Crustal structures of Southmost Mariana Trench revealed by wide-angle seismic profile TS01
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摘要:
马里亚纳海沟是西太俯冲系统的重要组成部分, 因复杂的地质环境和活跃的俯冲, 长期以来是地球科学领域关注和研究的热点区域.广角地震测线TS01位于马里亚纳俯冲系统的最南段, 横穿卡罗琳海底高原、马里亚纳海沟和弧前地块.本文基于测线上9个OBS台站所记录的高质量反射、折射震相, 通过震相走时拟合和正演模拟的方法, 获得了TS01测线的精细速度结构.此外, 本文还通过卫星重力数据和多波束水深数据, 计算获得了布格重力异常, 反映了区域的地壳厚度变化; 并沿测线进行了重力模拟, 与速度模型相互验证、共同约束了TS01测线的壳幔结构.最终结果显示, 弧前地块呈现楔状, 海沟内壁速度值较低, 呈现了构造侵蚀的特征; 俯冲板片的地壳速度在4.0~7.2 km·s-1之间, 厚12~13 km, 该厚度介于正常洋壳和海底高原类型地壳之间; OBS台站丰富的反射震相约束了上下板片交界面、俯冲板片中地壳和莫霍面等关键地质界面.重力模拟揭示了上地幔顶部的蛇纹石化现象.本文研究结果为揭示海底高原附近的俯冲动力学机制提供了有力支撑.
Abstract:Mariana Trench of Western Pacific Subduction System possesses complicated tectonic settings and vigorous subduction dynamics. Wide-angle seismic profile TS01 traverses the Caroline Plateau, Southmost Mariana Trench and Forearc. We obtained a velocity structure of TS01 via forward modeling based on reflection and refraction phases recorded by 9 OBSs. Furthermore, Bouguer gravity anomaly was calculated based on satellite gravity and multi-beam bathymetric data, and is used to analyze the variations of crustal thickness. Gravity modeling was further conducted to constrain the crustal and uppermost mantle structure. Our velocity model shows a wedge-shaped forearc crust, and relatively low velocities at inner trench slope, which may have arisen from subduction erosion. The velocities of subducting crust range from 4.0 to 7.2 km·s-1. The crustal thickness of the subducting plate is approximately 12~13 km, thicker than typical oceanic crust but thinner than many oceanic plateaus. Three interfaces (interfaces between upper and lower plates, mid-crustal interface and Moho of subducting slab) are constrained by reflection phases of OBSs. Gravity modeling shows a low-density area beneath the trench, indicating serpentinization in the uppermost mantle of the downgoing plate. Our results lay a solid foundation for revealing the subduction dynamics near an oceanic plateau.
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图 1
马里亚纳海沟南段的地理位置、构造单元和水深特征
Figure 1.
Location, tectonic and bathymetric map of Southmost Mariana Trench
图 2
OBS01台站的地震剖面、震相识别和射线追踪结果
Figure 2.
Seismic records, seismic phases, and raytracing of OBS01
图 3
OBS02台站的地震剖面、震相识别和射线追踪结果
Figure 3.
Seismic records, seismic phases, and raytracing of OBS02
图 4
OBS05台站的地震剖面、震相识别和射线追踪结果
Figure 4.
Seismic records, seismic phases, and raytracing of OBS05
图 5
OBS06台站的地震剖面、震相识别和射线追踪结果
Figure 5.
Seismic records, seismic phases, and raytracing of OBS06
图 6
OBS09台站的地震剖面、震相识别和射线追踪结果
Figure 6.
Seismic records, seismic phases, and raytracing of OBS09
表 1
正演模型的震相参数
Table 1.
Seismic phases of the forward modeling
震相种类 震相数量 RMS/ms 卡方值 拾取误差/ms Pg 2825 60 1.444 50 PtopP 198 63 1.104 60 PmP 372 81 1.838 60 PcP 312 66 1.213 60 Pn 256 111 1.935 80 总计 3963 67 1.476 - 
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表 2
重力模型参数
Table 2.
Parameters of the gravity model
模型区域 密度/(g·cm-3) 海水层 1.03 沉积层 2.2 地壳 2.64~2.90 弧前破碎水化地壳 2.58 干燥地幔 3.2~3.3 蛇纹石化地幔 2.93
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