Post-orogenic evolution of the Dabie Shan, eastern China, from (U-Th)/He and fission-track thermochronology

Abstract

The Dabie Shan of eastern China is a ϳ200 kilometers wide mountain range with nearly 2 kilometers of relief and is an archetype of deep ultrahigh-pressure metamorphic rock exhumation. Despite its regional and petrologic importance, little is known about the low-temperature and post-orogenic evolution of the Dabie Shan. Here we present apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and apatite fission-track (AFT) cooling ages from the Dabie Shan that constrain the patterns and history of exhumation over the last ϳ115 myr. On the scale of the whole orogen, ZHe and AHe ages are inversely correlated with mean elevation and are systematically younger in the core of the range. These cooling ages were converted to exhumation rates assuming steady-state erosion and accounting for topographic effects. These results indicate that, since the Eocene, flanks of the range have eroded at rates as low as 0.02 km/myr, while the range core has eroded at about 0.06 km/myr. Even in the core of the range, these recent exhumation rates are at least 10 to 20 times slower than those estimated for the initial stages of exhumation in the Triassic-Jurassic. In a 1.4 kilometer vertical transect in the core of the range, all ages are positively correlated with elevation, with ZHe ages increasing from 76 to 112 Ma, AFT from 44 to 70 Ma, and AHe from 24 to 43 Ma. We present a simple model for topographic correction of thermochronometric ages in vertical transects, using the admittance ratio (ratio of isotherm relief to topographic relief). Applied to the AHe age-elevation relationship, this yields Tertiary exhumation rates of 0.05 to 0.07 km/myr in the core of the Dabie Shan, in good agreement with regional exhumation rate patterns. Finally, age-elevation relationships for all three chronometers in the vertical transect are consistent with a constant exhumation rate of 0.06 ؎ 0.01 km/myr since the Cretaceous, with a possible modest increase in exhumation rates (as high as 0.2 km/myr) between 80 to 40 Ma. These data show no evidence for significant variations in exhumation rates over the last ϳ115 myr, as might be expected for decay of old topography or tectonic reactivation of old structures. introduction Collisional orogenies typically produce topographic and geophysical anomalies persisting several hundred million years. The post-orogenic evolution of mountain ranges and their responses to erosion and subsequent tectonic events provides insights to a variety of problems, including the deep crustal architecture of orogens, dynamics of lithospheric roots, and the erosional decay of topographic anomalies. Typically, the topographic and structural decay of an ancient mountain range is not monotonic, and

A linear inversion method to infer exhumation rates in space and time from thermochronometric data

“…The northeastern extent of the range continues to exhume at ∼ 0.09 km Myr −1 . The time interval from 60-30 Ma shows slower exhumation rates at the front of the range close to the Tan-Lu fault, in agreement with Reiners et al (2003). There is also a decrease in exhumation rate in the eastern region.…”

“…We specify a vertical velocity with an exhumation rate of 0.07 km Myr −1 within a rectangular region in the center of the model domain and 0.05 km Myr −1 outside of this rectangle. We predict AHe, AFT and ZHe ages with the Dodson approximation, at the same spatial locations as in Reiners et al (2003). We assign the same measurement errors as in the reported data.…”

“…The last major phase of tectonics was during the early to middle Cretaceous. Reiners et al (2003) reported ages from a vertical transect at the core of the range and from isolated samples collected around the flanks of the range, using three thermochronometric systems, (U-Th)/He ages from aper | Discussion Paper | Discussion Paper | Discussion Paper | zircon (ZHe) and from apatite (AHe), and AFT dating (Reiners et al, 2003). The ages across the region show a positive AER in the core of the range and older ages around the flanks (Reiners et al, 2003).…”

“…We now apply our method to the measured ages of Reiners et al (2003). As with the synthetic data, the model is initiated at 120 Ma and the exhumation history is discretized into 30 Myr time intervals.…”

Closure temperature of (U-Th)/He system in apatite obtained from natural drillhole samples in the Tarim basin and its geological significance

“…The apatite (U-Th)/He thermochronometry is largely significant in studying the late period of basin-mountain uplifting. Bullen [31] studied the cooling rate of the uplifting event in 3-10 Ma in the Kyrgyz Tianshan using apatite (U-Th)/He thermochronometry and Reiners et al [31,32] demonstrated the cooling rate of the uplift event in 23-40 Ma in the Dabie Mountain. Recently, we have used the apatite (U-Th)/He ages first to study the South Tianshan and then revealed the rapid uplift during the Neogene [33].…”