Formation of chocolate-tablet boudins in a foreland fold and thrust belt: A case study from the external Variscides (Almograve, Portugal). G. Zulauf, G. Gutiérrez-Alonso, R. Kraus, R. Petschick, S. Potel. 2011, Journal of Structural Geology, 33, 1639-1649. DESRCARGAR-DOWNLOAD
We have studied chocolate tablet structures which that are restricted to steep limbs of isoclinal D1-folds exposed in three dimensions along the SW coast of Portugal. Boudinage of Carboniferous quartzite worked in a slaty matrix under low differential stress, the latter supporting extension fracture in the neck domains and growth of fibrous quartz in veins. There are two generations of necks, the older subvertical, the younger subhorizontal, both of which developed under similar conditions and occasionally coevally: (1) the quartz veins of both sets of necks show stretched crystal fibers that have formed by repeated opening and sealing, probably due to a cyclic change in pore-fluid pressure; (2) the aspect ratio of the boudins is the same in vertical (3.0 ±1.4) and in horizontal sections (2.9 ±1.2); (3) temperatures obtained from fluid-inclusion data of the fibrous quartz of both sets of necks are similar within uncertainties (200 ±20° for vertical and 230 ±22°C for horizontal veins); (4) the deformation microstructures of the quartz fibers are similar. As microthermometry of fluid inclusions yielded similar temperatures like those obtained by illite crystallinity (ca. 200 – ca. 250°C), the chocolate tablet boudins formed close to the metamorphic peak by two stages of necking and fracturing during the late phase of shortening. The first initial folding stage led to vertical necks after while the limbs of the iscolinal D1-folds had attained their steep attitude and when the orientation of the least and intermediate principal stresses were interchanged. Subsequently, the initial (folding) local stress field was restored and formation of the horizontal necks led to the chocolate tablet structures, although some necks of the vertical set kept opening at the same time. The direction of the greatest regional principal stress was constant from the initial buckling stage via isoclinal folding to the final stages of boudinage.