Apatite Fission-track Dating By LA-ICP-MS And Neogene Expansion Of The Qilian Shan,North Tibetan Plateau

Apatite can provide us multi-dating methods and multi-elements analysis to investigate processes that operate from the surface of the earth down to the lower crust.Especially,the apatite fission track is popular for studying the lower crust processes.However,the external detector method is limited by the irradiation of the reactor.So,to avoid the limitations of the reactor,promote the study of annealing behaviour,obtain provenance information and improve the test efficiency of fission-track(FT)dating,this study attempted to establish a technique combining FT dating with measurements of rare earth element(REE)and chlorine concentrations by laser ablation-inductively coupled plasma-mass spectrometry(LA-ICPMS).An experiment was carried out with a 193 nm laser coupled with a quadrupole ICP-MS.By analysing Durango and Fish Canyon Tuff apatites,we obtained apatite ages and REE concentrations that are consistent with those of published data.To overcome the high first ionization,a calibration line between the chlorine signal intensity and chlorine concentration was established.To determine accurate chlorine concentrations,this study focused on decreasing background noise and increasing the chlorine signals from samples to improve the chlorine signal-to-background ratio.For this purpose,high-purity carrier gas and long sweep times were used to minimize the background.In addition,high laser output energy and long dwell times were used to enhance the chlorine signals from samples.This work successfully demonstrates the simultaneous determination of apatite FT ages and REE and chlorine concentrations within the same material.The Qilian Shan,at the northeastern frontier of the Tibetan Plateau,is a key area for studying the growth history and expansion mechanism of the Tibetan Plateau.Although previous thermochronology and paleomagnetic studies indicate Neogene(~17-2 Ma)northward expansion of the northern Qilian Shan,there is a distinct gap in the tectonic history of the middle and southern Qilian Shan.This has hindered a more complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan and its context within broader continental dynamics.To study the growth history of the Qilian Shan,apatite fission track(AFT)data have been acquired from middle and southern Qilian Shan.From the middle Qilian Shan,we report new constraints on the timing of accelerated uplift of the Tuolai Shan,one of the elongated mountain ranges in the middle-northern Tibetan Plateau.New apatite fission-track data from an elevation transect in the Tuolai Shan provide a definitive tie to rapid cooling that began at 17–15 Ma.We attribute this rapid cooling to accelerated exhumation resulting from thrusting in the hanging wall of the Haiyuan fault in response to progressive northward growth of the plateau.Combining these fission-track data and the published geologic,sedimentological,and thermochronologic data from the northern Qilian Shan and Hexi Corridor,we propose a progressively north-northeastward growth model for the middle-northern part of Tibet,suggesting that deformation in the inner Qilian Shan occurred synchronously in the middle Miocene,and subsequently,increasingly further north.Form the southern Qilian Shan,we collected samples from Zongwulong Shan to study the growth history of the southern Qilian Shan.Thermal history modeling of AFT data indicates the commencement of a rapid cooling episode at ~18-11 Ma,which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan.The previous study of the Huaitoutala section,detrital grains up-section shows progressively decreasing peak AFT ages followed by an age increase from midsection,implying that a sediment-recycling event occurred at ca.7±2 Ma.We interpret this change as a response to initial Late Miocene uplift of the northern Qaidam Basin.Combining our results with previous studies on the timing of deformation timing along the southern Qilian Shan,our findings suggest that the southern Qilian Shan gew southward during the Neogene.The tectonic movement plays the first order control factor on the changing of exhumation rate,so obtaining the evolution of the exhumation rates of the entire Qilian Shan can provide us a macroscopic view on understanding the deformation pattern of the northeastern of Tibetan Plateau.Based on the collected 197 thermochronometric data of zircon Helium(ZHe),apatite fission track(AFT),and apatite Helium(AHe),we quantified the exhumation rate using the program GLIDE.Results show that rapid exhumation has initiated at the middle Miocene.Two rapid exhumation phases occurred during the middle and late Miocene,mainly along the margin of the Qilian Shan.Our results show that no evident deformation occurred short after the India-Eurasia collision,the topography and geomorphology of the northeastern of Tibetan Plateau mainly built since the middle Miocene.In conclusion,our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions at 18-11 Ma and 7±2 Ma.The synchronous outward expansion may have been triggered by the removal of mantle beneath north Tibet.