Structure and organization of the Porma Mélange: Progressive denudation of a submarine Nappe toe by gravitational collapse

Abstract

The Porma Mélange is a unit up to 1 km thick underlying the Forcada Nappe, in the Variscan foreland of the Iberian Peninsula. Most of the Porma Mélange consists of a highly deformed, boudinaged sequence, with extension values over 300 percent. The boudins or mélange blocks are the result of bed-parallel extension in two perpendicular directions giving rise to equidimensional boudins in plan view (chocolate tablet structure). In cross-section view, the boudins show different shapes (rectangular, rhomboid, triangular, trapezoid, or with arrowhead edges) due to various combinations of extension and shear fractures. Both the provenance of the mélange blocks, mostly derived from the upper part of the Bodón Nappe, that overlies the Forcada Nappe, and flattening-type strain ellipsoid shapes support the interpretation that hanging-wall gravitational collapse of these nappes was the cause of the extensional deformation. The current distribution of the mélange blocks from different nappe formations implies progressive unroofing of the nappe stack through subsequent submarine slumps. The internal structure of the mélange blocks depends on the degree of sediment lithification during slumping and illustrates the significance of the so-called "native" and "exotic" blocks. The source of most of the mélange material (shaly matrix and "native" blocks or boudins) was the semilithified syntectonic sediments of the upper part of the Bodón Nappe, which underwent brittle-ductile deformation (boudinage) during slumping. However, a few and usually larger "exotic" blocks also came into the basin as individual slip blocks from competent well-lithified formations, originally located at the lower part of the nappe stack, with higher shear resistance and less suitable for spreading. Thus, the arrangement of major nappe fragments, traditionally named klippes and showing sizes that may reach up to several kilometers, does not agree with the geometric rules of thrusting, and are reinterpreted as megablocks resulting from gravitational sliding during denudation of advancing nappes.