洛南、山阳盆地沉积充填特征及其对东秦岭中—新生代陆内演化的约束

晚三叠世后东秦岭造山带进入陆内造山阶段,其陆内演化过程主要表现为显著的隆升冷却和剧烈的岩浆活动,形成了一系列的山间断陷盆地。作为东秦岭陆内演化的重要载体,这类盆地的发育在很大程度上揭示了造山带的演化。以东秦岭地区的洛南、山阳山间断陷盆地为对象,通过实测剖面、构造要素测量,分析其沉积充填特征,并厘定其地层构造变形期次,从而为东秦岭地区中—新生代陆内演化提供约束。研究结果显示:(1)晚白垩世,洛南与山阳盆地主控断裂发生总体N—S向拉张兼具左行走滑运动,盆地基底由此开始进入以N—S向拉张断陷为主的活动期。受盆地发生拉张断陷作用的影响,两盆地的物源区岩体在该时期经历了强烈的隆升剥蚀冷却过程,剥蚀下来的碎屑颗粒被就近搬运到相应的盆地中,在盆地内部形成扇三角洲、湖泊及旱地冲积扇等沉积体系,形成了非常典型的山间断陷盆地。(2)从古新世开始到渐新世盆地沉积规模由大变小,磷灰石裂变径迹计算结果和构造变形证据共同表明,控制盆地发展的深大断裂拉张兼走滑作用持续减弱,推测此时的洛南与山阳盆地可能受到其它非东西方向上的挤压作用,抑制了拉张走滑运动。

Sedimentary Filling Characteristics of Luonan and Shanyang Basins and Constraints on Mesozoic-Cenozoic Intracontinental Evolution in East Qinling

The east Qinling Orogen had experienced intracontinental orogenic tectonics since the Late Triassic, which formed a series of intermountain basins. The intracontinental evolution involved mainly intensive uplifting and magmatism. We took the Luonan and Shanyang basins in the east Qinling as a case study to analyze the sedimentary filling characteristics, to determine the phases of deformation on stratigraphic structures, and to constrain the Mesozoic-Cenozoic intracontinental evolution of the east Qinling Orogen. The results show that: (1) In Late Cretaceous,major faults of the Luonan and Shanyang basins were subjected to N-S-directed sinistral transtensional movement. Therefore, the basin basement had been activated via the N-S-trending fault depression. Intrusions in the detrital provenance region had been strongly uplifted, and then denudated and cooled, and the detritus were transported to the nearby basins. Various sedimentary systems (fan delta, lake and alluvial fan)were developed in the basins.(2)During Paleocene to Oligocene, sedimentation in the basins gradually waned. The deep faults had controlled the basinal development, as supported by structural evidence and apatite fission-track calculation results.Transtentional tectonics continued to wane, and we proposed that the Luonan and Shanyang basins may have been affected by non-E-W-directed compression, which suppressed the transtentional movement.