Along- and across-axis variations in crustal thickness and structure at the Mid-Atlantic Ridge at 5°S obtained from wide-angle seismic tomography: Implications for ridge segmentation
Along- and across-axis variations in crustal thickness and structure at the Mid-Atlantic Ridge at 5°S obtained from wide-angle seismic tomography: Implications for ridge segmentation
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从广角地震层析成像获得的南纬 5°大西洋中脊地壳厚度和结构的沿轴和跨轴变化：对脊分割的影响
DOI：
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
T. Reston
中科院分区：
文献类型：
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作者：
L. Planert;E. Flueh;T. Reston
在位于5°S的中大西洋海脊相邻的两个扩张段，观察到两种端元地壳增长类型：集中增长到段中心，北段地壳迅速变薄，南段地壳增厚，向大洋核杂岩转化。我们的结果是通过对沿三条相交剖面采集的广角地震反射和折射数据进行层析反演而得到的。5°S断裂带以北的地段具有发育的中谷和明显的海底隆起。在该凸起下方2.5公里处的−深度处，有一小部分速度异常偏低(−为0.4~0.5 km/S，相对于平均离轴结构)，可能与温度升高有关，也可能与少量部分熔融有关。这表明这一段目前正处于神奇的活跃期，观察到的新鲜熔岩流动和海底持续的高温热液活动证实了这一点。在现今扩张轴附近，地壳迅速变薄，从分段中心以下8.5公里到转换断层下方不到3公里，这表明这里的熔体供应强烈地集中在分段中心。地壳厚度的减薄几乎完全由指示地震第三层的速度部分减薄所适应。转换断层的特征是整个地壳剖面的速度梯度更加均匀，上地幔速度非常低，为7.2-7.3公里/S，表明在3.5公里深度，蛇纹岩作用可达25%。相反，北段∼4.1Ma的老地壳从段中心到段端部只有轻微的减薄。在这里，转换的特征是正常的地震层2/3过渡，表明在这些时间段末端有强劲的熔体供应。在相邻的南段，地壳从大洋核杂岩侧翼下的∼2.5公里处增厚到段边界处的∼5.0公里。观测到的地壳厚度变化显示出熔体供应在时间和横向上的显着变化，表明地壳侵位过程比集中熔体供应到分段中心的模型所预测的更为复杂。
Two end‐member styles of crustal accretion are observed at two adjacent spreading segments at the Mid‐Atlantic Ridge at 5°S: focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment. Our results were obtained by tomographic inversion of wide‐angle seismic reflection and refraction data collected along three intersecting profiles. The segment north of the 5°S fracture zone is characterized by a well‐developed median valley with a pronounced seafloor bulge in the segment center. A discrete portion of anomalously low velocities (−0.4 to −0.5 km/s relative to average off‐axis structure) at depths of ∼2.5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt. This suggests that this segment is currently in a magmatically active period, which is confirmed by the observation of fresh lava flows and ongoing high‐temperature hydrothermal activity at the seafloor. Close to the current spreading axis, the crust thins rapidly from 8.5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center. The reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3. The transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7.2–7.3 km/s indicating that serpentinization could be as much as 25% at 3.5 km depth. In contrast, ∼4.1 Ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end. Here, the transform is characterized by a normal seismic layer 2/3 transition suggesting robust melt supply to the segment end at those times. In the adjacent southern segment, the crust thickens from ∼2.5 km beneath the flank of an oceanic core complex to ∼5.0 km at the segment boundary. The observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers.
影响因子：
3.5
作者：
Haase, K. M.;Petersen, S.;Weber, S.
通讯作者：
Weber, S.