Deformation in the mantle wedge associated with Laramide flat‐slab subduction
Deformation in the mantle wedge associated with Laramide flat‐slab subduction
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DOI：
10.1002/2016gc006361
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发表时间：
2013-12
期刊：
影响因子：
3.7
通讯作者：
W. Behr;Douglas P. Smith
中科院分区：
文献类型：
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作者：
W. Behr;Douglas P. Smith
美国中西部落基山脉的Laramide地壳变形通常被认为与Farallon板片的浅俯冲的狭窄部分有关，但对于如何适应板幔岩石圈界面的变形沿着没有共识。在这里，我们研究了与北美板块底部接触的平板上方的水化和剪切相关的地幔岩石变形。我们关注的岩石是变形的、水合的、超镁铁质的包裹体，它们位于科罗拉多高原中部纳瓦霍火山区的diatremes内，在Laramide火山活动的衰退阶段爆发。我们记录了一系列变形结构，包括粒状橄榄岩，碎斑橄榄岩，糜棱岩和碎裂岩，我们解释为反映不同的接近板幔界面剪切带。矿物组合和化学性质将变形限制在温度范围为1550 -750°C的含水条件下。尽管存在的含水层状硅酸盐的模态百分比高达30%，变形主要是由位错蠕变橄榄石。糜棱岩在橄榄石中表现出不常见的晶格择优取向（LPO），称为B型LPO，其中a轴垂直于流动方向排列。这些组构形成的低温、水合环境与实验室实验一致，实验表明B-型LPO在高应力和高含水量条件下形成;此外，这些LPO的地幔楔背景与许多现代俯冲带上方地幔楔中的海沟平行各向异性的观察结果一致。用古压差法估算的糜棱岩的差应力值在290 - 444 MPa之间，用湿橄榄石流动定律计算的有效粘度约为1019 - 1023 Pa·s。这些岩石中记录的高应力幅值、高有效粘度和高应变与模型一致，该模型调用显著的基底剪切牵引力，作为对大陆内部Laramide隆起和收缩的贡献。
Laramide crustal deformation in the Rocky Mountains of the west‐central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab‐mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated with hydration and shear above the flat‐slab at its contact with the base of the North American plate. The rocks we focus on are deformed, hydrated, ultramafic inclusions hosted within diatremes of the Navajo Volcanic Field in the central Colorado Plateau that erupted during the waning stages of the Laramide orogeny. We document a range of deformation textures, including granular peridotites, porphyroclastic peridotites, mylonites, and cataclasites, which we interpret to reflect different proximities to a slab‐mantle‐interface shear zone. Mineral assemblages and chemistries constrain deformation to hydrous conditions in the temperature range ∼550–750°C. Despite the presence of hydrous phyllosilicates in modal percentages of up to 30%, deformation was dominated by dislocation creep in olivine. The mylonites exhibit an uncommon lattice preferred orientation (LPO) in olivine, known as B‐type LPO in which the a‐axes are aligned perpendicular to the flow direction. The low temperature, hydrated setting in which these fabrics formed is consistent with laboratory experiments that indicate B‐type LPOs form under conditions of high stress and high water contents; furthermore, the mantle wedge context of these LPOs is consistent with observations of trench‐parallel anisotropy in the mantle wedge above many modern subduction zones. Differential stress magnitudes in the mylonitic rocks estimated using paleopiezometry range from 290 to 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 1019−1023 Pa s. The high stress magnitudes, high effective viscosities, and high strains recorded in these rocks are consistent with models that invoke significant basal shear tractions as contributing to Laramide uplift and contraction in the continental interior.