The Red beds of the South Atlasic Front (Goulmina, Morocco) : a lithostragraphic unit with a different burial history involved in a thrust and belt kinematic sequence

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Influence of P/T conditions on the style of normal fault initiation and growth in limestones from the SE-Basin, France

International audienc

Influence of P/T conditions on the style of normal fault initiation and grouth in limestones from the SE-Basin, France

A conventional and cathodoluminescence microscopy, stable isotope, and fluid inclusion study was undertaken on normal fault zonesexposed on vertical cliffs in Tithonian limestones of the French SE-Basin. The aim was to determine the chronology of deformation, and factors controlling normal fault nucleation and evolution in carbonate rocks. Two models are proposed for fault zone development depending on the depths at which deformation takes place. At depths of 3–3.5 km, incipient shear zones consist of en-e´chelon veins, filled by fibrous calcite and confined to limestone layers bounded by thin clayey interlayers. As deformation increases and the layers rotate, slip occurs along the clay-rich interlayers. A system of pull-aparts forms allowing displacement to increase. Pull-aparts are commonly filled by fibrous calcite crystals. At depths of 1.5–2 km, faults initiate as small slip surfaces, early nucleating in some limestone layers. Their propagation is impeded by thin clayey interlayers, which enhances fracturing and bending of the limestone layers. As slip surfaces propagate, they overlap and can connect with one another by means of dilational jogs that are filled by large euhedral calcite crystals. In the first case, pull-aparts are coupled with the solution seams that reuse preexisting, rotated clay-rich interlayers. As they formed, they progressively filled and the fluid circulation remained local. In the second case, longer slip planes connect by dilation jogs, that are not immediately filled, which enhance the circulation of fluids, which show chemical variations. It appears that P/T conditions strongly control the relative role of brittle processes and solution/crystallization mechanisms, and the style of the early stages of normal faulting in the same limestone rocks. q 2005 Elsevier Ltd. All rights reserved

Detailed mineralogy and petrology of manganese oxyhydroxide deposits of the Imini district (Morocco)

Manganese ore in the Cenomanian-Turonian dolostone of the Imini district (south of the High Atlas, Morocco) displays a high Mn content due to the occurrence of pyrolusite, cryptomelane, hollandite sensu stricto, coronadite, romanechite, and lithiophorite. The orebodies occur mainly as three stratabound layers along the ~25-km-long ore belt following a WSW-ENE direction. Using scanning electron microscopy (SEM-EDS) coupled with X-ray diffraction (XRD), we here refine the mineralogy and petrology of Mn oxides and oxyhydroxides in order to constrain the paragenetic sequence and define the main processes of ore formation. Deposition and concentration of Mn oxyhydroxides follow a two-step sequence after dolomitization: (1) replacement of dolomite fabric (dolomite micrite and dolomite rhombs) by hollandite group minerals leading to a textural conservation and then (2) neoformation of collomorphous aggregates in an opened and brecciated system. These observations are consistent with a multistage evolution including multiple reworking and brecciation. Pyrolusite is the main Mn oxide observed in the Imini district during both early and late stages. The superficial environment provides supergene conditions suitable for the deposition of only Mn oxyhydroxides. This supergene environment and the occurrence of multiple bands of hollandite group minerals indicate an external migration (allochthonous supply) of Mn and associated elements, contemporaneous to an in situ chemical and mechanical dissolution of the host dolostone. The lithological heterogeneities of dolostone and its chemical environment compared to less permeable surrounding rocks may have provided the conditions to concentrate Mn oxides and oxyhydroxides.SCOPUS: ar.jinfo:eu-repo/semantics/publishe