

Kinematic evolution of fold-and-thrust belts in the Yubei-Tangbei area: Implications for tectonic events in the southern Tarim Basin

The Yubei-Tangbei area in the southern Tarim Basin is one of the best-preserved Early Paleozoic northeast-southwest trending fold-and-thrust belts within this basin. This area is crucial for the exploration of primary hydrocarbon reservoirs in northwestern China. In this study, we constructed the structural geometric morphology of the Yubei-Tangbei area using geophysical logs, drilling, and recent two- and three-dimensional (2-D and 3-D) seismic data. The Early Paleozoic fault-propagation folds, the Tangnan triangle zone, fault-detachment folds, and trishear fault-propagation folds developed with the detachment of the Middle Cambrian gypsum–salt layer. According to a detailed chronostratigraphic framework, the growth strata in the Upper Ordovician–Lower Silurian layer formed by onlapping the back limb of the asymmetric fault-propagation folds, which therefore defines the timing of deformations. The changes in kink band hinges and amplitudes in the Permian–Carboniferous and Cenozoic folding strata suggest that the evolution of the fold-and-thrust belts followed a sequential evolution process rather than a simultaneous one. Above the pre-existing Precambrian basement structure, the Yubei-Tangbei fold-and-thrust belts can be divided into four tectonic evolution stages: Late Cambrian, Late Ordovician to Early Carboniferous, Carboniferous to Permian, and Cenozoic. The northwestern-verging Cherchen Fault is part of the piedmont fold-and-thrust system of the southern Tarim foreland basin. We interpreted its strata as a breakthrough trishear fault-propagation fold that developed in three phases: Mid–Late Ordovician, Silurian to Middle Devonian, and Triassic to present. These tectonic events are responses of the Altyn-Tagh and Kunlun collisional orogenic belts and the Indian-Eurasian collision. The inherited deformation and structural modification in the southern Tarim Basin may be an indicator of the growth and evolution of peripheral orogens.

Subsurface Modeling of Digboi‒Dilli Ghat Oil Bearing Structures (India) Using 2D Move

The first discovery of commercial hydrocarbon in India was in 1889 in a structural trap at Digboi in Assam. Ever since the discovery, geologists have mapped and modeled the subsurface structure of this 50 km long anticlinal fold-thrust belt. However, never before, the existence of a hanging wall syncline north of Jaipur anticline, near the Kusijan oil and gas field, was considered. In this paper, we have proposed a structural model for the regional fold-thrust structures that successfully explains the existence of this syncline adjacent to the Naga thrust. Trishear Fault Propagation folding followed by a complex breakthrough pattern of the thrust at later stages is proposed as most suitable model to explain the outcrop pattern and sub-surface data. We have prepared two northwest-southeast trending balanced structural cross sections using this kinematic model. Total shortening percentages across the southern and northern cross sections are estimated to be 44 and 30 respectively. A complementary play with locations of prospective boreholes is suggested. Based on the dip data in the hanging wall structures of Naga thrust, we predict medium- to small-scale concealed imbricate structures in the sub-thrust. These zones of local thrust imbrications may prove to be prospective leads for concealed petroleum traps.

Seismic modeling and expression of common fold-thrust structures

We have conducted seismic modeling of common fold-thrust structures to understand the common geologic parameters influencing seismic data and to understand the common pitfalls associated with interpreting prestack time migration (PSTM) and prestack depth migration (PSDM) data. Mode 1 fault-bend folds are generally well-imaged in PSTM data, provided the correct migration velocities are used for the dipping back and front limbs. Seismic pull-ups of the footwall related to lateral velocity variations can result in problems in interpreting the fault geometry and the subthrust area underlying the crest. Fault-tip fault-propagation folds also show significant footwall pull-ups and show poor to no imaging of the steep front limbs. The geometry of trishear fault-propagation folds is dependent on the maximum slip on the fault (S) and the fault propagation to slip ratio (P/S ratio). We found that the slip has a strong influence on the dip of the front limb and therefore the quality of imaging whereas the P/S ratio, which controls the degree of folding versus thrust faulting, has only a secondary effect. For the front limb, only the area near the synclinal axial plan is well-imaged, so that the fault geometry and extent of propagation are typically difficult to interpret. The front limb dips are also sensitive to the accuracy of the rms velocity model used for migration. Lower velocities result in steeper dipping reflectors, whereas higher velocities result in shallower dips. In general, PSDM provides better imaging of the structures; however, the accuracy and quality of the image are dependent on the velocity models and interpretation derived from the PSTM data.

