Isostatic Compensation of a Paleozoic Orogen: Wide-angle Reflection Studies of the Blue Ridge Province, Southern Appalachians

古生代造山带的等静压补偿：阿巴拉契亚山脉南部蓝岭省的广角反射研究

• 批准号: 0124249
• 负责人: Robert Hawman
• 金额: $ 9.7万
• 依托单位: University of Georgia Research Foundation Inc
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-01-01 至 2004-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0124249&HistoricalAwards=false
• 关键词: Isostatic; Compensation; Paleozoic; Orogen; Wide

The main goal of this research project is to place constraints on the mechanism of isostatic compensation for topographic and intracrustal loads beneath a portion of the southern Appalachians. Although it is frequently asserted that the Appalachians possess no crustal root, the Moho beneath the highest elevations in the Blue Ridge and Valley & Ridge provinces remains poorly imaged.Recent studies in the Sierra Nevada and other young mountain belts indicate that they are supported by density heterogeneities within the upper mantle rather than by thickening of the crust. This suggests that a significant amount of interaction can occur between the crustal and mantle components of the lithosphere during mountain building. Seismic profiles across older mountain belts, however, indicate a variety of mechanisms for isostatic compensation. Profiles across some Archean-age orogenic belts in the Baltic and and Canadian shields reveal significant relief on the Moho, suggesting crustal roots may persist for over 1.5 billion years. In contrast, seismic profiles across many Paleozoic orogens show a relatively flat Moho, presumably in reponse to subsequent collapse, extension, and erosion. A notable exception is the Ural Mountains, a late Paleozoic orogen that escaped major extension, leaving the collisional architecture largely intact. Extensive seismic profiling across the southern Urals indicates a crustal root anywhere from 6 to 15 km thick. The root appears to be much thicker than required for compensation of the existing topography. In spite of extensive seismic profiling, very little is known about the crustal velocity structure or the configuration of the Moho beneath the southern Appalachians. New seismic refraction/wide-angle reflection profiles are being used to model the transition in crustal seismic-wave velocity structure from accreted crust beneath the Carolina Terrane to North American craton beneath the Blue Ridge. In particular, the new data are being used to: 1) image the Moho by taking advantage of elevated reflection coefficients near the critical angle; 2) map variations in averaged P- and S-wave velocities to constrain variations in the bulk composition of basement rocks across the study area; 3) test models for elastic bending of the continental lithosphere by incorporating the new estimates for relief on the Moho; 4) isolate the crustal contribution to the observed gravity by modeling the gravity field predicted by seismically determined variations in composition (density structure) and crustal thickness (this places constraints on any density variations within the uppermost mantle that help to support topographic and intracrustal loads); and 5) determine the extent to which the original deep structure of this orogen survived the effects of subsequent collapse and extension. The results are being used to assess the long-term stability of individual terranes/crustal blocks within Paleozoic orogens.The experiments are targeting the regional gravity low associated with the Blue Ridge province. Both instantaneous blasts at dimension-stone quarries and delay-fired blasts at crushed-stone quarries are being used as seismic sources. The use of quarry blasts, a valuable but largely untapped energy source for refraction/wide-angle reflection profiling, eliminates the high costs of drilling and explosives normally associated with crustal-scale, active-source experiments. The new experiments are helping to link the crustal images and velocity estimates from previous studies in the Carolina Terrane and associated East Coast gravity high, the Tennessee Valley & Ridge, and the Inner Piedmont and associated gravity gradient. The combined results provide high-frequency control on crustal velocity structure across an important part of the orogen, allowing future passive-source experiments to focus on related structures within the uppermost mantle.

该研究项目的主要目的是在阿巴拉契亚岛一部分的一部分下方对地形和围内载荷的等静力补偿机理施加限制。 尽管经常断言阿巴拉契亚族人没有地壳根，但在蓝岭和山谷和山脊省的最高海拔高度的莫霍人仍然很差成像。对内华达山脉和其他年轻的山地腰带进行了研究，这表明它们是由上层架子内的密度多样性所支撑的，而不是厚厚的。 这表明在山间建筑期间，岩石圈的地壳和地幔成分之间可能发生大量相互作用。 但是，较旧山带的地震曲线表明了等静止补偿的多种机制。 波罗的海和加拿大盾牌中一些高尚年龄造山带上的剖面揭示了莫霍的明显缓解，表明地壳根可能会持续超过15亿年。 相比之下，许多古生代植物的地震特征表现出相对平坦的Moho，大概是对随后的崩溃，延伸和侵蚀的反应。 一个值得注意的例外是乌拉尔山（Ural Mountains），这是一种晚古生代的造山基​​因，逃脱了主要的延伸，使碰撞建筑基本上完好无损。 整个南部乌拉尔的广泛地震谱分析表明，地壳根在6到15公里的厚度内。 根似乎比现有地形补偿所需的厚度要大得多。尽管进行了广泛的地震分析，但对地壳速度结构或阿巴拉契亚岛南部下方的莫霍人的配置知之甚少。 新的地震折射/广角反射曲线被用来建模地壳地震波速度结构的过渡，从在卡罗来纳州地形下方的堆积地壳到蓝色山脊下方的北美克拉通。 特别是，新数据用于以下方式：1）通过利用临界角度附近的反射系数升高来对MOHO进行图像； 2）平均P和S波速度的图形变化，以限制整个研究区域地下岩石的整体组成的变化； 3）通过在Moho上的新估计值纳入大陆岩石圈弹性弯曲的测试模型； 4）通过对观察到的重力的贡献隔离地壳贡献，通过建模由地震层（密度结构）和地壳厚度的变化所预测的重力场（这将在上层内的任何密度变化上构成约束，从而有助于支持地形和内部内部载荷载荷）； 5）确定该造成造成的原始深层结构在多大程度上在随后的崩溃和延伸的影响下幸存下来。 结果用于评估古生代植物中各个地层/地壳块的长期稳定性。实验针对与蓝岭省相关的区域重力。 两种瞬时爆炸在尺寸岩石采石场和碎石采石场的延迟爆炸都被用作地震来源。采石爆炸的使用是一种有价值但在很大程度上未开发的折射能源/广角反射分析，消除了通常与地壳尺度，活跃源实验相关的钻井和爆炸物的高成本。 新的实验正在帮助将Carolina Terrane和相关东海岸重力高，田纳西河谷和山脊以及Inter Piedmont和Inter Piedmont及相关重力梯度的先前研究中的地壳图像和速度估计联系起来。 组合的结果为跨根部重要部分的地壳速度结构提供了高频控制，从而使未来的被动源实验专注于最上层内的相关结构。