The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions
The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions
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东太平洋隆起的轴向顶地堑和横截面形状作为轴向岩浆房和近期火山喷发的指标
DOI：
10.1016/0012-821x(88)90051-9
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发表时间：
1988
影响因子：
5.3
通讯作者：
P. J. Fox
中科院分区：
文献类型：
--
作者：
K. Macdonald;P. J. Fox
东太平洋海隆（EPR）的轴沿走向在30-200 km的距离上上下波动数百米，深部区域出现在转换断层和其他脊轴不连续处，如重叠扩张中心（OSC）。我们已经提出，隆起深度和横截面形状的系统变化是局部轴向岩浆收支沿着给定脊段变化的指标[1]。最近收集的多道地震（MCS）数据[2]与我们的Sea Beam和SeaMARC II数据的比较使我们能够测试和推进这一假设。沿着EPR从9°到13°N有一个很好的相关性之间的三个参数，都直接关系到一个岩浆循环沿着一个给定的脊段的阶段：上升的横截面形状，存在或不存在一个轴向的首脑地堑，和存在或不存在一个浅轴向岩浆房（从MCS数据解释）。在轴向岩浆房存在的地方，脊的横截面形状是宽的，并且沿轴向沿着识别出轴向顶地堑。相反，上升的横截面形状是狭窄的三角形，轴向岩浆房没有检测到，轴向首脑地堑是缺席。这些脊轴特征倾向于沿给定脊段的较深部分沿着出现，通常在脊轴不连续处附近。我们认为，这些系统的脊轴形态（横截面形状）和结构（存在或不存在的轴向地堑）的变化反映了空间和时间的变化在脊轴的岩浆预算。在岩浆收支不断增加的地方，地壳和下伏上地幔中的浅层岩浆库会膨胀，形成一个宽的轴向隆起，顶部形成地堑。在岩浆收支减少的地方，地壳岩浆房很小（< 2 km宽）或不存在，洋脊轴的特征是狭窄的三角形构造，缺乏明确定义和连续的轴向地堑。由于下面的轴向岩浆储集层不足以产生显著的塌陷结构或破火山口，因此缺少一个山顶地堑。这种形状、结构和岩浆参数的相关性仅在脊的沿着两个短部分失效。在这些地区，有证据表明有一个轴向岩浆房和上升有一个广泛的横截面形状，但没有首脑地堑。然而，底部照片和潜水结果表明，在这些地区的上升波峰覆盖着非常新鲜的熔岩断层未中断，这表明山顶地堑最近已填补了熔岩流，和火山构造崩塌的山顶地堑（或线性破火山口）的发展尚未发生。
The axis of the East Pacific Rise (EPR) undulates up and down hundreds of meters over distances of 30–200 km along strike, the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers (OSCs). We have suggested that systematic variations in depth and cross-sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment [1]. A comparison of recently collected multichannel seismic (MCS) data [2] with our Sea Beam and SeaMARC II data have allowed us to test and advance this hypothesis. Along the EPR from 9° to 13°N there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment: the cross-sectional shape of the rise, the presence or absence of an axial summit graben, and the presence or absence of a shallow axial magma chamber (as interpreted from MCS data). Where the axial magma chamber is present, the cross-sectional shape of the ridge is broad and an axial summit graben is recognized along the axis. In contrast, where the cross-sectional shape of the rise is narrow and triangular, an axial magma chamber is not detected and an axial summit graben is absent. These ridge axis characteristics tend to occur along deeper portions of a given ridge segment, often near ridge axis discontinuities. We suggest that these systematic variations in ridge axis morphology (cross-sectional shape) and structure (presence or absence of an axial graben) reflect spatial and temporal variations in the magmatic budget of the ridge axis. Where the magmatic budget is waxing, shallow-level magma reservoirs in the crust and underlying upper mantle swell, creating a broad axial bulge with a summit graben. Where the magmatic budget is diminished, the crustal magma chamber is small (< 2 km wide) or absent and the ridge axis is characterized by a narrow triangular edifice that lacks a clearly defined and continuous axial graben. A summit graben is missing because the underlying axial magma reservoir is not large enough to produce a significant collapse structure or a caldera.This proposed correlation of shape, structure and magmatic parameters fails along only two short portions of the ridge. In these areas there is evidence for an axial magma chamber and the rise has a broad cross-sectional shape, but there is no summit graben. Bottom photographs and submersible results, however, show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting, suggesting that the summit graben has been recently filled in by lava flows, and the development of a summit graben (or a linear caldera) by volcano-tectonic collapse has not yet occurred.