利用双差层析成像方法反演阿拉斯加地区岩石圈速度结构

Lithospheric velocity structure of Alaska revealed by double difference tomography

阿拉斯加地区由不同地质时期的地体向北增生而成,经历了漫长的构造演化,地质构造复杂。ANF(Array Network Facility)网站新近提供的来自USArray地震台网记录的地震台站观测数据填补了阿拉斯加地区西部和北部的观测空白,本文选取该区域中345个台站记录的5638个地震事件的P波、S波到时数据,采用区域双差地震层析成像方法反演得到了该地区的岩石圈三维P波速度模型和地震重定位结果。研究结果显示:阿拉斯加西部太平洋板块的俯冲倾角较大,深部地幔楔表现为P波低速异常,推测由俯冲板块顶部脱水产生的流体释放到地幔楔并触发部分熔融所致,这些熔融物质上升到达地表形成阿留申火山岛链;中部亚库塔特(Yakutat)地体与太平洋板块发生耦合,俯冲倾角减小,一方面使地壳压应力增加,引起地壳增厚和楚加奇(Chugach)山脉隆升,另一方面导致该处地幔楔降温从而使产生的熔体减少,并随着地壳压应力的增加部分地壳裂隙闭合,阻断了熔体上升至地表,从而形成迪那利(Denali)火山空区;亚库塔特地体与东部兰格尔(Wrangell)火山区之间存在较明显的分界,兰格尔火山区下方的低速区(与岩浆活动对应)集中于西北侧,火山区的岩浆来源可能与环形地幔流沿太平洋—亚库塔特板块边缘的上升流相关。这些结果表明,阿拉斯加地区深部复杂的地球动力学过程导致了其地表复杂的地质构造。

Alaska region is formed by the northward accretions of terranes from different geological periods and has experienced extensive internal deformation and metamorphism,and the geological structure is complex.The Array Network Facility(ANF)website recently provides new observational data from the seismic network of USAarray,filling the observation gap in the west and north of Alaska.Based on P and S wave arrivals of 5638 events recorded by 345 stations from ANF,this study relocates earthquakes and images the 3D lithospheric P-wave velocity structure beneath Alaska simultaneously by regional double difference tomography.The results reveal larger dip angle of subducting Pacific Plate and low-vP anomalies in the mantle wedge beneath western Alaska.These observations reflect the subduction process,during which the dehydration of the subduting Pacific Plate releases fluids into the mantle wedge,triggers partial melting,and generates melts,which was then transported to the surface by the upwelling flow so as to form Aleutian volcanic arc.In central Alaska,the coupling between the Yakutat terrane and the Pacific Plate reduces the subduction dip.On the one hand,the shallow subduction of the Yakutat terrane increases the compressive stress of the crust,causing the crustal thickening and uplifting of Chugach mountains.On the other hand,it cools the mantle wedge reducing magma generation,which are then combined with the closure of crust fractures resulted from the increase of the crustal stress,blocking the supply of melt to the surface,and finally leading to the formation of Denali volcanic gap.In addition,there is a clear boundary between Yakutat terrane and Wrangell volcanic field in the east,and the low velocity zone corresponding to magmatic activity in the region is concentrated in the northwest.The magma source may be related to upwelling of the toroidal mantle flow around the Pacific-Yakutat slab edge.These results suggest that the complex geodynamic processes in deep Alaska lead to the complex geological structure on the surface.