Exhumation And Paleotopography Reconstruction In Dabie Orogen Since Late Cretaceous

The low-temperature thermochronological closure depth is subjected to significantly influence by factors such as heat advection, topography, exhumation rate, etc, because the low-temperature thermochronological data record the exhumation history of the shallow crust that is only several miles below the surface. These factors limit the accurate estimation of exhumation rates and paleotopography reconstruction. Alternative interpretation methods are therefore needed to unravel further defaulted information.Since the main structure of Dabie orogen has been strongly affected by the Cretaceous-Cenozoic tectonic events, study on the present UHP-HP units alone would not be enough to reconstruct the whole process of uplift and exhumation of the orogen. Therefore, late-orogenic and the post-orogenic tectono-thermal evolution history and exhumation process will provide key information for the understanding of the orogenic evolution in a large. By the quantitative study on the exhumation history and paleotopography based on the low-temperature thermochronological data of Dabie orogen, this article presents a more integrative and visual view on the evolution of post-orogenic exhumation and paleotopography. This analysis should not only deepen our present understanding on the late-orogenic and the post-orogenic tectonic evolution, but should also clarify the relationship between the uplift and exhumation history in Dabie orogen and sediment filling processes in adjacent basins.The fission track ages of the samples analysed in this study fall between 46Ma and 69Ma, and the average confined track lengths all exceed 13μm. The regional distribution of fission track collected and studied here shows that the fission track ages in Dabie orogen's core, the south of Tanlu fault zone, and the southwest of Dabie orogen are smaller than those in the peripheral areas. The study shows that rapid exhumation occurred during 90～60Ma. Differential strike-slip movement between Tanlu and Shangcheng-Macheng fault zones resulted in the intense push-up of the core of Dabie orogen. During the period between 60 and 75Ma, post-orogenic tectonic evolution of Dabie orogen was jointly controlled by the closure of Neo-Tethys in the far southwest and the convergence of Paleo-Pacific plate with Eurasia plate, which resulted in the shifting from the fast cooling stage to the slow cooling stage. Sediments in the adjacent basins of Dabie orogen between 60 and 30Ma might mainly come from Dabie orogen. Dabieshan orogen has reached the steady state between 60 and 30Ma. This study also highlights the importance of isostatic rebound in the post-orogenic uplift of Dabie orogen.The modelling exhibits the following the exhumation pattern of Dabie orogen since Cretaceous, i.e. the exhumation rates (0.08～0.10km/Ma) in the Tiantangzhai region, in the south of Tanlu fault zone and the southwest of Dabie orogen are larger than those of the peripheral areas (0.04～0.07km/Ma) since Cretaceous. Thus, the relatively fast exhumation of the core of Dabie orogen may be related to the transtensional movements along the Tanlu fault. Due to the differential strike-slip movement between Tanlu and Shangcheng-Macheng fault zones, the core of Dabie orogen experienced intense push-up and thus resulted in the larger exhumation rates. The differential exhumation caused by the NNE-trending faults (since Late Cretaceous) probably continued all the way from the 180℃isothermal surface, through 110℃isothermal surface to the 70℃isothermal surface.At 90Ma, Dabie orogen's average height of topography, compared with the current sea level, reached 1.45 km (when geothermal gradient is chosen as 25℃/km) or 1.75 km (when geothermal gradient is chosen as 20℃/km), which is 4(when geothermal gradient is chosen as 25℃/km) or 5(when geothermal gradient is chosen as 20℃/km) times of the height of today's surface topography in simulated region. If the tectonic uplift is not considered, the relief of Dabie orogen will reach at least 3km in Late-Cretaceous. As to the value of the relief reduction factor in Dabie orogen at 90Ma, the largest value appeared in northeast region, followed consecutively by the core and southeast Tanlu fault zone, with the smalleast value appeared in northwest and southwest region. The results reflect the strong uplift difference between core and North Huaiyang flank in Dabie orogen during doming extension period. Relief in the core region and southeast Tanlu fault zone is higher than those in the peripheral regions, northwest and southwest flanks are more stable than other regions.