Active Flexural‐Slip Faulting: Controls Exerted by Stratigraphy, Geometry, and Fold Kinematics

Abstract

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.

Cumulative and Coseismic (During the 2016 Mw6.6 Aketao Earthquake) Deformation of the Dextral‐Slip Muji Fault, Northeastern Pamir Orogen

“…The Pamir thrust system, characterized by multiple east-trending thrust fault splays (Figure 1b), localizes most of the Cenozoic convergence between the northern Pamir and Asia (Burtman & Molnar, 1993). Presently, it continues to accommodate significant regional convergence, manifested by strong active thrusting and folding deformation (Arrowsmith & Strecker, 1999;Li et al, 2012Li et al, , 2013Li et al, , 2015Li et al, , 2017Thompson Jobe et al, 2017), a shortening rate of up to 10-15 mm/a from GPS velocities (Zubovich et al, 2010), and a high level of seismicity (e.g., the 1974 M w 7.1 Markansu, the 1985 M w 6.9 Wuqia, and 2008 M w 6.6 Nura earthquakes ; Burtman & Molnar, 1993;Li et al, 2019;Sippl et al, 2014;Teshebaeva et al, 2014;Figure 1b al., 2013;Thomas et al, 1994). The Tajik thrust system has accommodated a total shortening of~30 km since middle Miocene (Chapman et al, 2017), whereas the displacement of the Darvaz Fault has not been well constrained.…”

Along‐Strike and Downdip Segmentation of the Pamir Frontal Thrust and Its Association With the 1985Mw6.9 Wuqia Earthquake

“…(a) Major Cenozoic structures and seismicity and (b) geologic map of the Pamir foreland thrust system and adjacent areas (modified from T. Li et al, ). Lines A to G depict locations of geological cross sections in Figures .…”

Active Bending‐Moment Faulting: Geomorphic Expression, Controlling Conditions, Accommodation of Fold Deformation

“…In areas dominated by active folds, flexural‐slip faults (FSFs) and bending‐moment faults (BMFs) can slice through the land surface and produce a series of subparallel, closely‐spaced geomorphic scarps (Figure c; e.g., Audin et al, ; T. Li et al, , ; Philip & Meghraoui, ; Yeats et al, ). Although the sense, orientation, and displacement of these faults cannot be extrapolated to those of the underlying causative thrust, they can provide useful information on folding kinematics (e.g., Ismat, ; T. Li et al, ; Salvini & Storti, ) and the paleoearthquake history of the underlying thrust (e.g., Gutiérrez et al, ; Kelsey et al, ; Livio et al, ; McCalpin, ). FSFs and BMnFs, as two typical secondary fault styles accommodating folding deformation, have been commonly observed in rock outcrops (e.g., Bazalgette et al, ; Fischer & Jackson, ; Gutiérrez‐Alonson & Gross, ; Horne & Culshaw, ; Tanner, ).…”

“…The Pamir‐western Kunlun and southern Tian Shan regions, northwestern China, are dominated by active thrusting and folding in their piedmonts and major intermontane basins (e.g., Allen et al, ; Goode et al, ; Huang et al, ; Hubert‐Ferrari et al, ; T. Li et al, , ; Saint‐Carlier et al, ; Scharer et al, ; S. C. Thompson et al, ; Thompson‐Jobe et al, ). Previous studies (Avouac & Peltzer, ; Heermance et al, ; Huang et al, ; T. Li et al, , ) and our new surveys document numerous sets of well‐developed FSF scarps and bending‐moment normal fault (BMnF) scarps (Figure ). The majority of FSF scarps are present in the western corner of the Tarim Basin where the Pamir and southern Tian Shan are converging at a high rate of ~7–11 mm/a (T. Li et al, ; Thompson‐Jobe et al, ), whereas the majority of BMnF scarps are present on the western Kunlun piedmont, where the modern convergent rate is <2.0 mm/a.…”

“…The majority of FSF scarps are present in the western corner of the Tarim Basin where the Pamir and southern Tian Shan are converging at a high rate of ~7–11 mm/a (T. Li et al, ; Thompson‐Jobe et al, ), whereas the majority of BMnF scarps are present on the western Kunlun piedmont, where the modern convergent rate is <2.0 mm/a. Through our recent work on prominent FSF scarps at eight sites (T. Li et al, , ), their geomorphic expression, diverse outcrop settings, and favorable conditions for formation (including bed lithology and dip, slip‐surface spacing, and folding kinematics) have been investigated and systematically summarized. In this study we focus on the BMnF scarps at four sites: Slik, Cele, Haermodun, and Atushi.…”