Whittaker Joanne M, Müller R Dietmar, Roest Walter R, Wessel Paul, Smith Walter H F
Earthbyte Group, School of Geosciences, Building F09, The University of Sydney, Sydney, New South Wales 2006, Australia.
Nature. 2008 Dec 18;456(7224):938-41. doi: 10.1038/nature07573.
Seafloor roughness varies considerably across the world's ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming below a half-spreading rate threshold of 30-35 mm yr(-1) (refs 4, 5), as well as on the local interaction of mid-ocean ridges with mantle plumes or cold-spots. Here we present a global analysis of marine gravity-derived roughness, sediment thickness, seafloor isochrons and palaeo-spreading rates of Cretaceous to Cenozoic ridge flanks. Our analysis reveals that, after eliminating effects related to spreading rate and sediment thickness, residual roughness anomalies of 5-20 mGal remain over large swaths of ocean floor. We found that the roughness as a function of palaeo-spreading directions and isochron orientations indicates that most of the observed excess roughness is not related to spreading obliquity, as this effect is restricted to relatively rare occurrences of very high obliquity angles (>45 degrees ). Cretaceous Atlantic ocean floor, formed over mantle previously overlain by the Pangaea supercontinent, displays anomalously low roughness away from mantle plumes and is independent of spreading rates. We attribute this observation to a sub-Pangaean supercontinental mantle temperature anomaly leading to slightly thicker than normal Late Jurassic and Cretaceous Atlantic crust, reduced brittle fracturing and smoother basement relief. In contrast, ocean crust formed above Pacific superswells, probably reflecting metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, is not associated with basement roughness anomalies. These results highlight a fundamental difference in the nature of large-scale mantle upwellings below supercontinents and superoceans, and their impact on oceanic crustal accretion.
海底粗糙度在全球各大洋盆地中差异很大，对于控制海洋中的热量循环和混合以及消散涡动能至关重要。基于对活跃大洋中脊分析得出的模型表明，海底粗糙度取决于海底扩张速率，在半扩张速率低于30 - 35毫米/年的阈值时会形成更粗糙的基底（参考文献4、5），同时也取决于大洋中脊与地幔柱或冷点的局部相互作用。在此，我们对白垩纪至新生代洋脊侧翼的海洋重力衍生粗糙度、沉积物厚度、海底等时线和古扩张速率进行了全球分析。我们的分析表明，在消除与扩张速率和沉积物厚度相关的影响后，大片海底仍存在5 - 20毫伽的残余粗糙度异常。我们发现，粗糙度作为古扩张方向和等时线方向的函数表明，观察到的大部分额外粗糙度与扩张倾斜度无关，因为这种影响仅限于非常高倾斜角度（>45度）的相对罕见情况。在之前被泛大陆超级大陆覆盖的地幔上形成的白垩纪大西洋海底，远离地幔柱处显示出异常低的粗糙度，且与扩张速率无关。我们将这一观察结果归因于泛大陆下超级大陆地幔温度异常，导致晚侏罗世和白垩纪大西洋地壳比正常情况略厚，脆性断裂减少，基底地形更平滑。相比之下，在太平洋超级地幔柱上方形成的洋壳，可能反映了温度仅略有升高的地幔之下的交代岩石圈，与基底粗糙度异常无关。这些结果突出了超级大陆和超级大洋之下大规模地幔上升流性质的根本差异，以及它们对洋壳增生的影响。
