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标是对阿巴拉契亚山脉南部部分地区下方地形和地壳内载荷的均衡补偿机制施加限制。 尽管人们经常断言阿巴拉契亚山脉没有地壳根部，但蓝岭省和谷岭省最高海拔下的莫霍面图像仍然很差。最近在内华达山脉和其他年轻山脉带的研究表明，它们得到了密度的支持上地幔内部的异质性，而不是地壳的增厚。 这表明造山过程中岩石圈的地壳和地幔成分之间可能发生大量的相互作用。 然而，古老山脉的地震剖面表明存在多种均衡补偿机制。 波罗的海和加拿大地盾中一些太古代造山带的剖面揭示了莫霍面的显着起伏，表明地壳根部可能存在超过 15 亿年。 相比之下，许多古生代造山带的地震剖面显示出相对平坦的莫霍面，可能是对随后的塌陷、伸展和侵蚀的反应。 一个值得注意的例外是乌拉尔山脉，这是一个晚古生代造山带，没有发生大规模扩张，使得碰撞结构基本完好无损。 乌拉尔南部的广泛地震剖面显示，地壳根部厚度为 6 至 15 公里。 根部似乎比补偿现有地形所需的粗得多。尽管进行了广泛的地震剖面分析，但人们对阿巴拉契亚山脉南部下方的地壳速度结构或莫霍面构造知之甚少。 新的地震折射/广角反射剖面被用来模拟地壳地震波速度结构从卡罗莱纳地体下方的增生地壳到蓝岭下方的北美克拉通的转变。 特别是，新数据被用于：1）利用临界角附近升高的反射系数对莫霍面进行成像； 2) 绘制平均纵波和横波速度的变化图，以限制整个研究区域基岩主体成分的变化； 3）通过结合莫霍面起伏的新估计来测试大陆岩石圈弹性弯曲模型； 4) 通过对由地震确定的成分（密度结构）和地壳厚度变化预测的重力场进行建模，隔离地壳对观测到的重力的贡献（这对最上地幔内有助于支撑地形和地壳内载荷的任何密度变化施加了限制） ; 5）确定该造山带原始深层结构在随后塌陷和扩张的影响下幸存下来的程度。 结果用于评估古生代造山带内各个地体/地壳块的长期稳定性。这些实验的目标是与蓝岭省相关的区域重力低点。 石料采石场的瞬时爆炸和碎石采石场的延迟爆炸都被用作地震源。采石场爆破是一种宝贵但尚未开发的折射/广角反射剖面能源，它的使用消除了通常与地壳规模主动源实验相关的钻探和爆炸的高昂成本。 新实验正在帮助将卡罗莱纳地体和相关东海岸重力高位、田纳西河谷和山脊以及内皮埃蒙特和相关重力梯度先前研究的地壳图像和速度估计联系起来。 综合结果为造山带重要部分的地壳速度结构提供了高频控制，使未来的被动源实验能够集中于最上地幔内的相关结构。