Similar Fold

Key Takeaways

• The axial-surface-parallel thickness constancy of similar folds distinguishes them geometrically from parallel folds and has direct implications for how the fold depth and closure can be extrapolated downward from seismic data or surface measurements: in a parallel fold, the constant layer thickness perpendicular to bedding means that the fold amplitude measured on a shallower layer is a reliable predictor of the amplitude at deeper levels (the fold amplitude tends to decrease predictably with depth as the decreasing radius of curvature becomes geometrically unsustainable), but in a similar fold, the layer maintains the same wave shape at all depths (the amplitude does not decrease with depth as predicted by parallel fold geometry) because the fold's similar shape is maintained by internal shearing rather than by mechanical continuity of the layer; from an exploration perspective, a similar fold in a deep reservoir may have greater amplitude (more structural closure) than the equivalent parallel fold geometry would predict from the shallow seismic expression, but the structural interpretation must account for the similar fold's distinctive geometry to correctly estimate the depth and area of the closure.
• Fold mechanisms producing similar geometry include simple shear (layer-parallel shear strain in which successive layers slide over each other by different amounts depending on their position in the fold, distorting the layer shape into the similar geometry) and pure shear (equal biaxial compression and extension in the fold plane that thins the limbs and thickens the hinges at the same rate), both of which are associated with ductile deformation at elevated temperatures and pressures typical of metamorphic and deeply buried sedimentary sequences: simple shear folding (also called passive folding) does not require mechanical contrast between layers (all layers deform together as a homogeneous ductile continuum), while flexural slip folding (which produces parallel folds) requires weak layer boundaries along which slip can occur; the transition from parallel (flexural slip) to similar (passive shear) fold geometry with increasing depth and temperature in fold-and-thrust belts reflects the changing deformation mechanism as the rocks pass through the brittle-ductile transition zone at 10 to 15 kilometers depth and temperatures of 200 to 350 degrees Celsius, with structural studies of deeply eroded fold belts (the Appalachians, the Alps, the Himalayan fold belt) showing the transition from parallel anticlines in the upper crust to similar folds with subvertical limbs in deeper exposures.
• Salt diapir-associated folds commonly exhibit similar geometry in the overburden above and flanking salt bodies because the ductile salt deforms and flows plastically, imposing simple shear-dominated deformation on the overburden sediments that produces similar folds rather than the parallel folds expected from flexural uplift over a rigid body: in the flanks of rising salt diapirs, the overburden is dragged upward by the salt and simultaneously compressed laterally, creating fold geometries that are transitional between parallel and similar depending on the brittleness and competence of the overburden layers; the interpretation of similar folds adjacent to salt bodies is complicated by the salt's ability to pierce and intrude the fold limbs, creating complex geometries that combine diapir intrusion, fold deformation, and salt withdrawal that require three-dimensional seismic interpretation to resolve; the similar fold geometry in salt-associated structures can create traps for hydrocarbons in the fold hinge zones and on the flanks of the structure where crestal closure or four-way dip closure of similar-fold anticlines provides hydrocarbon accumulation geometry.
• Seismic interpretation of similar folds requires awareness that amplitude anomalies on reflectors may partly reflect the thickness variations inherent in similar fold geometry rather than fluid-related acoustic impedance contrasts: because similar folds thin the limbs and thicken the hinges (relative to the same layer in parallel fold geometry), the seismic reflection amplitude from a thin-bedded reservoir in a similar fold's limb will be lower than the amplitude from the same reservoir in the hinge zone, creating an amplitude pattern that might be misinterpreted as a hydrocarbon-related DHI (direct hydrocarbon indicator) when it is actually a consequence of the fold geometry's layer thickness variation; the correct interpretation requires integrating the seismic amplitude pattern with the structural interpretation of the fold type (similar versus parallel), using the layer thickness variation predicted by similar fold geometry to separate the geometrically induced amplitude variation from the fluid-related amplitude variation; well control in the hinge zone (where the thicker layer gives the higher amplitude expected for a productive reservoir) and on the limb (where the thinner layer gives the lower amplitude that might be misinterpreted as a fluid effect) provides the tie-point needed to calibrate the amplitude interpretation and correctly identify which amplitude variations reflect hydrocarbon fill versus fold geometry effects.
• Restoration of similar folds in balanced cross-section construction uses area-preservation methods (equal area before and after deformation) rather than the line-length-preservation methods appropriate for parallel folds, because the similar fold's layer thickness variation means that individual bed lengths cannot be preserved during folding (the limb material was transported to the hinge during hinge thickening): the area-preservation constraint for similar folds states that the area of each stratigraphic layer in cross-section (the product of the layer thickness and the length along the layer) must be the same before and after deformation, allowing the pre-deformation geometry to be reconstructed by spreading the fold's material back to the original undeformed state while conserving total area; the balanced cross-section for a similar fold-and-thrust belt system must be internally consistent (no material created or destroyed during restoration) and must replicate the deformation mechanism (simple shear with appropriate shear direction and magnitude) that would produce the observed similar fold geometry from the pre-deformation stratigraphic column.