A gravity reconnaissance of the island of Oahu
A gravity reconnaissance of the island of Oahu
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欧胡岛重力勘测
DOI：
10.1029/tr032i003p00358
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发表时间：
1951
期刊：
影响因子：
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通讯作者：
G. Woollard
中科院分区：
文献类型：
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作者：
G. Woollard
1948 年秋，欧胡岛建立了由 30 个重力站组成的网络。对这些重力数据以及美国海岸和大地测量局在岛上的重力数据以及 Vening Meinesz 在邻近深海区域的观测进行的研究表明：（1）该岛块直至海底的平均比重为 2.3。 (2) 经海洋站海水和岛屿熔岩质量差异校正后的区域布格异常值显示，该岛异常为-75 mgals。 (3) 这种吸引力的不足，部分可能与结晶地壳层变薄有关，因为岩浆物质在深处撤退，以提供熔岩和可能在深处的岩浆房。 (4) 异常的平衡可能与地表沉降有关，地表沉降使比重为 2.3 的熔岩和沉积物与比重为 2.95 的洋壳物质并置。 (5) 迄今为止，根据上述基础，该岛的区域沉降量已达到 8,000 至 10,600 英尺。 (6) 为了实现均衡平衡（消除 +50 毫克的区域均衡异常），将需要大约 7,000 英尺的额外沉降。 (7) 古老的 Koolau 和 Waianae 火山喷口上空出现 (+) 110 mgals 的局部异常，但没有发现明显的异常与钻石头、Koko、Punch Bowl 或盐湖等较新的火山喷口有关。对这种现象的解释似乎在于早期和晚期喷口的相对大小的差异以及存在的喉部物质的岩石学差异及其与周围火山流物质的密度对比。 (8) 计算火山喉物质和流动物质之间存在 0.6 gm/cc 的最大密度对比以满足 110 mgals 异常的深度，表明沉降到海底以下 9,000 英尺的深度。该值与区域异常（8,000 至 10,600 英尺）分析确定的沉降量基本一致。 (9) 由于早期熔岩不太可能呈多孔状，因此这些沉降程度的数字可能代表最小值。 (10) 艾里和普拉特均衡理论下实现均衡的经典机制对于此类地质结构似乎都无效。
A network of 30 gravimeter stations was established on the island of Oahu in the fall of 1948. A study of these gravity data in conjunction with those of the U. S. Coast and Geodetic Survey on the island and the observations of Vening Meinesz in the adjacent deep-sea areas indicates the following: (1) The mean specific gravity of the island block down to the ocean floor is 2.3. (2) The regional Bouguer anomaly values corrected for the mass differential of the sea water and island lavas at the sea stations show an anomaly of −75 mgals on the island. (3) Of this deficiency in attraction, part may be related to a thinning of the crystalline crustal layer because of the withdrawal of magmatic material at depth to furnish the lavas and a possible magma chamber at depth. (4) The balance of the anomaly presumably is related to surface subsidence whereby lavas and sediments of 2.3 specific gravity are brought into juxtaposition with oceanic crustal material of 2.95 specific gravity. (5) Regional subsidence of the island to date on the above basis has amounted to between 8,000 and 10,600 ft. (6) To achieve isostatic equilibrium (remove the +50 mgals regional isostatic anomaly) an additional subsidence of approximately 7,000 ft will be required. (7) Local anomalies of (+) 110 mgals occur over the ancient Koolau and Waianae volcanic vents, but no appreciable anomaly is found to be associated with the more recent volcanic vents as Diamond Head, Koko, the Punch Bowl, or Salt Lake. The explanation of this phenomenon appears to lie in the difference in the relative sizes of the early and late vents and the difference in the petrology of the throat material present and its density contrast with the surrounding volcanic flow material. (8) Computations as to the depth to which a maximum density contrast of 0.6 gm/cc exists between volcanic throat material and flow material to satisfy the 110 mgals anomaly indicate subsidence to a depth of 9,000 ft below the ocean floor. This value is in general agreement with the subsidence determined from the analysis of the regional anomalies (8,000 to 10,600 ft). (9) Since it is improbable that the early lavas were vesicular, these figures on degree of subsidence probably represent minimum values. (10) None of the classical mechanisms for achieving isostatic equilibrium under the Airy and Pratt theories of isostasy appear valid for this type of geologic structure.