Effects of fault slip distribution on the geometry and kinematics of the southern Junggar fold-and-thrust belt, northern Tian Shan

At the foreland edge of fold-and-thrust belts (FTBs), tectonic shortening could either be accommodated by thick-skinned faulting and/or structural wedges developed near range-fronts, or be transferred along detachment layers further into basins, forming thin-skinned structures. Although the geometry and kinematics of each structure have been well-investigated, studies that characterize slip distribution along a specific fault as well as the effects of variations in magnitude of fault displacement on the development of fault-related folds are still lacking. In this study, we present a case study of the southern Junggar Thrust (SJT), a seismically-active fault developed in the southern Junggar FTB, northern Tian Shan with an overall vergence to the north. The results of structural interpretations of seismic reflection profiles combined with the age constraints provided by previous magnetostratigraphic studies demonstrate that: 1) The kinematics of the SJT vary along strike from purely structural wedging to classical fault-bend folding. Besides these end-member models, intermediate cases exist between them, in which fault slip is partitioned by two-stage structural deformation, i.e., fault-bend folding subsequent to structural wedging at the range front. As a result, part of the displacement is transferred northward, forming the Tugulu anticline. 2) Along-strike variations in displacement along the Tugulu fault (frontal fault of the SJT) also affect the geometry and kinematics of the Tugulu anticline, varying from detachment folding, trishear fault-propagation folding with a pure-shear fault-bend folding back-limb, and classical trishear fault-propagation folding to fault-bend folding with increasing displacement. 3) The SJT has an average Quaternary slip rate of 3.9 ± 0.4 mm/yr, serving as a principal structure accommodating tectonic shortening in the southern Junggar FTB. The results of this study are helpful for seismic hazard assessments of the study area, which may also shed light on structural evolution of fault-related folds in other FTBs.

Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya

Pure strain is an important component of the total displacement vector in fold-thrust belts like the Himalayan orogenic belt. We describe a methodology to compute cataclastic strain from external thrust sheets using the Bootstrapped Modified Normalized Fry Method and validate it in two different structural settings in the frontal Himalaya. In the frontal imbricate zone of the Dharan salient in the Darjiling Himalaya we measured the highest strain from a central imbricate thrust (T3) and found that strain decreased away from the fault zones within individual thrust sheets. Modelling of our results indicated that there was significant layer-perpendicular flattening in the fault zones while layer parallel shortening related strain dominated the thrust sheets. Also, fault parallel shear decreased away from the fault zones. In contrast, layer-perpendicular flattening was largely absent in the fault zone associated with the Main Frontal thrust (MFT) and the MFT sheet in the Mohand Range of the Dehradun recess. Here the strain distribution pattern was consistent with a trishear fault propagation monocline which is our preferred model for the structure of the MFT sheet in the recess. Modelling of our results suggested that fault parallel shear decreased away from the MFT fault zone like in the thrust sheets from the Dharan salient and that the fault propagation folding was accomplished by flexural slip folding. We contend that our methodology can be effectively used to quantify and study the pure strain part of the total displacement vector in external thrust sheets from fold-thrust belts worldwide.

3D geometric modeling for the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, China: Oblique thrusting kinematic implications

Three-dimensional (3D) subsurface seismic data acquisition and modeling are rarely undertaken; consequently, the complex effects of oblique thrusting on the 3D morphology and kinematics of folds are still poorly understood. We use 3D seismic data to produce five 3D geometric models and compare our results with structural models of the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, to investigate the effects of oblique thrusting on 3D fold evolution. 3D geometric modeling results reveal that oblique thrusting may cause fold asymmetry, vertical axis rotation, and a curved fold axis. 3D seismic interpretations and structural modeling suggest that the Yanjinggou anticline developed as a trishear fault-propagation fold and experienced a maximum shortening amount of 1.2 ± 0.1 km. The 3D structural and geometric models are consistent with paleomagnetic results, suggesting that the anticline was affected by oblique thrusting at an angle of 55° above a listric thrust fault with three fault ramps (19°, 30°, and 40°), causing the fold to rotate and develop a curved morphology. The integrated method used in this study to investigate the effects of oblique thrusting on 3D fold morphology and kinematics can be applied to regional oblique thrusting in the Longmen Shan and other fold-and-thrust belts worldwide.

Detection of a fracture zone using microtremor array measurement

We have conducted microtremor array measurements to estimate shallow S-wave velocity ([Formula: see text]) structures at two sites (the 921 Earthquake Museum of Taiwan and the Taiwan Provincial Consultative Council) located near surface ruptures of the Chelungpu Fault. Ten stations, consisting of three different-aperture triangles and a central station, are adopted for each array deployment. Using the array data, we calculate dispersion curves of Rayleigh waves using the frequency-wavenumber spectrum method and then estimate [Formula: see text] structures by the surface-wave inversion technique. The obtained 2D [Formula: see text] profiles could clearly show compressive and flexural deformation structures with the surface ruptures located at relatively weak (low [Formula: see text]) zones. This indicates compressive buckling as the most likely mechanism for surface rupturing along these low [Formula: see text] zones. Importantly, this study successfully depicts strata disturbances in a fault fracture zone using microtremor array measurements and forward numerical modeling of trishear fault-propagation folds.

