三维数值模拟研究俯冲区火山的时空活动性

摘要/Abstract

摘要： 俯冲板块的深部脱水使得上覆地幔含水, 从而降低含水地幔的熔点, 导致上覆地幔部分熔融。部分熔融的地幔柱一旦喷发到地表就是俯冲带火山, 也形成新的地壳。相对于周围的地幔来讲, 具有较小密度和黏度的部分熔融地幔的时空活动性就控制着俯冲带火山的时空分布特征。本文主要回顾近年来运用三维热力学岩石力学模型数值模拟研究与板片脱水相关的俯冲带火山活动的时空分布特性。结果表明, 部分熔融地幔的有效黏度和密度是影响俯冲板片之上的三维地幔柱横向分布特征的主要因素。高黏度的部分熔融地幔(10²⁰~10²¹ Pa·s )易于形成近平行于海沟的、长波长(70~100 km)的、薄的波状地幔柱; 低黏度(10¹⁸~10¹⁹ Pa·s )的熔融地幔易于形成平行于海沟的, 短波长(30~50 km)的蘑菇状地幔柱和垂直于海沟的山脊状地幔柱。当部分熔融地幔和周围地幔的密度相差小于50 kg/m³时, 在俯冲板片之上只能形成长波长低幅度(宽50~100 km, 高10~15 km)的地幔山丘。岩浆产率随着时间的变化反映了火山活动的生命周期性。板块俯冲速度会影响地幔柱形成的深度和范围大小。高效率熔融提取会增加新地壳增长总量。低的板块俯冲速度和低的熔融提取效率会增加上地壳(花岗岩质)和中地壳(英安岩质)化学成分的比例。数值模拟结果可以很好地解释如日本东北、新西兰、南阿拉斯加俯冲区火山的横向分布特征。

关键词: 火山活动, 俯冲带, 板片脱水, 地幔柱, 三维数值模拟

Abstract: Slab dehydration hydrates overlying mantle atop subducting slab, which lowers mantle melting points and causes partially melting of the hydrated mantle. Once melts in the partially melting mantle are extracted to surface, volcano explodes and new crust forms at the surface. Therefore, spatial and temporal activities of the partially melting mantle with relatively small density and viscosity in the mantle wedge controls volcanisms in space and time. We used 3-D petrological-thermomechanical model to simulate spatial and temporal volcanisms closely related to the slab dehydration in subduction zone. Our results show that effective viscosity and density of partially melting mantle are the main factors for different patterns of the 3-D mantle plumes atop the subducting slab. In high-viscosity models (10²⁰~10²¹ Pa·s ) trench-subparalleled plumes become rather sheet-like/wave-like, and the spacing between them increases to 70~100 km, while in low-viscosity models (10¹⁸~10¹⁹ Pa·s) the typical spacing of finger-like/ridge-like plumes is about 30~50 km. In contrast, a low density contrast of 0~50 kg/m3 between partially melting mantle and ambient mantle (i.e. high to complete melt removal) suppresses pronounced plumes and is associated with low-amplitude (50~100 km wide and 10~15 km high) domal structures. Periodic melt productivity (5 Myrs) shows the periodicity of volcanism in subduction zone. The higher subduction velocity favors bigger partially melting mantle at bigger depths ( i.e.120 km), and the higher melt extraction efficiency increases the total amount of new crust. The lower subduction velocity and lower melt extraction efficiency increase the ratio of upper (granitic) and middle (dacitic) crust to lower(basaltic) crust. Our results may explain the periodicity and clustering of volcanic activity observed in magmatic arcs such as in North-East Japan, New Zealand and Southern Alaska.

Key words: Subduction zone, Slab dehydration, Mantle plumes, Volcano, 3-D numerical modeling

中图分类号:

P317

朱桂芝, Gerya Taras, Tackley Paul. 三维数值模拟研究俯冲区火山的时空活动性[J]. 地震, 2013, 33(4): 97-104.

ZHU Gui-zhi, Gerya Taras, Tackley Paul. Three-dimensional Numerical Modeling of Spatial and Temporal Evolution of Volcanism in Subduction Zones[J]. EARTHQUAKE, 2013, 33(4): 97-104.

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