内蒙古大青山基底晚中生代以来的差异隆升-剥蚀过程及其构造意义——来自磷灰石裂变径迹低温热年代学的约束

Differential uplift-denudation of the basement in the Daqingshan Mountains,Inner Mongolia,since the Late Mesozoic:constraints from the apatite fission track thermochronology

地形地貌和沉积学研究表明,在内蒙古大青山地区发生过早白垩世晚期、晚白垩世和新生代三期隆升-剥蚀事件,但在大青山北部前寒武纪基底岩石中,磷灰石裂变径迹研究仅识别出晚白垩世早期和中新世以来等两期隆升-剥蚀事件。为了探讨大青山南部晚中生代以来的隆升-剥蚀过程,本文报道了6件古元古代花岗岩样品的AFT结果。这6件样品的最大高差约700m,AFT年龄介于119±8~79±5Ma之间,AFT年龄与高程之间无显著关联。平均封闭径迹长度在13.2±2.2~11.8±1.8μm之间,呈单峰负偏斜的分布特征。热史模拟结果显示,晚中生代以来,大青山南部基底岩石的隆升-剥蚀过程中存在3期重要事件:早白垩世晚期(约120~110Ma)、晚白垩世早期(约100~90Ma)和新生代以来(约34~15Ma以来)。结合区域构造、沉积和地形地貌资料,并对比南部和北部基底岩石样品的AFT结果,早白垩世晚期(约120~110Ma)事件时间上与韧性剪切带的隆升-剥蚀过程相当,很可能具有相同的构造成因;晚白垩世早期(约100~90Ma)事件很可能是阴山-燕山陆内造山带整体隆升-剥蚀的结果;新生代以来(约34~15Ma以来)的事件很可能反映了南侧山前正断层的活动,且南部基底新生代以来的隆升-剥蚀事件很可能稍早于北部基底,暗示北侧山前正断层启动时间稍晚于南侧山前正断层。通过对比分析北山、北大山、狼山、内蒙古高原中部、燕山及大兴安岭等地的资料,大青山早白垩世晚期快速隆升-剥蚀事件很可能主要与蒙古-鄂霍茨克洋闭合造山后的垮塌有关;晚白垩世早期快速隆升-剥蚀事件很可能受太平洋板块俯冲的影响所致;而新生代以来的快速隆升-剥蚀事件则是印度-欧亚板块碰撞的远程效应。

Geomorphological and sedimentological data indicate that the Daqingshan Mountains experienced uplift-denudation in the late Early Cretaceous,the early Late Cretaceous,and the Cenozoic since the Late Mesozoic;However,previous apatite fission track(AFT)research only revealed two stages of upliftdenudation events for the basement of the northern Daqingshan Mountains during the early Late Cretaceous and since the Miocene.AFT results were reported in this study performed on six Paleoproterozoic granitic samples,in order to clarify the uplift-denudation processes for the basement of the southern Daqingshan Mountains.Six AFT ages vary between 119±8and 79±5 Ma with altitude difference of ca.700m,but show no clear correlation with altitude.Mean track lengths range from 13.2±2.2to 11.8±1.8μm with unimodal distributions.Thermal history modeling results indicate three stages of uplift-denudation events for the basement of the southern Daqingshan Mountains:the late Early Cretaceous(from ca.120Ma to ca.110Ma),the early Late Cretaceous(from ca.100Ma to ca.90Ma),and the Cenozoic(since ca.34 Ma to ca.15 Ma).Combined with structural,sedimentary,and geomorphological data in the region,together with the comparisons to the basement of the northern Daqingshan Mountains,the timing of the late Early Cretaceous event agrees well with that of the rapid cooling of the ductile shear zone,indicating the same driving mechanism;the early Late Cretaceous event corresponds well with the contemporary whole uplift-denudation of the Yinshan-Yanshan intracontinental orogenic belt;the Cenozoic event is likely to be related to the activities of the southern piedmont normal fault.The event for the basement of the southern Daqingshan Mountains since the Cenozoic is likely to be earlier than that for the basement of the northern Daqingshan Mountains,suggesting that the northern piedmont normal fault start later than the southern piedmont normal fault.Compared to data in the Beishan,Beidashan,Langshan,Yanshan,Greater Hinggan Mountains,and central Inner Mongolia Plateau,the rapid uplift-denudation event in the late Early Cretaceous in the Daqingshan Mountains is more likely related to the post-orogenic collapse after the closure of the Mongolian-Okhotsk Ocean;the rapid uplift-denudation event in the early Late Cretaceous is more likely affected by the subduction of the Pacific plate;the distant effect of the collision between the Indian and Eurasian plates probably plays a major role in the rapid uplift-denudation in the Cenozoic.