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Abstract We present 230 field observations on the location, trending, and displacement vectors of the coseismic surface cracks induced by the 2016 Meinong earthquake in the Guanmiao area, SW Taiwan. Coseismic surface cracks trends from the north to northeast. The coseismic deformed region moved toward the W-WNW. In Guanmiao town, surface cracks were mainly distributed on both limbs of the Guanmiao syncline. The preseismic deformation was also observed along the axial trace of the Guanmiao syncline. These results give clues to high structural activities in SW Taiwan. We argue that Guanmiao syncline is an active fold with both coseismic activity and interseismic creeping, which induced nonnegligible micro-geohazards because of the continual loss. We report a new case of the normal bending-moment fault, the Luosianliao fault, which locates between the Guanmiao syncline and Chungchou anticline. However, the linkage between the shallow Luosianliao fault and the deep-seated causative fault of Guanmiao aftershocks are not known yet. We demonstrate the kinematic process of coseismic surface deformation and argue that the bending-moment fault could provide an opportunity to understand the recurrence interval of folding. The mechanism of earthquake-induced folding amplification through high fluid-pressure rocks may play a critical role in assessing earthquake hazard risks in regions with similar geology to SW Taiwan.

We present 230 field observations on the location, trending, and displacement vectors of the coseismic surface cracks induced by the 2016 Meinong earthquake in the Guanmiao area, SW Taiwan. Coseismic surface cracks trends from the north to northeast. The coseismic deformed region moved toward the W-WNW. In Guanmiao town, surface cracks were mainly distributed on both limbs of the Guanmiao syncline. The preseismic deformation was also observed along the axial trace of the Guanmiao syncline. These results give clues to high structural activities in SW Taiwan. We argue that Guanmiao syncline is an active fold with both coseismic activity and interseismic creeping, which induced nonnegligible micro-geohazards because of the continual loss. We report a new case of the normal bending-moment fault, the Luosianliao fault, which locates between the Guanmiao syncline and Chungchou anticline. However, the linkage between the shallow Luosianliao fault and the deep-seated causative fault of Guanmiao aftershocks are not known yet. We demonstrate the kinematic process of coseismic surface deformation and argue that the bending-moment fault could provide an opportunity to understand the recurrence interval of folding. The mechanism of earthquake-induced folding amplification through high fluid-pressure rocks may play a critical role in assessing earthquake hazard risks in regions with similar geology to SW Taiwan. Key points: Active Guanmiao syncline induced continual and nonnegligible micro-geohazards. We report a bending-moment fault induced by the Meinong earthquake. Earthquake-induced folding amplification is critical to assess hazards in SW Taiwan. [ABSTRACT FROM AUTHOR]

Abstract We investigated the relationship between contractional deformation of sedimentary mass in the upper crust and the geodetic strain rate in a high-strain zone (part of the Niigata–Kobe Tectonic Zone) of the mid-Niigata region, Central Japan. We observed numerous examples of layer-parallel slip (bedding-plane slip) generated by folding. The slip layers have an average spacing of ~ 3.6 m (measured normal to bedding planes) at the Katagai site. In shallow sedimentary units of the upper crust, numerous bedding-plane slip events act to reduce the mechanical strength (effective elastic thickness) of sedimentary rocks under contractional deformation. The results support an existing model, based on geodetic observations, of mechanical decoupling between the weak sedimentary layers and basement. On the layer-parallel slip, we measured the friction coefficient of gouge generated by bedding-plane slip and of mud around non-slip surfaces using double-direct shear tests, and found no difference in frictional coefficient between slip and non-slip surfaces.

