Coeval micro-polygonal faulting and orthogonal jointing in Oligocene Boom Clay (Belgium)
Boris Dehandschutter1, Manuel Sintubin1, Noël Vandenberghe2, Sara Vandycke3, Patrick Gaviglio4, Laurent Wouters5 & Jeroen Mertens5
1 Structural Geology & Tectonics Group, K.U.Leuven, Redingenstraat 16, 3000 Leuven, Belgium
2
Stratigraphy Group, K.U.Leuven, Redingenstraat 16, 3000 Leuven, Belgium3
Faculté Polytechnique de Mons, 9 rue de Houdain, 7000 Mons, Belgium4
Lab. de Géosciences, Université de Franche-Comté, 16 route de Gray, 25030 Besançon cédex, France5
NIRAS/ONDRAF, Kunstlaan 14, 1210 Brussel, Belgiumboris.dehandschutter@geo.kuleuven.ac.be
A multitude of variably trending microfaults with cm-scale lengths and mm-scale offsets have been observed in the Oligocene Boom Clay Formation (Rupelian), outcropping in central Flanders (Belgium). They can be related to an orthogonal set of vertical joints (m-scale). Although chaotic at first sight, a detailed analysis of fault geometry, paleostrain and paleostress reconstructions learn that they can be related to an extensional faulting regime with near-radial tensile stresses, however showing a distinct extension direction. This rather unexpected result puts forward a new type of polygonal faulting mechanism, which is driven by a combination of compaction-induced extension and tectonic forcing.
The Boom Clay formation is a weak clay, composed of rhythmically alternating silt- and clay-rich beds, deposited in the Oligocene southern North Sea at a depth of about 100 m. Nowadays, the beds dip 2° to the northeast. Microstructural analysis in five quarries covering an area of 200 kmē showed that this mechanically weak and plastic clay (slightly overconsolidated, porosity ~30%, frictional angle ~15°, Young Modulus ~500 MPa, Poisson ratio ~0.4) microscopically behaves transitionally between ductile and brittle. SEM analysis of fault-zone microfabrics shows ductile fault tips without dilatation, and areas of clear dilatant brittle failure.
This brittle-ductile rheology complicates the interpretation of the regularly spaced (m-scale) vertical orthogonal cross-joints and normal faults. Although pore-fluid expulsion (shrinkage) would be a plausible endogenous driving mechanism generating horizontal tensile stresses, the distinct NW trend of one of the joint sets suggests an exogenous influence of the Shmin trend, perpendicular to the main (NW trending) joint direction. The perpendicular (NE trending) joint direction is interpreted to result from an elastic post-rupture response causing temporal inversion of the s3 direction.
Analysis of the microfaults also indicates regularity in the extension direction, which is NNE to NE. We again interpret this dominant extension direction to be caused by regional tectonism, as there is a fundamentally common direction of principal minimum compression, directly controlled by the regional stress field. The driving force can still be compaction-driven extension, which is facilitated in the tectonic direction of s3.
The orthogonal joint set as well as the polygonal faults lack distinct cross-cutting regularities, and seem to be coeval in origin. Relations between faults and joints also suggest simultaneous formation. The timing of the deformation must be post-depositional and situated in the Late Oligocene- Early Miocene, as the overlying sandy Miocene sediments (with erosive contact) are less tilted and contain reworked blocks of faulted Boom clay material. We relate this timing to the onset of uplift of the Brabant and Ardenne massifs, along with an intensified activity in the Rhine graben at the end of the Oligocene. It are those northeast-verging tectonic forces that are held responsible for the observed deformation.