A COMPARISON OF BOX FOLD SHAPE AND DEVELOPMENT: BUCKLING VS. DETACHMENT FOLDING

Fold shape can be defined by a number of classic geometric methods, but these methods do not typically allow the worker to quantify the deformation history of the structure. An alternate classification mechanism, which has a more quantitative approach, is by the use of Bezier curves, describing shapes such as chevron, sinusoidal, parabolic, and double-hinged folds. Previous numerical modeling has shown that box folds, that is, folds which are double-hinged, develop from a variety of initial perturbation geometries and material properties in the center of a true multilayer stack. The modeling further showed that in this scenario, box folds formed in a progression from sinusoidal folds to parabolic folds to box folds for 20-50% of total shortening. However, analog models indicate that box folds can also form above ductile detachments, at as little as 5% bulk shortening, forming geometries very similar to those observed in natural situations such as the Jura (Switzerland). This study consists of a series of analog models shortened to between 5-15% total shortening. Each analog model is made up of 5mm silicon polymer, to simulate a ductile salt detachment, overlain by 15mm of finely layered colored sand to simulate brittle cover rocks. Sidewall photographs are used, despite the risk of distortion, and measurements of aspect ratio, P (where P = 2*amplitude/wavelength) and dip angle at the inflection point are made. The relationship between these two parameters is used to track the development of different fold shapes and to confirm the presence of box folds. For comparison, the same measurements are made across a series of cross-sections of each finished model. Preliminary results indicate that box folds forming due to detachment folding do not follow the same shape progression as box folds in multilayers, instead initiating as parabolic structures and rapidly developing into box folds, then becoming asymmetric and developing thrust faults in one limb. These data indicate that field observations of box folds have to be carefully analyzed as the different deformation mechanisms, i.e., buckling vs. detachment folding, result in different box fold shapes at different times throughout the deformation history.