Megaflap formation and Damage recording: the case of the Cotiella Megaflap, South-Central Pyrenees.

Salt tectonics is responsible for typical structures associated with salt structures margins development: (i) minibasin subsidence, (ii) basin edge backfolding of basin margins, forming plurikilometric steep or overturned structures along the salt structures or their equivalent welds, called megahooks and megaflaps and (iii) smaller-scale halokinetic drape folding and composite halokinetic sequences (CHS). Mega-halokinetic structures like the megaflaps are of particular interest to us as they have been recently defined and their kinematics are poorly understood compared to those of CHS. They develop either during halokinetic drape folding, or during contractional squeezing of the diapirs or during some combination of both processes. It seems megaflaps form early in the salt reliefs development as opposed to more mature structures allowing the CHS development. Because of their geometry, megaflaps have also implications for reservoirs geometries and fluid pressures distribution, critical for successful exploration or potential storage. Megaflaps seem to have the same behaviour as detachment folds and could present same kinematics and deformations. Characterizing the multi-scale damage records, using fracturation network and matrix damage analyses, may allow us to reconstruct the megaflaps formation dynamics and to establish relationships between reservoir properties and structural evolution.

In the Cotiella Basin, recent studies have shown the role of salt tectonics associated with gravity in the creation of various minibasins during the post-rift system between the Cenomanian and the Santonian. The Cotiella minibasin s.s. presents a megaflap with vertical to completely overturned layers. The calcarenites composing this megaflap present numerous joints, veins and stylolites. The first observations and analyses show several stages of deformation, from Layer Parallel Shortening (LPS) to Late Stage Fold Tightening (LSFT) as observed in the folds. Moreover, preliminary results of matrix damage, using AMS, also indicate a record of LPS and even late deformation, LSFT, in the rocks. A detailed scenario of the damage acquisition chronology, from the multi-scale damage, is under contruction to understand the formation of this megaflap. We will then be able to compare the damage, the type of megaflap and the causal relationships such as geodynamic context and lithology with others such as the Sivas megaflap (Turkey).