Post-orogenic topographic evolution of the Dabie orogen, Eastern China: Insights from apatite and zircon (U-Th)/He thermochronology

Highlights

• The Dabie orogen forms a tectonic and climatic boundary in Eastern China.

• Thermochronological data collected along horizontal transects and vertical profiles

• Transect/profile thermochronology results used for topography modeling

• Rapid exhumation from 70 to 40 Ma along NE 2-D transect segment linked to faulting

• Rapid exhumation from 50 to 30 Ma along SE 2-D transect segment linked to climate change

Abstract

This paper presents an approach to improve the previously proposed 2-D topography modeling (Ding et al., 2019) that is primarily based on inversion of low-temperature thermochronological data. The new approach requires thermochronological data from both horizontal transect and vertical profiles to yield a more comprehensive modeling of topographic evolution. This modeling allows for (1) reconstruction of paleotopography evolution and (2) calculation of spatial distribution of paleo-geothermal gradients over time; therefore, it is possible to deduce more reliable exhumation histories even for non-steady state topographic evolution. The deduced exhumation histories are used to re-evaluate the topography modeling results. This approach when applied to the Dabie orogen in Eastern China suggests an uneven topographic evolution along the Xishui-Lu'an section and comparable exhumation histories for the Meichuan, the Tianzhushan, the Mingtangshan and the Tiantangzhai vertical profiles since the Late Cretaceous. The modeling results suggest that the location of the highest elevation in the NE-SW transect has been almost identical for the last 80 Ma although the elevation of the peak has reduced by ~3.3 km over the same period. The elevation reduction in the NE segment during 70–40 Ma is caused by extensional activity of the Xiaotian-Mozitan fault. The elevation reduction in the SW segment during 50–30 Ma is probably related to climate change, where the Paleogene topography change in the Dabie orogen played an important role introducing humid and warm air from the Pacific Ocean.

Introduction

Topography represents surface expressions derived from complex interactions between tectonics, climate, and surface processes. Paleotopography, is a fundamental component of geomorphology because it provides the most direct description of surface expression in the past, and has been quantitatively studied using various tools of paleoaltimetry (Meyer, 2007; Kohn and Dettman, 2007; Sahagian and Proussevitch, 2007; Riihimaki and Libarkin, 2007; Reiners, 2007; Quade et al., 2011). For an orogen-scale reconstruction of paleotopography (commonly in the scale of ~100s of kilometers in length; 100s to 1000s of meters in elevation; and over 10s to 100s of Myrs in time), low-temperature thermochronometry is most widely applied (Ehlers and Farley, 2003; Reiners and Shuster, 2009; Valla et al., 2010); in some cases in combination with heat-transfer modeling in the Earth's crust (van der Beek et al., 2010; Guillaume et al., 2013; Wang et al., 2016). Although this approach is primarily limited to the sensitivity of available thermochronometers to temperature (therefore to different depths or elevations), it can provide a good first-order approximation of large-scale topographic evolution. In this paper, we show how thermochronometry can be used to simultaneously constrain 2-D paleotopographic evolution and exhumation histories of vertical sections.

Low-temperature thermochronology, primarily involved with fission track and (U-Th)/He dating, provides information about thermal history at temperatures corresponding to the shallow section (i.e. a few kms) of the Earth's crust (Reiners, 2007). A given set of thermochronological data can be used in numerical modeling implemented in computer software, such as HeFTy (Ketcham, 2005), QTQt (Gallagher, 2012), and Low-T Thermo (Ding, 2017), to simulate thermal histories. The conversion of the resulting thermal histories to topographic change has been a major challenge in reconstructing paleotopography because of the uncertainty of paleo-isotherms. After the pioneering studies to establish simple 1-D modeling in the early 2000's (Gleadow and Brown, 2000; Braun, 2002; Montgomery and Brandon, 2002), more advanced 3-D paleotopography modeling regarding the effects of the surface topographic change and exhumation rate on the shape of the paleo-isotherms became available (Braun, 2003, Braun, 2005; Braun et al., 2012). These approaches require high computing power for paleotopography reconstruction at variable time scales. The inversion modeling developed by Ding et al. (2019) requires low-T thermochronological data along a horizontal transect, and allows for reconstruction of the paleo-elevation variations along the same transect. In this first application of the method, Ding et al. (2019) showed how the thermochronological data (apatite and zircon (U-Th)/He), collected along a ~200 km-long horizontal transect cross-cutting the coastal mountain system (CMS) in China, can be used to model thermal history and 2-D topographic evolution (Fig. 1). This method, which requires only modest computing power, allows for reconstruction of paleotopography evolution and local relief variations along a cross-section. The inverse modeling yielded a rapid elevation reduction during 80–40 Ma as well as uneven topographic evolution of the CMS, which are consistent with the climatic changes observed in the study area.

In low-temperature thermochronology, two major sampling strategies are frequently employed: vertical sampling and horizontal cross-sectional sampling. Traditionally, the vertical sampling has been widely used to unravel the detailed exhumation history of mountain ranges (e.g., Reiners et al., 2003; Fitzgerald et al., 2006; Huntington et al., 2007; Restrepo-Moreno et al., 2009; van der Beek et al., 2010) or sedimentary basins (e.g., Green, 2002; Johnson and Gallagher, 2000; House et al., 2002; Zhang et al., 2019). This approach is very efficient to document a local exhumation history, but its applications to a regional area are seriously limited due to the localized sampling. To model the regional topographic evolution of a mountain range, it is essential to collect samples covering the entire mountain range, at least from a single transect preferentially perpendicular to the major structural orientations. Horizontal cross-sectional sampling is normally undertaken across tens to hundreds of kilometers long at similar elevations (House et al., 1998; Foeken et al., 2007), or at various elevations on the surface along a linear transect for sampling convenience (Ding et al., 2019). Such a sampling strategy warrants regional scale topographic modeling, although it does not guarantee a detailed exhumation history for individual sampling locations. One potential drawback of this sampling strategy is that the samples are sparsely collected along a transect (e.g., every 10 km in distance); therefore, the modeling can be easily biased if one or two samples yielded incorrect thermochronological data. In this paper, we demonstrate that combining these two sampling strategies (vertical and horizontal) can improve the previously developed topography modeling (Ding et al., 2019).

To demonstrate this new approach, we used the example of the Dabie orogen because it is not only one of the most important watersheds in East China, but also it is located at the tectonic boundary between the North and South China Blocks. Thus, it provides an ideal opportunity to deconvolute the relative roles of tectonics and climate in topographic evolution. We obtained apatite and zircon (U-Th)/He data from a ~200 km cross-section covering the Dabie orogen, and performed inverse modeling to reconstruct 2-D post-orogenic topography evolution. In addition, we determined apatite (U-Th)/He ages from three vertical sections near the 2-D cross-section, and more detailed exhumation histories of all the vertical sections are simulated using the paleo-geothermal gradients calculated from the 2-D topography modeling. This information about paleo-geothermal gradients is crucial to model the exhumation history for vertical profiles.

It is noteworthy that our objective is reconstructing the paleotopographic evolution, which is the final outcome of numerous geologic, climatic and other surface processes. Therefore, these processes do not need to be included in the modeling as input parameters. Rather, the modeling results can be used to investigate these causal processes that formed the paleotopography. For example, previous topography modeling across the CMS yielded contrasting topographic evolutions for the NW and SE regions, which are bounded by a large fault system (Ding et al., 2019). This illustrates that the topography modeling can provide clues to understanding the causal processes (e.g., faulting) that formed the paleotopography.