Late Cretaceous – Cenozoic thermal structure and exhumation of the Eastern Tibetan Plateau margin: A doubly-vergent orogenic wedge

Highlights

• Low temperature thermochronology data for four vertical sections (outcrop and drill-hole), and linking surface samples, along a NW-SE transect across the 400 km-wide Eastern Tibetan Plateau margin, are presented and interpreted.

• Two episodes of exhumation have been identified during Late Cretaceous (~90–70 Ma) and Late Cenozoic (~27 Ma – Present-day) across the Eastern Tibetan Plateau margin.

• The total exhumation across the Eastern Tibetan Plateau margin partitions into Late Cretaceous (20%) and Late Cenozoic (80%).

• The Longmen Shan Thrust Belt in the transect we examined between the Longriba Fault System and the Anxian-Guanxian Fault is inferred to be a doubly-vergent orogenic wedge.

Abstract

The current structural model for the Longmen Shan (Mountain) belt, which is peripheral to the Eastern Tibetan Plateau, is an asymmetric orogenic wedge with a series of northwest-dipping listric thrust faults that sole-out into a sub-horizontal mid-crustal level detachment. Current debate, stemming from newly published data about the metamorphic petrology of greenschist and higher-grade basement rock facies within the Longmen Shan, coupled with numerical ages, centers around how exhumation of this margin has been partitioned into Cretaceous versus Late Cenozoic components. Here we report extensive new low-temperature thermochronology data for samples across the margin obtained from four vertical sections, one in outcrop and three in deep (6–7 km) drill-holes (including drill-hole HC-1 on the plateau), and interpret them, together with comparable data published for another two steep vertical sections in outcrop (Xuelongbao and Pengguan massifs). The results highlight the disrupted paleo-thermal structure of the upper crust underlying the margin, from which we map in space and time two episodes of exhumation, one during the Late Cretaceous (~90–70 Ma), and the other during the Late Cenozoic (~27 Ma – present day). Some 80% of the total exhumation occurred during ~27–0 Ma and the balance during ~90–70 Ma. A new feature in the distribution of Late Cenozoic exhumation across the margin is the surprisingly large amount southeast of the Longriba Fault System (LFS), which is located immediately east of the high plateau. This new component of exhumation diminishes the asymmetric character of the current structural model of the margin. We interpret the LFS as southeast dipping back-thrusts to the northwest dipping listric thrust faults within the eastern Longmen Shan. We therefore conclude that the Longmen Shan Thrust Belt is a doubly-vergent orogenic wedge peripheral to the Eastern Tibetan Plateau.

Introduction

The Eastern Tibetan Plateau margin, with mean surface elevations of 4–4.5 km above sea level, is boarded on its eastern side by a 400 km-wide system of mountain ranges with peaks reaching up to 6 km above sea level (Fig. 1, Fig. 2). Topographically and structurally it has some similarities with the southern margin of the Tibetan Plateau and its peripheral Himalayan Mountain Range system (Fig. 2; transects P1-P2 and P3-P4). Mean surface elevation falls to the east across the eastern mountain range system, most dramatically over the last 30 km of it, to a few hundred metres above sea level in the Sichuan Basin. The Longmen Shan is the name for the outer mountain ranges closest to the Sichuan Basin.

The structure of the upper crust beneath the mountain belt system involves several major northwest-dipping listric thrust faults considered to sole-out into a horizontal mid-crustal detachment (Hubbard and Shaw, 2009). This structural interpretation is consistent with other analyses of the 2008 Wenchuan Earthquake (de Michele et al., 2010; Qi et al., 2011; Fielding et al., 2013), although variations of it have been reported by Robert et al. (2010), Li et al. (2013) and Feng et al. (2016). The overall tectonic structure appears to conform to an intra-continental orogenic wedge. We use the informal term “Eastern Tibetan Plateau margin” to refer to this wedge as well as the easternmost part of the high Tibetan Plateau, and the western part of the Sichuan Basin that contains thrust faults associated with the wedge, as far as the Longquan Shan (range) within the basin. The Longmen Shan Thrust Belt (LSTB) has historically been used for the part of the imbricate wedge between the Sichuan Basin and the Wenchuan-Maoxian Fault (WMF), located some 40–50 km into the mountain belt from the Sichuan Basin. Here, we extend the LSTB as a name for the mountain range system as far west as the Longriba Fault System, which is the first major fault system east of the high plateau, such that the LSTB encompasses the whole of the wedge deformation zone. Our study is limited to the part of the eastern Tibetan Plateau margin, north of the Xianshuihe Fault, which is the northernmost of the major transcurrent faults that facilitate tectonic extrusion of the upper crust underlying the plateau towards the South China Block (Fig. 1).

