Lizarralde Daniel, Axen Gary J, Brown Hillary E, Fletcher John M, González-Fernández Antonio, Harding Alistair J, Holbrook W Steven, Kent Graham M, Paramo Pedro, Sutherland Fiona, Umhoefer Paul J
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
Nature. 2007 Jul 26;448(7152):466-9. doi: 10.1038/nature06035.
Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.
裂谷型大陆边缘的结构限制以及由此类裂谷作用产生的岩浆活动，有助于深化我们对岩石圈强度、下地幔状态以及从裂谷作用到海底扩张转变的理解。裂谷的一个重要结构分类是依据宽度，窄裂谷被认为是由颈缩不稳定性形成（即伸展速率超过热扩散速率），而宽裂谷则被认为需要一种抑制局部化的机制，比如在高热流环境下的下地壳流动。对裂谷作用产生的岩浆活动的观测结果范围广泛，从岩浆活动是熔融模型预测值两到三倍的火山边缘，到几乎没有裂谷期或裂谷后期岩浆活动的非火山边缘。这种岩浆活动的变化通常归因于地幔温度的变化。在此，我们描述了加利福尼亚湾南部的佩斯卡多尔地震实验结果，并展示了横跨三个裂谷段的地壳尺度图像。在短距离的横向范围内，我们观测到裂谷样式和岩浆活动存在巨大差异——从伴有少量同期岩浆活动的宽裂谷到岩浆活动强烈段的窄裂谷。但是，许多被认为在裂谷作用期间控制结构演化和岩浆活动的因素（伸展速率、地幔位势温度和热流）往往在更大的长度尺度上变化。我们反而得出结论，地幔亏损而非低地幔温度，是观测到的宽且贫岩浆边缘的原因，并且地幔富度以及可能的沉积隔热而非高地幔温度，是观测到的强烈裂谷期和裂谷后期岩浆活动的原因。
