Underplating-related finite-strain patterns in the Gran Paradiso massif, Western Alps, Italy: heterogeneous ductile strain superimposed on a nappe stack

A finite-strain study in the central and northern Gran Paradiso massif of the Italian Western Alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the Alps. The <em>R</em>_f/φ and Fry methods were used on feldspar porphyroclasts from 98 samples of the Gran Paradiso unit (upper tectonic unit of the Gran Paradiso massif) and 12 samples from the underlying Erfaulet unit (lower unit of the Gran Paradiso massif). Microstructures and thermobarometric data show that feldspar ductility at temperatures higher than <em>c</em>. 450 °C occurred only during high-pressure metamorphism, when the rocks were underplated beneath the overriding Adriatic plate. Therefore, the finite-strain data can be related to high-pressure metamorphism in the Alpine subduction zone. The augen gneiss was heterogeneously deformed and axial ratios of the strain ellipse in <em>xz</em> sections range from 2.1 to 69.8. The long axes of the finite-strain ellipsoids trend west–WNW and the short axes are subvertical, associated with a subhorizontal foliation. The strain magnitudes do not increase towards the nappe contacts. The data indicate flattening strain type in the Gran Paradiso unit and constrictional strain type in the Erfaulet unit and prove deviations from simple shear. We conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that ductile strain was superimposed on and modified the nappe structure during high-pressure underplating in the Alpine subduction zone. The accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the Gran Paradiso massif.

针对意大利西阿尔卑斯山格兰帕拉迪索地块（Gran Paradiso massif）中北部开展的有限应变研究，旨在阐明韧性应变是否与推覆体接触面存在关联，并厘清阿尔卑斯地区片麻岩推覆体典型的近水平面理的成因本质。研究采用Rf/φ法与Fry法，对取自格兰帕拉迪索地块上部构造单元——格兰帕拉迪索岩系的98件样品，以及取自该地块下部构造单元——埃尔福莱（Erfaulet）岩系的12件样品中的长石残斑进行了分析。显微构造与温压数据显示，长石韧性变形仅发生于约450℃以上的温度条件下，且对应于岩石在仰冲的亚得里亚板块下方发生底垫作用时的高压变质作用阶段。因此，本次有限应变数据可与阿尔卑斯俯冲带内的高压变质作用建立关联。眼球状片麻岩发生了非均匀变形，xz截面内的应变椭圆轴比介于2.1至69.8之间。有限应变椭球的长轴呈西-西西北向展布，短轴近垂直，与近水平面理相伴生。应变强度并未朝向推覆体接触面递增。数据表明，格兰帕拉迪索岩系发育压扁应变型变形，而埃尔福莱岩系则呈现收缩应变型变形，且两类变形均偏离简单剪切机制。研究认为，推覆体堆叠作用于俯冲早期可能通过脆性叠瓦作用完成，而韧性应变在阿尔卑斯俯冲带的高压底垫作用过程中叠加并改造了推覆构造。底垫作用过程中韧性应变的积累并非通过简单剪切机制实现，且伴随垂直缩短分量，这一过程造就了格兰帕拉迪索地块内的近水平面理。