Three‐dimensional mantle convection beneath a segmented spreading center: Implications for along‐axis variations in crustal thickness and gravity
Three‐dimensional mantle convection beneath a segmented spreading center: Implications for along‐axis variations in crustal thickness and gravity
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DOI：
10.1029/93jb02397
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发表时间：
1993-12
影响因子：
--
通讯作者：
D. Sparks;E. Parmentier;J. Morgan
中科院分区：
文献类型：
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作者：
D. Sparks;E. Parmentier;J. Morgan
单个区段内的分段和沿轴变化表明，大洋扩张中心下方地幔上涌和熔融具有固有的三维性质。数值对流实验被用于探究局部浮力对分段扩张中心下方的上涌和熔体产生的影响。实验是在一个由热学定义的刚性岩石圈以及覆盖在高粘度地幔半空间之上的均匀粘度软流圈组成的区域内进行的。施加了一种周期性的板块边界几何形状，它由扩张区段和转换断层偏移组成。浮力是由热膨胀以及由于部分熔体的提取而导致的成分密度降低引起的。地幔流的浮力和板块驱动分量的相对大小由扩张速率和地幔粘度控制，在较低的扩张速率和粘度下，浮力流更为重要。扩张轴下方的浮力流会放大洋脊 - 转换断层交汇处附近上涌的沿轴变化，并使这种变化沿整个扩张轴分布。因此，浮力流可能是慢速扩张中心更具三维特征的原因。在远离扩张轴的地方，热浮力驱动对流涡旋，这些涡旋与板块运动方向一致，其沿轴波长由规定的软流圈厚度控制。然而，涡旋的位置和稳定性受分段几何形状的影响。在扩张中心几何形状不允许涡旋形成稳定配置的情况下，流动是随时间变化的。上涌的沿轴变化导致熔体产生的变化，这意味着地壳厚度有很大变化，而这种变化主导着地表重力信号。这些数值实验所暗示的地壳厚度分布在扩张区段上方产生了以牛眼形状为特征的负地幔布格异常，正如在几个扩张中心所观测到的那样。异常的幅度随着扩张速率的降低而增大。
Segmentation and along-axis variations within individual segments indicate the inherently three-dimensional nature of mantle up welling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate-driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations along the entire spreading axis. Buoyant flow may thus be responsible for the more three-dimensional character of slow spreading centers. Away from the spreading axis, thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along-axis wavelength controlled by the prescribed thickness of the asthenosphere. However, the position and stability of rolls are influenced by the segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time-dependent. Along-axis variations in upwelling cause variations in melt production, which imply large variations in crustal thickness that dominate the surface gravity signal. The crustal thickness distributions implied by these numerical experiments produce bulls-eye-shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate.