Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)
Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)
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DOI：
10.1029/2019jb018052
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发表时间：
2019-10-29
影响因子：
3.9
通讯作者：
Menegon, L.
中科院分区：
文献类型：
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作者：
Campbel, L. R.;Menegon, L.
要了解下地壳在地震周期中承受应力和应变率瞬时变化的能力，需要对深部地壳断层岩的变形机制和流变学进行详细研究。在这里，我们表明，下地壳pseudotachyte轴承剪切带能够适应短期的高应变率和高应力变形的加速粘性蠕变，其次是减少应力的一些环境变形条件。挪威罗弗敦未变形麻粒岩中含有假玄武玻璃的剪切带中的石英微结构表明，在快速应变速率和分别类似于10（-9）s（-1）和类似于100 MPa的高应力下的粘性蠕变过程中发生了动态重结晶。较低应力的微结构（即，泡沫纹理）也记录在剪切带中，指示在变形循环期间应力和应变速率的空间和时间变化。高应力和低应力石英重结晶都发生在650 ℃-750 ℃和0.7-0.8 GPa的麻粒岩相条件下，代表了下地壳内高度局部化的粘性蠕变记录。这意味着，下地壳的假玄武玻璃体有可能在强的下地壳内形成极其局部化的薄弱地带，从而对应力和应变率的扰动产生深层的力学响应，例如在地震周期中经历的扰动，例如孕震加载，随后是震后松弛。
Understanding the ability of the lower crust to support transient changes in stresses and strain rates during the earthquake cycle requires a detailed investigation of the deformation mechanisms and rheology of deep crustal fault rocks. Here, we show that lower crustal pseudotachylyte-bearing shear zones are able to accommodate short-term episodes of high strain rate and high stress deformation by accelerated viscous creep, followed by a reduction in stresses to some ambient deformation condition. Quartz microstructure within pseudotachylyte-bearing shear zones in otherwise undeformed granulites from Lofoten, Norway, indicates that dynamic recrystallization occurred during viscous creep under rapid strain rates and high stresses of similar to 10(-9) s(-1) and similar to 100 MPa, respectively. Lower stress microstructures (i.e., foam textures) are also recorded in the shear zones, indicating spatial and temporal variations of stress and strain rate during deformation cycles. Both the high and lower stress quartz recrystallization took place under granulite facies conditions of 650 degrees C-750 degrees C and 0.7-0.8 GPa and represented a record of highly localized viscous creep within the lower crust. This implies that lower crustal pseudotachylytes are potentially able to form extremely localized weak zones within strong lower crust, enabling a deep mechanical response to perturbations in stress and strain rate such as those experienced during the seismic cycle, for example, seismogenic loading followed by subsequent postseismic relaxation.