Quantitative Measures of the Antiquity and Evolution of Cratonic Surfaces

9805132石侵蚀夷平面是全球南极地区的主要地貌，覆盖所有大陆数百万平方公里。 这些地表的大部分区域被深层风化剖面所覆盖，这些剖面掩盖了基岩地质并隐藏了下面的矿藏。 在许多情况下，它们的夷平面化可以追溯到第三纪早期和中生代，在某些情况下，甚至更早。 这些表面随后的向下磨损和暴露历史通常不太受约束。 保存的平面形状和山顶高度一致性，以及在近地表环境中形成的古老的深风化地幔和硬壳的生存表明侵蚀率极低。 相反，物理、生物和化学过程造成的积极剥蚀的证据以及对岩石表面侵蚀的宇宙成因同位素初步研究结果表明，向下磨损的速度为每百万年几米，这很难与它们的生存相吻合。 将通过用宇宙成因同位素测定选定表面在过去1-5 Ma的侵蚀速率，并将其与发掘由相同表面的古代风化形成的定年氧化锰组合所需的侵蚀速率进行比较，来探讨这种明显的冲突。（如硅结砾岩、红土中的石英脉）代表了选定用于检查的大尺度地貌表面。 在这些测定中平均的时间周期将随侵蚀速率而变化，从1 Ma（如果侵蚀速率超过2 m/Ma）到最大值5-6 Ma（如果侵蚀速率接近于零）。 这些测量的目的是测量第四纪侵蚀的幅度和空间变异性，将通过对相关深风化剖面中沉淀的含钾氧化锰矿物进行40 Ar/34 Ar测年，获得相同表面的风化年龄。 K-Mn氧化物沉淀深度的上限（从现代类比和水化学的限制），结合K-Mn氧化物年龄，将rpovide自风化时间以来的平均剥蚀率的上限。 澳大利亚Mt伊萨地区的氩测年结果表明，从阶梯式夷平面上获得5 Ma至65 Ma时间尺度上的平均剥蚀率是可能的。 已从这些表面收集了宇宙成因同位素测量样品，但尚未进行分析，将扩大伊萨山地区的初步工作，在西澳大利亚的皮尔巴拉区块和哈默斯利盆地以及巴西地盾上的卡拉哈斯地区和Quadrilatero Ferrifero进行研究。 所有这三个地区都呈现出阶梯状的形态，核心是高高耸立的、被切割的高原，依次位于较低和较年轻的插入平原之上。 这些地区的景观演化可以追溯到中生代和早第三纪。比较四个研究地点的短期和长期剥蚀速率将：（i）判断第四纪侵蚀速率的大小和空间变化是否与第四纪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否增加或减少，或持续以类似于第三纪平均值的速率。 根据这些结果，它可能是必要的，以开发地貌演化模型的古平面的前体继承的平面形式和首脑会议的高度一致性，但随后的侵蚀显着降低和均匀的帐户，为古地形地貌。 结合表面重建，结果还将：（iii）提供地表向下磨损与陡崖后退的相对速率的限制，这是发展所研究地区的阶梯地貌的一个关键比率，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的重要性，当地古气候和全球尺度的强迫因素，如大气CO2的变化和第四纪气候恶化，在确定侵蚀历史。 这项工作将提供一个详细的了解地貌的演变和最近的速率的地貌活动，在低梯度，低救济的南极洲。 这一结果对产于这些地区但通常被深风化盖层所掩盖的矿床的找矿具有重要意义。 重建沉积物的生产和扩散途径，通过时间，和历史的幕式风化和侵蚀，将提供一个解释框架，为未来的地球化学勘探。

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.