Jabal Hafit anticline (UAE and Oman) formed by décollement folding followed by trishear fault-propagation folding

Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the décollement, to accommodate WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.

A trishear model for the deformation of the Sudbury Igneous Complex, Canada

The Sudbury Igneous Complex (SIC), Canada, is an impact-induced layered sheet of crystalline rocks deformed into an asymmetrical fold basin, the Sudbury Basin. The basin geometry at depth is largely unknown as few attempts were made to quantify displacement and rotation of layer contacts. We propose that the dip of layer contacts and foliation surfaces in the southern SIC can be approximated by trishear fault propagation folding. Trishear deformation accounts for: (1) angular discordances between upper and basal SIC contacts, (2) local overturning of the SIC, (3) progressive steepening of foliation surfaces from NW to SE, (4) strain gradient in the Sudbury Basin sedimentary rocks, and (5) thickness variations in SIC layers. Moreover, the South Range Shear Zone, a zone of moderately strong metamorphic fabrics, coincides with the surface manifestation of the proposed trishear zone.We demonstrate the use of structural data together with forward kinematic modelling to identify the strain distribution within the SIC, rotation of SIC contacts and thickness changes of SIC layers. Backward kinematic modelling provides information on the initial geometry of the SIC and is used to restore the shape of the igneous sheet, showing that the SIC was shortened by approximately 10 km in NW-SE diameter. Most of the shortening was accommodated by 40% reduction in the thickness of the upper SIC layer.

High precision analysis of an embryonic extensional fault-related fold using 3D orthorectified virtual outcrops: The viewpoint importance in structural geology

Image-based 3D modeling has recently opened the way to the use of virtual outcrop models in geology. An intriguing application of this method involves the production of orthorectified images of outcrops using almost any user-defined point of view, so that photorealistic cross-sections suitable for numerous geological purposes and measurements can be easily generated. These purposes include the accurate quantitative analysis of fault-fold relationships starting from imperfectly oriented and partly inaccessible real outcrops. We applied the method of image-based 3D modeling and orthorectification to a case study from the northern Apennines, Italy, where an incipient extensional fault affecting well-layered limestones is exposed on a 10-m-high barely accessible cliff. Through a few simple steps, we constructed a high-quality image-based 3D model of the outcrop. In the model, we made a series of measurements including fault and bedding attitudes, which allowed us to derive the bedding-fault intersection direction. We then used this direction as viewpoint to obtain a distortion-free photorealistic cross-section, on which we measured bed dips and thicknesses as well as fault stratigraphic separations. These measurements allowed us to identify a slight difference (i.e. only 0.5°) between the hangingwall and footwall cutoff angles. We show that the hangingwall strain required to compensate the upward-decreasing displacement of the fault was accommodated by this 0.5° rotation (i.e. folding) and coeval 0.8% thickening of strata in the hangingwall relatively to footwall strata. This evidence is consistent with trishear fault-propagation folding. Our results emphasize the viewpoint importance in structural geology and therefore the potential of using orthorectified virtual outcrops.

Combining multiple quantitative structural analysis techniques to create robust structural interpretations

Carefully selected 2D transects contain an abundance of structural information that can constrain 3D analyses of petroleum systems. Realizing the full value of the information in a 2D transect requires combining multiple, independent structural analysis techniques using fully interactive tools. Our approach uses quantitative structural geologic software that instantaneously displays structural computations and analyses, eliminating time-intensive manual measurements and calculations. By quickly testing multiple hypotheses, we converged on an optimal solution that is consistent with available data. We have combined area-depth-strain (ADS) analysis, structural restoration, and forward modeling of a structural interpretation of a fault-propagation fold in the Niger Delta. These methods confirmed the original interpretation and furthermore quantified displacement, strain, detachment depth, and kinematic history. ADS analysis validated the interpreted detachment depth and revealed significant layer-parallel strain (LPS) that varied systematically with stratigraphic depth. The stratigraphic distribution of the LPS was diagnostic of structural style and, in this example, discriminated against fixed-axis and constant-thickness fault-propagation folding. A quantitative forward model incorporating backlimb shear and trishear fault-propagation folding accurately reproduced folding and faulting in the pregrowth section and folding in the growth section. The model-predicted strain distributions were consistent with those from ADS analysis. The highest local strains on the back limb of the structure were spatially coincident with two backthrusts, which accommodated these strains. Animations of a more complete model including the backthrusts revealed that the backthrusts formed sequentially as rock passed through the main fault bend.

