The heterogeneous upper mantle low velocity zone
The heterogeneous upper mantle low velocity zone
复制标题
DOI：
10.1016/j.tecto.2005.11.021
复制
发表时间：
2006-04
期刊：
影响因子：
2.9
通讯作者：
H. Thybo
中科院分区：
文献类型：
--
作者：
H. Thybo
上地幔低速带（LVZ）是一个与周围深度段相比地震速度略有降低的深度段。在世界上大多数大陆地区，无论是在平均速度高的地壳运动区还是在平均速度低的构造活动区，该带都存在于相对恒定的100公里深度以下。低速区的证据来自受控和天然震源地震学，包括表面波研究、边界界面体波的一次和多次反射研究、接收函数计算和绝对速度层析成像。现有的数据表明，S波的地震速度和Q值比P波更明显地降低，以及LVZ中的高电导率。地震波被该带强烈散射，这表明存在小尺度非均匀性。在世界上寒冷、稳定的电离层区域，LVZ底部的深度比炎热、活跃的区域要浅。由于其全球性的发生低于100公里的相对恒定的深度，LVZ不能解释的变质或成分变化和流变学的变化。计算的上地幔温度表明，岩石接近固相线的间隔与可变厚度低于100公里的深度，只要岩石含有水和二氧化碳。地幔岩中即使存在少量的这种流体，也会使固相线降低几百度，并在80-100公里深度附近的固相线曲线上引入特征性的扭结。接近固相线或含有少量部分熔融的岩石，其地震速度和Q值显著降低。因此，LVZ可以解释为上地幔温度接近固相线在100公里以下的深度间隔。假设岩石只含有有限数量的流体，这种机制可以解释低速，Q值和电阻率，以及固有的散射，和低速带厚度的特征变化。
The upper mantle low velocity zone (LVZ) is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals. The zone is present below a relatively constant depth of 100 km in most continental parts of the world, both in cratonic areas with high average velocity and tectonically active areas with low average velocity. Evidence for the low velocity zone arises from controlled and natural source seismology, including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces, calculations of receiver functions, and absolute velocity tomography. The available data indicates a more pronounced reduction in seismic velocity and Q-value for S-waves than P-waves as well as high electrical conductivity in the LVZ. Seismic waves are strongly scattered by the zone, which demonstrates the existence of small-scale heterogeneity. The depth to the base of the LVZ is systematically shallower in cold, stable cratonic areas than in hot, active regions of the world. Because of its global occurrence below a relative constant depth of 100 km, the LVZ cannot be explained by metamorphic or compositional variation and rheological changes. Calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth, provided that the rocks contain water and carbon dioxide. The presence of, even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80–100 km depth. The seismic velocities and Q-values are significantly reduced of rocks, which are close to the solidus or contain small amounts of partial melt. Hence, the LVZ may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km. Assuming that the rocks contain only limited amounts of fluids, this mechanism may explain the low velocities, Q-values, and resistivity, as well as the intrinsic scattering, and the characteristic variation in thickness of the low velocity zone.