A direct method for modelling and unfolding developable surfaces and its application to the Ventura Basin (California).

n order to draw precise geological structures using scattered data, it is relevant to use criteria that integrate the kinematic properties of the structure. For example, developable folds can be selected by the observation of constant-length deformation markers. By establishing an accurate drawing of these folds, it is possible to precisely determine the geometrical limits of basin. We have developed a program, DEVELOPABLE-MESH, that integrates these developability criteria: this program first builds a 3D mesh using criteria that are based on the geometrical properties of developable surfaces; it then ‘slightly' modifies the mesh to improve its developability; this mesh is finally unfolded. This program has been tested on a natural example of the lateral evolution of a thrust system to a fold structure (Red Mountain area in the Ventura basin). This particular structure has been already balanced by a trial and error method (UNFOLD program). This test confirms the validity of our approach: the DEVELOPABLE-MESH program builds near perfect developable folded surfaces. Therefore, it is geometrically coherent and can unfold these surfaces in a very short time. Moreover, the program is faster and can construct more complex developable surfaces than other conventional unfolding techniques based on a common interpolator