Long-term exhumation of a Palaeoproterozoic orogen and the role of pre-existing heterogeneous thermal crustal properties: a fission-track study of SE Baffin Island

New apatite fission-track data from SE Baffin Island exhibit central ages that range from just under 200 Ma to 440 Ma, and mean track lengths that vary between c. 12 and 13.3 µm. First-order analysis of the data (a plot of central age v. mean track length) reveals an approximate ‘boomerang’ trend, typical of samples that have experienced contemporaneous cooling from an array of initial temperatures. One-dimensional inverse thermal modelling of single samples suggests that cooling through the partial annealing zone (c. 120–60 °C) occurred over discrete periods ranging from 100 to 300 Ma. Modelling the 3D exhumation of a heterogeneous crust with flat topography demonstrates that some of the variability in observed fission-track ages could be attributed to heterogeneity in crustal heat production and thermal conductivity. The remaining variability in the observed dataset is attributed here to differential erosion from a variable initial topography. However, age discontinuities over short distances require other explanations such as faulting and/or unidentified compositional effects. Collectively, these results suggest that the observed data are consistent with a simple exhumation scenario where the present-day high topography is a remnant of that created during Palaeoproterozoic orogenies. The new data do not require any recent (Cenozoic) periods of exhumation.

来自巴芬岛东南部的全新磷灰石裂变径迹（apatite fission-track）数据显示，其中心年龄介于近200 Ma至440 Ma之间，平均径迹长度则在约12 µm至13.3 µm范围内波动。对该数据开展的一阶宏观分析（以中心年龄与平均径迹长度绘制的散点图）呈现出近似“回旋镖”形的分布趋势，这一特征是经历过从一系列初始温度同步冷却的样品的典型表现。针对单样品的一维热反演模拟（one-dimensional inverse thermal modelling）结果显示，地壳冷却通过部分退火带（partial annealing zone，约120~60℃）的过程发生在100 Ma至300 Ma的多个离散时段内。对具有平坦地形的非均匀地壳开展三维剥露模拟的结果表明，观测到的裂变径迹年龄的部分差异，可归因于地壳生热率与热导率的非均质性。本数据集剩余的年龄差异，则被归因于初始地形起伏导致的差异侵蚀作用。不过，短距离内出现的年龄间断现象，还需要诸如断裂作用或未被识别的成分效应等其他解释。综合来看，上述结果表明，本次观测数据与一个简单的剥露模型相符：现今的高地地形是古元古代造山运动所形成地形的残留体。本次新数据并不支持存在新生代（Cenozoic）以来的剥露事件。