3-D symbolization of L–S fabrics as an aid to the analysis of geological structures

Abstract

The digital capture and management of primary field observations has expanded the potential to represent site-specific geological information in a more intuitive manner, ultimately optimizing the interpretation of map-related geological field observations. In structural mapping, for example, the geometry and kinematics of large-scale structures needs to be reconstructed from local measurements and observations of linear and planar fabric elements. The traditional methods for fabric representations are inherently restrictive because they do not allow simultaneous representation of the orientation and nature of the fabric with respect to the structural level of observation. To overcome these limitations we represent planar (S) fabric elements as the flat portion of a 3-D ellipsoid or ‘surfboard’ symbol according to their strike/dip attributes. The associated linear (L) fabric elements are represented by the elongation direction of the ‘surfboard’, thus reflecting the pitch of the linear element in the fabric plane. The resulting composite linear–planar fabric (L–S) representation is positioned in 3-D space according to its location in ( x , y , z ) space. By adjusting the principal axis of the ‘surfboard’, variations in the shape of the elements can be used as an additional symbolization of the strain-state of the fabric, ranging from pure L to L>S, S>L to pure S fabrics. Various colour mappings of the ‘surfboard’ can be used to indicate the degree of co-planarity between lineations and planar fabric orientations. This 3-D visualization of L–S fabrics aids in first-order interpretation of regional scale ductile structures, such as thrusts and low-angle detachment faults. An application is presented from the Mulhacen metamorphic complex of the Internal Zone of the Betic Cordillera, SE Spain, demonstrating the 3-D fabric visualization method. This method facilitates recognition of variations in lineation trends with respect to the structural level within a ductile nappe structure and subsequent overprinting deformation events. The approach is considered relevant to all other applications where local field-based measurements and descriptions are fundamental to the reconstruction of macro-scale geologic processes.