Denudation pattern across the Longriba fault system and implications for the geomorphological evolution of the eastern Tibetan margin

Highlights

• We document the denudation pattern across the Longriba fault system, east Tibet, with 10Be concentrations in river sediments

• A threefold increase in denudation rates across the LFS highlights its importance as a geomorphological boundary

• The LFS restrains the headward propagation of the drainage network into the low relief of the Tibetan Plateau

Abstract

Following the 2008 Wenchuan earthquake (Sichuan, China), the dextral strike-slip Longriba fault system (LFS) has been recognized as a main intracontinental structural boundary within the eastern Tibetan Plateau. While numerous studies have focused on the Longmen Shan frontal range to constrain the dynamics of the eastern Tibetan margin, little is known on the LFS, particularly on its eventual influence on the geomorphological evolution of the latter. Here, we provide a new data set of denudation rates derived from beryllium-10 concentrations in river sediments from 33 medium-sized catchments. Our sampling area covers the frontier between the dissected margin and the low relief interior plateau. Our results reveal a sharp increase of denudation across the LFS, from < 0.1 mm/y in the Ruoergai basin to 0.3 mm/y toward the Longmen Shan range. Such denudation pattern indicates a major morphotectonic control of the fault system on the eastern Tibetan margin evolution. Additional topographic analysis confirms the role of the LFS as an important geomorphological boundary, restraining the westward propagation of river incision into the low-relief areas, thus partly preventing the dismantling of the eastern Tibetan Plateau.

Introduction

Landscape evolution is intimately controlled by the competing influences of external and internal forcing mechanisms such as crustal deformation and climatic changes. For that reason, it is postulated that landscape morphology and the rates and patterns of surface processes can deliver important information about geodynamic processes (e.g., Seeber and Gornitz, 1983, Anderson, 1986, Burbank and Pinter, 1999, Kirby et al., 2003, Wobus et al., 2006, Bishop, 2007, Zhang et al., 2011, Kirby and Whipple, 2012). Quantification of denudation rates, derived from cosmogenic nuclide data in river sediments, can provide such clues on the recent evolution of tectonically active mountain belts. For instance, several studies have shown that the denudation patterns inferred from such inventories are primarily controlled by rock uplift distribution (Safran et al., 2005, Wittmann et al., 2007, Cyr et al., 2010, Kirby and Ouimet, 2011, Gudmundsdottir et al., 2013, Godard et al., 2014, Scherler et al., 2014, Morell et al., 2015). Moreover, the combination of these denudation rates with geomorphological analysis of hillslope and fluvial processes can also inform about features of landscape evolution such as drainage reorganization.

The eastern margin of the Tibetan Plateau (Fig. 1) has been a key research area over the last two decades for the study of crustal deformation mechanisms (Royden et al., 1997, Royden et al., 2008, Clark and Royden, 2000, Tapponnier et al., 2001, Clark et al., 2005, Hubbard and Shaw, 2009, Qi et al., 2011, Tian et al., 2013, Zhang, 2013) and their links with surface processes (Kirby et al., 2000, Kirby et al., 2003, Kirby and Ouimet, 2011, Zhang et al., 2011). Indeed, the apparently peculiar setting of this plateau margin attracted much attention as its elevation rise is about 3500 m over only 50 km (Fig. 2), but only accommodates ~ 3 mm/y of convergence over geodetic timescales (Gan et al., 2007, Shen et al., 2009). The Longmen Shan range, located at the very edge of the margin, directly west of the Sichuan basin, demonstrated its high seismogenic potential thanks to two major and devastating earthquakes, which respectively occurred in 2008 (Mw 7.9) and 2013 (Mw 6.6) (e.g., Liu-Zeng et al., 2009, Xu et al., 2009, De Michele et al., 2010, Lin et al., 2010) (Fig. 1). This further increased the interest for a better understanding of the geodynamics and geomorphological evolution of this region.

Because of its geographical location and the occurrence of these earthquakes, the Longmen Shan range has been postulated to be a major feature in the eastern Tibetan margin evolution (e.g., Chen and Wilson, 1996, Arne et al., 1997) (Fig. 1). However, recent geodetic results and field observations have highlighted the importance of the Longriba fault system (LFS), located 200 km northwest of the Longmen Shan, as a major structural boundary between crustal blocks, with an important role in stress and strain field distribution (Xu et al., 2008, Shen et al., 2009, Ren et al., 2013a). While some results are available concerning the tectonic activity of the LFS, from a geomorphological perspective little is known about its contribution to the evolution of the eastern Tibetan margin.

In this paper we interpret and integrate combined morphological analysis and denudation rates derived from beryllium-10 concentrations in quartz (¹⁰Be_qtz) from river sediments to assess the geomorphological importance of the LFS at the scale of the plateau margin. We are specifically interested in documenting the denudation pattern in the area located west of the Longmen Shan range and studying its evolution across the LFS in relation with the regional and local tectonic context.