Fold scarps, a type of geomorphic scarp formed by folding mechanisms of hinge migration or limb rotation, serve to delineate both fault-bend characteristics and folding histories, which can, in turn, illuminate tectonic processes and seismic hazards associated with thrust systems. Because the subsurface geometry of folds is commonly difficult to determine, existing fold-scarp models, which rely on both the fold type and its causative fault geometries, remain uncertain with respect to the kinematic evolution of a given fold. In this paper, we develop a model to illustrate that, irrespective of specific fold type and subsurface geometries, fold-scarp growth in the mechanism of hinge migration can be successfully reconstructed based on analyses of bed geometry. This model reveals that the underlying bed dips and the ratio of hinge migration distance/hinge width control the fold-scarp shape and slope. During initial growth (ratio1), the slope reaches a maximum, which solely depends on underlying bed dips. The scarp height, however, is independent of the hinge width and can be used to quantify folding magnitude. Application of our model to fold scarps in the Mingyaole anticline in the southern foreland of Chinese Tian Shan indicates that the modeled fold-scarp geometry can roughly match with field observations. The Mingyaole shortening rate is estimated to be ≥5.0 mm/a since ~15ka, such that this single fold has accommodated about half of the regional convergence during the Holocene. Key Points Folding through curved hinges creates a distinctive fold-scarp geometry Scarp slope relies on ratio of hinge migration/width, but scarp height is not Shortening rate of the Mingyaole fold is estimated to be ~5.0 mm/a since ~15 ka, such that this single fold has accommodated about half of the regional convergence during the Holocene.

The flexural-slip fault (FSF), a type of secondary fault generated by bed-parallel slip, occurs commonly and plays an important role in accommodating fold growth. Although the kinematics and mechanics of FSFs are well studied, relatively few field observations or geometric models explore its geomorphic expression. In the Pamir-Tian Shan convergent zone, NW China, suites of well-preserved FSF scarps displace fluvial terraces in the Mingyaole and Wulagen folds. Integrating interpretations of Google Earth images, detailed geologic and geomorphic mapping, and differential GPS measurements of terrace surfaces, we summarize geomorphic features that typify these faults and create kinematic models of active flexural-slip faulting. Our study indicates the following: (i) FSF scarps commonly occur near synclinal hinges, irrespective of whether (a) the dip direction of beds on either side of the hinge is unidirectional or in opposite directions, (b) the hinge is migrating or fixed, or (c) the hinge shape is narrow and angular or wide and curved. (ii) Active FSFs are likely to produce higher scarps on steeper beds, whereas lower or no topographic scarps typify gentler beds. (iii) Tilt angles of the terrace surface displaced above FSFs progressively decrease farther away from the hinge, with abrupt changes in slope coinciding with FSF scarps; the changes in tilt angle and scarp height have a predictable geometric relationship. (iv) Active FSFs can accommodate a significant fraction of total slip and play a significant role in folding deformation. (v) Active FSFs may be used to assess seismic hazards associated with active folds and associated blind thrusts.

We investigated the relationship between contractional deformation of sedimentary mass in the upper crust and the geodetic strain rate in a high-strain zone (part of the Niigata–Kobe Tectonic Zone) of the mid-Niigata region, Central Japan. We observed numerous examples of layer-parallel slip (bedding-plane slip) generated by folding. The slip layers have an average spacing of ~ 3.6 m (measured normal to bedding planes) at the Katagai site. In shallow sedimentary units of the upper crust, numerous bedding-plane slip events act to reduce the mechanical strength (effective elastic thickness) of sedimentary rocks under contractional deformation. The results support an existing model, based on geodetic observations, of mechanical decoupling between the weak sedimentary layers and basement. On the layer-parallel slip, we measured the friction coefficient of gouge generated by bedding-plane slip and of mud around non-slip surfaces using double-direct shear tests, and found no difference in frictional coefficient between slip and non-slip surfaces. [ABSTRACT FROM AUTHOR]

