西南印度洋中脊斜向扩张分段特征及构造成因探讨

doi:10.13745/j.esf.sf.2020.12.7

摘要/Abstract

摘要：

斜向扩张是超慢速扩张洋中脊独特的构造特征,其地形分段特征明显区别于经典的快速-慢速端元洋中脊模型,是理解超慢速扩张洋中脊地质过程的重要切入点。基于西南印度洋中脊Indomed-Gallieni和Shaka-DuToit段多波束地形数据,分析了不同斜向扩张角度(α)洋中脊的地形分段样式。其中,46.5°~47.5°E(α=5°)、16°~25°E(α=10°)和48.5°~52°E(α=15°)为近正向扩张段,发育雁列式叠置的中央火山脊;47.5°~48.5°E(α=50°)和16°~25°E(α=60°)为斜向扩张段,仅在洋脊段中部形成中央火山脊。利用有限差分+颗粒法(FD+MIC)数值模拟技术研究了洋中脊应变分布特征对不同α值的响应,结合地形分析,认为斜向扩张角度和温度异常分布共同控制了洋中脊地形分段样式。近正向扩张洋中脊(α＜20°)在温度异常处形成地壳伸展应变的集中区,有利于岩浆汇聚,发育雁列式叠置的中央火山脊,其位置随温度异常分布的变化而改变;斜向扩张洋中脊(α>20°)地壳伸展应变集中区的位置受斜向扩张几何样式控制,在洋脊段中部发育中央火山脊,对温度异常不敏感,形成位置长期固定的岩浆活动中心。

关键词: 斜向扩张, 超慢速扩张洋中脊, 地形分段样式, 多波束地形, 数值模拟

Abstract:

Oblique spreading is a characteristic feature of ultra-slow spreading ridges. As its bathymetric segmentation pattern is distinct from classical fast-slow spreading ridge models, it can provide insight into the geological process of ultra-slow spreading ridges. Based on multibeam bathymetric data, the segmentation patterns (with variable spreading obliquity (α)) of the Southwest Indian Ridge (SWIR) Indomed-Gallieni and Shaka-DuToit zones are analyzed. Results show that the SWIR segments, between 46.5°E and 47.5°E (α=5°), 16°E and 25°E (α=10°), and 48.5°E and 52°E (α=15°), respectively, are quasi-orthogonal spreading segments, which have developed en échelon axial volcanic ridges. Along the axis of the oblique spreading SWIR, between 47.5°E and 48.5°E (α=50°) and between 16°E and 25°E (α=60°), only one axial volcanic ridge has formed at the center of the axis. The distribution of the strain rates along the mid-ocean ridges with variable obliquity (α) is calculated by a ‘finite difference+markers in cells’ (FD+MIC) technique. Combined with bathymetry analysis, two factors influencing the segmentation pattern of oblique spreading results, namely, the spreading obliquity and distribution of temperature disturbance, are proposed. Along the quasi-orthogonal spreading segments (α<20°), temperature disturbance led to localization of extensional strain and development of en echelon axial volcanic ridges. In the oblique spreading segments (α>20°), strain localization was controlled by oblique geometry so the axial volcanic ridges only developed in the middle of the segment. These volcanic ridges were insensitive to temperature disturbance, therefore, they could form lasting fixed local magmatic centers.

Key words: oblique spreading, ultra-slow spreading ridges, bathymetric segmentation, multibeam bathymetry, numerical modeling

中图分类号:

P736
P542

张华添, 李江海, 陶春辉. 西南印度洋中脊斜向扩张分段特征及构造成因探讨[J]. 地学前缘, 2021, 28(2): 271-283.

ZHANG Huatian, LI Jianghai, TAO Chunhui. Discussions on the bathymetric segmentation and tectonogenesis of the oblique spreading Southwest Indian Ridge[J]. Earth Science Frontiers, 2021, 28(2): 271-283.

