科科斯板块驱动力研究及其对洋中脊扩张速率的影响

洋中脊是横贯在全球大洋底部的巨型张裂和板块增生系统。地幔物质在洋中脊之下上涌，并冷却凝固成新的海洋岩石圈，从而形成地球的新生板块。俯冲带是地球板块消亡的场所，新生的海洋岩石圈在此向下俯冲潜没，重新回到地幔中去。洋中脊和俯冲带分别作为板块的出生地与消亡地，记录了板块的运动学与动力学过程，驱动着全球尺度的构造活动和演化，然而目前对这两种系统动力学关系的联合讨论的研究甚少。

扩张速率是决定洋中脊构造特征的一阶影响因素。板块重构结果表明，洋中脊扩张速率与周围俯冲带密切相关，其本质上受控于板块运动的驱动力。因此，定量研究板块运动驱动力及其对洋中脊扩张速率的影响，对于探讨全球洋中脊扩张速率差异性的成因具有重要意义，为从洋中脊-俯冲带系统的角度阐述板块运动的驱动机制问题提供新见解。

本研究以科科斯板块为研究区，基于全球板块运动模型、全球俯冲板片深度和厚度数据以及全球海底水深数据，定量化计算板块驱动力，获取东太平洋海隆的洋脊推力和中美俯冲带的板片拖拽力，并分析这两种板块驱动力与洋中脊全扩张速率之间的关系。此外，选取超慢速-中速扩张的大西洋脊和印度洋脊开展全球类比研究，以期进一步揭示板块驱动力的机制。

研究结果表明：板片拖拽力的量级大小为10¹² N/m，而洋脊推力的量级则仅为10¹¹ N/m，前者比后者大一个数量级；板片拖拽力与洋中脊全扩张速率具有强相关性，全扩张速率> 90%受控于板片拖拽力，而剩余< 10%的贡献来自洋脊推力，因此板片拖拽力的大小是影响洋中脊全扩张速率快慢的决定性因素；洋中脊和俯冲带系统存在相互作用，俯冲带俯冲时产生的板片拖拽力可能拉动邻近的洋中脊，从而产生洋脊推力，导致进一步加速俯冲过程。

Mid‐ocean ridges are giant rifts and plate accretion systems in the global oceans. Thermal mantle material upwells beneath mid‐ocean ridges, cools and solidifies into new oceanic lithosphere, thus forming the Earth's new plate. Subduction zones are places where plates demise (i.e., the oceanic lithosphere subducts downward and returns to the mantle). Mid‐ocean ridges and subduction zones, as the birthplace and extinction sites of plates, respectively, record the kinematic and dynamical processes of plates, and drive the tectonic activities and evolution on a global scale. However, there are few researches on the joint discussion of the dynamical relationship between these two systems.

Spreading rates are the first order factor that determine the tectonic characteristics of mid‐ocean ridges. The results of plate reconstruction show that spreading rates of mid‐ocean ridges are closely related to the surrounding subduction zones, which are essentially controlled by driving forces of plate motions. Therefore, the quantitative research on driving forces of plate motions and their effects on spreading rates of mid‐ocean ridges is of great significance to explore the causes of the differences in spreading rates of global mid‐ocean ridges, and provides new insights into the driving mechanism of plate motions from the perspective of mid‐ocean ridges and subduction zones.

In this research, the Cocos plate was taken as the research area. Based on the global plate motion model, global slab depth and thickness, and global seafloor bathymetry, we quantitatively calculated plate driving forces to obtain the ridge push of the East Pacific Rise and the slab pull of the middle America subduction zone. Then, we analyzed the relationship between these two plate driving forces and full spreading rates of mid‐ocean ridges. Furthermore, the ultraslow to intermediate spreading Atlantic and Indian ridges were selected for global comparative research to further reveal the mechanism of plate driving forces.

The results showed that the magnitude of the slab pull is 10¹² N/m, while the magnitude of the ridge push is only 10¹¹ N/m, which is one order of magnitude smaller than that of slab pull. Slab pull is strongly correlated with full spreading rates of mid‐ocean ridges, with more than 90% of full spreading rates controlled by slab pull and less than 10% of full spreading rates contributed by ridge push. Therefore, slab pull is the decisive factor affecting full spreading rates of mid‐ocean ridges. Moreover, there is an interaction between mid‐ocean ridges and subduction zones: the slab pull generated by subduction zones may have driving force on adjacent mid‐ocean ridges, thus generating ridge push and further accelerating the subduction process.

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