TrishearCreator: A tool for the kinematic simulation and strain analysis of trishear fault-propagation folding with growth strata

TrishearCreator is a platform independent web program constructed in Flash, which enables fold modeling, numerical simulation of trishear fault-propagation folding and strain analysis, etc. In the program, various types of original strata, such as folds and inclined strata can be easily constructed via adjusting shape parameters. In the simulation of trishear fault-propagation folding, growth strata and strain ellipses are calculated and displayed simultaneously. This web-based program is easy to use. Model parameters are changed by simple mouse actions, which have the advantage of speed and simplicity. And it gives an instant visual appreciation of the effect of changing the parameters that are used to construct the initial configuration of the model and the fold-propagation folding. These data can be exported to a text file, and be shared with other geologists to replay the kinematic evolution of structures using the program.

Three Dimension Construction and Magnetic Fabric Analysis of the Yanjinggou Fault-propagation Fold in Western Sichuan, China

Recently, there is a transition trend of the research on the fault-related fold from 2D to 3D. Based on ARCGIS, DISCOVERY and GOCAD software platform, we built a 3D geological model of the Yanjinggou anticline using 3D seismic data in the area. It was found that Yanjinggou anticline was a typical trishear fault-propagation fold. Because the structural interpretation of the 3D model only is of non-uniqueness and uncertainty due to geometric constraint, the finite strain analysis was also applied in the structural interpretation. 184 oriented samples have been drilled at 18 sampling sites in Yanjinggou, western Sichuan. AMS of these samples disclosed: (1) that most of the magnetic fabrics were the initial deformation fabric; (2) that deformation in the forelimb was relatively stronger than that in the rearlimb; and (3) that the finite strain was strong within the trishear zone of the fault-propagation fold, in accordance with demonstration in the 3D geometric model. The tectonic stress field indicated by the magnetic fabrics suggested a NW-SE compression and shortening, which was consistent with the prediction of the 3D geometric model.

Inference of trishear-faulting processes from deformed pregrowth and growth strata

Near surface strata deformed by propagation of a fault situated underneath these strata hold explicit records of fault activities. The delicate deformed strata exposed in investigation trenches for paleoseismologic study conform to fault-propagation folds growing above a blind thrust. Growth strata deposited subsequently across the coseismic fold scarps are preserved by onlap and unconformities. Information from these sedimentary and folding processes was used in conjunction with inverse modeling using the trishear model technique proposed by Allmendinger (Allmendinger, R.W., 1998. Inverse and forward numerical modeling of trishear fault-propagation folds. Tectonics 17, 640–656) in estimating the times and amounts of slips. This methodology becomes necessary especially when the extent of a deformed profile is insufficiently exposed to observation or the location of the fault tip is invisible. The proposed method can deal with growth strata that can be selectively removed during fold restoration. The interpretations on the deformation history obtained by inverse analysis and trench investigation were then compared. The combination of these two approaches seemed to enhance and complement each other and allows a better and more complete understanding of the trishear-faulting processes and is useful in seismic hazard assessment.

Paleoseismic evidence for coseismic growth-fold in the 1999 Chichi earthquake and earlier earthquakes, central Taiwan

The 1999 Chichi earthquake ruptured along previously unrecognized traces of the Chelungpu fault, because the traces were covered with thick-bedded fluvial, alluvial, and colluvial deposits. The earthquake created a 95-km-long surface rupture and associated fault-related fold scarps. This study focused on the fault-related fold at two locations, where the fold scarp is characterized with trench excavation and shallow cored boring results. The structural characteristics revealed by the two trench sites are consistent with a trishear fault-propagation fold growth above a blind thrust. Several characteristics of the fold observed in the Holocene deposits show smoothly rounded fold-hinges, unconformities, onlapped structure, and downward steepening of forelimb strata. Results from these structures suggest that the fold grows by progressive limb rotation of growth strata in sequential coseismic growth episodes. The growth strata show several unconformable contacts as indicated by paleosoil horizons developed on event horizons. Based on the syntectonic sedimentary structure, three events are revealed at the Siangong-Temple site and two paleoearthquake events on the Shijia site. Integration with the two trenches and the previous studies suggests the three paleoearthquake events occurred 300–430, 710–800, and 1710–1900 yr B.P. These data on the two trench sites indicate that the average slip rate is 4.2 and 4.5 mm/yr, respectively.