We report the results of a field study on Sikinos Island in the Aegean extensional province of Greece and propose a hinge zone controlling incipient bivergent extension in the southern Cyclades. A first deformation event led to top-S thrusting of the Cycladic Blueschist Unit (CBU) onto the Cycladic basement in the Oligocene. The mean kinematic vorticity number (W_m) during this event is between 0.56 and 0.63 in the CBU, and 0.72 to 0.84 in the basement, indicating general-shear deformation with about equal components of pure and simple shear. The strain geometry was close to plane strain. Subsequent lower-greenschist-facies extensional shearing was also by general-shear deformation; however, the pure-shear component was distinctly greater (W_m = 0.3–0.41). The degree of subvertical pure-shear flattening increases structurally upward and explains alternating top-N and top-S shear senses over large parts of the island. Along with an increased coaxial deformation component, the strain geometry became oblate. Published quantitative data from nearby Ios Island are similar and both data sets define an extensional hinge zone between top-N extensional deformation across large parts of the central and northern Cyclades and top-S extensional deformation at the southern and western fringe of the archipelago. This extensional hinge zone is an important large-scale structure forming early in the history of lithospheric extension due to southward retreat of the Hellenic slab.

中文翻译：

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南基克拉迪群岛双分支扩张的几何学和运动学：约束希腊爱琴海锡基诺斯岛的扩张铰链区

我们报告了对希腊爱琴海伸展省锡基诺斯岛的实地研究结果，并提出了控制基克拉泽斯南部初期双辐伸展的铰链区。第一次变形事件导致基克拉迪蓝片岩单元 (CBU) 顶向 S 冲撞到渐新世基克拉迪基底上。该事件期间的平均运动涡度数 ( W _m ) 在 CBU 中介于 0.56 和 0.63 之间，在基底中介于 0.72 到 0.84 之间，表明纯剪切和简单剪切分量大致相等的一般剪切变形。应变几何形状接近平面应变。随后的低绿片岩相伸展剪切也是一般剪切变形；然而，纯剪切分量明显更大（W _m = 0.3–0.41）。近垂直纯剪切变平的程度在结构上向上增加，并解释了该岛大部分地区的顶部 N 和顶部 S 剪切感交替。随着同轴变形分量的增加，应变几何形状变得扁平。来自附近伊奥斯岛的已发布定量数据相似，两个数据集都定义了基克拉泽斯中部和北部大部分地区的 top-N 拉伸变形与群岛南部和西部边缘的 top-S 拉伸变形之间的拉伸铰链区。该伸展铰链带是早期由于希腊板块向南退缩而形成的岩石圈伸展历史上的重要大尺度构造。