Wilson David, Aster Richard, West Michael, Ni James, Grand Steve, Gao Wei, Baldridge W Scott, Semken Steve, Patel Paresh
Department of Earth and Environmental Science and Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
Nature. 2005 Feb 24;433(7028):851-5. doi: 10.1038/nature03297.
A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.
在美国西南部里奥格兰德裂谷地区进行的一项高分辨率区域被动地震实验，产生了上地幔速度结构和壳幔地形的新图像。将这些结果与地球化学及其他地球物理证据相结合，揭示出高度对称的下地壳和上地幔岩石圈伸展变形，这表明里奥格兰德裂谷存在纯剪切裂谷作用机制。下地壳的伸展分布在一个区域，其宽度是裂谷地表表现宽度的四倍。我们在此提出，横向分布的纯剪切伸展是裂谷形成初期低应变率和区域地热升高的综合效应，可能还受到了先前存在的岩石圈结构的影响。与更局部化的变形相比，下地壳和地幔中的分布式伸展导致地幔垂直上涌的集中程度较低，小规模对流也较弱。这种缺乏高度集中的地幔上涌现象，可能解释了与其他主要裂谷系统（如肯尼亚裂谷）相比，里奥格兰德裂谷中与裂谷相关的火山活动较少的原因。
