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

本研究项目的主要目标是对阿巴拉契亚山脉南部部分地区地形和地壳内载荷的均衡补偿机制进行限制。尽管人们经常断言阿巴拉契亚山脉没有地壳的根基，但在蓝岭和山谷岭省海拔最高的地方，莫霍人的图像仍然很差。最近对内华达山脉和其他年轻山带的研究表明，它们是由上地幔内部的密度不均匀性支撑的，而不是由地壳的增厚支撑的。这表明，在造山过程中，岩石圈的地壳和地幔组分之间可能发生大量的相互作用。然而，古山带的地震剖面显示了多种均衡补偿机制。在波罗的海和加拿大盾地的一些太古代造山带的剖面显示，莫霍上有明显的起伏，表明地壳的根可能持续了超过15亿年。相比之下，许多古生代造山带的地震剖面显示出一个相对平坦的莫霍，可能是对随后的崩塌、伸展和侵蚀的反应。乌拉尔山脉是一个明显的例外，它是晚古生代的造山带，躲过了大规模的伸展，碰撞建筑基本上完好无损。乌拉尔山脉南部广泛的地震剖面显示，地壳根部厚度在6至15公里之间。树根似乎比补偿现有地形所需的要厚得多。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构造知之甚少。新的地震折射/广角反射剖面被用于模拟地壳地震波速度结构从卡罗莱纳地体下的增生地壳到蓝岭下的北美克拉通的过渡。特别是，新数据被用于：1)利用在临界角附近提高的反射系数来成像莫霍图；2)绘制平均纵波和横波速度的变化，以约束整个研究区基底岩石体组成的变化；3)结合新的莫霍起伏估计，检验大陆岩石圈弹性弯曲模型；4)通过模拟由地震确定的成分（密度结构）和地壳厚度变化预测的重力场，分离出地壳对观测重力的贡献（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)确定该造山带的原始深部构造在随后的崩塌和伸展作用下的幸存程度。研究结果被用于评估古生代造山带内单个地体/地壳块体的长期稳定性。实验的目标是与蓝岭省相关的区域重力低。大型石矿采石场的瞬时爆破和破碎石矿采石场的延时爆破都被用作震源。对于折射/广角反射剖面来说，采石场爆炸是一种宝贵但尚未开发的能源，它的使用消除了通常与地壳尺度有源实验相关的钻井和炸药的高昂成本。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、田纳西河谷和山脊、内皮埃蒙特和相关重力梯度的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的地壳速度结构的高频控制，使未来的被动源实验能够集中在上地幔内的相关结构上。

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.