Multidisciplinary characterization of diapiric basins integrating field examples, numerical and analogue modelling: Central High Atlas Basin (Morocco)

Abstract

The complexity of the interplay between tectonics and sedimentation increases when salt tectonics is involved because of the ductility of salt and its ability to flow. Discrimination between extensional tectonics and salt-related processes is problematic; especially where salt-related rift basins were inverted as occurred in the Central High Atlas in Morocco. The aim of the present work was to analyse and understand the dynamics of the Central High Atlas diapiric basin during the Early Jurassic rift and subsequent post-rift periods using a multidisciplinary workflow integrating fieldwork, analogue models and subsidence and thermal numerical modelling. Two regions were examined to assess the effects of salt tectonics in the evolution of the basin; the Djebel Bou Dahar platform-basin system represented the fault domain of the rift basin where diapiric activity was not described and the Tazoult-Amezraï area and Imilchil diapiric province corresponding to the unstable domain of the basin characterised by the presence of diapiric salt ridges and minibasins. Results from analogue models highlighted the intrinsic interrelation between extension, diapirism and sedimentation that characterised the diapiric domain of the Central High Atlas. Longitudinal and transverse sedimentary progradations and their timing had a strong impact in the migration of ductile layers, in the growth of diapirs and in their lateral structural variations; triggering well-developed passive diapirs in the proximal domains and incipient reactive diapirs or poorly developed roller-like and passive diapirs in the distal domains of the sediment source. Analogue models including post-diapiric compression fairly reproduced the observed structure in the studied areas. Modelling with 6% and 10% of shortening, slightly lower than the Atlas one, produced the progressive close-up of the two flanks of salt walls and their final welding as well as the steepening of their outward flanks, with dips increasing from 8o-17o prior to compression to 30o-50o after compression. Subsidence curves varied depending on the analysed localities of the rift basin. Djebel Bou Dahar showed long-term and low-rate tectonic and total subsidence (0.06 and 0.08 mm yr-1, respectively). The roughly parallel evolution of both total and tectonic subsidence curves indicates the main extensional tectonic influence on subsidence pattern, as corroborated by the syndepositional activity of the outcropping Sinemurian-Pliensbachian normal faults. In the unstable domain, Amezraï minibasin centre showed tectonic and total subsidence rates between 0.06-0.32 and 0.19-0.98 mm yr-1, rates one order of magnitude higher than in the Djebel Bou Dahar. These subsidence rates were up to two-fold their equivalent rates in the Tazoult salt wall (0.01-0.27 and 0.09-0.74 mm yr-1). In the Imilchil diapiric province lateral shifts of the main subsiding depocenters were recorded during Toarcian to Callovian times (tectonic and total subsidence rates up to 0.23 and 0.90 mm yr-1). The subsidence of the unstable domain was caused by the combination of normal fault extension and salt withdrawal from beneath the minibasins during rifting, being the salt-related subsidence predominant during the post-rift and masking the expected subsidence pattern for a rift-post rift transition. For the first time, 27 new vitrinite reflectance data were used to build the thermal evolution and associated geohistory of the Central High Atlas. Thermal models, with heat flows of 105 mW/m2 (from 189 to 140 Ma) followed by 60 mW/m2 and 70 mW/m2 (from 189 to 182.7 Ma) followed by 60 mW/m2, pointed to a post-Middle Jurassic evolution characterised by long-term and low-rate subsidence and an overburden between 1200-2400 m on the Tazoult-Amezraï area. The comparison of subsidence curves from this study with Saharan Atlas and Tunisian Atlas showed that peak of subsidence in these salt-related domains became younger to the east.

La discriminació entre processos associats a tectònica extensiva i a tectònica salina es problemàtica; especialment en conques diapíriques extensives invertides com és el cas del Alt Atles Central de Marroc (CHA). L’objectiu d’aquesta tesis és analitzar i entendre els processos que interaccionaren a la conca diapírica del CHA durant el rift Juràssic i el subseqüent període post rift, utilitzant una metodologia multidisciplinària que integra treball de camp, models analògics i models numèrics. S’han estudiat dos dominis de la conca: no diapiric i diapiric (diapirisme durant el Juràssic Inferior i Mitjà). Els models analògics mostren que progradacions longitudinals i transversals i el moment quan s’inicien tenen un gran impacte en la migració dels nivells dúctils, en el mode de creixement diapiric i les seves variacions laterals. Els models amb compressió post-diapírica mostren que dita compressió produeix la reducció progressiva de l’amplada de les estructures diapíriques fins al seu tancament complet, així com a un increment dels cabussaments dels flancs com s’ha observat a les zones d’estudi. Les corbes de subsidència varien segons el domini analitzat. El domini no diapiric es caracteritza per un període llarg de baixes taxes de subsidència tectònica i total (0.06 i 0.08 mma-1). El domini diapiric registra taxes de subsidència tectònica i total fins a un ordre de magnitud majors que en el domini no diapiric (0.23 i 0.90 mma-1) i una migració dels depocentres subsidents. La subsidència del domini diapiric s’interpreta, durant el rift Juràssic Inferior, com una combinació d’activitat de falles normals i migració salina, sent aquesta darrera la predominant durant la fase post-extensiva i emmascarant el patró de subsidència esperat en un estadi de transició rift-post rift. Per primera vegada, es presenta la geohistòria de la part central del CHA. Els models tèrmics emprats per a la seva construcció, avaluats amb 27 noves dades de reflectància de vitrinites de la zona d’estudi, suggereixen una evolució post-Juràssic Mitjà caracteritzada per un període llarg de baixa subsidència que hauria enterrat la regió entre 1200-2400 m, en comptes de una complexa historia que inclouria diversos esdeveniments d’exhumació com s’ha enregistrat en altres zones del Alt Atles Marroquí.