| 摘要: | 地球上的隱沒帶為單邊隱沒;在隱沒帶中只有單一板塊隱沒到上覆板塊之下。然
而,數值模型中的隱沒帶大多傾向發展出雙邊隱沒,也就是在隱沒帶兩側的板塊一同隱
沒。在本研究中,我們將探討在數值模型 中設置低黏滯性地幔楔如何幫助生成單邊隱沒
系統,及其對隱沒帶周遭地函流的影響。
在數值模型中模擬單邊隱沒可以透過幾種技術達成,包括在模型中設定板塊移動
速度(Tan et al., 2002)
、非牛頓流變性(Billen and Gurnis, 2001)或是自由表面(Geryaet
al., 2008)
。但是這些技術都有它們各自的缺點。設定板塊移動速度需要特殊的力學邊界
條件,可能使模擬出的地函流場與浮力不一致。使用非牛頓流變性及自由表面時,需要
解算非線性方程式,會消耗大量計算資源。
在本研究中,我們的地函熱對流數值模型避免了前述的缺點,採用牛頓黏度流體
和自由滑動表面。我們以參數化的方式,來模擬生成海洋地殼行脫水作用並促使其上方
的地函蛇紋岩化、形成黏滯性極低的楔型區域。如下圖所示,在我們的數值模型中設置
了大量的追踪子(tracers)以代表含水的海洋地殼。當追踪子隨著岩石圈隱沒至一定深
度時,我們於其上方設置一個參數化的低黏滯性楔型體(Low viscosity wedge, LVW)
以代表地幔楔(Manea and Gurnis, 2007)。
我們測試了隱沒運動對不同地幔楔參數的敏感度,包含地幔楔的深度上、下界及
其黏滯性。並計算各個模型中上覆板塊及隱沒板塊各別相對於海溝的表面速度以判別是
否產生單邊隱沒系統。除此之外,我們還假設含水的海洋地殼會有較低的黏滯性,這有
助於形成板塊間的低黏滯性交界層(Low viscosity layer)。
我們的模擬結果顯示,低黏滯性地幔楔對於單邊隱沒帶的形成而言是必要的,以
更有效的降低板塊間的摩擦,使隱沒作用順利進行。另一方面,板塊的年紀(同時影響
到板塊本身的厚度與黏滯性)對於隱沒速度也有極高的影響力。;Subduction on Earth is one-sided, where one oceanic plate sinks beneath the other.
However, subduction zones in most numerical models tends to develop two-sided subduction,
where both plates sink to the mantle. In this study, we use numerical model to find out how
the existence of low viscosity wedge (LVW) can enable single-sided subduction and affects
the flow in the subduction system. At the mantle wedge, water released from dehydrated
oceanic crust serpentinized the mantle, which forms the LVW. LVW is an important part of
the subduction system and provides efficient lubricant between the subducting slab and
overriding lithosphere.
Single-sided subduction can be generated in numerical models by different techniques,
including prescribed plate velocity (Tan et al., 2002), non-Newtonian rheology (Billen and
Gurnis, 2001), and free surface (Gerya et al., 2008). These techniques either requires
kinematic boundary condition, which produces mantle flow inconsistent with the buoyancy, or
costs great amount of computational resources when solving nonlinear equations. In this study,
we tried to generate single-sided subduction with Newtonian viscosity and free slip surface. A
set of tracers representing hydrated oceanic crust are placed near the surface. As the tracers
subducted with the lithosphere, we assume that the oceanic crust becomes dehydrated and
serpentinizes the mantle wedge above. A parameterized LVW is placed above the subducted
tracers in the models. (Manea and Gurnis, 2007). We also decrease the viscosity of the
hydrated oceanic crust to represent a low viscosity layer (LVL).
We test with different upper and lower depth limits of the LVW and the viscosity of the
LVW. Both overriding plate and subducting plate′s surface velocity relative to the trench is
calculated in order to determine whether the subduction is single-sided.
Results of our numerical models show that not only the LVW above the slab is essential
for the formation of single-sided subduction, a LVL between the two tectonic plates is also
needed to efficiently lubricate the plate interface after the subduction started. Additionally, the
plate′s age, which effects the plate thickness and viscosity, strongly influence the speed of
subduction. |
