Sara Vandycke

Palaeostress records in Cretaceous formations in NW Europe: extensional and strike–slip events in relationships with Cretaceous–Tertiary inversion tectonics

Abstract The Chalk formations in NW Europe have been investigated in terms of brittle tectonics and palaeostress analysis. Studies of mesofaults and joints reveal that extension has prevailed since the Cretaceous. The palaeostresses recorded in the Chalk formations of the Isle of Wight, Sussex, Kent, Boulonnais, the Mons Basin and NE Belgium are characterised by an extensional regime interrupted periodically by compressional events related to regional inversions. Four principal extensional events have been distinguished. The early synsedimentary Cretaceous extensional event is present in the whole studied domain. It is related to basin subsidence. The E–W and N–S extension regimes are related to the opening dynamics of the English Channel and the North Sea. The younger NE–SW extension is the most important system of faults, in terms of amount and extent. It is active since the Late Cretaceous and related to recent tectonics. In Boulonnais and the Mons Basin, strike–slip events are related to right-lateral motion on the regional crustal-scale North Artois Shear Zone (NASZ). In Boulonnais and Kent, strike–slip events are Cenomanian in age (the so-called Subhercynian phase). In the Mons Basin and NE Belgium, Early Maastrichtian compressional events are related to the Laramide inversion phase. In Sussex and the Isle of Wight, faulting is principally related to Tertiary (Eocene–Oligocene) inversion. A strike–slip system in N–S compression is dominant in Sussex. Successive strike–slip fault and reverse fault system have been identified in the Isle of Wight flexure. The palaeostress field evolution in NW Europe recorded in Chalk formations is complex but representative of a relay zone between the Atlantic opening and the Tethysian dynamics, where compressional events along crustal regional structures periodically interrupted a regional extensional regime.

Dynamics and inversion of the Mesozoic Basin of the Weald-Boulonnais area: role of basement reactivation

Abstract The geometry and dynamics of the Mesozoic basins of the Weald–Boulonnais area have been controlled by the distribution of preexisting Variscan structures. The emergent Variscan frontal thrust faults are predominantly E–W oriented in southern England while in northern France they have a largely NW–SE orientation. Extension related to Tethyan and Atlantic opening has reactivated these faults and generated new faults that, together, have conditioned the resultant Mesozoic basin geometries. Jurassic to Cretaceous N–S extension gave the Weald–Boulonnais basin an asymmetric geometry with the greatest subsidence located along its NW margin. Late Cretaceous–Palaeogene N–S oriented Alpine (s.l.) compression inverted the basin and produced an E–W symmetrical anticline associated with many subsidiary anticlines or monoclines and reverse faults. In the Boulonnais extensional and contractional faults that controlled sedimentation and inversion of the Mesozoic basin are examined in the light of new field and reprocessed gravity data to establish possible controls exerted by preexisting Variscan structures.

Brittle tectonic structures and palaeostress analysis in the Isle of Wight, Wessex basin, southern U.K.

Abstract Brittle tectonic analysis of Cretaceous–Paleogene sediments at a total of 17 sites located in the Isle of Wight (U.K.) enables four main tectonic events that occurred prior to and after the folding to be identified and successive palaeostress tensors to be determined using the inversion method. Three of the events can be shown to have occurred prior to the folding: (1) a syn-sedimentary extension of Upper Cretaceous age; (2) a strike-slip faulting regime with an ESE–WNW direction of compression; (3) a compressional regime, marked by strike-slip faulting, with an NNE–SSW to N–S direction of compression. The fourth and last compressional event took place after the folding and is characterised both by reverse and strike-slip faulting, with a dominant N–S direction of compression. Syn-folding faults also developed between the third and fourth events. All four events can be connected to the extensional tectonics and different steps of structural inversion, both of which were integral to the development and evolution of the Wessex basin.

Palaeostress analysis and geodynamical implications of Cretaceous-Tertiary faulting in Kent and the Boulonnais

Detailed studies of fracture patterns and palaeostress reconstruction based on fault slip data analysis allow identification of five main palaeostress fields in the Cretaceous formations of the Boulonnais and Kent areas. These stress patterns correspond to six successive tectonic events: (1) a N–S compression/E–W extension of early Cenomanian age, (2) an E-W extension of late Cenoma-nian age, and (3) four post-Santonian events with N–S, E–W, NW–SE and NE–SW extensions, respectively. The first event, interpreted from conjugate strike-slip faults is related to the beginning of the Sub-Hercynian phase and linked to a right-lateral motion of the Nord-Artois Shear Zone. The first E-W extensional event and the N-S extensional event could be related to the last rifting episodes of the North Sea graben and English Channel (Wessex basin). The others post-Santonian extensional events are related to Late Cretaceous, Tertiary and Recent tectonic episodes.

