全球三大洋底高原重力异常与地壳厚度特征及对比研究

Characteristics of gravity anomaly and crustal thickness of three largest oceanic plateaus

翁通爪哇高原、凯尔盖朗高原与沙茨基海隆是全球三大洋底高原,是大量岩浆喷发到地表的结果,火山面积分别达1.90×10^6、1.25×10^6、0.53×10^6km^2。本文详细分析了该三大洋底高原的地形、剩余地幔布格重力异常(residual mantle Bouguer anomaly, RMBA)与重力反演的相对地壳厚度,并结合地质与地球化学特征约束进行对比研究。结果显示,翁通爪哇高原、凯尔盖朗高原与沙茨基海隆分别高出周围海底约4.3、5、4km,相应的地幔布格重力异常最大变化值分别为250、330、200mGal,以及相应的相对地壳厚度变化分别为11、13、9km,表明形成三大洋底高原的岩浆量远远大于正常洋中脊的岩浆量。此外,三大洋底高原皆形成于洋中脊附近。Nd、Pb、Hf同位素比值分析表明,翁通爪哇高原的玄武岩组分为洋岛玄武岩;凯尔盖朗高原大部分类似于洋岛玄武岩,并含有洋中脊玄武岩组分;沙茨基海隆的玄武岩组分主要为东太平洋海隆正常洋中脊玄武岩,却又存在少量位于全球洋岛玄武岩范围内。这些特征揭示了三大洋底高原可能形成于"地幔柱-洋中脊相互作用"。对此本文提出了两种模式:一为洋中脊被地幔柱拖拽至其上方;二为洋中脊之下的软流圈受到地幔柱影响,从而产生超常熔融与超厚地壳。

The Ontong Java Plateau, Kerguelen Plateau, and Shatsky Rise are the three largest oceanic plateaus on Earth, exhibiting voluminous magmatism with areas of 1.90×10~6, 1.25×10~6, and 0.53×10~6 km^2, respectively. In this study, we conducted a detailed comparative analysis of bathymetry, residual mantle Bouguer anomaly(RMBA), and gravity-derived relative crustal thickness of the three plateaus, correlating results with geological and geochemical constraints. Relative to adjacent normal seafloors, the Ontong Java Plateau is elevated by 4.3 km;the Kerguelen Plateau, by 5 km;and the Shatsky Rise, by 4 km. The corresponding peak-to-trough amplitudes in RMBA are about 250, 330, and 200 mGal, while the relative crustal thickness variations are of 11, 13, and 9 km, respectively, implying that the magma output forming these three oceanic plateaus is far more than that generated by a normal mid-ocean ridge. Moreover, all three plateaus were formed near mid-ocean ridge or ridge triple junction settings. Isotopic analyses of Nd, Pb, and Hf reveal that the Ontong Java Plateau is characterized by ocean island basalts(OIB);most of the Kerguelen Plateau is predominantly OIB, but is mixed with some mid-ocean ridge basalts(MORB);and most of the Shatsky Rise volcanism shows East Pacific Rise-type MORB, with a few exceptions of OIB-like components. These features imply that these oceanic plateaus were probably formed by "plume-ridge interaction". Two models are proposed herein:(1) the peripheral mid-ocean ridge is dragged onto the top of the mantle plume, and/or(2) the asthenosphere is fed by plume material nearby, resulting in melting anomalies and the resultant thickened crust along the mid-ocean ridge.