Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A, Mid-Atlantic Ridge)
Crystallization depth beneath an oceanic detachment fault (ODP Hole 923A, Mid-Atlantic Ridge)
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大洋滑脱断层下方的结晶深度（ODP 孔 923A，大西洋中脊）
DOI：
10.1002/2015gc006027
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Lissenberg C
中科院分区：
文献类型：
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作者：
Lissenberg C
人们越来越认识到，在缓慢和超低速扩张的洋中脊的海底扩张过程中，海洋剥离断层发挥着不可或缺的作用，并对海洋岩石圈的结构产生了重大影响。虽然熔体供应被认为对海洋分离体的形成和演化起着关键的控制作用，但人们对熔体和断层如何相互作用和相互影响知之甚少。对于支队附近熔体就位的位置和深度，几乎没有直接的约束条件。在ODP 923A孔钻探的辉长岩靠近中大西洋脊和凯恩变换断层（23°N； MARK区域）的交汇处，代表岩浆进入了这样一个拆离断层的下盘，并被它所覆盖。我们在这里给出了这些辉长岩和伴生闪长岩脉的U - Pb锆石日期，结合该地区的构造重建，我们可以计算出熔体结晶的深度。三个样品的Th校正锆石U - Pb年龄范围为1.138±0.062至1.213±0.021 Ma。我们发现结晶深度为6.4 +1.7/−1.3 km，并估计辉长岩的母体熔体最初位于1.5 km深的地方，在海底以下<8 km处。构造重建表明，导致该层序暴露的滑脱断裂可能在深部越过洋脊轴线，表明滑脱断裂下盘的融侵是一个重要的过程。我们发现在~ 1.2 Ma与“拆离模式”扩张相关的深侵深度似乎明显大于该地区当前“岩浆模式”扩张期间的岩浆储层深度，这表明北MARK段保留了最近在两种时间上截然不同的扩张模式之间的转换，这些模式具有根本不同的岩石圈结构。
Oceanic detachment faults are increasingly recognized as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid‐ocean ridges, with significant consequences for the architecture of the oceanic lithosphere. Although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments, much less well understood is how melts and faults interact and influence each other. Few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available. Gabbros drilled in ODP Hole 923A near the intersection of the Mid‐Atlantic Ridge and the Kane transform fault (23°N; the MARK area) represent magmas emplaced into the footwall of such a detachment fault and unroofed by it. We here present U‐Pb zircon dates for these gabbros and associated diorite veins which, when combined with a tectonic reconstruction of the area, allow us to calculate the depths at which the melts crystallized. Th‐corrected single zircon U‐Pb dates from three samples range from 1.138 ± 0.062 to 1.213 ± 0.021 Ma. We find a crystallization depth of 6.4 +1.7/−1.3 km, and estimate that the melts parental to the gabbros were initially emplaced up to 1.5 km deeper, at <8 km below the seafloor. The tectonic reconstruction implies that the detachment fault responsible for the exposure of the sampled sequence likely crossed the ridge axis at depth, suggesting that melt emplacement into the footwall of oceanic detachment faults is an important process. The deep emplacement depth we find associated with “detachment mode” spreading at ∼1.2 Ma appears to be significantly greater than the depth of magma reservoirs during the current “magmatic mode” of spreading in the area, suggesting that the northern MARK segment preserves a recent switch between two temporally distinct modes of spreading with fundamentally different lithospheric architecture.