The extensional Messaria shear zone and associated brittle detachment faults, Aegean Sea, Greece

Structural, thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional Messaria shear zone and the associated brittle Messaria and Fanari detachment faults, which exhumed their footwall from mid-crustal depths on the island of Ikaria in the Aegean. Thermobarometric data indicate that the Messaria shear zone formed at 350–>400 °C and 3–4 kbar (i.e. at a depth of c. 15 km). Normal faulting was accompanied by the intrusion of two granites, which together with the thermobarometric data indicate a relatively high thermal field gradient of 25–35 °C km−1. Zircon and apatite fission-track and apatite (U–Th)/He ages demonstrate rapid cooling in the footwall of the Messaria detachment from c. 400 °C to c. 40 °C between 11 and 3 Ma. Age–distance relationships of the data suggest that the Messaria shear zone and the Messaria detachment slipped at apparent rates of c. 6–9 km Ma−1. Kinematic indicators show a consistent top-to-the-NNE shear sense for the extensional faults. However, at the southern part of the Messaria detachment some late-stage shear-sense indicators are top-to-the-SSW and are assumed to be associated with updoming of the footwall. Numerous deformed pegmatite veins in the Messaria shear zone allow the reconstruction of deformation and flow parameters. The mean kinematic vorticity number ranges from 0.13 to 0.80, indicating that shearing deviated significantly from simple shear; that is, extensional shearing was associated with vertical ductile thinning, which contributed to tectonic exhumation. Finite strain shows oblate geometries and axial ratios of the finite-strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1.8 to 19.9. We calculate, using a 1D numerical model, that vertical ductile thinning contributed c. 20% to exhumation during extensional shearing. Normal faulting was the major agent exhuming the footwall from c. 15 km depth.本数据集涵盖构造、热年代学与变质数据，旨在阐明韧性伸展型梅西阿剪切带（Messaria shear zone）及伴生的脆性梅西阿、法纳里拆离断层的构造性质与演化历程——上述构造将爱琴海伊卡里亚岛的下盘从地壳中部深度剥露至地表。温压数据显示，梅西阿剪切带形成于350~>400℃、3~4千巴（对应约15公里深度）。正断层作用伴随两期花岗岩侵入，结合温压数据可推知，当时区域地热梯度高达25~35℃·km⁻¹。锆石、磷灰石裂变径迹以及磷灰石(U–Th)/He测年结果表明，梅西阿拆离断层下盘在11~3 Ma期间经历快速冷却，温度从约400℃降至约40℃。基于年龄-距离关系数据推算，梅西阿剪切带与梅西阿拆离断层的表观滑动速率约为6~9 km·Ma⁻¹。运动学标志显示，该类伸展断层的剪切指向整体为北东上盘（top-to-the-NNE）；但在梅西阿拆离断层南段，部分晚期剪切指向标志为南南西上盘（top-to-the-SSW），推测其与下盘上隆作用相关。梅西阿剪切带内发育大量变形伟晶岩脉，可用于重建变形与流动参数：其平均运动学涡度数值介于0.13~0.80之间，表明剪切作用显著偏离简单剪切，即伸展剪切伴随韧性垂直减薄，该过程对构造剥露具有明确贡献。有限应变结果显示，在平行于构造运移方向且垂直于糜棱叶理的切片中，有限应变椭圆呈扁椭球形态，轴率介于1.8~19.9之间。通过一维数值模型计算可知，伸展剪切过程中韧性垂直减薄约贡献了20%的剥露量，而正断层作用才是将下盘从约15公里深度剥露至地表的主要营力。