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–&gt;400 °C and 3–4 kbar (i.e. at a depth of <em>c</em>. 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 <em>c</em>. 400 °C to <em>c</em>. 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 <em>c</em>. 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 <em>c</em>. 20% to exhumation during extensional shearing. Normal faulting was the major agent exhuming the footwall from <em>c</em>. 15 km depth.

本数据集依托构造学、热年代学与变质学数据，旨在阐明爱琴海伊卡里亚岛（Ikaria）韧性伸展型梅萨里亚剪切带（Messaria shear zone）及伴生脆性梅萨里亚与法纳里拆离断层（Fanari detachment fault）的构造属性与演化历史——上述构造将其下盘地壳从中地壳深度剥露抬升。热压测温数据显示，梅萨里亚剪切带形成于350~>400 ℃与3~4 kbar的温压条件下，对应埋藏深度约15 km。正断层活动伴随两期花岗岩侵入，结合热压测温数据可知，研究区当时存在25~35 ℃·km⁻¹的相对较高地热梯度。锆石与磷灰石裂变径迹（fission-track）定年、磷灰石（铀-钍）/氦（(U–Th)/He）定年结果表明，梅萨里亚拆离断层下盘在11~3 Ma期间经历了从约400 ℃快速冷却至约40 ℃的过程。数据的年龄-距离关系显示，梅萨里亚剪切带与梅萨里亚拆离断层的视滑移速率约为6~9 km·Ma⁻¹。运动学标志显示，该类伸展断层具有统一的北北东向剪切指向。但梅萨里亚拆离断层南部区域的部分晚期剪切指向标志呈南南西向，推测其与下盘地壳的上隆作用相关。梅萨里亚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。其运动学平均涡度值介于0.13~0.80之间，表明剪切作用显著偏离纯简单剪切：即伸展剪切伴随韧性垂向减薄，该过程对构造剥露具有贡献。有限应变呈扁椭球形，且在平行于构造运移方向、垂直于糜棱叶理（mylonitic foliation）的切片中，有限应变椭圆（finite-strain ellipse）的轴比介于1.8~19.9之间。我们通过一维数值模型计算得出，伸展剪切过程中，韧性垂向减薄对剥露的贡献约为20%。正断层作用则是将下盘地壳从约15 km深度剥露抬升的主要营力。