Lithospheric Extension Near Lake Mead, Nevada: A Model for Ductile Flow in the Lower Crust
Lithospheric Extension Near Lake Mead, Nevada: A Model for Ductile Flow in the Lower Crust
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DOI：
10.1029/90jb02621
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发表时间：
1991-03
影响因子：
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通讯作者：
S. Kruse;M. McNutt;J. Phipps-Morgan;L. Royden;B. Wernicke
中科院分区：
文献类型：
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作者：
S. Kruse;M. McNutt;J. Phipps-Morgan;L. Royden;B. Wernicke
重力异常和地形高程的微小变化，在经历了高度可变的上地壳减薄量的地区，可以令人满意地解释下地壳物质的韧性流动在适当的条件下。在这项研究中，我们研究之间的边界未扩展的科罗拉多高原和强烈扩展域在盆地和山脉省的米德湖（内华达州）地区。布格重力和地形数据表明，目前和伸展前的地壳厚度之间的变化，未扩展和扩展的地区是小的。分析通道流动模型表明，在下地壳通道，以减少地壳厚度的不连续性与变量的扩展所需的韧性流动的粘度是高度依赖于通道的厚度和流动所需的长度尺度。牛顿流动和幂律蠕变的有限元模拟表明，在东部盆地和山脉的长度尺度（500公里或更长）上的流动对应于上地壳伸展的1.4-3倍超过10百万年。对于10-25 km厚的韧性河道，要求有效粘度小于10^（18）-10 ^（20）Pa·s。在较短长度尺度（150 km）上的流动，有效粘度可高达10^（21）Pa·s。模拟表明，这些有效的粘度可能会持续较低的地壳物质变形在实验室推导的幂律蠕变速率。较长尺度的流动可能需要更高的地壳温度（超过700°C），这取决于假设的成分和物质特性。在本研究中假设的边界条件下，线性粘性流动模型产生令人满意的近似幂律蠕变变形。这项工作表明，下地壳中的流动可能是一种可行的机制，可以在强烈伸展和不太伸展的区域之间产生地壳总厚度的微小变化，从而解释这些区域之间重力和地形的相对均匀性。
Small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions. In this study we examine the boundary between the unextended Colorado Plateau and a strongly extended domain in the Basin and Range Province in the Lake Mead (Nevada) region. Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. Analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required. Finite element modeling of Newtonian flow and power law creep shows that flow over the length scale of the eastern Basin and Range (500 km or more) corresponding to upper crustal extension by a factor of 1.4–3 over 10 m.y. requires effective viscosities less than 10^(18)–10^(20) Pa s for ductile channels 10–25 km thick. Flow over shorter length scales (150 km) may be accommodated with effective viscosities as high as 10^(21) Pa s. Modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory-derived power law creep rates. The longer-scale flow may require elevated crustal temperatures (more than 700°C), depending on the composition and material properties assumed. Under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep. This work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions, and thereby explaining the relative uniformity in gravity and topography between such regions.