Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest
Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest
复制标题
东太平洋隆起顶部的重力异常、均衡和地幔流
DOI：
10.1029/93jb01551
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发表时间：
1993
影响因子：
--
通讯作者：
J. R. Cochran
中科院分区：
文献类型：
--
作者：
Xuejin Wang;J. R. Cochran
在南东太平洋隆起7°S至9°S的详细水文测量中获得的水深和重力数据用于研究快速扩展的洋中脊轴线上的均衡。特别地，我们研究了水深峰顶高地的支撑方式，以及这种支撑如何沿着160公里长的7°12′s -8°38′s山脊段的轴线变化。峰顶高地高出相邻山脊两侧约400米，在140公里的距离内具有几乎恒定的最小轴向深度。峰顶宽而平，整个山脊段都有一个轴向峰顶破火山口。然而，波峰高的宽度和横截面形状沿轴线有系统地变化。它是宽阔的，在中段有平缓的斜坡，但在山脊段的末端逐渐变得狭窄和陡峭。脊顶的自由空气重力异常宽约15-20 km，振幅为10-15 mGal。地幔布格异常沿轴有系统地变化，最小值出现在段的中心附近。因此，轴向地幔布格异常并不反映轴向深度，而是与波峰等深高横截面积的变化相关。从测深和自由空气异常中消除了远离轴线的冷却和沉降的影响，从而隔离了与脊顶相关的残余地形和重力异常。残余波峰测深高被建模为一个弯曲特征，这是由轴下中心的低密度材料区域的向上浮力载荷引起的。岩石圈被视为一个破碎的板块，要么具有恒定的弯曲刚度，要么具有与距地轴距离的平方根成正比的有效弹性厚度。恒刚度情况下Te的最佳拟合值在0.3 ~ 0.6 km范围内。对于生长板模型，Te以0.2-0.3 km / 2的速率增加。重力限制了两种岩石圈模型的质量缺陷延伸到20-30 km的深度。我们将这种低密度物质解释为部分熔体向脊轴输送岩浆的区域。最佳拟合密度异常表明，在峰顶高度下存在4-9%的熔体分数。因此，向地轴上涌的熔体被限制在距地轴约10公里的狭窄区域内。质量不足导致的部分熔体上涌并不是沿脊轴均匀分布，而是集中在脊段的中心部分。因此，在缓慢扩张和快速扩张脊上观测到的沿轴重力和深度模式的差异似乎不是由缓慢扩张脊的三维集中上升流向快速扩张脊的二维片状上升流转变的结果。相反，快速和缓慢扩张脊在轴向重力和深度上的差异反映了沿轴浅层岩浆分布效率的差异。
Bathymetry and gravity data obtained during a detailed Hydrosweep survey of the southern East Pacific Rise from 7°S to 9°S are used to investigate isostasy at the axis of a fast spreading mid-ocean ridge. In particular, we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160-km-long 7°12'S-8°38'S ridge segment. The crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km. The summit is broad and flat, and an axial summit caldera is present for the entire length of the ridge segment. However, the width and cross-sectional shape of the crestal high vary systematically along the axis. It is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment. The ridge crest is marked by a free-air gravity anomaly high about 15–20 km wide with an amplitude of 10–15 mGal relative to the ridge flanks. Mantle Bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment. The axial mantle Bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross-sectional area of the crestal bathymetric high. The effects of cooling and subsidence away from the axis were removed from the bathymetry and free-air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest. The residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis. The lithosphere was treated as a broken plate, either with a constant flexural rigidity or an effective elastic thickness Te which grows at a rate proportional to the square root of distance from the axis. The best fitting values of Te for the constant rigidity case are in the range of 0.3–0.6 km. For the growing plate model, Te increases at a rate of 0.2–0.3 km½. The gravity constrains the mass deficiency to extend to a depth of 20–30 km for both lithospheric models. We interpret this low-density material as a region of partial melt feeding magma to the ridge axis. The best fitting density anomalies imply that a 4–9% melt fraction is present beneath the crestal high. Upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis. The mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment. It thus appears that differences between the along-axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three-dimensional, focused upwelling at slow spreading ridges to two-dimensional sheetlike upwelling at fast-spreading ridges. Rather, the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along-axis magma distribution.
影响因子：
3.9
作者：
MCKENZIE, D;BICKLE, MJ
通讯作者：
BICKLE, MJ