3D GEOLOGICAL MODELLING OF THE VARISCAN / ALPINE FOLDS IN ALTO TAJO NATURAL PARK, IBERIAN RANGE (SPAIN)

An analysis of 90 Variscan and Alpine fold-shapes using orthogonal thickness (Ramsay 1967) and the s₁-s₂ diagram (Bastida et al. 2004) have been carried out to characterize fold geometry. Although the shapes of the Variscan and Alpine folds are quite similar, there is a subtle difference between the two geometries which is best seen using the s₁-s₂ diagram. Most of the Variscan and Alpine folds lie in the field 1C, but the latter are more widespread, lying in the 1A-1C, 1C-1A, 3-1C and 1A-3 fields. There are also some folds belonging to class 2.

Fitting fold profiles by conic functions has been extensively used to describe fold profile geometry (Aller et al. 2004). The h - α diagram relates the aspect ratio (h) to the normalized area (α). The use of the FOLD PROFILER program, a MATLAB^® code (Lisle et al. 2006), to analyze the Variscan and Alpine folds from the Alto Tajo Park allows us to obtain the h-α diagrams. Most of the shapes, both of Variscan and Alpine fold profiles, tend to sinusoidal and parabolic folds with some tendency towards chevron folds. Elliptical shapes are more common in the Variscan folds.

Fold geometry has important implications in folding mechanisms. Buckling and bending are kinematic mechanisms of this folded sequence, but in the Alpine folds buckling is the predominant mechanism. Another mechanism involved, which is characteristic of folds belonging to class 1C, is flattening of previous buckle folds, and occurs in the late stages of folding.

In this multilayer folded sequence made up of competent layers (limestone beds) separated by weakness zones (more incompetent layers), the strain is concentrated in the fold hinge zones. This is characteristic of folds with a neutral surface, and the suggested kinematic mechanism is tangential longitudinal strain which was operative in individual competent layers such as the limestone beds.

Finally, based on the geometric characteristics of this folded sequence, we have used GeoModeller^© (www.geomodeller.com) to generate two 3D geological models for the Variscan and for the Alpine folds. These models are built in a georeferenced system in a GIS, and use a DEM for the selected areas, the geological maps and the available field exposures. This has led to a three-dimensional representation which is a realistic approximation to the geometry of the folded surfaces in both cases.