Mantle Plumes beneath the Baikal Rift Zone and Adjacent Areas: Geophysical Evidence
Mantle Plumes beneath the Baikal Rift Zone and Adjacent Areas: Geophysical Evidence
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贝加尔裂谷带及邻近地区下方的地幔柱：地球物理证据
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
V. Kozhevnikov
中科院分区：
文献类型：
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作者：
Y. Zorin;E. Turutanov;V. Kozhevnikov
贝加尔裂谷带及邻近的东西伯利亚和蒙古中部地区的晚新生代碱性火山岩体积相对较小。然而，地球化学和同位素特征表明这些火山岩可能与地幔柱有关[1-3]。与此同时，一些研究人员认为地球化学特征并不能作为羽流存在的关键证据[4]。地球物理方法可能对地幔柱的识别和定位有很大帮助。本通讯讨论了有利于研究区域地幔柱发育的重力和地震证据。地幔柱的加热材料应具有较低的密度和地震速度。重力测量可能有助于羽流识别。在有关羽流的文献中普遍注意到[2、5 等]，羽流管道中的上升物质流对岩石圈产生动态影响，导致正重力异常与地形隆起相结合。自由空气中相关的正异常归因于地形质量影响的主导。此外，理论估计表明，上升流对岩石圈的影响仅对于具有均匀粘度的地幔模型才显着[6]。该模型考虑到软流圈的存在，其粘度比更深地幔中的粘度低两个数量级，假设上升流对岩石圈的动态影响明显减弱[6]。由于软流圈的阻尼效应，羽流管道的异常质量实际上不参与动态或静态平衡。由于密度降低，上述现象应产生负均衡异常，由于吸引物体的深层定位，该负均衡异常应相当广泛（即区域性）。冰岛上方的相对重力最小值和加拉帕戈斯群岛上方的绝对重力最小值[7]就是此类异常的例子。在计算均衡异常作为羽流通道重力效应的指导时，应该记住，羽流上方的地形隆起可能会通过岩浆底侵引起的地壳增厚和羽流头部替代导致的岩石圈变薄来平衡[5]。在这项工作中，我们计算了均衡异常，考虑到只有 40% 的地形质量通过莫霍面深度的变化进行补偿，其余部分 (60%) 通过岩石圈厚度的变化进行补偿。这些关系基于先前建立的西伯利亚东部和蒙古中部地壳结构的地形和重力场与地震数据的相关性[8, 9]。半径222 km平面区域的等静压补偿重力效应是由于异号地形体凝结形成的深层薄层的影响。与莫霍面深度变化相关的补偿质量在该间断面的平均深度（45 km）处凝结成薄层，而与地幔相关的补偿质量在岩石圈-软流圈边界的平均深度（120 km）处凝结成薄层。这些层的深度是根据该地区地壳和岩石圈厚度的地震数据估计的[8, 9]。从布格异常中减去半径 222 公里平面区域内的重力补偿效应。球形区域（半径超过 222 公里）的地形等静压校正是使用 Artem’ev 编制的校正图确定的 [10]。
Late Cenozoic alkaline basic volcanics in the Baikal Rift Zone and adjacent territories of eastern Siberia and central Mongolia are relatively small in volume. However, the geochemical and isotopic signatures indicate that these volcanics could be related to mantle plumes [1‐3]. At the same time, some researchers deem that the geochemical signature does not serve as critical evidence for the existence of plumes [4]. Geophysical methods may substantially help in the identification and localization of mantle plumes. The gravity and seismic evidence in favor of mantle plume development in the study region is discussed in this communication. A heated material of mantle plume should have lowered density and seismic velocity. Gravity measurements may be helpful in plume recognition. It is commonly noticed in the literature on plumes [2, 5, and others] that ascending material flows in plume conduits exert dynamic influence upon lithosphere, giving rise to positive gravity anomalies combined with topographic uplifts. The associated positive anomalies in free air are attributed to the domination of topographic masses influence. Moreover, theoretical estimates show that the influence of ascending flows on lithosphere is significant only for mantle models with uniform viscosity [6]. The models taking into account the existence of asthenosphere having a viscosity two orders of magnitude lower than the viscosity in a deeper mantle assume a marked weakening of the dynamic influence of an ascending flow on lithosphere [6]. Due to the damping effect of asthenosphere, the anomalous masses of plume conduit virtually do not participate in either dynamic or static equilibria. Since density is decreased, the above phenomenon should create a negative isostatic anomaly that should be rather wide (i.e., regional) owing to the deep localization of the attracting object. The relative gravity minimum above Iceland and the absolute minimum above the Galapagos Islands [7] are examples of such anomalies. Calculating the isostatic anomalies as guides for gravity effect of plume conduits, one should keep in mind that topographic uplifts above the plume may be equilibrated both by crust thickening caused by its magmatic underplating and by lithosphere thinning as a result of its replacement with a plume head [5]. In this work, we have calculated the isostatic anomalies taking into account that only 40% of topographic masses are compensated by variation of the Moho depth and the remainder (60%) is compensated by variation of lithosphere thickness. These relationships are based on the previously established correlation of topography and gravity field with seismic data on the crustal structure of eastern Siberia and central Mongolia [8, 9]. Gravity effects of isostatic compensation in plane zones with a radius of 222 km were attributed to the influence of deep-seated thin layers formed as a result of the condensation of topographic masses with opposite sign. The compensation masses related to variations of the Moho depth were condensed into a thin layer localized at the mean depth of this discontinuity (45 km), whereas the compensation masses related to the mantle were condensed into a thin layer at an average depth of the lithosphere‐asthenosphere boundary (120 km). The depths of these layers were estimated from seismic data on crust and lithosphere thickness in this region [8, 9]. The gravity compensation effects within the plane zones, 222 km in radius, were subtracted from the Bouguer anomalies. Topographic isostatic corrections for spherical zones (beyond the radius of 222 km) were determined using the correction map compiled by Artem’ev [10].