Overturned Synclinal and Anticlinal Bend Folds: Characteristics, Symbology and Possible Implications for Structural Interpretations

Brian Swanson, California Geological Survey

On-going preparation of geologic maps at the California Geological Survey (CGS) has revealed a class of overturned folds that has not been symbolized in the USGS FGDC cartographic standards. This class of fold is defined as having one upright limb and one overturned limb that dip in opposite directions, where the facing direction of the strata does not change across the fold hinge. The proposed designations for the subject folds are overturned synclinal bends or overturned anticlinal bends. The axial surface of this fold class is geometrically constrained to dip less than 45°, but the interlimb angle may be open or tight. These criteria contrast with the definition of standard overturned folds, where both limbs dip in the same direction, and with recumbent folds, which are defined by near-horizontal axial surfaces (e.g. Fleuty, 1964) rather than the relative orientation of the limbs, and are characteristically tight to isoclinal.

The symbology proposed for this fold shows arrow ticks on either side of the axial trace that point in the direction of bedding dip on the corresponding fold limbs. The symbolism is a slight modification of the symbol informally adopted by Stone et al. for the antiformal version (2009 – USGS SIM 3094). The Australia. Bureau of Mineral Resources, Geology and Geophysics has a similar symbol for recumbent folds. However, the arrow tick dip directions are contrary to the actual dip direction of the fold limbs in map view. The standard for inverted folds adopted by the USGS shows the arrow ticks consistent with the bedding dip on each limb in map view, regardless of facing direction, and this convention is preferred. The proposed symbology has been submitted to the USGS and is under consideration for future adoption.

Overturned fold bends are not a common fold class but may be locally important indicators of certain deformational regimes in moderately to severely deformed rocks. Where first recognized by the author in the southern Inyo Mountains, strata were initially folded along northeast-trending structures of the Last Chance Thrust System and then later overprinted by northwest-trending structures of the East Sierran Thrust System. Overturned fold bends have subsequently been recognized in localized areas of the western Transverse Ranges where inclined beds are cut by reverse faults, or at associated step-overs. Recognition of overturned fold bends and additional studies may reveal that these folds provide a diagnostic indicator of certain styles of progressive or multi-phased deformation.