Abstract :
[en] This study is the result of a rigorous brittle structural analysis of the c. 419 Ma ± 13 Ma [1] microdioritic intrusion of Lessines (Belgium). As for similar intrusions in Bierghes and Quenast, the latter belongs to the discontinuous intrusive microdiorite ribbon that is embedded along the southern part of the single-phase deformed Brabant Massif (Anglo-Brabant Deformation Belt) [2]. Because of the scarcity of outcrops in the host sedimentary rocks, the Lessines quarries provide an interesting field laboratory for tracing tectonic regimes in the study area.
We performed a systematic structural survey of joints and faults orientations with their associated slickensides and kinematic indicators. A total of 356 faults have been measured, covering all the study area. An inversion paleo-stress analysis has been carried out using TENSOR method [3] and semi-automatic separation of fault-slip data subsets.
In this scheme, five distinct paleo-stress tensors have been highlighted. Three of them created their own structures (P) while the two last one acted as reactivators of pre-existing brittle structures (S).
A first fault system consistent with a NW-SE compression/NE-SW extension in strike-slip tectonic regime is dominant. This one is characterized by high angular dispersion of faults strikes and by typical chlorite-pyrite hydrothermal fillings, represented by the Ermitage faults. On the other hand, a second faults system fits to a NE-SW compression/NW-SE extension tensor. For these two main strike-slip regimes, relative chronology is difficult to establish in the field. We thus relied on the geomechanical properties of the rock during brittle deformation. In this case, the high angular dispersion of faults strikes combined with the mineralogical nature of joint-filling phases, suggest certain precocity of the NW-SE compression/NE-SW extension tectonic phase. Another relevant population of faults and joints are distinguished by their low dips (ranging between 24° to 35° SSW-dipping). They are also characterized by systematic mineralization of a quartz-chlorite-sulfur assemblage, which is consistent with a late-magmatic age. Furthermore, crosscutting relationships on the field clearly place the formation of these brittle structures before the tectonic event forming the Ermitage faults. Although they are poorly marked by slickensides, their low dips and discrete conjugate faults (~ 30° NNE-dipping) suggests an inverse tectonic regime, with the main stress trending NNE-SSW.
Other faults strongly correlate with pre-existing structures and are attributed to subsequent basement reactivations. Two new main regimes could be distinguished. A first N-S compression has reactivated N010 to N030 faults in sinistral strike-slip regime. A second reactivation process is also marked as an NNE extensional stress, constrained by down-dip slickensides on WNW-ESE striking Ermitage faults, sometimes with chalk filling.
Finally, using statistical calculations we integrated all these brittle tectonics and paleo-stress records in a consistent chronological geodynamic scheme in agreement with preliminary studies.
[1] André, L. & Deutsch, S., (1984)
[2] Debacker, T. N. & Sintubin, M. (2008)
[3] Angelier, J. (1984)
