Fracturing, comminution and grain-size-sensitive creep as a record of coseismic loading in the middle-crust: Insights from the Urtiga mylonitic pluton (NE Brazil)

IF 2.6 2区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
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Abstract

Evidence for mid-crustal seismic slip is scarce due to the limited capacity of downward propagation of rupture damage imposed by the confining pressure. Nevertheless, structures indicative of elevated stresses occur throughout the crustal profile. To further investigate this setting, we have studied the fabrics of the Urtiga pluton, emplaced at the south Patos shear zone (northeast Brazil), a major Neoproterozoic crustal boundary. K-feldspar and plagioclase porphyroclasts are fractured, with fine-grained K-feldspar + plagioclase mixtures filling cracks. The edges of the clasts are rimmed by fine feldspar grains that form a fine-grained matrix. Quartz ribbons are parallel to the mylonitic foliation and show microstructures and a fabric indicating deformation via dislocation creep. Chemical compositions of feldspars are typically similar between porphyroclasts, fractures and matrix, with plagioclase grains locally being more albitic within fractures. Crystallographic preferred orientations (CPO) in grains within fractures are host-controlled by the adjacent porphyroclast, while fine grains rimming the clasts show a weak CPO and are mostly strain-free. These characteristics suggest that grain size reduction in the Urtiga mylonitic pluton occurred through fracturing and subsequent grain-size-sensitive creep under mid-crustal conditions, which were possibly attained via downward propagation of seismic rupture from the overlying seismogenic zone during transient, seismic slip episodes, giving rise to spatially related pseudotachylytes at the boundaries of the southern Patos shear zone.

断裂、粉碎和对晶粒尺寸敏感的蠕变是中间岩壳同震加载的记录:从 Urtiga mylonitic pluton(巴西东北部)获得的启示
由于约束压力造成的破裂破坏向下传播的能力有限,地壳中部地震滑动的证据很少。然而,在整个地壳剖面上都出现了表明应力升高的结构。为了进一步研究这一环境,我们研究了位于南帕托斯剪切带(巴西东北部)的 Urtiga 长岩的构造,该剪切带是新近新生代地壳的一个主要边界。钾长石和斜长石斑岩呈断裂状,细粒钾长石+斜长石混合物填充裂缝。岩屑边缘有细长石颗粒,形成细粒基质。石英带平行于麦饭石折线,显示出微观结构和构造,表明是通过位错蠕变发生变形的。长石的化学成分在斑岩、断裂和基质之间通常是相似的,断裂中的斜长石晶粒在局部地区更偏向于白垩系。裂缝内晶粒的晶体学优选取向(CPO)受相邻斑岩的寄主控制,而环绕斑岩的细晶粒则显示出微弱的 CPO,且大多无应变。这些特征表明,Urtiga岩浆岩柱的晶粒尺寸减小是通过断裂和随后的晶粒尺寸敏感蠕变发生的,这种蠕变发生在地壳中层条件下,可能是在瞬时地震滑动事件中通过上覆地震成因区的地震断裂向下传播实现的,从而在南部帕托斯剪切带的边界产生了空间上相关的假梯状岩。
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来源期刊
Journal of Structural Geology
Journal of Structural Geology 地学-地球科学综合
CiteScore
6.00
自引率
19.40%
发文量
192
审稿时长
15.7 weeks
期刊介绍: The Journal of Structural Geology publishes process-oriented investigations about structural geology using appropriate combinations of analog and digital field data, seismic reflection data, satellite-derived data, geometric analysis, kinematic analysis, laboratory experiments, computer visualizations, and analogue or numerical modelling on all scales. Contributions are encouraged to draw perspectives from rheology, rock mechanics, geophysics,metamorphism, sedimentology, petroleum geology, economic geology, geodynamics, planetary geology, tectonics and neotectonics to provide a more powerful understanding of deformation processes and systems. Given the visual nature of the discipline, supplementary materials that portray the data and analysis in 3-D or quasi 3-D manners, including the use of videos, and/or graphical abstracts can significantly strengthen the impact of contributions.
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