The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile
The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile
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DOI：
10.1016/j.jsames.2019.102355
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发表时间：
2020
影响因子：
1.8
通讯作者：
R. C. Ruthven;J. Singleton;N. Seymour;R. Gomila;G. Arancibia;D. Stockli;J. Ridley;J. F. Magloughlin
中科院分区：
文献类型：
--
作者：
R. C. Ruthven;J. Singleton;N. Seymour;R. Gomila;G. Arancibia;D. Stockli;J. Ridley;J. F. Magloughlin
帕波索断层带是阿塔卡马断层系统 (AFS) 的主要脆性-韧性链，记录了与智利北部白垩纪斜向俯冲相关的左旋剪切。新的详细地质测绘、宏观和微观结构数据以及锆石地质/热年代学揭示了帕波索断层南部结构演化的见解。帕波索断层的核心由约 50m 厚的富含伊利石的断层带界定，该断层陡峭地倾斜于 ESE，断层以西的断裂但未剥落的早侏罗世英云长岩与断层以东的糜棱岩晚侏罗世至早白垩世花岗岩并置。断层以东的糜棱岩带厚约0.7-1km，包括约100-500m厚的热液蚀变超糜棱岩带，源自晚侏罗世（146.5±1.5Ma）至早白垩世（138.5±1.6Ma）花岗闪长岩。在这个超糜棱岩带的西边，有一个约 150-350m 厚的较年轻的英英闪长岩带 (136.0±1.8Ma)，与帕波索断层平行，从原糜棱岩到糜棱岩，距其西缘泥岩带 <10m。糜棱岩叶理陡峭地倾斜至中度东南方向，线理通常倾斜约 10-30° 西南方向。大多数东南倾糜棱岩组构记录了斜向左旋反向剪切，这与帕波索断层泥带小规模脆性断层和S-C-C’组构的整体格局一致。然而，在热液蚀变糜棱岩带的几个部分，对称的微结构和S>L构造岩组构很可能记录了区域正常扁平化的重要组成部分。变形年龄上限被限制为比晚侏罗世/早白垩世花岗闪长岩更年轻。晚侏罗世/早白垩世花岗闪长岩的热液蚀变和高应变区的发育局部与不切割较年轻的原糜棱岩的镁铁质岩脉有关，表明大部分热液蚀变和糜棱岩应变发生在~139Ma和136Ma之间。根据粘土矿物学和伊利石库伯勒指数，帕波索断层泥区形成于 150°C 至 200°C 之间。凿岩形成的时间很可能与晚侏罗世/早白垩世花岗闪长岩的锆石 (U-Th)/He 日期为 116.6±6.2Ma 记录的冷却至约 180–190°C 以下重叠。这些数据共同将脆性和韧性变形限制在早白垩世，类似于沿 AFS 其他部分的变形年龄，并与同空间弧岩浆作用同时期。从区域上看，AFS 的帕波索段呈弓形，北端走向 NNW-SSE，南端走向 NNE-SSW。先前对沿帕波索断层北部部分的断层带的研究记录了左旋转张，而斜向左旋反向剪切和局部同轴压扁则记录了沿帕波索断层南部部分的左旋压裂。我们认为，沿 AFS 的变形和变形是由弓形几何形状控制的，并且两者都与沿 N-S 走向的岩浆弧的左旋简单剪切兼容。
The Paposo fault zone is a major brittle-ductile strand of the Atacama fault system (AFS), which records sinistral shear associated with Cretaceous oblique subduction beneath northern Chile. New detailed geologic mapping, macro- and microstructural data, and zircon geo/thermochronology reveal insight into the structural evolution of the southern portion of the Paposo fault. The core of the Paposo fault is defined by a ~50 m-thick zone of illite-rich gouge that dips steeply ESE and juxtaposes fractured but unfoliated Early Jurassic tonalite west of the fault against mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault. The mylonite zone east of the fault is ~0.7–1 km thick and includes a ~100–500 m-thick band of hydrothermally-altered ultramylonite derived from Latest Jurassic (146.5 ± 1.5 Ma) to Early Cretaceous (138.5 ± 1.6 Ma) granodiorite. West of this ultramylonite zone, a ~150–350 m-thick zone of younger tonalite (136.0 ± 1.8 Ma) parallels the Paposo fault and grades from protomylonite to mylonite <10 m from the gouge zone on its western margin. Mylonitic foliations dip steeply to moderately SE with lineations that typically plunge ~10–30° SW. Most SE-dipping mylonitic fabrics record oblique sinistral-reverse shear that is consistent with the overall pattern of small-scale brittle faults and S-C-C’ fabrics in the Paposo fault gouge zone. However, in several parts of the hydrothermally-altered mylonite zone, symmetric microstructures and S > L tectonite fabrics most likely record a significant component of zone-normal flattening. The upper age limit of deformation is constrained to be younger than the Late Jurassic/Early Cretaceous granodiorite. Hydrothermal alteration and development of high-strain zones in the Late Jurassic/Early Cretaceous granodiorite are locally associated with mafic dikes that do not cut the younger protomylonitic tonalite, indicating that most of the hydrothermal alteration and mylonitic strain occurred between ~139 Ma and 136 Ma. The Paposo fault gouge zone formed between 150 °C and 200 °C based on clay mineralogy and the illite Kübler index. The timing of gouge formation most likely overlaps with cooling below ~180–190 °C recorded by a zircon (U-Th)/He date of 116.6 ± 6.2 Ma from the Late Jurassic/Early Cretaceous granodiorite. Together these data constrain brittle and ductile deformation to the Early Cretaceous, similar to the age of deformation along other segments of the AFS and coeval with co-spatial arc magmatism. Regionally, the Paposo segment of the AFS is arcuate, trending NNW-SSE in the northern end and NNE-SSW in the southern end. Previous studies of fault strands along the northern portion of the Paposo segment document sinistral transtension, whereas oblique sinistral-reverse shear and local coaxial flattening record sinistral transpression along the southern portion of the Paposo fault. We propose that transtension and transpression along the AFS are controlled by the arcuate geometry, and both are compatible with sinistral simple shear along the N-S-trending magmatic arc.