On backflow associated with oceanic and continental subduction
On backflow associated with oceanic and continental subduction
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与海洋和大陆俯冲相关的回流
DOI：
10.1093/gji/ggab246
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发表时间：
2021
影响因子：
2.8
通讯作者：
Schmalholz, Stefan M
中科院分区：
文献类型：
--
作者：
Moulas, Evangelos;Brandon, Mark T;Vaughan Hammon, Joshua D;Schmalholz, Stefan M
一个流行的想法是，俯冲带沉积物的增生通常会导致俯冲通道的形成，该通道被认为是位于俯冲板块上方的狭窄区域，充满了剧烈循环的增生沉积物和外来块体。这种环流可以被视为强制对流，由于俯冲板块的夹带，在通道下部出现向下流动，在通道上部出现“回流”。回流经常被用来解释高压/低温变质岩从 30 至 50 公里深处折返的现象。先前对此问题的分析主要集中在限制流动的壁被人为固定和刚性的限制情况。一个关键问题是这种配置是否可以在地质相关的时间尺度上持续下去。我们使用一对耦合的角流来解决这个问题。前角负责俯冲板块正上方的吸积和变形，后角对应于上覆板块的可变形区域。两个角共享内侧边界，该边界完全耦合，但可以自由旋转和变形。我们的结果表明，在狭窄的前角 (<15°) 中维持稳定的循环流需要异常大的粘度比，μretro/μpro> 103。对于较低的粘度比，内侧边界将向后旋转，将最初狭窄的前角转换为钝角几何形状。对于稳定的狭窄拐角，我们表明拐角内的回流是由流入流向下收敛和相关的动态压力向下增加引起的，动态压力在拐角点达到最大值。因此，预计总压力将远大于使用静岩梯度预测的压力，这意味着必须相应地调整变质压力对深度的估计。此外，我们还表明，与强制角流和浮力辅助通道流相关的速度场几乎相同。因此，构造地质学研究不足以区分这两个过程。
A popular idea is that accretion of sediment at a subduction zone commonly leads to the formation of asubduction channel, which is envisioned as a narrow zone located above a subducting plate and filled with vigorously circulating accreted sediment and exotic blocks. The circulation can be viewed as a forced convection, with downward flow in the lower part of the channel due to entrainment by the subducting plate, and a ‘backflow’ in the upper part of the channel. The backflow is often cited as an explanation for the exhumation of high-pressure/low-temperature metamorphic rocks from depths of 30 to 50 km. Previous analyses of this problem have mainly focused on the restricted case where the walls bounding the flow are artificially held fixed and rigid. A key question is if this configuration can be sustained on a geologically relevant timescale. We address this question using a coupled pair of corner flows. Thepro-corneraccounts for accretion and deformation directly above the subducting plate, and theretro-cornercorresponds to a deformable region in the overlying plate. The two corners share amedial boundary, which is fully coupled but is otherwise free to rotate and deform. Our results indicate that the maintenance of a stable circulating flow in a narrow pro-corner (<15°) requires an unusually large viscosity ratio, μretro/μpro> 103. For lower viscosity ratios, the medial boundary would rotate rearwards, converting the initially narrow pro-corner into an obtuse geometry. For a stable narrow corner, we show that the backflow within the corner is caused by downward convergence of the incoming flow and an associated downward increase in dynamic pressure, which reaches a maximum at the corner point. The total pressure is thus expected to be much greater than predicted using a lithostatic gradient, which means that estimates of depth from metamorphic pressure would have to be adjusted accordingly. In addition, we show that the velocity fields associated with a forced corner flow and a buoyancy-assisted channel flow are nearly identical. As such, structural geology studies are not sufficient to distinguish between these two processes.
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