Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise
Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise
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DOI：
10.1029/93jb02872
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发表时间：
1994-05
影响因子：
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通讯作者：
G. Kent;A. Harding;J. Orcutt;R. Detrick;J. Mutter;P. Buhl
中科院分区：
文献类型：
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作者：
G. Kent;A. Harding;J. Orcutt;R. Detrick;J. Mutter;P. Buhl
利用共深度点反射剖面的偏移资料，我们发现沿加勒特断裂带以南的超快扩张（>150 mm/yr）东太平洋海隆的构造差异是二级的，表明地壳增生过程非常均匀。上升轴南部的加勒特转换是由一个狭窄的（<1.0公里）熔体透镜，显示出很大的沿走向的连续性。轴向熔融岩床深度约为1200 m，比沿着扩张较慢的东太平洋海隆（9°30′N）浅约400 m。这一观察结果加强了地壳速度反演顶部的深度取决于扩展速率的论点。熔融岩床宽度，但是，显示沿着东太平洋隆起的变化不大，这表明没有岩浆房的规模扩张速度的依赖。熔体储层的宽度朝向/跨越14°27′S脊轴不连续性减小了250-300 m，并且似乎在此特征上是连续的。考虑到轴向熔融透镜的小长宽比（1.0 km × 1.50 m ×几十km），先前记录的14°27′S偏移处MgO含量的跳跃可能是混合边界的结果，该混合边界通过对流中的沿走向阻抗而得以维持。源自地震层2A底部的广角反射，假定与喷出层一致，显示在隆起轴1-2公里范围内厚度增加了2 - 3倍（200-250米到500-600米）。在加勒特变换以南拍摄到的挤压增厚模式与在9°N沿沿着扩张较慢（110-120 mm/年）的东太平洋海隆观察到的模式相似。在新火山带之外，平均喷出厚度沿剖面和从剖面到剖面沿着相对不变。这意味着一定程度的时间稳定性的沿走向岩浆供应时，整合超过10千年，对应于新火山区的宽度。离轴平均喷出厚度的推断一致性与推测不一致，推测轴向体积向14°27′S不连续面减小是由于岩浆供应的长期减少。二阶差异的风格喷出加厚可能与结构差异的低速区的基础上的上升轴和/或内的应力场的变化，在上覆甲壳的上升轴附近的熔岩扩散侵位的结果。莫诺在横轴剖面上的图像可以追溯到熔融岩床边缘1.0 km以内;这一观测结果与上升峰模型一致，该模型通过轴向熔融透镜向下和向外的物质平流而不是通过大型岩浆房底部的累积沉积来生成下地壳剖面。
Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second-order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along-strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off-axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long-term reductions in magma supply. Second-order differences in the style of extrusive thickening may be related to structural differences within the low-velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross-axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.