Abstract
The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.
Highlights
Beginning with the reconstruction of [19], which documented the gravitational sedimentation of parts of the Chaotic Unit (CCU), we focus on the roles played by the Alpine subduction and collisional tectonics recorded by map- to micro-scale structures
We evaluate the impact of polyphasic Alpine tectonic deformation in reworking and reorganizing the primary fabric of Jurassic mass-transport and turbidite deposition, and resulting in the chaotic rock assemblage presently exposed in the CCU
Our findings demonstrate that the CCU primary stratigraphic succession was overprinted and reworked by a polyphase Alpine tectonic deformation and metamorphic recrystallization
Summary
In most orogenic belts and exhumed subduction-accretion complexes around the world, a strong morphological convergence of meso- to map-scale fabrics exists in the block-in-matrix fabric of different types of mélange (i.e., sedimentary, diapiric, tectonic, and polygenetic mélanges) [1,2,3].This close resemblance has led to a long-standing debate on the nature and mode of geological processes (i.e., gravitational vs. tectonics), that lead to the formation of chaotic rock assemblages, in areas of well-preserved, exhumed subduction-accretion complexes such as in the Western US Cordillera, the Circum-Pacific Region, and Circum-Mediterranean Region [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. In most orogenic belts and exhumed subduction-accretion complexes around the world, a strong morphological convergence of meso- to map-scale fabrics exists in the block-in-matrix fabric of different types of mélange (i.e., sedimentary, diapiric, tectonic, and polygenetic mélanges) [1,2,3] This close resemblance has led to a long-standing debate on the nature and mode of geological processes (i.e., gravitational vs tectonics), that lead to the formation of chaotic rock assemblages, in areas of well-preserved, exhumed subduction-accretion complexes such as in the Western US Cordillera, the Circum-Pacific Region, and Circum-Mediterranean Region [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].
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