Matrix strains along normal fault planes in the Campanian White Chalk of Belgium: structural consequences

Abstract Significant matrix strains were associated with normal faulting in the Campanian chalk of the Mons basin. These transformations, mainly due to pressure solution, located against the fault planes, brought about systematic changes in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the material. The measurements of other physical properties (elastic waves velocity, capillary rise, permeability) of the chalk provide good evidence for estimating the width of the transformed zone: it ranges between 100 and 150 mm. These transformations cannot be detected when the medium is observed at the grain scale: the most visible evidences of transformation are limited in a 50 mm thick fringe. The transformation may have occurred either in a closed or in an open system. If we follow the first assumption we can estimate the change in volume, and therefore the change in horizontal dimension of the faulted blocks: it is small compared to the horizontal extension. The second assumption is supported by a few evidences: tilted and striated rostra of belemnites along fault planes. In that case the geometrical change can be estimated too and reveals that the horizontal shortening is in the same range as the horizontal extension. It means that the extensional deformation of the Mons basin might have been partially hindered by chemically induced matrix strains.

Volumetric matrix strain related to intraformational faulting in argillaceous sediments

Soft-sediment deformation involves complex interactions between discrete fracturing and diffuse bulk strain, described in terms of volume change and shear strain in a critical state mechanics framework. This study reports on a mesoscale normal fault zone, intraformational in Oligocene argillaceous sediments from the Boom Formation (Belgium), containing several metre-scale normal fault strands. They form either discrete fault planes or decimetre-wide shear zones with internal fabric. The faults have been subjected to microtectonic and petrophysical analysis. Small but significant changes occur in the porous network of the argillaceous matrix approaching a fault or shear zone, indicating compactional strain in both footwall and hanging wall. Internal compaction associated with faulting is put forward as a ductile–brittle feedback mechanism in the kinematics of intraformational fracture systems. Small differential stress induced by compaction and minor regional tectonic forces (differential uplift and tilt) and subsequent gravitational forces (downslope shear stress) induce small shear bands in nearly critically stressed weak mud. Shear banding is accompanied by layer-parallel shortening and bulk volume loss. This provides an additional extension of endogenous origin, accommodated by further deformation. This ductile–brittle feedback mechanism eventually leads to commonly observed intraformational collapse structures called polygonal fault systems.

Reconstruction of neotectonic activity using carbonate precipitates: a case study from the northwestern extremity of the Isparta Angle (SW Turkey)

Abstract Fault- and joint-related calcite precipitates are investigated to reconstruct the neotectonic activity in the northwesternextremity of the Isparta Angle (SW Turkey). A preliminary palaeostress analysis allows us to frame the observed fault activity and joint development in the neotectonic history of SW Turkey. The mineralogical and geochemical study of the fault-related calcites indicates that fluids with a long residence time in the Lycean limestone were responsible for their precipitation. The joints, on the contrary, were passively filled with calcites precipitated from infiltrating near-surface meteoric water.

Tight Chalks - How Does Microtexture Affect Petrophysical and Geomechanical Properties?

High porosity chalks have been extensively studied over the past decades. Only recently, there has been an increasing interest in the understanding of low permeability chalks, forming potential unconventional reservoirs or intra-reservoir seals. In order to better understand the properties of those tight chalks, an integrated petrographical, petrophysical and geomechanical study was carried out on a set of 35 carefully selected outcrop samples, covering a wide range of lithotypes. The dataset gathered covers a broad spectrum of values with regards to determined petrophysical (e.g.porosities from 15 to 45%, pore throat diameters from 25nm to 1080nm) and geomechanical properties (e.g.: strengths from 3 to 50 MPa). The samples have also been characterized in terms of microtextures by integrating both geological and sedimentpetrological data. Tight chalks encompass different lithotypes, but the main factors controlling the microtexture are: (1) the non-carbonate content, either related to sedimentological settings (e.g. argillaceous chalks) or burial diagenesis (e.g. marl-seam chalks); (2) the degree of cementation, either eogenetic (e.g. hardgrounds) or mesogenetic. Those parameters strongly modify chalk microtexture and thus its porous network, reducing pore-throat and pore body sizes hence altering poroperm properties. Beyond petrophysical properties, cementation appears to be the main parameter controlling the strength of chalks.