Crustal accretion at mid-ocean ridges governs the creation and evolution of the oceanic lithosphere. Generally accepted models^1,2,3,4 of passive mantle upwelling and melting predict notably decreased crustal thickness at a spreading rate of less than 20 mm year⁻¹. We conducted the first, to our knowledge, high-resolution ocean-bottom seismometer (OBS) experiment at the Gakkel Ridge in the Arctic Ocean and imaged the crustal structure of the slowest-spreading ridge on the Earth. Unexpectedly, we find that crustal thickness ranges between 3.3 km and 8.9 km along the ridge axis and it increased from about 4.5 km to about 7.5 km over the past 5 Myr in an across-axis profile. The highly variable crustal thickness and relatively large average value does not align with the prediction of passive mantle upwelling models. Instead, it can be explained by a model of buoyant active mantle flow driven by thermal and compositional density changes owing to melt extraction. The influence of active versus passive upwelling is predicted to increase with decreasing spreading rate. The process of active mantle upwelling is anticipated to be primarily influenced by mantle temperature and composition. This implies that the observed variability in crustal accretion, which includes notably varied crustal thickness, is probably an inherent characteristic of ultraslow-spreading ridges.

大洋中脊的地壳增生控制着大洋岩石圈的形成和演化。普遍接受的被动地幔上涌和熔化模型^{1、2、3、4}预测地壳厚度会以小于20毫米年^-1的扩张速度显着减少。据我们所知，我们在北冰洋的加克尔海脊进行了首次高分辨率海底地震仪（OBS）实验，并对地球上传播最慢的海脊的地壳结构进行了成像。出乎意料的是，我们发现沿脊轴地壳厚度范围在 3.3 公里至 8.9 公里之间，并且在过去 5 Myr 的横轴剖面上，地壳厚度从约 4.5 公里增加到约 7.5 公里。地壳厚度的剧烈变化和相对较大的平均值与被动地幔上涌模型的预测不符。相反，它可以通过由熔体提取引起的热和成分密度变化驱动的浮力活动地幔流模型来解释。预计主动上升流与被动上升流的影响会随着扩散速度的降低而增加。预计活跃的地幔上涌过程主要受地幔温度和成分的影响。这意味着观察到的地壳增生的变化，包括显着变化的地壳厚度，可能是超慢速扩张脊的固有特征。