赫兹破裂:一种被忽视的构造

Hertzian fracture:A type of neglected structure

赫兹破裂在工程和材料领域一直备受关注,但其在地质学领域的应用还很少,远未像里德尔破裂那样熟知。从微观尺度到露头尺度,从板块尺度到行星尺度,赫兹破裂在自然界都广泛存在。赫兹破裂作为一种点接触破裂区别于常见的里德尔剪切破裂。它们均是自然界中特殊而又普遍发育的破裂形式,代表了不同的边界和应力条件。典型的赫兹破裂属于弹性变形范畴,由于点接触而在三维空间中形成特征的锥形破裂,在平面上则表现为放射状及环状破裂。在剖面上锥形破裂一般会出现一定角度的拐折,下段破裂与接触面的角度变大,是锥形破裂的典型特征之一。在地质学领域,赫兹破裂有着重要的指示意义,可以表征物质的韧性(toughness)、解释环状和放射状构造、解释锥状以及弧形破裂成因与样式、判断冰川以及断层的运动学特征、区分不同破裂机制形成的新月形构造等,也可以应用于恢复沉积环境以及判断成岩过程中或者构造变形过程中的古应力及其方向。板块碰撞过程中,在特定环境下也可形成赫兹破裂,因此该类型破裂将会有助于我们理解一些断层的成因、发展以及大陆的变形,同时为类地行星(卫星)表层的构造提供解释方案。

The Hertzian fracture has always been an important research field in engineering and material fields,but its application in geology is still rare,far from being as familiar as the Riedel fracture.However,the Hertz fracture is indeed widely developed in nature,ranging from micro scale to outcrop scale,from plate scale to planetary scale.The Hertzian fracture is distinguished from the Riedel shear fracture as a point contact fracture.They are both unique and commonly developed fractures in nature,representing different boundaries and stress conditions.The typical Hertzian fracture belongs to the category of elastic deformation.Because of the point contact,it forms a characteristic cone cracks in three-dimensional space,and turns into radial and ring fractures/cracks on the plane.The angle between the cone crack and the point action surface is closely related to the Poisson's ratio of the acted materials.On the profile,the cone crack generally has a certain angle of inflection,and the angle between the lower crack and the sample plane becomes larger,which is a typical feature of the cone crack.The Hertz fracture has many uses in the field of geology,such as measuring the toughness of materials,explaining circular and radial structures,explaining the origin and shape of cone and arc fractures/cracks,judging the kinematic characteristics of glaciers and faults,distinguishing crescentic structures formed by different fracture mechanisms,etc.It can help restore the sedimentary environments and judge the ancient stress field during the diagenesis or tectonic deformation.In the process of plate collision,the Hertz fracture can also be formed under specific circumstances.This type of fracture will help us understand the origin and development of some faults and the deformation of continents,and provide the interpretation of the deformation on the surface of terrestrial planets(satellites).