ABSTRACT

In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of ‘native’ rock slices wrenched from the overlying Dronero Unit, and ‘exotic’ rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

摘要

在Dora Maira Massif南部（西阿尔卑斯山）的Valmala区段，两个不同的榴辉岩相和蓝片岩相单元（即分别为Rocca Solei和Dronero单元）被剪切带（即Valmala构造单元）隔开，尤其是由片状的石屑岩和大陆岩石混合而成。一张1：10,000比例尺的详细地质图可以指出，瓦尔马拉构造单元内的构造层包括上覆的德罗内罗单元钳制的“天然”岩片和可能来自多拉·迈伊拉其他单元的“奇异”岩石并且来自大陆边缘和海洋盆地。相反，缺乏从底层的罗卡·苏莱伊部队获得的岩石切片。在与俯冲有关的早期变形阶段（即D1）之后，形成了整个构造叠层，以及随后两个与掘尸相关的变形阶段（即D2和D3）的普遍叠印。据推测，Valmala构造单元在俯冲过程中使南部的Dora Maira地块解耦，和/或驱动了邻近的Brossasco-Isasca单元中超高压岩石的掘出工作。