Megathrust slip enhanced by metasomatic actinolite in the source region of deep slow slip
Megathrust slip enhanced by metasomatic actinolite in the source region of deep slow slip
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深部慢滑移源区交代阳起石增强的逆冲滑移
DOI：
10.1016/j.lithos.2023.107115
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发表时间：
2023
期刊：
影响因子：
3.5
通讯作者：
Masuyama Haruna
中科院分区：
文献类型：
--
作者：
Nishiyama Naoki;Ujiie Kohtaro;Noro Kazuya;Mori Yasushi;Masuyama Haruna
陆地莫霍面下方的俯冲巨型逆冲断层会在稳定蠕动或偶发慢滑事件（SSE）中发生滑移。然而，导致抗震巨型逆冲滑动的变形方式和机制仍然未知。我们研究了日本九州的俯冲混杂岩，它由超镁铁质、镁铁质和沉积岩组成。混杂岩在绿帘岩-角闪岩相条件下在~470°C发生变形，与南开俯冲带地幔楔角附近的SSE的推断条件相当。混杂岩中俯冲相关的粘性剪切集中形成了以绿泥石和细粒阳起石为主的叶蛇纹岩和超镁铁质片岩，其特点是鳞片状片状网状结构和单键C组构。镁铁质和超镁铁质岩石在混杂岩中的混合引发了交代反应，导致交代的镁铁质岩石中释放出水，并在超镁铁质片岩中产生细粒阳起石。细粒交代阳起石表现出铝的化学分带以及通过溶解-沉淀蠕变平行于S表面的分带的截断。交代反应释放的水可能有助于溶解-沉淀蠕变。流变分析表明，超镁铁质片岩中细粒阳起石的溶解-沉淀蠕变分别在 0.3-5 MPa 和 10-40 MPa 的剪应力下适应板块收敛和 SSE，而叶蛇纹石可以在 ≤43-94 MPa 的剪应力下适应慢滑移速率，远高于主动俯冲 SSE 期间推断的剪应力 区。根据超镁铁质片岩中交代阳起石的稳定条件确定，交代反应的下倾极限为~40-50 km深度，与南开俯冲带SSEs区域的下限相当。我们认为，虽然叶蛇纹石仅适应抗震蠕变，但较弱的超镁铁质片岩中交代阳起石的溶解-沉淀蠕变可以产生更多样化的滑移行为，包括抗震蠕变和 SSE。板片-地幔界面处的镁铁质和超镁铁质岩石之间的交代反应可能是控制陆地莫霍面以下 SSE 下倾极限的因素之一。
Subduction megathrusts below the land Moho slip at either steady creep or episodic slow slip events (SSEs). However, deformation styles and mechanisms responsible for aseismic megathrust slip remain unknown. We examined the subduction mélange in Kyushu, Japan, which consists of ultramafic, mafic, and sedimentary rocks. The mélange deformed at ∼470 °C under epidote-amphibolite facies condition, comparable to the inferred conditions of SSEs near the mantle wedge corner in the Nankai subduction zone. Subduction-related viscous shear in the mélange was concentrated into antigorite serpentinite and ultramafic schist mainly composed of chlorite and fine-grained actinolite, which is characterized by anastomosing scaly foliations andSsingle bondCfabric. The mixing of mafic and ultramafic rocks in the mélange induced metasomatic reactions, resulting in the release of water from metasomatized mafic rock and the production of fine-grained actinolite in ultramafic schist. The fine-grained metasomatic actinolite exhibits chemical zoning of aluminum and truncation of the zoning parallel toSsurface through dissolution-precipitation creep. Water released by metasomatic reactions may assist the dissolution-precipitation creep. Rheological analysis indicates that the dissolution-precipitation creep of fine-grained actinolite in ultramafic schist accommodated plate convergence and SSEs at shear stresses of 0.3–5 MPa and 10–40 MPa, respectively, whereas antigorite serpentinite can accommodate slow slip rates at shear stresses of ≤43–94 MPa, much higher than inferred shear stresses during SSEs in active subduction zones. The down-dip limit of the metasomatic reactions, determined from the stable condition of metasomatic actinolite in the ultramafic schist, was ∼40–50 km depth, comparable to the lower limit of the SSEs region in the Nankai subduction zone. We suggest that while antigorite serpentinite only accommodated aseismic creep, dissolution-precipitation creep of metasomatic actinolite in weaker ultramafic schist can host more diverse slip behavior including aseismic creep and SSEs. The metasomatic reaction between mafic and ultramafic rocks at the slab-mantle interface is potentially one of the factors controlling the downdip limit of SSEs below the land Moho.
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--
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