Influence of structural position on fracture networks in the Torridon Group, Achnashellach fold and thrust belt, NW Scotland

Abstract

a b s t r a c tIn fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.In high curvature regions, such as forelimbs, fracture intensities are high and fractures are short and oriented parallel to fold hinges. In low curvature regions fractures have variable intensities and are longer. Fracture orientations in these regions are scattered and vary over short distances. These variations do not relate to strain; data suggests lithology may influence fracturing. The strain history of fold structures also influences fracturing; structures with longer deformation histories exhibit consistent fracture attributes due to moderate-high strain during folding, despite present day low curvature. This is in contrast to younger folds with similar curvatures but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural controls, whereas in low strain regions lithology becomes more important in influencing fracturing.

Distribution of joints in the hinge-line culmination of foreland-verging overturned anticlines: an example from the Montagna dei Fiori structure in the Central Apennines of Italy

“…This is coherent with the conceptual fold-related fracture models proposed by Price (1966), Stearns (1969) and more recently by Watkins et al (2018). Moreover, according to Watkins et al (2015), the longitudinal set, parallel to the fold hinge, may be associated with outer-arc stretching during folding, whereas joint sets cross-striking the fold hinge can be related to the hinge-line parallel extension developed in the culmination zone of doubleplunging anticlines. Transverse joints/veins are usually associated with an along-axis elongation recognized in fold-and-thrust belts by Price & Cosgrove (1990).…”

“…Price, 1966;Stearns, 1969;Ramsay & Huber, 1987;Pollard & Aydin, 1988; Barchi et al 1993;Cosgrove, 2015;Watkins et al 2015Watkins et al , 2018Francioni et al 2018a). Price (1966), Stearns (1969) and Watkins et al (2015Watkins et al ( , 2018 developed conceptual fold-related fracture models showing that fold geometry and kinematics are the most important factors that control the possible fracture sets expected in an anticline fold structure. These authors proposed fold-related fracture models including longitudinal and transverse joint sets striking longitudinally and transversely to the fold hinge, respectively, and two conjugate shear fracture sets developed obliquely with their obtuse angle intersecting the fold-axial trend.…”

Inter-well scale natural fracture geometry and permeability variations in low-deformation carbonate rocks

“…Conventional workflows for modelling permeability in fractured reservoir models can be based on extracting geometrical distributions from outcrops for stochastic Discrete Fracture Networks (DFNs) and upscaling to effective properties, where seismic-scale variability in fracture patterns is constrained by considering the resultant structural geometries and implications of kinematic and mechanical forward models (Sanders et al, 2004;Shackleton et al, 2009;Bond et al, 2013;Watkins et al, 2015;Ukar et al, 2016).…”

Data acquisition by digitizing 2-D fracture networks and topographic lineaments in geographic information systems: further development and applications

“…1-D approaches are relatively rapid but do not directly constrain parameters such as fracture length and connectivity. If the fracture network is anisotropic, which is commonly the case, the characterization is biased by the orientation of the scanline or the borehole (orientation bias; Singhal and Gupta, 2010;Zeeb et al, 2013;Watkins et al, 2015b).…”