龙门山冲断隆升及其走向差异的裂变径迹证据

大量的低温年代学研究用来讨论龙门山晚新生代的隆升,但很少涉及其走向差异和中生代隆升。本文分别沿龙门山北、中、南段3条剖面进行了锆石和磷灰石裂变径迹测试,结合已有的热年代学数据,以期揭示整个中-新生代期间龙门山隆升历史及其时空变化。中生代以来,龙门山主要有印支期(约200 Ma)、早白垩世末(约100 Ma)、早新生代(65～30 Ma)以及晚中新世(15～9 Ma)等或快或慢的冷却事件,总体上经历了中生代至早新生代的缓慢冷却和晚新生代快速冷却2个阶段,快速剥露开始于15～9 Ma,剥蚀速率由早期的0.1 mm/a增加到0.15～0.3 mm/a左右,局部可达0.9 mm/a左右。走向上,龙门山北段相对偏小的锆石裂变径迹年龄和相对偏大的磷灰石裂变径迹年龄反映其在中生代较中、南段隆升更快,而裂变径迹年龄总体上从北段向中、南段减小,表明中、南段在新生代发生了更快的隆升。倾向上,多种热年代学数据显示新生代期间在北川断裂和彭灌断裂两侧存在明显的差异剥露,这种差异在中、南段表现比北段更为突出。龙门山晚新生代快速隆升和剥露是青藏高原区域隆升背景上叠加的冲断活动所致,而非下地壳流动驱动。

Differential uplift driven by thrusting and its lateral variation along the Longmenshan belt, western Sichuan, China: Evidence from fission track thermochronology

A number of thermochronologic studies have been carried out to investigate the Cenozoic exhumation history of the Longmenshan range,which defines the eastern margin of the Tibet Plateau and is characterized by the paradox of high topographic gradients with but low convergence ratesHowever,the Mesozoic thermal history and the lateral variation of both Cenozoic and Mesozoic uplift of the Longmenshan are not yet well understoodIn this article,we present zircon(ZFT)and apatite(AFT)fission track data for samples collected from three transects along the northern,central and southern Longmenshan Thrust Belt(LMTB).There were several cooling events,revealed by the ZFT and AFT ages,in the Longmenshan region since the Indosinian deformation,ie.,the Late Triassic(～200 Ma),the end of Early Cretaceous(～100 Ma),the Early Cenozoic(30～65 Ma)and the Late Miocene(9～15 Ma)eventsAt a regional scale,the Longmenshan experienced a slow cooling between the Mesozoic and Early Cenozoic followed by a rapid cooling since the Middle Miocene,rapid cooling started at 9～15 Ma,with a sharp increase of exhumation rate from <0.1 mm/a to 0.15～0.3 mm/a,up to 0.9 mm/a locallyComparison of ZFT and AFT ages shows that the northern LMTB cooled more rapidly than central and southern LMTB during the Mesozoic,while younger AFT ages indicates more rapid exhumation in central and southern LMTB during the Late CenozoicMultiple thermochronologic data reveal that the Beichuan fault and Pengguan fault have accommodated significant differential exhumation across faults by thrusting activity and associated denudation,with a greater amount of differential exhumation across faults from north to southWe suggest that the major cause for the rapid exhumation of the Longmenshan range during the Late Cenozoic is the thrusting activities superimposed upon the regional uplift of the Tibet Plateau,not necessary the channel flow within the lower crust.