Reconstructing the original geometry of a high-pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, nappe-internal thrusts may disrupt it into several subunits. The Middle-CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans-Cycladic-Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi-petrological and geochronological data from the Middle-CBU nappe from the Thera and Ios islands (Greece). Using Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium-Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle-CBU nappe occurred during the Oligocene at different rates for different localities. The Middle-CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle-CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle-CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.

中文翻译：

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挖掘过程中高压装置的中断：基克拉迪蓝片岩装置示例（希腊锡拉岛、伊奥斯岛和纳克索斯岛）

重建高压构造单元的原始几何形状具有挑战性，但对于理解造山机制很重要。当单个推覆体被俯冲和挖出时，推覆体内部的逆冲作用可能会将其分裂成几个亚基。基克拉迪蓝片岩单元（希腊希腊俯冲造山带）的中 CBU 推覆带显示了沿着跨基克拉迪逆冲断层（TCT）的这种破坏的证据，但是，这种逆冲的时间尚不清楚。在这里，我们报告了锡拉岛和伊奥斯岛（希腊）中 CBU 推覆体的多种岩石学和地质年代学数据。使用金红石中锆测温法与石榴石中石英弹性气压法、平均 P-T 和相平衡热力学模型相结合，我们发现 Ios 中石榴石的生长发生在 6.7 ± 1.4 kbar 至 13.0 ± 1.6 kbar 和 326 的顺变质过程中。 ± 20°C 至 506 ± 13°C（2σ 不确定性），随后早期折返至 10.1 ± 0.6 kbar 和 484 ± 14°C，并在 5.7 ± 1.2 kbar 和 416 ± 14°C 处出现绿片岩相叠印。对于 Thera，我们将峰值 HP 条件限制为 7.6 ± 1.8 kbar 和 331 ± 18°C，然后使用平均 P-T 和相平衡热力学模型在 ~2 kbar 和 ~275°C 下进行折返和平衡。对于 Ios，铀-铅石榴石地质年代学提供的顺积变质年龄为 55.7 ± 5.0 Ma（2σ 不确定性），峰值 HP 变质作用年龄为 40.1 ± 1.4 Ma。将锡拉岛和伊奥斯岛的新 P-T-t 数据与纳克索斯岛的现有数据相结合，我们得出的结论是，所研究的推覆带段代表了以前连贯推覆带的残余物。 P-T-t数据定义始新世俯冲速率为2.1±1.0 km/Ma，明显慢于当前40-45 km/Ma的俯冲速率。俯冲后，中 CBU 推覆体在渐新世发生折返，不同地点的折返速度不同。纳克索斯岛的中 CBU 推覆体以约 6 km/Ma 的速率被折返，而艾奥斯岛计算的折返速率为约 3 km/Ma。这一结果表明，纳克索斯岩石的中 CBU 推覆体在渐新世期间被逆冲到了依奥斯岩石上。使用 P-T-t 数据并假设始新世和渐新世期间的真实俯冲角度，我们提出了这些岛屿中 CBU 推覆体的二维结构重建。这种重建有助于了解大陆边缘俯冲的机制及其在折返过程中的破坏。