20 Ma以来Mohns洋中脊的非对称扩张速率与地壳结构

超慢速扩张的Mohns洋中脊共轭两侧的地球物理场与地壳结构具有显著的非对称性.利用我国第五次北极科学考察采集的水深、重力与磁力数据，结合历史资料，我们计算了14条垂直Mohns洋中脊剖面的扩张速率、剩余水深、剩余地幔布格重力异常（RMBA）、地壳厚度和非均衡地形.对洋中脊共轭两侧以上计算结果的进一步对比发现，Mohns洋中脊两侧整体（下文均指同一地质时刻各剖面的平均值）的非对称性呈现明显的两段性：20~10.5 Ma，相比Mohns洋中脊东侧，西侧的扩张速率更慢、地壳更厚、非均衡地形更低；10.5~0 Ma，扩张速率、地壳厚度和非均衡地形的非对称的极性与20~10.5 Ma期间完全相反.后一阶段，整体扩张速率在西侧更快、剩余水深更浅，但是对应更薄的地壳和更高的非均衡地形.我们推断前者为冰岛沿Kolbeinsey洋中脊的作用增厚了Mohns洋中脊西侧地壳并使得洋中脊向西侧跳动，而后一阶段反映了岩浆供给减少后西侧集中的构造活动导致的更多的拉伸与隆升.沿各剖面上，10.5~0 Ma期间构造活动集中的洋中脊西侧均具有薄地壳和高非均衡地形，但构造拉伸的增加并不总是对应增快的扩张速率.岩浆在浅部更多地向东侧的分配以及洋中脊向西侧的跳动可能使得东西两侧具有相近的扩张速率.

Asymmetric spreading rates and crustal structures of the Mohns Ridge since 20 Ma

The ultraslow-spreading Mohns Ridge shows significant asymmetric geophysical signatures and crustal structures. The spreading rates, residual bathymetry, residual mantle Bouguer anomalies, crustal thickness, and non-isostatic topography along 14 profiles across the Mohns Ridge were calculated based on a combined dataset from the 5th Arctic expedition of China and open-source data. A further analysis of conjugate ridge flanks reveals that the asymmetric spreading of the Mohns Ridge can be divided into two distinct phases:phase I (20~10.5 Ma) and phase Ⅱ (10.5~0 Ma). During 20~10.5 Ma,the western flank was associated with slower spreading rates, thicker crust, lower non-isostatic topography in comparison to the eastern flank. During 10.5~0 Ma,the polarity of the asymmetry reversed, as the western flank was associated with faster spreading rates, higher topography, thinner crust, and higher non-isostatic topography. We infer that the phase I may reflect the transportation of the asthenospheric material from the Iceland hotspot to the Mohns Ridge through the Kolbeinsey Ridge, which might thicken the western flank and caused the westward ridge to jump. The phase Ⅱ may be related to concentrated tectonic activities on the western flank, which occurred under low-melt conditions and generated thinner crust and elevated topography. The tectonically-focused western flank was persistently associated with thinner crust and higher non-isostatic topography along all 14 profiles during 10.5~0 Ma, while this is not the case for the spreading rates. We attribute this phenomenon to the magma asymmetrically partitioned to the eastern flank and/or the ridge axis jumped westward consecutively.