Flexural unfolding of complex geometries in fold and thrust belts using paleomagnetic vectors

Ramón Ortiga, María José
Pueyo Morer, Emilio Luis (dir.) ; Pocoví Juan, Andrés (dir.) ; Briz Velasco, José Luis (dir.)

Universidad de Zaragoza, 2013

Resumen: Three-dimensional reconstructions of the subsurface are an important field in Earth Sciences due to their considerable socio-economic implications as exploration of petroleum resources. 3D reconstruction aims at providing a plausible image of the underground which entail the integration of discrete and heterogeneous datasets. They are based on geometric/mechanic laws and are designed to tackle areas with scarce and heterogeneous data. Restoration algorithms are an important tool to validate these 3D geological reconstructions of the subsurface. Restoration is the way back from the deformed to the undeformed states. Undo the deformation and achieve an initial surface with geological meaning is useful to validate the reconstruction of the folded structure and the deformation processes assumed. The main postulate in most restoration methods is the horizontality of the initial layers while restoration algorithms are based in several deformation processes as flexural slip or simple shear. We deepen in restoration techniques in next chapter but we want to emphasize the importance of a continuous feedback between reconstruction and restoration. This become especially important when complex deformation processes are implied and limited data is available. In addition, restoration tools may also be useful to predict deformation patterns for well characterized structures. However, existing restoration methods do not always succeed for complex structures like non-cylindrical, non-coaxial and/or areas undergoing vertical-axis rotations (out-of-plane motions). We suggest using paleomagnetic information, known in the undeformed (horizontal) and deformed states, as an additional and powerful constraint to improve restoration methods and to reduce the uncertainty of the results. The use of paleomagnetism in restoration tools was recommended in the early 1990¿s. So far, however, relatively few researchers have tried using paleomagnetic information to double-check the rotation inferred from restoration methods and hardly ever paleomagnetism is used as primary information of these tools. In this PhD we want to show how paleomagnetism can reduce the uncertainty in restoration tools when it is used as a constraint, particularly for structures with out-of-plane motions. The bedding plane is the basic 2D reference to relate the undeformed and deformed states, but never could be a real 3D indicator. Our proposal is the usage of paleomagnetism together with the bedding plane as references known in both states. The bedding plane determines the horizontal rotation and paleomagnetism the vertical axis rotation. Paleomagnetic vectors are the record of the ancient magnetic field at the time of the rock formation and we assume that it behaves as a passive marker during the deformation process. Its original orientation can be known in the undeformed surface, and it is represented by the paleomagnetic reference vector. If we see the deformation mechanisms, paleomagnetism allow reducing the number of variables, since it is a passive marker that records the internal deformation and provides us with information of vertical axis rotation. Because accurate paleomagnetic data is necessary to improve results we also work on a good data acquisition. Paleomagnetism may be incorporated in many restoration tools, particularly; we centre our study in geometrical surface unfolding algorithms valid for globally developable surfaces. Developable surfaces are those with Gaussian curvature equal to zero everywhere. These surfaces in geology are stratigraphic horizons folded under flexural conditions that have minimum internal deformation. That implies surfaces isometrically folded with preservation of lengths and angles and consequently with preservation of area. By globally we mean that these constraints are valid almost everywhere but there are areas where internal deformation is possible. We can find this kind of structures in the fold and thrust belts of competent layers at crustal levels. In order to test the restoration methods we develop analog models of complex structures. Laboratory-scale models are based on non-coaxial structures of External Sierras (Pyrenees). These analogs are digitalized with photogrametry and X-Ray CT scanner. In this way, models are completely characterized before and after deformation. This allows the calculus of real deformation of the folded surface and the comparison of the restored surface with the initial one.

Pal. clave: paleomagnetismo ; geología estructural ; reconstrucción tridimensional

Área de conocimiento: Geodinámica interna

Departamento: Ciencias de la Tierra

Nota: Presentado: 12 07 2013

Nota: Tesis-Univ. Zaragoza, Ciencias de la Tierra, 2013