南海西南次海盆扩张期后岩浆活动及其残留扩张中心的纵横波速度结构

发布时间：2018-01-07 05:10

本文关键词：南海西南次海盆扩张期后岩浆活动及其残留扩张中心的纵横波速度结构　出处：《浙江大学》2016年博士论文　论文类型：学位论文

【摘要】：西南次海盆是渐进式扩张的海盆,在其扩张停止后仍然存在着强烈的岩浆活动,使得西南次海盆包含了边缘海构造演化的各个阶段,成为了边缘海演化研究的热点区域。目前扩张期后的岩浆活动是大家关注的热点问题,但其内部结构尚未揭示,极大的影响了我们对扩张期后岩浆活动的深入研究。而西南次海盆海底扩张最后阶段研究较少的现状,使海底扩张到扩张停止之间缺少衔接的依据。因此本文从速度结构的角度出发,以西南次海盆轴部海山和残留扩张中心的结构为切入点,探讨海盆扩张期后岩浆活动和海底扩张最后阶段的岩浆和构造活动。本文基于2010年南海西南次海盆所采集的OBS数据,首先利用射线追踪正演方法得到西南次海盆东北段残留扩张中心和轴部海山的纵横波速度结构。为了能够更好的控制模型结果,我们同时利用地震层析成像的反演方法开展了残留扩张中心纵波速度结构的研究,获得了以下成果：(1)基于速度等值线形态与岩浆加载机制的关系,提出了轴部海山是岩浆喷发占主导的新认识。利用海山速度、结构和多波束地形地貌计算扩张停止后龙南海山岩浆的加载量获得了岩浆喷发/侵入比1.92。(2)基于洋壳厚度与岩浆供应的关系,通过分析西南次海盆洋壳厚度变化,揭示了西南次海盆岩浆供应量时空变化的特点。在磁异常条带C6n和C5En期间整个西南次海盆内部的供给量相似,都表现出较多的岩浆活动。从磁异常条带C5En到C5Cr期间,海盆局部区域内的岩浆供给量减少,表现为局部区域洋壳厚度的减薄。扩张中心处的洋壳要比两侧的薄,表明海底扩张最后阶段的岩浆供量相对较少。扩张停止后西南次海盆东北部的岩浆活动明显强于西南部,表明了岩浆供给量的强烈不均一。(3)通过地震学特征对比和构造环境分析,在西南次海盆残留扩张中心的NW侧发现拆离断层的存在,这条拆离断层的存在对海盆扩张最后阶段的演化具有重要的意义。西南次海盆残留扩张中心两侧具有不对称的纵波速度结构,局部伴随着莫霍面的强烈抬升和洋壳的减薄。而且西南次海盆是一个慢速扩张的海盆,其轴部洋脊段的半扩张速率小于40 mm/yr,此处构造环境适合断层发育且地震学特征也与断层一致。(4)依据扩张期和扩张期后岩浆活动情况,将残留扩张中心重新划分为三类：(1)扩张最后阶段及扩张停止后岩浆量相对增加,致使轴部洋壳厚于离轴洋壳；(2)扩张最后阶段岩浆量相对减少,而扩张停止后岩浆量相对增加,致使轴部洋壳厚度先小于离轴洋壳,增生后又厚于离轴洋壳；(3)扩张最后阶段及扩张停止后岩浆少,致使轴部洋壳厚度始终薄于离轴洋壳。
[Abstract]:The southwest subbasin is a gradual spreading basin, and there are still strong magmatic activities after its expansion stops, which makes the southwest subbasin include each stage of the tectonic evolution of the marginal sea. It has become a hot spot in the study of marginal sea evolution. At present, magmatic activity after expansion period is a hot issue of concern, but its internal structure has not been revealed. This has greatly affected our in-depth study of post-expansionist magmatic activity. However, there is little research on the last stage of seafloor expansion in the southwest subbasin. There is no basis for the connection between the expansion of the sea floor and the stop of the expansion. Therefore, from the point of view of the velocity structure, the paper takes the structure of the seamounts in the axis of the southwest subbasin and the residual spreading center as the starting point. In this paper, the magmatic and tectonic activities in the late stage of sea basin expansion are discussed. This paper is based on the OBS data collected from the southwest subbasin of the South China Sea in 2010. First, the longitudinal and shear wave velocity structures of the residual spreading center and axial seamounts in the northeast part of the southwest subbasin are obtained by using the ray tracing forward method in order to better control the model results. At the same time, we use the inversion method of seismic tomography to study the longitudinal wave velocity structure of residual spreading center. The following results are obtained: 1) based on the relationship between velocity isoline shape and magmatic loading mechanism. It is proposed that the axial seamounts are dominated by magmatic eruptions. Structure and multi-beam topographic geomorphology calculation of magma loading in Longnan seamounts after the stop of expansion the magmatic eruption / intrusion ratio of 1.92. 2) based on the relationship between the thickness of oceanic crust and magmatic supply. By analyzing the thickness variation of the oceanic crust in the southwest subbasin, the characteristics of the temporal and spatial variation of magma supply in the southwest subbasin are revealed. The supply quantity in the whole southwest subbasin is similar during the period of the magnetic anomaly strip C6n and C5En. During the period from C _ 5E _ n to C _ 5Cr, the magma supply in the local area of the basin decreased. The oceanic crust at the center of expansion is thinner than that on both sides. The magmatic activity in the northeast of the southwest subbasin is obviously stronger than that in the southwest after the end of the expansion. It shows that the strong heterogeneity of magma supply. 3) through the comparison of seismological characteristics and tectonic environment analysis, the detachment fault is found in the NW side of the residual extension center of the southwest subbasin. The existence of this detachment fault is of great significance to the evolution of the basin in the last stage of its expansion. There is an asymmetric longitudinal wave velocity structure on both sides of the residual spreading center of the southwest subbasin. The strong uplift of the Moho surface and the thinning of the oceanic crust are accompanied locally. The southwest subbasin is a slow-spreading basin with a semi-spreading rate of less than 40 mm/yr at the axis of the ridge. The tectonic environment here is suitable for fault development and the seismological characteristics are consistent with the fault. 4) according to the magmatic activity during and after the spreading period. The residual spreading center is divided into three classes: 1) the magma volume increases relatively at the last stage of the expansion and after the expansion stops, resulting in the thickness of the axial oceanic crust than that of the off-axis oceanic crust; (2) the magmatic volume decreases relatively in the last stage of the expansion, but the magma volume increases relatively after the expansion stops, which results in the thickness of the axial oceanic crust being smaller than that of the off-axis oceanic crust, and then thicker than the off-axis oceanic crust after the expansion; 3) the thickness of the axial oceanic crust is always thinner than that of the off-axis oceanic crust.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P736.1

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