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Sinistral shear during Middle Jurassic emplacement of the Matancilla Plutonic Complex in northern Chile (25.4° S) as evidence of oblique plate convergence during the early Andean orogeny
Journal of South American Earth Sciences ( IF 1.8 ) Pub Date : 2022-09-27 , DOI: 10.1016/j.jsames.2022.104047
S.P. Mavor, J.S. Singleton, G. Heuser, R. Gomila, N.M. Seymour, S. Williams, G. Arancibia

Arc magmatism in a continental subduction zone facilitates rheological weakening of the rigid upper plate, and can accommodate the partitioned trench-parallel component of oblique subduction into an intra-arc shear zone. We document a shear zone at latitude 25.4° S near Taltal, Chile that was associated with intrusion of the Matancilla Plutonic Complex at ∼169 Ma to evaluate intra-arc deformation and possible tectonic plate configurations during this time period. Polyphase folding of Paleozoic metasedimentary rocks is overprinted by mylonitic fabrics that are most extensive in a zone up to 1.4 km wide in the thermal aureole of the granodioritic Matancilla pluton, where contact metamorphic andalusite porphyroblasts are synkinematic with fabric development. Mylonite in metasedimentary rocks is overprinted by a ∼130 Ma granodiorite (zircon U–Pb) and by ∼133 Ma postkinematic monazite (U–Pb). Within the Jurassic Matancilla granodiorite, pervasive ductile shear occurs along the intrusive contact while centimeter-scale discrete high-strain zones throughout the pluton are associated with focused hydrothermal alteration and reaction weakening. Mylonitic foliation in the metasedimentary rocks and within the pluton strikes N- to NE and dips steeply, while stretching lineations are subhorizontal on average. Kinematic indicators record dominantly sinistral shear, though some dextral or symmetric indicators and S > L fabrics suggest a component of coaxial strain and flattening. Sinistral strike-slip kinematics in the Matancilla shear zone may indicate that Middle Jurassic convergence had sinistral obliquity that was locally partitioned into the contemporaneous magmatic arc. Sinistral-oblique convergence would require the Phoenix-Farallon spreading center to be north of ∼25° S in the Middle Jurassic, providing a constraint to plate reconstructions during the early Andean orogeny.



中文翻译:

智利北部(25.4°S)中侏罗世马坦西拉深成复合体就位期间的左旋切变,作为早期安第斯造山运动期间倾斜板块汇聚的证据

大陆俯冲带的弧岩浆作用有利于刚性上板块的流变弱化,可以将倾斜俯冲的分区沟槽平行分量容纳到弧内剪切带中。我们记录了智利塔尔塔尔附近纬度 25.4° S 的剪切带,该剪切带与大约 169 Ma 的 Matancilla 深成复合体的侵入有关,以评估此期间的弧内变形和可能的构造板块配置。古生代变沉积岩的多相褶皱被糜棱岩组织叠印,糜棱岩组织在花岗闪长岩Matancilla pluton的热光环中 1.4 公里宽的区域中最为广泛,该区域接触变质红柱石。porphyroblasts 与织物发育是同步运动的。变沉积岩中的糜棱独居石(U-Pb)叠印。在侏罗纪 Matancilla 花岗闪长岩中,沿侵入接触发生普遍的韧性剪切,而整个岩体中厘米级的离散高应变区与集中的热液蚀变和反应减弱有关。变沉积岩和岩体内部的糜棱线状平均而言是亚水平的。运动学指标主要记录左旋剪切,尽管一些右旋或对称指标和 S > L 织物表明同轴应变和展平的成分。Matancilla 剪切带的左旋走滑运动学可能表明中侏罗世辐合具有左旋倾角,该倾角局部划分为同期岩浆弧。左旋斜辐合要求凤凰-法拉隆扩张中心位于侏罗纪中期~25°S以北,为安第斯早期造山运动期间的板块重建提供了约束。

更新日期:2022-09-27
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