Quantification of three‐dimensional folding using fluvial terraces: A case study from the Mushi anticline, northern margin of the Chinese Pamir

Abstract

[1] Fold deformation in three dimensions involves shortening, uplift, and lateral growth. Fluvial terraces represent strain markers that have been widely applied to constrain a fold's shortening and uplift. For the lateral growth, however, the utility of fluvial terraces has been commonly ignored. Situated along northern margin of Chinese Pamir, the Mushi anticline preserves, along its northern flank, flights of passively deformed fluvial terraces that can be used to constrain three-dimensional folding history, especially lateral growth. The Mushi anticline is a geometrically simple fault-tip fold with a total shortening of 740 ± 110 m and rock uplift of~1300 m. Geologic and geomorphic mapping and dGPS surveys reveal that terrace surfaces perpendicular to the fold's strike display increased rotation with age, implying the fold grows by progressive limb rotation. We use a pure-shear fault-tip fold model to estimate a uniform shortening rate of 1.5 + 1.3/À0.5 mm/a and a rock-uplift rate of 2.3 + 2.1/À0.8 mm/a. Parallel to the fold's strike, longitudinal profiles of terrace surfaces also display age-dependent increases in slopes. We present a new model to distinguish lateral growth mechanisms (lateral lengthening and/or rotation above a fixed tip). This model indicates that eastward lengthening of the Mushi anticline ceased by at least~134 ka and its lateral growth has been dominated by rotation. Our study confirms that terrace deformation along a fold's strike not only can constrain the lateral lengthening rate but can serve to quantify the magnitude and rate of lateral rotation: attributes that are commonly difficult to define when relying on other geomorphic criteria.

Active flexural‐slip faulting: A study from the Pamir‐Tian Shan convergent zone, NW China

“…Since late Miocene‐Pliocene times, deformation along the eastern margin of the Pamir ceased [ Cowgill , ; Sobel et al ., , ], and the Pamir welded together with the Tarim Basin to collide with the Tian Shan. This tectonic transformation has caused deformation to transfer from both the Pamir's eastern margin and its northwestern extension to the Pamir‐Tian Shan convergent zone, which includes both the Pamir Frontal Thrust, a fault formed by northward propagation of the Pamir with a Quaternary shortening rate of ~6–7 mm/a [ Li et al ., , ], and the Kashi‐Atushi fold‐and‐thrust system, which formed by southward propagation of the southern Tian Shan with a Quaternary shortening rate of ~5 mm/a [ Chen et al ., ; Scharer et al ., ; Heermance et al ., ]. Active thrust faulting and folding is widely distributed across this zone (Figure ).…”

Hinge‐migrated fold‐scarp model based on an analysis of bed geometry: A study from the Mingyaole anticline, southern foreland of Chinese Tian Shan

“…To the west, the Atushi‐Kashi fold belt interferes with the Pamir Frontal Thrust, a fault formed by northward propagation of the Pamir and accommodating a Quaternary shortening rate of ~5–7 mm/a [ Li et al ., ; Thompson et al ., ]. This interference zone is characterized by widely distributed active thrust faulting and folding [ Li et al ., , , , ]. A geodetically defined convergent rate of ~7–10 mm/a [ Yang et al ., ; Zubovich et al ., ] and strong earthquake activity [ Feng , ] reflect the ongoing and concentrated convergence.…”

“…Four samples taken from the T3 terrace give ages in a range of ~12–16 ka (Table ), and we use the average age of 14.6 ± 2.6 ka as the terrace age. This age correlates with the age of regional Last Glacial Maximum terrace of ~14–18 ka [ Li et al ., , ; Thompson , ] and implies that the T3 terrace formed in response to climate change. Ages of the two samples taken from the T1 surface are similar, and their average age of 4.5 ± 0.8 ka is used to represent the terrace age.…”

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

“…Because the underlying thrust ramp is parallel with the hanging wall beds, fault slip increments can be calculated from formula to be ~31 and ~9 m since abandonment of the T3a and T2a terraces, respectively. The terrace age has not been determined; however, existing dated terraces in the Pamir and Tian Shan regions indicate that spatially extensive strath terrace formation is typically climatically controlled, and terraces are most commonly abandoned during glacial‐interglacial transitions (e.g., Huang et al, ; Li et al, ; Lu et al, ; Molnar et al, ; Pan et al, ). If we assign an age of ~15 ka (transition from Marine Isotope Stages 2 to 1; Lisiecki & Raymo, ) to the T2 terrace (similar to the age of terrace T3 at Kalangoulvke South and terrace T3 at Wulagen), the slip rate of the thrust controlling the Bashjiqike fold is estimated to be ~0.6 mm/a.…”

“…Due to feedback, these secondary structures will influence fold development, for example, the fold geometry, the causative fault plane formation, and upward propagation (Mitra, ; Niño et al, ; Roering et al, ). Diverse mechanisms work together to produce various folding‐related geomorphic expressions, such as fold scarps (e.g., Hubert‐Ferrari et al, ; Le Béon et al, ; Li et al, ; Thompson et al, ), land‐surface tilting and warping (e.g., Daëron et al, ; Li et al, ; Rockwell et al, ; Saint‐Carlier et al,; Simoes et al, ), and flexural‐slip and bending‐moment fault scarps (e.g., Huang et al, ; Li et al, , ; Rockwell et al, ; Yeats et al, ), as well as an assemblage of different patterns (e.g., Ishiyama et al, ; Kelsey et al, ; Li et al, ; Philip and Meghraoui, ).…”