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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Internal dynamics of a paleoaccretionary wedge: insights from combined isotope tectonochronology and sandbox modelling of the South-Central Chilean forearc
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Internal dynamics of a paleoaccretionary wedge: insights from combined isotope tectonochronology and sandbox modelling of the South-Central Chilean forearc

机译：古增生楔的内部动力学：智利中南部前同位素结合的同位素构造年代学和沙箱建模的见解

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摘要

Forearc accretionary wedges are cyclic systems in which material is frontally and/or basally accreted. Material cycling involves underthrusting, subduction, underplating, exhumation, erosion, transfer to the trench and underthrusting again. In this study we present a novel, tectonochronologic approach to constrain long-term exhumation rates of basally accreted wedge complexes, based on isotopic dating of structural features, on petrological data and sandbox analogue simulations. Congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics. For the present-day surface material of the paleoaccretionary wedge of South-Central Chile (Valdivia area, 40°S), published U–Pb ages of detrital zircon place a maximum age of 278 Ma for subduction. Prograde metamorphism at transitional greenschist to blueschist facies conditions (420 °C, 8–9 kbar) was immediately followed by progressive penetrative deformation associated with basal accretion, dated at 250–245 Ma using Rb/Sr internal mineral isochrons. The accretion process involved duplex tectonics and antiformal stacking, with formation of near-horizontal mylonitic shear zones at around 241 Ma. Continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels. Both semi-ductile, small-scale extensional shear zones and post-kinematic vein mineralizations yield Rb/Sr ages of 235 Ma. Tension gashes, representing the latest isotopically dateable stage of structural evolution, were formed at 210 Ma, at conditions of 230 °C at 1.5–3 kbar, as constrained by fluid inclusion data. Zircon fission track data indicate final cooling to below 200 °C at 186±24 Ma. The results suggest continuous basal accretion for at least 50 Ma, with long-term average exhumation rates of 0.6±0.2 mm/a, most probably outbalanced by similar long-term average erosion rates. Changing plate boundary conditions at about 210–200 Ma terminated the accretion process, as evident from a dramatic decrease of exhumation rates at that time. Since then, the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the Andean active margin. The tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high-temperature increments of P–T trajectories.
机译：前额增生楔形是循环系统，其中材料是正面和/或基底增生的。材料循环涉及下冲，俯冲，下冲，掘尸，侵蚀，转移到沟槽并再次下冲。在这项研究中，我们基于构造特征的同位素测年，岩石学数据和沙箱模拟模拟，提出了一种新颖的构造年代学方法来限制基生的楔形复合物的长期发掘速率。大自然中的结构清单与按比例缩放的沙箱实验中生成的结构之间的一致性使对楔形动力学的深入了解成为可能。对于当今智利中南部古增生楔（瓦尔迪维亚地区，40°S）的表层物质，已公布的碎屑锆石的U–Pb年龄为俯冲的最大年龄为278 Ma。在过渡绿片岩向蓝片岩相的条件下（420°C，8–9 kbar）进行渐进变质，随后伴随着基底增生的渐进性渗透变形，使用Rb / Sr内部矿物等时线在250–245 Ma发生。吸积过程涉及双重构造和反形式堆积，并在241 Ma左右形成了近水平的绵状剪切带。深度不断的基底增生在较高的构造水平上引起了伸展构造。半延性的，小规模的伸展剪切带和运动学后的静脉矿化带的Rb / Sr年龄均为235 Ma。受流体包裹体数据约束，在210 Ma，230°C，1.5–3 kbar的压力下，形成了代表结构演变的最新同位素年代学阶段的张力间隙。锆石裂变径迹数据表明在186±24 Ma时最终冷却至200°C以下。结果表明，至少50 Ma的持续基底增生，长期平均尸体发掘速率为0.6±0.2 mm / a，很可能因类似的长期平均侵蚀率而失衡。大约210–200 Ma的板块边界条件发生变化，终止了吸积过程，这从当时的尸体发掘率急剧下降就可以看出。从那时起，尽管古增生楔在安第斯活动边缘内的精细大地构造位置，仍稳定地保持在原位。构造年代学方法是对发掘研究的热年代学和地貌学方法的补充，因为它为质量通量率提供了直接约束，即使是在P–T轨迹的高温情况下也是如此。

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