Variations of Lateral Bedrock Erosion Rates Control Planation of Uplifting Folds in the Foreland of the Tian Shan, NW China

Abstract

Fluvial planation surfaces, such as straths, commonly serve as recorders of climatic and tectonic changes and are formed by the lateral erosion of rivers, a process that remains poorly understood. Here we present a study of kilometer‐wide, fluvially eroded, low‐relief surfaces on rapidly uplifting folds in the foreland of the southwestern Tian Shan. A combination of field work, digital elevation model analysis, and dating of fluvial deposits reveals that despite an arid climate and rapid average rock‐uplift rates of 1–3 mm/yr, rivers cut extensive (>1–2 km wide) surfaces with typical height variations of <6 m over periods of >2–6 kyr. The extent of this “beveling” varies in space and time, such that different beveling episodes affect individual structures. Between times of planation, beveled surfaces are abandoned, incised, and deformed across the folds. In a challenge to models that link strath cutting and abandonment primarily to changes in river incision rates, we demonstrate that lateral erosion rates of antecedent streams crossing the folds have to vary by more than 1 order of magnitude to explain the creation of beveled platforms in the past and their incision at the present day. These variations do not appear to covary with climate variability and might be caused by relatively small (much less than an order of magnitude) changes in sediment or water fluxes. It remains uncertain in which settings variations in lateral bedrock erosion rates predominate over changes in vertical erosion rates. Therefore, when studying fluvial planation and strath terraces, variability of both lateral and vertical erosion rates should be considered.

Temporal changes in rock uplift rates of folds in the foreland of the Tian Shan and the Pamir from geodetic and geologic data

“…Our data might indicate ongoing uplift along the South Tian Shan Thrust, but the Main Pamir Thrust, as well as the Tian Shan foreland north and east of the Atushi fold (including the Mutule fold), appear to be inactive (Figure 2). Modest earthquakes (M ≤ 5) that occurred between 2003 and 2010 and depths of 9-10 km ( Figure 2) are neither known nor expected to have produced surface displacements larger than the sensitivity of the InSAR (>4-10 cm over 7 years) [Bonilla, 1982;Biasi and Weldon, 2006]. Whereas slip rate variations during a seismic cycle could introduce uncertainties [Thatcher, 1984;Cattin and Avouac, 2000;Hilley et al, 2009], the observed uplift pattern is consistent with constraints on longer-term geologic uplift rates (Figure 3), as well as inferred basinward propagation of the deformation front over the past 25 Myr [Heermance et al, 2008;Chen et al, 2011;Thompson et al, 2015].…”

“…We compiled published initiation ages (0.4-1.6 Myr) and measured total structural relief (0.6-5.0 km) on published structural cross sections for the Mingyaole [Chen et al, 2005;Li et al, 2015], Kashi [Chen et al, 2007], Atushi [Scharer et al, 2004;Heermance et al, 2008], Mushi [Li et al, 2013;Thompson, 2013], and Mutule [Bufe et al, 2017] folds (Table S3 and Figure S8). As an exception, initiation of the Mutule fold was estimated from the published initiation age of the Atushi fold to the west [Chen et al, 2002;Scharer et al, 2004;Heermance et al, 2008] on the basis of interference between the two structures (Table S3).…”

“…Even structures lacking propagating tips might accelerate after initiation and decelerate toward the cessation of deformation [Gourmelen et al, 2011] or vary in a complex sequence of accelerations and decelerations [Chevalier et al, 2005;Gunderson et al, 2014]. Such gradual changes can only be detected in high-resolution uplift rate data sets obtained, for example, from well-dated terraces [Bourgois et al, 2007]. The single most likely scenario is shown in red.…”

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

“…The T3 tread, as the most spatially extensive surface, can be traced ~4 km along the fold transect (Heermance et al, ). According to cosmogenic 10 Be depth profile dating, the T3 tread was abandoned 70–90 ka (Bufe et al, ). Despite the presence of terrace levels, the river channel creating them was not been preserved and perhaps was diverted to flow along the fold's northern flank and finally joins the Boguzi River (Figure a), in response to continuing uplift of the fold.…”

“…Because the T3 tread does not span the entire fold width, this shortening represents a minimum shortening since the terrace abandonment. A combination with the terrace age of 70–90 ka (Bufe et al, ) yields a shortening rate of ≥0.3 mm/a. This rate appears to be much lower than the average Quaternary shortening rate of ~3.3 mm/a (Y. Chen et al, ).…”