Late Cretaceous-Cenozoic Exhumation History of the L(u|¨)liang Mountains,North China Craton:Constraint from Fission-track Thermochronology

The Lüliang Mountains, located in the North China Craton, is a relatively stable block, but it has experienced uplift and denudation since the late Mesozoic. We hence aim to explore its time and rate of the exhumation by the fission-track method. The results show that, no matter what type rocks are, the pooled ages of zircon and apatite fission-track range from 60.0 to 93.7 Ma and 28.6 to 43.3 Ma, respectively; all of the apatite fission-track length distributions are unimodal and yield a mean length of -13 μm; and the thermal history modeling results based on apatite fission-track data indicate that the time-temperature paths exhibit similar patterns and the cooling has been accelerated for each sample since the Pliocene （c.5 Ma）. Therefore, we can conclude that a successive cooling, probably involving two slow （during c.75-35Ma and 35-5Ma） and one rapid （during c.5 Ma-0 Ma） cooling, has occurred through the exhumation of the Liiliang Mountains since the late Cretaceous. The maximum exhumation is more than 5 km under a steady-state geothermal gradient of 35℃/km. Combined with the tectonic setting, this exhumation may be the resultant effect from the surrounding plate interactions, and it has been accelerated since c.5 Ma predominantly due to the India-Eurasia collision.

Late Cretaceous-Cenozoic Multi-Stage Denudation at the Western Ordos Block： Constraints by the Apatite Fission Track Dating on the Langshan

The apatite fission track dating of samples from the Dabashan （i.e., the Langshan in the northeastern Alxa Block） by the laser ablation method and their thermal history modeling of AFT ages are conducted in this study. The obtained results and lines of geological evidence in the study region indicate that the Langshan has experienced complicated tectonic-thermal events during the the Late Cretaceous-Cenozoic. Firstly, it experienced a tectonic-thermal event in the Late Cretaceous （-90-70 Ma）. The event had little relation with the oblique subduction of the Izanagi Plate along the eastern Eurasian Plate, but was related to the Neo-Tethys subduction and compression between the Lhasa Block and Qiangtang Block. Secondly, it underwent the dextral slip faulting in the Eocene （-50-45 Ma）. The strike slip fault may develop in the same tectonic setting as sinistral slip faults in southern Mongolia and thrusts in West Qinling to the southwest Ordos Block in the same period, which is the remote far-field response to the India-Eurasia collision. Thirdly, the tectonic thermal event existed in the late Cenozoic （since -10 Ma）, thermal modeling shows that several samples began their denudation from upper region of partial annealing zone （PAZ）, and the denudation may have a great relationship with the growth of Qinghai-Tibetan Plateau to the northeast. In addition, the AFT ages of Langshan indicate that the main body of the Langshan may be an upper part of fossil PAZ of the Late Cretaceous （-70 Ma）. The fossil PAZ were destroyed and deformed by tectonic events repeatedly in the Cenozoic along with the denudation.

Study on Late Cretaceous-Cenozoic exhumation of the Yanji area,NE China: insights from low-temperature thermochronology

The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.