Thermal contraction and the state of stress in the oceanic lithosphere
Thermal contraction and the state of stress in the oceanic lithosphere
复制标题
海洋岩石圈的热收缩和应力状态
DOI：
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发表时间：
1988
期刊：
影响因子：
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通讯作者：
E. Parmentier
中科院分区：
文献类型：
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作者：
W. Haxby;E. Parmentier
海洋岩石圈在扩张中心轴上形成后冷却和变厚时的不均匀收缩导致复杂的三维偏应力状态，该状态可分为两部分：由于垂直温度分布变化引起的热弯曲应力和由于垂直平均温度横向变化引起的热收缩应力。本文研究了由扩散中心轴约束的半无限长矩形薄板由于温度变化而产生的热收缩应力。两个无应力边界（代表转换或断裂区）定义了接骨板宽度或脊段长度L。由于弹性板的底部由规定的温度限定，板随着年龄的平方根而变厚，正如由于垂直传导冷却而产生的热边界层所预期的那样。在这个规定的温度以上，弹性应力被假定为迅速松弛。当最初无应力的材料被加到冷却增厚板的底部时，通过适当地考虑垂直平均应力的累积速率来获得应力。由该模型计算的热收缩应力状态的特征在于板的边界处的大的拉应力和板内部的相对低的应力。在脊轴处，平行于脊的拉应力约为300 MPa，与不允许板在该方向上收缩的情况相同。沿着变形边界，最大变形平行拉伸应力出现在距离脊-变形交叉点L/2处，其大小与脊轴处的应力相当。板边界处的切向应力随着离边界的距离而迅速减小;在离脊轴L/4的距离处，脊轴平行应力是其脊轴大小的十分之一。应力大小与扩张速率和脊段长度无关。大的变换平行拉伸应力可以控制变换偏移的长度。脊轴附近的大的热收缩应力影响热弯矩。然而，短距离的山脊，这些时刻达到其自由水平收缩值，以前的研究表明，造成可观察到的弯曲的板块和大地水准面异常的断裂带。由于热弯矩引起的挠曲将使弯曲应力集中在距断裂区一定距离处，该距离由板的挠曲长度确定，从而提供控制变形间距的自然长度尺度。
Nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three-dimensional state of deviatoric stress which can be separated into two parts: a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature. We examine thermal contraction stresses due to temperature changes in a thin, semi-infinite rectangular plate bounded by the spreading center axis. Two stress-free boundaries, representing transforms or fracture zones, define the plate width or ridge segment length L. With the bottom of the elastic plate defined by a prescribed temperature, the plate thickens as the square root of age, as expected for a thermal boundary layer due to vertical conductive cooling. Above this prescribed temperature, elastic stresses are assumed to relax quickly. Stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress-free material is added to bottom of the cooling, thickening plate. The state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior. At the ridge axis, ridge-parallel tensile stresses are about 300 MPa, the same as if the plate were not allowed to contract in this direction. Along the transform boundary, the maximum transform-parallel tensile stress occurs at a distance L/2 from the ridge-transform intersection, where its magnitude is comparable to the stress at the ridge axis. The tangential stresses at the plate boundary decrease rapidly with distance from the boundary; at a distance of L/4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude. The stress magnitudes are independent of both spreading rate and ridge segment length. A large transform-parallel tensile stress may control the length of transform offsets. Thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis. However, a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones. Flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate, thus providing a natural length scale controlling the spacing of transforms.