Quantitative Measures of the Antiquity and Evolution of Cratonic Surfaces

石质夷平面是世界各地克拉通地区的主要地貌，覆盖所有大陆数百万平方公里。这些地表的大片区域被深厚的风化剖面所覆盖，这些剖面掩盖了基岩地质，掩盖了下面的矿藏。在许多情况下，它们的夷平面可以追溯到第三纪和中生代早期，在某些情况下，甚至更早。这些表面随后的磨损和曝光历史通常较少受到很好的约束。保存完好的平面形状和顶峰高度的整合，以及在近地表环境中形成的古老风化地幔和硬锈的存留表明，侵蚀率极低。相比之下，物理、生物和化学过程的活动剥蚀的证据以及对岩石表面侵蚀的初步宇宙成因同位素研究的结果表明，每百万年的损耗率为每百万米，这与它们的存活率很难调和。这种明显的冲突将通过用宇宙成因同位素测定过去1-5 Ma选定表面的侵蚀速率来探索，并将它们与相同表面古老风化形成的过时氧化锰组合所需的侵蚀速率进行比较。宇宙成因核素测量将对代表所选大尺度地貌表面的抗性和硬化露头(例如红土中的硅酸盐、石英脉)进行宇宙核素测量。这些测定的平均时间周期将随着侵蚀速率的变化而变化，如果侵蚀速率超过2m/Ma，从1 Ma到最大5-6 Ma，如果侵蚀速率接近于零。这些测量旨在测量第四纪侵蚀的规模和空间变异性。通过对相关深部风化剖面中沉淀的含钾氧化锰矿物进行40Ar/34Ar测年，将获得相同表面的风化年龄。K-Mn氧化物沉淀深度的上限(来自现代地质和水化学约束)，与K-Mn氧化物年龄相结合，将提供自风化时间以来平均剥蚀速率的上限。澳大利亚Mt Isa地区的Ar测年结果表明，在5 Ma至65 Ma的时间尺度上获得阶梯状夷平面的平均剥蚀速率是可能的。已经从这些表面采集了用于宇宙成因同位素测量的样本，但尚未进行分析。伊萨山地区的初步工作将扩大，包括西澳大利亚的皮尔巴拉地块和哈默斯利盆地，以及巴西盾上的加拉加斯地区和Quadrilatero Ferrifero。这三个地区都表现出阶梯状的地貌，高位的剥离高原的核心依次位于较低和较年轻的镶嵌平原之上。这些地区的景观演化可以追溯到中生代和第三纪早期。比较四个研究点的短期和长期剥蚀率将：(I)表明第四纪侵蚀速率的大小和空间变异性是否与克拉通夷平面的保存相一致。(Ii)揭示这些表面上的侵蚀速率在第四纪是增加了还是减少了，还是以与第三纪的平均值相似的速率持续存在。根据这些结果，可能有必要建立克拉通地体的地貌演化模型，该模型考虑了从古代平面前兆继承而来的平面形状和峰顶高度一致性，但由于随后的侵蚀而显著而均匀地降低。结合地表重建，结果还将：(Iii)提供地表下降与陡峭撤退的相对速率的界限，这是待研究地区阶梯景观发展的关键比率。四个地理区域之间的比较将揭示海平面和构造基准面变化、局部古气候和全球尺度强迫因素(如大气二氧化碳变化和第四纪气候恶化)在确定侵蚀历史方面的重要性。这项工作将对低梯度、低起伏的克拉通地体的地貌演化和最近的地貌活动速率提供详细的洞察。这一结果对寻找赋存于这些地体中、但通常被深度风化覆盖层所掩盖的矿床具有重要意义。随着时间的推移，沉积物产生和扩散路径的重建，以及幕式风化和侵蚀的历史，将为未来的化探提供解释框架。

9805132StoneErosional planation surfaces are a predominant landform of cratonic regions worldwide, covering millions of square kilometers across all continents. Large areas of these surfaces are mantled by deep weathering profiles that mask bedrock geology and conceal underlying mineral deposits. Their planation can be dated, in many instances, to the early Tertiary and Mesozoic eras, and in some cases, even earlier. The subsequent downwearing and exposure history of these surfaces is generally less well constrained. Preserved planar forms and summit-height concordances, and the survival of ancient deeply-weathered mantles and duricrusts formed in the near-surface environment point to extremely low rates of erosion. In contrast, evidence of active denudation by physical, biological and chemical processes and results of initial cosmogenic isotope studies of rock surface erosion point to downwearing rates of metres per million years, which are difficult to reconcile with their survival. This apparent conflict will be explored by determining erosion rates of selected surfaces over the past 1-5 Ma with cosmogenic isotopes, and comparing them to the erosion rates required to exhume dated Mn oxide assemblages formed by ancient weathering of the same surfaces.Cosmogenic nuclide measurements will be made on resistant and indurated outcrops outcrops (e.g. silcrete, quartz veins within laterite) representative of the broad-scale geomorphic surfaces selected for examination. The time period averaged in these determinations will vary with erosion rate, from 1 Ma if erosion rates exceed 2 m/Ma, to a maximum of 5-6 Ma if erosion rates approach zero. These measurements are aimed at measuring both the magnitude and spatial variability of Quaternary erosion.Weathering ages for the same surfaces will be obtained by 40Ar/34Ar dating of K-bearing manganese oxide minerals precipitated in related deep-weathering profiles. Upper limits to the depth of K-Mn oxide precipitation (from modern anlogues and hydrochemical constraints), combined with the K-Mn oxide ages, will then rpovide upper limits on the average denudation rate since the time of weathering. Argon dating results from the Mt Isa district of Australia indicate that it will be possible to obtain average denudation rates over timescales ranging from 5 Ma to 65 Ma from stepped planation surfaces. Samples for cosmogenic isotope measurement have been collected from these surfaces but not yet analysed.Preliminary work in the Mt Isa area will be extended with studies in the Pilbara Block and Hamersley Basin of Western Australia, and the Carajas region and Quadrilatero Ferrifero on the Brazilian Shield. All three regions show a stepped morphology, with cores of high-standing, dissected plateaus standing above successively lower and younger inset plains. Landscape evolution in these areas can be traced back to the Mesozoic and early tertiary.Comparison of the short-and long-term denudation rates at the four study sites will: (i) Indicate whether the magnitude and spatial variability of Quaternary erosion rates are compatible with the preservation of cratonic planation surfaces. (ii) Reveal whether erosion rates on these surfaces have increased or decreased in the Quaternary, or persisted at rates similar to average values through the Tertiary. Depending on these outcomes, it may be necessary to develop geomorphic evolution models for cratonic terranes that account for planar forms and summit height concordances inherited from ancient planar precursors but lowered significantly and uniformly by subsequent erosion. combined with surface reconstructions, the results will also: (iii) Provide limits on the relative rates of surface downwearing versus scarp retreat, a key ratio in the development of the stepped landscapes characteristic of the areas to be studied.Comparisons between the four geographic areas will reveal the importance of eustatic and tectonic base level changes, local paleoclimate and global-scale forcing factors such as atmospheric CO2 changes and Quaternary climatic deterioration in determining erosional histories. This work will provide a detailed insight into the geomorphic evolution and recent rates of geomorphic activity in low-gradient, low-relief cratonic terranes. The results will be of importance to mineral exploration for ore deposits which occur in these terranes but are commonly concealed by deeply weathered cover. Reconstruction of sediment production and dispersal pathways through time, and the history of episodic weathering and erosion, will provide an interpretive framework for future geochemical prospecting.