低温热年代学在沉积盆地研究中的应用:以四川盆地北部为例

Application of low-temperature thermochronology to sedimentary basins:case studies in the northern Sichuan Basin

低温热年代计,如磷灰石与锆石裂变径迹、(U-Th)/He等矿物年代计,具有较低的封闭温度(约60~250℃),是研究沉积盆地埋藏-剥蚀历史的重要工具之一。四川盆地地层向西逐渐变年轻,这一现象指示盆地可能经历了显著的后期剥蚀,但是剥蚀过程及时限不确定。四川盆地北部钻井样品碎屑锆石(U-Th)/He(ZHe)数据显示上三叠统至中侏罗统下部ZHe年龄已经发生了部分乃至完全重置。最小的两个年龄分布峰值(约80~100 Ma)已经完全重置,制约了川北最大埋深时代下限。这一解释与该区残余的最晚沉积记录以及已经发表的AFT数据一致。河坝1井AFT和(U-Th)/He数据联合反演模拟的结果显示川北地区的剥蚀速度可能在约32 Ma左右加快。这一加速剥蚀很可能是区域现象,但也可能反映了局部的新生代构造活动。

Low-temperature thermoehronology, such as apatite and zircon fission-track and （U-Th）/He, is one of the most important tools for uncovering the burial and erosional histories of sedimentary basins due to its relatively low closure temperatures of -60 - 250 ℃. The westward younging direction of the strata distribution in the Sichuan Basin indicates that the basin might have experienced significant denudation. However, it remains uncertain when and how the denudation had occurred and when the basin reached its maximum burial depth. Detrital zircon （U-Th）/He （ZHe） data suggests that the upper Triassic-lower middle Jurassic strata have been once heated to the temperatures of the ZHe partial retention zone （130 - 180 ℃）. The age spectra show two peaks at -80- 100 Ma, formed probably by total resetting of the initial ZHe system at that time. The age peak thus dates the maximum timing of the maximum burial depth of the northern Sichuan Basin. This interpretation is consistent with the relicts of the latest sedimentary record （early Cretaceous strata） and the published apatite fission-track data, which suggests that cooling and erosion started before 80 Ma. Thermal history modelling of the apatite fission-track data and （U-Th）/He data from the Heba-1 borehole show enhanced cooling and erosion at -32 Ma, possibly resulted from either regional denudation or local Cenozoic shortening.