Disruption of a high-pressure unit during exhumation: Example of the Cycladic Blueschist unit (Thera, Ios and Naxos islands, Greece)

IF 3.5 2区 地球科学 Q1 GEOLOGY
Alexandre Peillod, Clifford G. C. Patten, Kirsten Drüppel, Aratz Beranoaguirre, Armin Zeh, Dominik Gudelius, Simon Hector, Jarosław Majka, Barbara I. Kleine-Marshall, Andreas Karlson, Axel Gerdes, Jochen Kolb
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Abstract

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.

Abstract Image

出露过程中高压单元的破坏:基克拉泽斯蓝岩单元实例(希腊泰拉岛、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,但对于了解造山机制非常重要。虽然一个单一的岩层经过俯冲和隆起,但岩层内部的推力可能会将其破坏成几个子单元。基克拉泽斯蓝晶岩单元(希腊海伦内德俯冲造山带)的中基克拉泽斯蓝晶岩岩脉显示了沿跨基克拉泽斯推力(TCT)发生这种破坏的证据,但是,这种推力发生的时间尚不清楚。在此,我们报告了来自希腊泰拉岛和伊奥斯岛的中CBU岩层的多岩石学和地质年代学数据。通过使用Zr-in-rutile热量测定法、石英-in-石榴石弹性气压测定法、平均P-T和相平衡热力学建模,我们表明伊奥斯岛的石榴石生长发生在6.7 ± 1.4千巴至13.0±1.6千巴、326±20°C至506±13°C(不确定性为2σ),随后是早期掘起至10.1±0.6千巴和484±14°C,以及5.7±1.2千巴和416±14°C的绿泥石面叠加。对于Thera,我们利用平均P-T和相平衡热力学建模,将峰值HP条件约束在7.6 ± 1.8千巴和331 ± 18°C,然后在~2千巴和~275°C进行出露和平衡。对于 Ios,铀铅石榴石地质年代学提供了 55.7 ± 5.0 Ma 的顺行年龄(不确定性为 2σ)和 40.1 ± 1.4 Ma 的峰值 HP 变质作用年龄。结合特雷拉岛和伊奥斯岛的新 P-T-t 数据以及纳克索斯岛的现有数据,我们得出结论,所研究的岩层片段代表了以前连贯岩层的残余。P-T-t数据确定始新世的俯冲速率为2.1 ± 1.0 km/Ma,明显慢于目前40-45 km/Ma的俯冲速率。在俯冲作用之后,中CBU岩浆岩在渐新世以不同的速度在不同的地方发生了隆起。纳克索斯的中-CBU岩层以大约6 km/Ma的速度被掘出,与伊奥斯计算出的大约3 km/Ma的掘出速度形成鲜明对比。这一结果表明,在渐新世时期,纳克索斯岩石的中CBU岩层被推到了伊奥斯岩层上。利用P-T-t数据,并假设始新世和渐新世期间的俯冲角度符合实际情况,我们提出了这些岛屿的中CBU岩层的二维结构重建。这种重建有助于理解大陆边缘的俯冲机制及其在掘起过程中的破坏。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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