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

这个研究项目的主要目标是把约束的地形和地壳内负荷下的部分南阿巴拉契亚山脉的均衡补偿机制。 尽管人们经常断言阿巴拉契亚山脉没有地壳根，但蓝岭和谷岭地区最高海拔以下的莫霍面的成像仍然很差。最近对内华达州山脉和其他年轻山脉带的研究表明，它们是由上地幔内的密度不均匀性而不是地壳增厚所支持的。 这表明，在造山过程中，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，穿越较老的山脉带的地震剖面显示了各种均衡补偿机制。 在波罗的海和加拿大地盾的一些太古代造山带的剖面显示，莫霍面有明显的起伏，这表明地壳的根源可能持续了15亿年以上。 相反，许多古生代造山带的地震剖面显示莫霍面相对平坦，这可能是对随后的崩塌、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，它逃脱了大规模的伸展，留下了大部分完整的碰撞结构。 横跨乌拉尔山脉南部的广泛地震剖面显示，任何地方都有6至15公里厚的地壳根。 根部似乎比补偿现有地形所需的厚得多。尽管广泛的地震剖面，很少有人知道的地壳速度结构或配置的莫霍面下的阿巴拉契亚山脉南部。 目前正在使用新的地震折射/广角反射剖面图来模拟地壳地震波速度结构从卡罗莱纳地体下的增生地壳向蓝岭下的北美克拉通的过渡。 特别是，新的数据正被用于：1）利用临界角附近的高反射系数，对莫霍面进行成像; 2）绘制平均P波和S波速度的变化图，以限制整个研究区域基底岩石整体成分的变化; 3）通过纳入对莫霍面起伏的新估计，测试大陆岩石圈弹性弯曲模型; 4）通过模拟由地震确定的成分变化预测的重力场，分离地壳对观测重力的贡献（密度结构）和地壳厚度（这限制了最上层地幔内有助于支持地形和地壳内负荷的任何密度变化）;（5）确定该造山带原始深部结构在随后的崩塌和伸展作用下幸存的程度。 这些结果正被用来评估古生代造山带内个别岩石/地壳块体的长期稳定性，实验的目标是与蓝岭省有关的区域重力低。 尺寸石采石场的瞬时爆破和碎石采石场的延迟爆破都被用作地震源。采石场爆破是折射/广角反射剖面测量的一种宝贵但基本上尚未开发的能源，使用采石场爆破消除了通常与地壳规模的有源实验有关的高昂的钻探和爆炸费用。 新的实验有助于将以前在卡罗莱纳地体和相关的东海岸重力高压、田纳西谷脊、内皮埃蒙特和相关的重力梯度中进行的研究中的地壳图像和速度估计联系起来。 综合结果提供了高频控制地壳速度结构的一个重要组成部分的造山带，使未来的被动源实验专注于最上层地幔内的相关结构。

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.