利用远震接收函数揭示的喜马拉雅东构造结台阵下方地壳结构及其动力学意义

在喜马拉雅造山带的东缘,雅鲁藏布江缝合带在这里发生急剧转折,南迦巴瓦变质体快速隆起,然而关于东构造结的形成机制一直未有定论.利用围绕南迦巴瓦峰的48个宽频带地震台站记录的远震数据提取P波接收函数,采用改进的H-κ叠加方法和共转换点叠加方法综合研究了东构造结的地壳厚度、波速比分布和地壳结构特征.结果表明:研究区平均地壳厚度为64.03 km,大部分台站介于60.48~66.55 km范围;平均波速比为1.728,主要集中范围为1.696~1.742.东构造结地壳厚度横向变化剧烈,构造结西端和北端厚而中间薄,东构造结核部Moho面呈现上隆的构造形态,东西向上隆幅度约为6~7 km,南北向的上隆超过9~10 km.东构造结核部地壳上隆减薄可能由高密度、高波速的岩石圈撕裂残片拆沉到上地幔软流圈后重力失衡所致.平均波速比超过1.8的高值异常展布于东构造结的两侧,推测为环东构造结的壳内部分熔融体.东构造结地壳上隆减薄和壳内部分熔融的存在很可能均与幔源热物质的上涌有关,而软流圈地幔的上涌则可能由印度板片的撕裂引起.

Crustal structure under the eastern Himalayan Syntaxis seismic array and its geodynamic implications derived from receiver functions

On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism of the Eastern Himalaya Syntaxis (EHS) is still debated. We used the teleseismic data observed by 48 broadband seismic stations around the Namche Barwa Mountain to estimate P-wave receiver functions and study crustal thickness, V_P/V_S ratio and crustal structure based on improved H-κ analysis and common conversion point (CCP) stacking method. The results demonstrate that the average crustal thickness is about 64.03 km with most stations ranging from 60.48 km to 66.55 km and the average V_P/V_S ratio is about 1.728 with most stations ranging from 1.696 to 1.742. The crust thickness exhibits drastic lateral variation beneath the EHS, which is thicker in the western and northern area and thinner in the middle area, thus the crust of the EHS presents an uplifting structural feature. The Moho boundary is uplifted about 6~7 km in the W-E direction, and more than 9~10 km in the N-S direction. The uplifted Moho boundary is formed because high-density and high-velocity lithospheric tearing slab could be delaminated and sank into asthenosphere in the upper mantle and result in unbalanced gravity. The anomaly of the average V_P/V_S ratio, which is more than 1.8, is distributed on both sides of the EHS, which can be interpreted as a crustal partial melting body around the EHS. The uplifting Moho beneath the EHS and the existence of partial melting in the crust might be associated with the upwelling of hot mantle material, and the upwelling of the asthenosphere may be caused by a tear of the India plate.