Quantification of crustal shortening rates across foreland fold-and-thrust belts is essential for evaluating how convergence is accommodated throughout the orogenic belt and analyzing seismic hazard in neighboring cities. In this study, we focused on active fold along the western portion of the Qilian Shan Foreland Thrust Belt in NE Tibet. The shortening rate across the Hujiatai Anticline (HA) along the eastern segment of the Fodongmiao-Hongyazi Fault (FHF) is 0.8 ± 0.2 mm/a based on terrestrial cosmogenic nuclide (TCN) dating of deformed Late Pleistocene fluvial terrace. When this rate is combined with the shortening rates of 1.1–1.4 mm/a and 0.7–1.2 mm/a along the western and central segments obtained by previous studies, it is apparent that crustal shortening rates along the FHF gradually decrease to the east. This eastward decrease in fault activity may be a result of the northward propagation of the FHF or partitioning of strain off to nearby faults or folds. Furthermore, tectonic geomorphology and trenching suggest that several thrust faults around the HA ruptured during the historical 1609 earthquake. Combining the shortening rate of 0.6–0.8 mm/a across the north side of the Jiudong basin with our rate, we suggest that the total crustal shortening rate across the basin is about 1.3–2.2 mm/a assuming little internal deformation, which accounts for roughly 20–40% of the amount of crustal shortening deformation across the Qilian block. These results demonstrate that the Qilian Shan Foreland basin plays an important role in accommodating the crustal deformation of northeastern Tibet. [ABSTRACT FROM AUTHOR]

Abstract: Bending‐moment faults and flexural‐slip faults (FSFs), as two basic fault styles due to bending‐related tangential longitudinal strain, extensively and prominently crop out as surface scarps in the Pamir‐western Kunlun and southern Tian Shan regions, northwestern China. Characteristic geomorphic expression, favorable formation conditions, and the role in folding accommodation of active FSFs have been systematically summarized in our recent studies. Here we investigate similar properties for well‐developed bending‐moment normal fault (BMnF) scarps at four sites. Our study concludes the following: (i) BMnF scarps are relatively sinuous compared to FSF scarps and probably trend obliquely to the fold hinge. A group of BMnF scarps can delineate a single asymmetric graben or create grabens alternating with horsts. (ii) BMnF scarps primarily overlie poorly‐layered conglomerates. The fold's interlimb angle can range from ~160° to <40°, and the folding kinematics can vary from active‐hinge migration to fixed‐hinge rotation. (iii) The fault‐zone width, fault spacing, and efficiency in folding accommodation significantly decrease with (a) thinner conglomerate beds, (b) a smaller interlimb angle, and (c) the transition of the hinge from migrated to fixed. (iv) Different bed lithologies and fold geometries beneath the surface account for the predominance of BMnF scarps on the western Kunlun piedmont and FSF scarps in the Pamir‐Tian Shan convergent zone. (v) Presence of BMnF scarps on the western Kunlun piedmont indicates that ~4 km of fault slip is transferred northward along a detachment at the base of the Cenozoic and is ultimately absorbed by the Mazatagh Thrust in the Tarim Basin. [ABSTRACT FROM AUTHOR]

Ongoing deformation processes combining fault propagation and folding are the cause of diffuse seismicity in many areas of the world. A detailed understanding of the structural evolution of tectonically active folds is crucial for the evaluation of seismic hazard. This paper proposes an integrated analysis of an active fold, consisting in the development of a 3D model by combination of geomorphological observations, field geological-structural data and seismic reflection sections. Our case study is the Tsaishi anticline, located at the southwestern tip of the Rioni Basin uplifted area, at the foothill of Greater Caucasus (Western Georgia). We recognized that the fold started to form since the beginning of the middle Miocene, although preliminary data suggest the possibility of initial local uplift in the Oligocene. Folding process continues up to nowadays giving rise to a south-vergent anticline, as shown by upwarped late Quaternary river deposits. The fold backlimb is affected by three main back-thrusts, whereas at the foot of the forelimb a main north-dipping thrust comes very close to the surface based on seismic sections. Here, field data show the presence of a 13-km-long fault scarp (or fold scarp). Along the scarp is located the epicenter of the strongest earthquake to date: the M S 6.0 Tsaishi earthquake that struck the area in 1614 CE. Based on our results, we propose that the overall structure can be classified as an active fault-propagation fold. The recognition of its very recent growing associated with a major, underlying active fault, represents also a major contribution to the seismic hazard assessment of this populated area. [ABSTRACT FROM AUTHOR]