Numerous low-temperature thermochronological studies have been undertaken within the LSTB during the past 25 years, attracted by the opportunity to constrain the timing and extent of exhumation of the LSTB. These studies have established two key features: (i) the greatest amount of exhumation has been associated with the highest elevation mountain ranges, and (ii), the Late Cenozoic timing of substantial exhumation (e.g. Arne et al., 1997; Xu and Kamp, 2000; Wang et al., 2012) conforms with the timing of substantive exhumation along the southern margin of the Tibetan Plateau and Himalaya Mountain Range (e.g. Copeland et al., 1995; Lee et al., 2011; Nadin and Martin, 2012; Carrapa et al., 2014). Numerous low-temperature thermochronological studies of the Eastern Tibetan Plateau margin, and in particular the Longmen Shan Thrust Belt (LSTB), as previously defined, have subsequently been published (e.g. Tian et al., 2013; Tan et al., 2014, Tan et al., 2019; Ansberque et al., 2018; see a review in Supplementary material online, which includes a comprehensive reference list), which have fleshed-out important details about the amounts, timing, and rates of Late Cenozoic exhumation within and between the major thrust faults of the LSTB.

Modern rates of geodetic strain markedly decrease across the Longriba Fault System (Guo et al., 2015) where the high plateau transitions from passive uplift, with minimal exhumation, into the imbricate wedge structure of the LSTB, characterized by pervasive uplift and exhumation. The Sichuan Basin developed as a 3–5 km deep foreland basin during the Late Triassic and Early Jurassic as a result of loading due to shortening of prior passive margin northwest-dipping normal faults and overthrusting of the Songpan-Ganzi Terrane across the zone of inverted structures (Chen et al., 1994; Burchfiel et al., 1995). Late Cenozoic shortening has accentuated the imbrication and steepened the faults close to the surface, resulting in limited further flexure of the basin and sediment accumulation. Various models have been proposed to accommodate the shortening within the lithosphere beneath the mid-crustal detachment fault. We do not comment upon this debate here, because low-temperature thermochronology data cannot uniquely discriminate between exhumation due to imbricate thrusting in the upper crust versus any passive exhumation of the upper crust inferred to have been driven from below it.

New petrological data on greenschist and higher-grade metamorphic rocks within the eastern part of the LSTB, coupled with new ⁴⁰Ar/³⁹Ar geochronological data, has started a debate about the proportion of Cretaceous versus Late Cenozoic exhumation across the LSTB (Tian et al., 2016; Xue et al., 2017, Xue et al., 2021; Airaghi et al., 2017a, Airaghi et al., 2017b, Airaghi et al., 2018a, Airaghi et al., 2018c; Zhang et al., 2020). However, most published low-temperature thermochronology studies within the eastern part of the LSTB have yielded Late Cenozoic ages, reflecting the lower closure temperatures of those systems. Here, we take two approaches to contribute to this debate: (i) we present data for samples from sites in western and eastern areas peripheral to the LSTB, both up on the plateau and in the western Sichuan Basin, and (ii), we present new data for four vertical sections in those peripheral areas together with linking surface samples. We integrate these data with published data for two additional vertical sections, one up the Pengguan Massif and the other up the Xuelongbao Massif (Wang et al., 2012; Furlong et al., 2021). This extended data set enables us to partition, in space and time, the Cretaceous versus Late Cenozoic components of exhumation across the whole of the Eastern Tibetan Plateau margin. In doing so, we appeal to interpretation of biotite Ar ages from samples of the Xuelongbao Massif to constrain the total amount of Late Cretaceous exhumation there. Of the vertical sections with new data, three are deep drill holes (6.6–7.2 km deep; HC-1, LS-1 & CK-1) and one is an outcrop vertical section (Yanggonghai granitoid). All of the apatite and zircon fission track and most of the zircon (U-Th)/He data for the six vertical sections (including the Pengguan and Xuelongbao massifs) and the linking surface samples were generated by the same analysts in the University of Waikato (New Zealand) laboratory.