Vertical movements of the Paris Basin (Triassic-Pleistocene): from 3D stratigraphic database to numerical models

Abstract

Abstract A 3D stratigraphic database has been constructed from the inspection of 1100 wells and outcrops in the Paris basin. The database contains 88 surfaces correlated at high temporal resolution using sequence stratigraphy. For each well and each surface, the present-day depth, the depositional environment and the lithology between two layers are available. This database provides a key to quantify the tectonics associated with this intracratonic basin and to model the thermal and mechanical processes at the origin of the tectonics. Three types of numerical modelling have been carried out in order (1) to better constrain the long-term thermal subsidence and its cause, (2) to characterize the spatial and temporal evolution of the crustal tectonics during the ‘extensional’ period and (3) to test a lithospheric folding origin during the end-Cretaceous to present-day compressional period. The philosophy of these three models are different. The Chablis model for the lithospheric thermal evolution is used to predict the long-term subsidence of the Paris Basin. The thermal evolution of the lithosphere is computed, taking account of a constant temperature or heat flow at the base of the lithosphere, temperature- and pressure-dependent thermal characteristics, metamorphism in the crust, top-crustal erosion and phase transition in the mantle. The long-term subsidence of the Paris basin results from the decay of a thermal anomaly initiated during late Variscan times. The subsidence data can be explained by short- (Stephano-Autunian) as well as long- (Stephano-Triassic) lasting extension. These hypotheses both implicitly refer to extensional collapse of the Variscan belt. The characterization of the spatial and temporal evolution of the crustal tectonics during the thermal relaxation period has been need to quantify the local effect of the sediment load on vertical crust movements. From sedimentary thickness and bathymetric data, maps of relative tectonics have been drawn at a time scale around 500 ka. These maps show two different tectonic behaviours: (1) narrow regions with a high horizontal gradient of tectonics (faults), and (2) domains with a diffuse subsidence correlated with topographic domes and high rates of sedimentation. The geometrical and temporal characteristics of the regions of diffuse subsidence are compatible with a model of flow of the lower crust if the thickness of the flowing channel is at least 20 km with a viscosity of 10 20 Pas. The Tertiary characteristics of the Paris Basin could be the record of large-scale lithospheric folding. The numerical experiments demonstrate that extremely low (0.2 mm a −1 ) shortening rates are largely sufficient to induce large-scale low-amplitude folding under low maximum values of tectonic stresses ( c. ∼50 MPa). These values suggest that alpine compression is largely sufficient to activate this deformation. From the data collected in this database and from the models described here, the evolution of the Paris Basin is better understood. The Paris Basin Meso-Cenozoic evolution can be described as a long-term thermal subsidence, inherited from the Permian extension and perturbed by intraplate deformations in reaction to the geodynamic events occurring in western Europe, i.e. the Ligurian Tethys opening and closure, and the Atlantic opening. Those tectonic events modify in space and time both subsidence and facies distributions. The Paris Basin was initially an ‘extensional’ basin which progressively evolved into a compressional one, temporarily (lower Berriasian and late Aptian) and then permanently (late Turonian to present day). The present-day geometry of the Paris Basin is the consequence of lithospheric folding occurring mainly during the Tertiary. In consequence, (1) the Paris Basin is not still a subsiding basin but an uplifted area, and (2) during the Jurassic and part of the Cretaceous, the surrounding present-day outcropping basement massifs were subsiding areas flooded by the sea.