Flexural slip plays an important role in accommodating fold growth, and its topographic expression, flexural-slip fault (FSF) scarps, may be one of the most commonly occurring secondary structures in areas dominated by active thrusts and folds. Where FSF scarps are present and what factors control their occurrence, however, are typically poorly known. Through an investigation of clearly expressed FSF scarps, well-preserved fluvial terraces, and well-exposed bedrock at eight sites in the Pamir-Tian Shan convergent zone and Kuche fold belt, NW China, we summarize the most favorable conditions for active flexural-slip faulting. Our study yields six key results. First, flexural slip operates commonly in well-layered beds, although uncommonly can occur in massive, poorly layered beds as well. Second, in well-layered beds, the slip surface is commonly located either (a) close to the contact of competent and incompetent beds or (b) within thin incompetent beds. Third, FSF scarps are always found overlying steep beds with dips of ~30-100°. Fourth, slip surfaces are typically spaced between ~10 and 440 m but can reach up to ~600 m. Fifth, FSF scarps at most sites can be observed far away from the hinge-migrated fold scarps, suggesting that compared to hinge migration, limb rotation is generally required to accumulate flexural slip and produce associated topographic scarps. Finally, a higher regional convergent rate seems to facilitate the creation of FSF scarps more often than lower rates, whereas well-preserved, old terraces capped by thin deposits are more likely to record FSF scarps than unevenly preserved, young terraces with thick sedimentary caps. [ABSTRACT FROM AUTHOR]

Abstract: This paper uses high-resolution images and field investigations, in conjunction with seismic reflection data, to constrain active structural deformation in the Kashan region of Central Iran. Offset stream beds and Qanats indicate right-lateral strike slip motion at a rate of about 2mm/yr along the NW–SE trending Qom-Zefreh fault zone which has long been recognized as one of the major faults in Central Iran. However, the pattern of drainage systems across the active growing folds including deep incision of stream beds and deflected streams indicate uplift at depth on thrust faults dipping SW beneath the anticlines. Therefore, our studies in the Kashan region indicate that deformation occurs within Central Iran which is often considered to behave as a non-deforming block within the Arabia–Eurasia collision zone. The fact that the active Qom-Zefreh strike-slip fault runs parallel to the active folds, which overlie blind thrust faults, suggests that oblique motion of Arabia with respect to Eurasia is partitioned in this part of Central Iran. [Copyright &y& Elsevier]

The east-west trending Qiulitage anticline is one of the most active elements of the Kuqa fold-and-thrust belt located along the southern margin of the Tianshan Mountains. Subsurface data and the well-preserved tectonic geomorphology provide a natural laboratory for exploring the detailed spatio-temporal evolutionary processes of a "mature fold". We present a study of fluvial/alluvial terraces which passively record the progressive deformation of active folds across the rapidly uplifting Qiulitage anticline. The vertical incision and lateral erosion rates of the Kezile river derived from 10Be exposure dating are ~1.3–2.0 mm/a and >1.3 mm/a, respectively, while those of the Bositankelake River, located near the central part of the fold, are only ~0.8 ± 0.04 mm/a and <0.8 ± 0.04 mm/a, respectively. This difference indicates that the lateral propagation rate of active fold increases gradually away from its central peak to its tip. In the meanwhile, a comprehensive analysis of stratigraphy, landforms, dominant faults at depth, and inherited drainage patterns reveal that the eastern segment of the Qiulitage anticline initially develop from the linkage of two smaller fold, i.e. the Kuqadawu anticline and the East Qiulitage anticline, and subsequently this fold assemblage has propagated eastward. Although there is an apparent topographic-relief transfer relationship between the eastern segment of the Qiulitage anticline and the adjacent Yaken anticline, a long-term spatio-temporal variation in surficial erosion may have accentuated this relationship by significantly reducing the elevation of the Qiulitage anticline near its central peak. [Display omitted] • Study of fold lateral growth, Kuqa fold-thrust belt, Tianshan Mountains • Fluvial terraces record progressive surface deformation of the Qiulitage anticline. • 10Be exposure dating shows spatial and temporal erosion patterns across the anticline. • Geomorphology and erosion data suggest anticline growth via linkage of smaller folds. • Erosion reduces fold elevation over time, supporting tectonic fold growth models. [ABSTRACT FROM AUTHOR]