Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge
Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge
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地堑的形成与最近的堤坝侵入和洋中脊火山喷发有关
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
R. Embley
中科院分区：
文献类型：
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作者：
W. Chadwick;R. Embley
地堑已被绘制成紧邻最近爆发的海底熔岩流的胡安德富卡海岭（JdFR）的裂缝和同轴段。地堑宽10-100米，深5-15米，位于供给水流的喷发口的上游和/或下游。我们解释说，这些结构形成（或被重新激活）直接在堤防，饲料喷发，因为它是入侵的表面。这些地堑结构是新侵入岩脉冷却过程中海底扩散热液排放的主要通道。东太平洋海隆（EPR）上某些位置的轴向顶峰“地堑”或“破火山口”与JdFR上观察到的地堑尺寸相似，我们认为它也可能是一个由堤坝引起的地堑结构（尽管经常被随后的喷发所掩埋）。JdFR和EPR上的地堑明显比在陆地（冰岛和阿法尔）上缓慢扩展的裂谷上形成的有据可查的岩脉侵入期间形成的地堑更窄更深。力学模拟表明，狭窄的地堑会形成时，岩墙是在浅的深度，它施加了高应力扰动的环境应力状态，造成断层。因此，JdFR和EPR上的狭窄地堑意味着垂直于山脊通常存在相对较高水平的水平压应力，这必须通过高岩脉引起的扰动来克服，从而导致断层。这可能是因为相对于板块扩张速率，岩脉侵入事件的频率很高，因此板块的逐渐扩张很少有足够的时间来显著降低压缩应力。侵入这种环境的岩脉必须具有相对较高的内部岩浆压力。因此，在中间到快速扩张的山脊表面形成的岩脉诱发地堑的大小和特征反映了这样一个事实，即火山作用在调节局部应力状态方面优于构造作用，而局部应力状态是可获得强大岩浆供应的。
Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10–100 m wide and 5–15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit “graben” or “caldera” on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike-induced graben structure (although often buried by subsequent eruptions where it is narrowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well-documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dike-induced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike-induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.