【作者】 江国明；

【导师】 张贵宾； 赵大鹏；

【作者基本信息】 中国地质大学（北京） ， 地球探测与信息技术， 2008， 博士

【摘要】 西太平洋俯冲带是世界上最典型、最活动的俯冲带,已成为地学家们研究的一个热点。本研究以堪察加和日本地区为代表详细讨论了西太平洋俯冲板块的形态分布和速度结构。利用从16个地震台站拾取的768条远震到时和赵大鹏远震层析成像方法研究了堪察加地区下方从莫霍面至700 km深度范围内的三维速度结构。成像结果清楚地显示出两大速度异常特征,一是高波速的太平洋板块在堪察加地区南部下方一直俯冲到660-km不连续面以下,而且由南自北俯冲深度逐渐变浅,在阿留申-堪察加汇合带附近几乎消失;二是低波速的软流圈高温物质存在于堪察加的北部和汇合带的下方。在地幔过渡带内和过渡带的下方发现了两块高速异常体,分析认为它们分别是2 Ma前脱落的太平洋板块岩石圈和10 Ma前俯冲的Komandorsky板块。结合前人的研究,太平洋板块边缘处的岩石圈拆沉可能是由其周围高温地幔物质的消融和剪切作用引起的。此外,俯冲的明治海山群不但对太平洋板块的拆沉发挥了重要作用,而且使得靠近汇合带处的板块俯冲角度减小。尽管许多学者对日本列岛下的太平洋俯冲板块做了大量的研究,但板块的精细结构仍然不太清楚,主要包括板块厚度、板块内地震波速度随深度的变化、洋壳的俯冲情况以及橄榄石亚稳态楔是否存在等。本研究利用日本台网收集到的远震和近震的高精度到时数据探讨上述问题。采用三维射线追踪正演模拟法,首先利用333个远震计算得到了太平洋板块的平均厚度为85 km。接着利用3283个近震(震源深度大于40 km)分段测试了板块内的速度异常分布,结果表明速度异常随深度的增加而减小,这与地幔内的温度变化有关。然后在前者计算结果的基础上利用40-300 km深度范围内的近震测试得到日本东北和北海道地区下方洋壳俯冲的深度均为110 km,洋壳平均厚度分别为7.5 km和5 km,速度异常分别为1%和-3%。这说明洋壳在俯冲至110 km以深时,由于受温度和压力的影响,逐渐脱水、变质,直至与板块融合,而且通过分析震源与洋壳的位置关系认为靠近板块上边界的地震是由洋壳脱水变脆触发的。最后利用23个深震测试日本海地区和小笠原地区太平洋板块内的橄榄石亚稳态楔结构,结果显示在太平洋板块内约400 km深度附近的确存在一个低速异常体(-3%),该异常体被解释为橄榄石的亚稳态楔。通过分析深发震源与亚稳态楔的位置关系,发现大部分深震发生在亚稳态楔的内部。据此,可用相态转换断层理论解释深震的发震机制。更多还原

【Abstract】 The western Pacific region is the most typical and most active subduction zone on Earth, and so it has been one of the most studied regions by many geoscientists since the advent of plate tectonics. In this study, we have investigated the morphology and seismic velocity structure of the subducting Pacific slab under Kamchatka and Japan Islands.We determined a 3-D P-wave velocity structure of the mantle down to 700 km depth under the Kamchatka peninsula by applying teleseismic tomography to 678 P-wave arrival times recorded by 16 seismic stations. The results show two significant structural features. One is the high-velocity subducting Pacific slab, which is visible in the upper mantle and extends below the 660-km discontinuity under southern Kamchatka, while it shortens toward the north and terminates near the Aleatian-Kamchatka junction. The other is a low-velocity anomaly interpreted as the asthenospheric flow, which is imaged beneath northern Kamchatka and under the junction. Two isolated high-velocity anomalies are imaged in and below the mantle transition zone, which are interpreted as the Pacific slab detached about 2 Ma ago and the Komandorsky lithosphere subducted about 10 Ma ago. Combining with many previous results, we conclude that the slab loss occurring under northern Kamchatka may be caused by slab-edge pinch-off by the asthenospheric flow. In addition, the subducted Meiji seamounts may have played an important role in the detachment of the Pacific slab, which cause the Pacific plate to subduct under Kamchatka with a lower dip angle near the junction.Although many studies have been made to image the subducting Pacific slab in and around the Japan Islands, details of the slab structure (such as the slab thickness, the relation between seismic velocity and depth variation, the subducting oceanic crust and the metastable olivine wedge) are still unclear. In this study, we have addressed these issues by adopting a forward-modeling approach with a 3-D ray-tracing technique and using arrival times from teleseismic, local and regional events recorded by the seismic network on the Japan Islands. Firstly, we use 333 teleseismic events and find that the average thickness of the Pacific slab beneath Japan is 85 km. Secondly, we use 3283 local and regional earthquakes with focal depths greater than 40 km to study the vertical distribution of velocity anomaly in the slab. Our result shows that the amplitude of velocity perturbation decreases with depth, which is related to the variation in temperature. Thirdly, we use the local and regional events with focal depths from 40 to 300 km to study the subducting oceanic crust beneath Northeast Japan and Hokkaido. Our results display that the oceanic crust extends down to 110 km depth under both regions, the average thickness of the oceanic crust is 7.5 and 5 km, and the velocity perturbation in the oceanic crust relative to the 1-D model is 1% and -3%, respectively. These results can be interpreted that the oceanic crust has dehydrated and metamorphosed gradually because of the increasing temperature and pressure with depth. After analyzing the relationship between the hypocenters and the oceanic crust, we consider that the earthquakes near the upper slab boundary are caused by the dehydration embrittlement of the oceanic crust. Finally, we use 23 deep earthquakes to investigate the metastable olivine wedge within the subducting Pacific slab beneath the Japan Sea and Izu-Bonin region. The results indicate that a low-velocity anomaly (-3%) indeed exists within the slab below 400 km depth, which is interpreted as the metastable olivine wedge. After careful earthquake relocation, we find that most deep earthquakes occurred within the wedge, suggesting that deep earthquakes may be caused by the phase transformational faulting.更多还原