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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Brittle deformation during Alpine basal accretion and the origin of seismicity nests above the subduction interface
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Brittle deformation during Alpine basal accretion and the origin of seismicity nests above the subduction interface

机译：高山基础增生期间的脆性变形和俯冲界面上方的地震性巢穴的起源

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Geophysical observations on active subduction zones have evidenced high seismicity clusters at 20-40 km depth in the fore-arc region whose origin remains controversial. We report here field observations of pervasive pseudotachylyte networks (interpreted as evidence for paleo-seismicity) in the now-exhumed Valpelline continental unit (Dent Blanche complex, NW. Alps, Italy), a tectonic sliver accreted to the upper plate at c. 30 km depth during the Paleocene Alpine subduction. Pre-alpine granulite-facies paragneiss from the core of the Valpelline unit are crosscut by widespread, mm to cm-thick pseudotachylyte veins. Co-seismic heating and subsequent cooling led to the formation of Ti-rich garnet rims, ilmenite needles, Ca-rich plagioclase, biotite microliths and hercynite micro-crystals. Ar-39-Ar-40 dating yields a 51-54 Ma age range for these veins, thus suggesting that frictional melting events occurred near peak burial conditions while the Valpelline unit was already inserted inside the duplex structure. In contrast, the base of the Valpelline unit underwent synchronous ductile and brittle, seismic deformation under water-bearing conditions followed by a re-equilibration at c. 40 Ma ( 39(Ar)-Ar-40 on retrograded pseudotachylyte veins) during exhumation-related deformation. Calculated rheological profiles suggest that pseudotachylyte veins from the dry core of the granulite unit record upper plate micro-seismicity (M-w 2-3) formed under very high differential stresses (500 MPa) while the sheared base of the unit underwent repeated brittle-ductile deformation at much lower differential stresses (40 MPa) in a fluidsaturated environment. These results demonstrate that some of the seismicity clusters nested along and above the plate interface may reflect the presence of stiff tectonic slivers rheologically analogous to the Valpelline unit acting as repeatedly breaking asperities in the basal accretion region of active subduction zones. (C) 2018 Elsevier B.V. All
机译：主动俯冲带地球物理观测在20-40公里深度的弧前地区，其起源一直存在争议证明高地震群。我们在这里报告在现在挖出瓦尔佩利内欧陆单位普遍存在假玄武网络的实地观察（解释为古地震证据）（凹痕布兰奇复杂，NW，阿尔卑斯山，意大利），一个构造条子位于C增生到上板。 30公里深度古高山俯冲期间。预高山粒岩相从瓦尔佩利内单元的核心副片麻岩被广泛，毫米到厘米厚的假玄武静脉横切。同震加热和随后的冷却导致富Ti的石榴石轮辋，钛铁矿针，钙富斜长石，黑云母和石器铁铝尖晶石微晶体的形成。 AR-39-AR-40约会产生用于这些静脉一个51-54马年龄范围，从而表明，虽然瓦尔佩利内部已经双链体结构内插入摩擦熔化的事件发生在靠近峰埋藏条件。与此相反，瓦尔佩利内单元的基座进行含水条件，随后在℃的再平衡下同步韧性和脆性，地震变形。 40马（39（Ar）的-Ar-40上回生假玄武静脉）折返相关变形期间。计算流变学特性建议从下非常高的应力差所形成的粒单元记录上板的微地震活动的干燥芯（MW 2-3），该假玄武静脉（大于500兆帕），而单元的剪切基后行重复brittle-在fluidsaturated环境;在低得多的应力差（40兆帕＆LT）韧性变形。这些结果表明，一些沿和板界面上方嵌套在地震簇的可能反映僵硬构造条子流变学上类似于充当活性俯冲带的基底吸积区域多次开断凹凸的瓦尔佩利内单元的存在。 （C）2018爱思唯尔B.V.所有

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