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

这项研究项目的主要目标是对阿巴拉契亚山脉南部部分之下地形和壳内负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，但蓝岭和山谷山脊省最高海拔以下的莫霍面仍然成像不佳。最近在内华达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支持的，而不是由地壳增厚支持的。这表明，在造山过程中，岩石圈的地壳和地幔成分之间可能会发生大量的相互作用。然而，跨越老山带的地震剖面表明，均衡补偿的机制多种多样。波罗的海和加拿大地盾中一些太古代造山带的剖面显示，莫霍面上有显著的起伏，这表明地壳根源可能持续了15亿年以上。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，推测是对随后的坍塌、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，没有受到主要的伸展，使碰撞结构基本完好无损。横跨乌拉尔南部的广泛地震剖面显示，地壳根在任何地方都有6到15公里厚。根部似乎比补偿现有地形所需的厚度要厚得多。尽管进行了广泛的地震剖面分析，但对阿巴拉契亚山脉南部的地壳速度结构或莫霍面的形态知之甚少。正在使用新的地震折射/广角反射剖面来模拟从卡罗莱纳地体之下的增生壳到蓝脊之下的北美克拉通的地壳地震波速度结构的过渡。具体地说，这些新数据正被用于：1)利用临界角附近反射系数升高的优势对莫霍面成像；2)绘制研究区域内平均P波和S波速度的变化图，以约束整个研究区基岩的整体成分变化；3)通过结合莫霍面新的起伏估计，测试大陆岩石圈的弹性弯曲模型；4)通过模拟由地震确定的成分变化(密度结构)和地壳厚度预测的重力场，分离地壳对观测重力的贡献(这限制了最上地幔内有助于支持地形和地壳内载荷的任何密度变化)；5)确定该造山带的原始深部结构在多大程度上经受了随后的坍塌和伸展作用。这些结果正被用来评估古生代造山带内个别地体/地壳区块的长期稳定性。实验的目标是与蓝脊省相关的区域重力低。维石采石场的瞬时爆炸和碎石采石场的延时爆炸都被用作震源。采石场爆炸是折射/广角反射剖面的一种宝贵但基本上尚未开发的能源，它的使用消除了通常与地壳规模的主动源实验相关的高昂的钻探和炸药成本。新的实验正在帮助将以前在卡罗莱纳地体和相关东海岸重力高、田纳西山谷和山脊以及内山前和相关重力梯度研究中得到的地壳图像和速度估计联系起来。综合结果提供了对造山带重要部分地壳速度结构的高频控制，使未来的被动源实验能够专注于最上面地幔内的相关结构。

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.