晚白垩世以来川东北地区的剥蚀历史——多类低温热年代学数据综合剖面的制约

低温热年代学数据是一个与热历史过程紧密相关的资料类型,与高温年代学不同,低温热年代学表观年龄本身在很多情况下没有直接的地质意义.当且仅当样品线性持续冷却的情况下,表观年龄才可以被直接解释为样品经过其封闭温度的大致时间.因此,只有结合地质约束通过对低温热年代学数据进行热历史模拟才能更好地揭示其所蕴含的地质信息.对川东北地区现有裂变径迹数据的统计显示,露头样品的表观年龄主要集中在60~80 Ma,其为冷却年龄并无直接的地质意义.前人利用这些数据对川东北地区热历史进行了模拟,然而不同研究者的研究结果却不甚一致,争议主要集中在四川盆地最后一期剥蚀开始的时间上.这体现了单一低温热年代学指标应用范围局限的缺陷.为解决这一问题,本文介绍了一种多类低温热年代学数据剖面联合解释的方法:首先根据低温热年代学动力学模型对诸多种可能的热历史进行正演模拟,然后将正演模拟的结果与观测结果相比较,因此,通过对比正演模拟结果与实测结果的拟合程度便可从诸多种可能的热历史中选择出最可能的一种.本文利用此方法对四川盆地东北部已发表的诸多可能的冷却/剥蚀历史进行了正演模拟,并将这些正演模拟与实测磷灰石裂变径迹和(U-Th)/He综合深度剖面数据进行比较,更好地制约了四川盆地的热历史:~100 Ma和~30 Ma之间冷却速度为0.57 ℃/Ma,~30 Ma以来冷却速度加快(~1.67 ℃/Ma).在假设川东北地区100 Ma内地温梯度大致与现今20 ℃/km的地温梯度相近的前提下,其剥蚀历史可计算为:在~100 Ma和~30 Ma之间剥蚀速度为29 m/Ma,~30 Ma以来剥蚀速度加快(~83 m/Ma),川东北地区自晚白垩世以来总剥蚀量约为5 km.

Post-Early Cretaceous denudation history Of the northeastern Sichuan Basin: constraints from low-temperature thermochronology profiles

Previously reported apatite fission track (AFT) data from surface samples of the northeastern Sichuan Basin yield apparent ages range between 60~80 Ma, which as is often the case, cannot be linked directly to any obvious geological events. Thermal modeling based on those data; however, provide some constraints on the cooling history of the northeastern Sichuan Basin. However, the details of the reconstructed cooling histories reported, vary significantly, especially with regard to the timing of the latest episode of cooling. This variation can be attributed to the lack of geological constraints for modeling purposes and the limited thermal resolution of AFT thermochronology in the lower temperature range (<~60℃). To address these issues, we have acquired new borehole AFT and apatite (U-Th)/He (AHe) data, which when combined with previously reported AFT surface and borehole data, allows synthetic AFT and AHe age and AFT length profiles for the basin to be constructed, and improves thermal history modeling to be carried out. This strategy includes, forward modeling based on candidate thermal histories to obtain predicted low-temperature thermochronology profiles and achieving a good match between the predicted and observed results. Using this strategy, we tested three possible cooling histories for the northeastern Sichuan Basin: linear cooling since 100 Ma, enhanced cooling since 40 Ma following slow cooling since 100 Ma and enhanced cooling since 30 Ma following slow cooling since 100 Ma. Results suggest that the most likely post-Early Cretaceous cooling history for the basin is - cooling at a rate of ~0.57 ℃/Ma between ~100 and 30 Ma, followed by a rate of ~1.67 ℃/Ma since ~30 Ma. Assuming that cooling resulted from denudation (as suggested from previous work) and occurred under a constant paleothermal gradient of ~20 ℃/km (similar to that of the present day), the denudation history of the basin is calculated - denudation at rate of ~29 m/Ma between ~100 Ma and ~30Ma and at ~83 m/Ma since ~30 Ma. The total post Early Cretaceous denudation for the northeastern Sichuan Basin is estimated at ~5 km.