图/表 10

图1 西南印度洋大地构造简图转换断层编号:1—Bouvet; 2—Islas Orcadas; 3—Shaka; 4—DuToit; 5—Andrew Bain; 6—Marion; 7—Prince Edward; 8—Eric Simpson; 9—Discovery II;10—Indomed; 11—Gallieni; 12—Gazelle; 13—Gauss; 14—Atlantis II; 15—Novara; 16—Melville。

Fig.1 Tectonic sketch map of the Southwest Indian Ocean

图2 西南印度洋中脊Indomed-Gallieni段地形分段样式(a)及轴部地形剖面图(b) a—根据洋中脊地形分段特征,自东向西划分为S-Ⅰ、S-Ⅱ和S-Ⅲ三部分,斜向扩张角度分别为15°、50°和5°。将洋中脊垂向上轴部凹陷最低点/中央火山脊最高点的连线作为洋中脊轴部,黑色曲线为中央火山脊轴部,白色围限区域为轴部凹陷。洋中脊编号据文献[41];b—黑色曲线为中央火山脊地形剖面,灰色曲线为轴部凹陷地形剖面。

Fig.2 Segmentation pattern (a) and bathymetric profile (b) of SWIR Indomed-Gallieni

图3 西南印度洋中脊Shaka-DuToit段地形分段样式(a)及轴部地形剖面图(b) 地形据多波束海底地形数据库(Multibeam Bathymetry Database, MBBDB),空间分辨率约150 m。16°~25°E为近正向扩张,斜向扩张角度α=10°;9°~16°E为斜向扩张,斜向扩张角度α=60°。a—黑色曲线为中央火山脊,白色区域为NTDs,垂直于洋中脊的曲线为离轴脊构造的迹线,是持续岩浆活动中心的遗迹;b—黑色剖面为中央火山脊轴部地形,灰色为NTDs轴部地形。

Fig.3 Segmentation pattern (a) and bathymetric profile (b) of SWIR Shaka-DuToit

表1 模型物质参数列表(据文献[42],物质材料选取下地壳辉长岩)

Table 1 Parameters used for numerical modeling of gabbro from lower crust (adapted from [42])

物理量		数量	物理量		数量
密度(ρ)	标准密度(ρ₀)	3 000 kg/m³	比热容(C_p)		1 000 J/kg
	热膨胀系数(α)	3×10^-5 K^-1	黏滞系数η	物质常数(A_D)	3.3×10^-4 MPa^-ⁿ•s^-1
	压缩系数(β)	1×10^-11 Pa^-1		应力指数(n)	3.2
热导率(κ)	标准热导率(κ₀)	1.18 W•m^-1•K^-1		活化能(E_a)	238 kJ/mol
热导率(κ)	热导率系数(α)	474 W/m		活化体积(V_a)	0 J/Pa
剪切模量(μ)		2.5×10¹⁰ Pa	放射产热率(H_r)		2.5×10^-7 W/m³

图4 斜向扩张洋中脊模型的初始设定左图:几何模型与边界条件,斜向扩张角度α为洋中脊轴部与N90°E方向的夹角,逆时针为正;右图:初始温度模型,白色箭头指示初始温度异常的位置。

Fig.4 Initial setup of oblique spreading model

图5 斜向扩张洋中脊模型计算结果 a—正应力(σzz);b—剪应力(σzx);c—第二应变不变量(εII);d—体应变速率(lgεkk);e—南北方向速度梯度(dvz/dz);f—东西方向速度梯度(dvx/dx)。模型设定α=10°,计算时间为0.06 Ma。

Fig.5 Results of oblique spreading model

图6 不同扩张角度洋中脊体应变速率分布图 a至f分别对应α=5°、10°、15°、20°、30°、45°下的体应变速率分布;模型计算时间为0.06 Ma;将z方向0~20 km和60~80 km区域裁掉,以放大显示洋脊轴应变分布特征。

Fig.6 Volume strain rate of mid-ocean ridge with various spreading obliquityies

图7 不同扩张角度洋中脊体应变速率分布图(无初始温度扰动) a至f分别对应α=5°、10°、15°、20°、30°、45°下的体应变速率分布;模型计算时间为0.06 Ma;将z方向0~20 km和60~80 km区域裁掉,以放大显示洋脊轴应变分布特征。

Fig.7 Volume strain rate of mid-ocean ridge with various spreading obliquities (no initial temperaure disturbance)

图8 西南印度洋中脊28和29段地形图(上)及构造解译图(下)

Fig.8 Bathymetry (upper panel) and structural interpretation (lower panel) of SWIR segments 28 and 29

图9 斜向扩张对洋中脊地形分段性的影响示意图 a—圆圈为温度扰动;b—应变集中区分布样式;c—中央火山脊和NTD分布,双线为中央火山脊,箭头指示扩张方向,椭圆为NTDs;d—地幔整体向西运动背景下,中央火山脊位置向西移动示意图,离轴位置线段为脊构造及其最高点相连形成的迹线。

Fig.9 Schematic diagram showing the influence of oblique spreading on mid-ocean ridge segmentation pattern

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