Abstract
Intracratonic basins tend to subside much longer than the timescale predicted by thermal relaxation of the lithosphere. Many hypotheses have been suggested to explain their longevity, yet few have been tested using quantitative thermo-mechanical numerical models, which capture the dynamic of the lithosphere. Lithospheric-scale geodynamic modelling preserving the tectono-stratigraphic architecture of these basins is challenging because they display only few kilometres of subsidence over 1000 of km during time periods exceeding 250 Myr. Here we present simulations that are designed to examine the relative role of thermal anomaly, tectonics and heterogeneity of the lithosphere on the dynamics of intracratonic basins. Our results demonstrate that initial heterogeneity of accretionary continental lithosphere explains long-term subsidence and the arches-basins architecture of Saharan type intracratonic basins at first order. The simulations show that initially heterogeneous lithospheres inherited from accretion are strong enough to resist local isostatic re-equilibration for very long period of time. Indeed, the lateral density variations store potential gravitational energy that is then slowly dissipated by differential erosion and slow vertical movements. For relatively well-accepted coefficient of erosion of 10−6 m2/s, the subsidence last longer than 250 Myr. Extensional tectonic forcing and thermal anomalies both result in an effective strength drop of the lithosphere, which allows a temporal acceleration of local isostatic re-equilibration. Periodic changes in far field tectonic forcing from extension to compression complicate the tectono-stratigraphic architecture (intra-basin arches, sub-basins) introducing stratigraphic unconformities between different neighbouring basins such as the ones observed in North Africa.
Highlights
Intracratonic basins called “cratonic basins”, “interior cratonic basins” or “intracontinental sags” host most of freshwater aquifers, minerals resources and hydrocarbon reserves of the world (Allen and Allen, 2013)
During the Paleozoic, the structures of these basins are mainly controlled by N-S sinistral or dextral high dip (> 60°) normal faults forming horst and graben network associated with forced folds (Fig. 3) weakly inverted and/or reactivated through time
In order to measure the effect of structural inheritance on subsidence, we compare models with laterally homogeneous composition (P and A) with a model M that is largely inspired from the geodynamic setting of the Saharan platform (Perron et al, 2018) where both the age and the geometry of different terranes control the architecture of the basins and arches (Figs. 2 and 3)
Summary
Intracratonic basins called “cratonic basins”, “interior cratonic basins” or “intracontinental sags” host most of freshwater aquifers, minerals resources and hydrocarbon reserves of the world (Allen and Allen, 2013). Subsidence (Fig. 1), which are very well documented on the Saharan Platform (Perron et al, 2018) Driven by this geological example, the present contribution introduces alternative hypothesis to thermal subsidence in context of accretionary lithosphere and aims to test these different hypotheses by the mean of thermo-mechanical simulations in order to circumscribe what first-order mechanisms can maintain the low, long-term subsidence rate of the intracratonic basins through the geological time and what second-order forcing can explain local acceleration and inversion of subsidence. During the Paleozoic, the structures of these basins are mainly controlled by N-S sinistral or dextral high dip (> 60°) normal faults (i.e. transtension to transpression) forming horst and graben network associated with forced folds (Fig. 3) weakly inverted and/or reactivated through time (see tectonic history and stresses orientation in Zazoun, 2001; Haddoum et al, 2001; Perron et al, 2018). The thickness reaches 1.7 to 7.1 km in the Ahnet Basin which is in average higher than the Mouydir type basins (Beuf et al, 1971; Conrad, 1984; Wendt et al, 2006, 2009; Zieliński, 2012)
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