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This study aims at further documenting the mechanisms of shortening at the front of fold‐and‐thrust belts. We focus on an actively growing anticline located at the deformation front of the Taiwan fold‐and‐thrust belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the Tainan anticline is a pure‐shear fault‐bend fold growing above a 38–45° west dipping back thrust, the Houchiali fault, rooted on a 3.8‐km‐deep detachment. The cumulative shortening is estimated at 2–3 km since 310 ± 50 ka, including ~30–50% of horizontal compaction shortening. The significance of the fold is little in terms of total shortening at the scale of the mountain piedmont, yet the Holocene shortening rate of 10.3 ± 1.0 mm/a accounts for 25% of the present‐day shortening rate across the piedmont. Earthquake scaling relationships applied to the Houchiali fault predict Mw~6 earthquakes that would occur a lot more frequently than indicated from historical earthquake catalogs. Hence, the aseismic slip behavior observed from geodetic measurements since two decades is a representative behavior of the fault at least at the scale of a few centuries. Our results bear out the dominance of pure‐shear folding at the front of fold‐and‐thrust belts and support horizontal compaction as a significant shortening mechanism. In contrast, the back thrust wedge structure and the aseismic slip are peculiar characteristics that likely arise from the combination of low friction and high‐pore pressure related to the thick mudstone formation hosting the wedge and of high syntectonic sedimentation rates. Plain Language Summary: The toe of active mountain belts often concentrates human population, fertile land, and economic centers. They are also the location of active faults that expose the population to seismic hazard. This study investigates how compressive tectonic forces are consumed at the front of mountain belts. We focus on an actively growing anticline located at the front of the Taiwan mountain belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the fold grows by thickening of the geological strata due to the westward displacement of a wedge structure defined by a west dipping fault and a 3.8‐km‐deep detachment (i.e., a subhorizontal bedding‐plane fault). The fold has little significance when considering the history of deformation at the scale of the mountain belt; however, it has been a major player since 10,000 years, as it concentrates 25% of the tectonic activity across the piedmont. The lack of destructive earthquakes in historical accounts during the last 300 years shows that the tectonic forces are mainly consumed by slow and nearly steady free slip on the fault, as observed from geodetic methods during the last two decades, rather than by earthquakes. Key Points: Seismic reflection profiles and present‐day crustal deformation indicate a shear fold growing on a back thrust rooted on a shallow detachmentArea‐depth analysis and porosity gradients help us estimate the shortening components due to folding and to horizontal compactionStresses due to rapid Holocene and present‐day shortening rates of ~12 mm/a are likely released by transient aseismic slip [ABSTRACT FROM AUTHOR]

The inference of fault geometry from suprajacent fold shape relies on consistent and verified forward models of fault-cored folds, e.g. suites of models with differing fault boundary conditions demonstrate the range of possible folding. Results of kinematic (fault-parallel flow) and mechanical (boundary element method) models are compared to ascertain differences in the way the two methods simulate flexure associated with slip along flat-ramp-flat geometry. These differences are assessed by systematically altering fault parameters in each model and observing subsequent changes in the suprajacent fold shapes. Differences between the kinematic and mechanical fault-fold relationships highlight the differences between the methods. Additionally, a laboratory fold is simulated to determine which method might best predict fault parameters from fold shape. Although kinematic folds do not fully capture the three-dimensional nature of geologic folds, mechanical models have non-unique fold-fault relationships. Predicting fault geometry from fold shape is best accomplished by a combination of the two methods. [Copyright &y& Elsevier]

Analog modeling of a flat-ramp-flat fault system was performed and its geometry and displacement field were compared to those of different kinematic models such as classical fault bend folding, fault parallel flow, incline-shear, curvilinear hinge, and backlimb trishear. To obtain the displacement vectors of the analog experiment, a Particle Image Velocimetry was performed. All analyzed kinematic models could explain the general configuration of the fault bend folding. However, only backlimb trishear could represent the geometry, directions of particle displacements, and relations between the displacements' vectors. We propose in this paper that the combination of different asymmetry angles and different apical angles of the backlimb trishear model for each bend in a fault bend fold could be a very versatile and general kinematic model for simulating fault bend folds. Backlimb trishear apical angle can be used to control the shape of the hinges of a fold, while the asymmetry can be used to convolve the velocity of the particles above the fault. Both apical angle and asymmetries different from zero imply thickness changes. Fault bend folds with high inclination forelimbs can be reproduced with high positive asymmetries in the anticline bends of the fault. • Analog model was performed to obtain the velocity field during deformation. • Documentation of velocity vectors comes from particle image velocimetry. • Backlimb trishear can represent the geometry and directions of particles velocities. • Steeper-dipping forelimbs can be reproduced using high positive asymmetries. • Backlimb trishear apical angle can be used to control the shape of the hinges. [ABSTRACT FROM AUTHOR]