Analogue models of basins affected by differential deformation in extensional and compressional regime s Jara, Pamela 1,2; Likerman, Jeremías 3,4; Cristallini, Ernesto 3,4, Ghiglione, Matías 3,5, Pinto, Luisa 1, Reynaldo Charrier 1 and Jara, Carlos 1. 1 Departamento de Geología, FCFM, Universidad de Chile, Plaza Ercilla 803, Casilla 13518, Correo 21, Santiago, Chile. 2 Departamento de Ingeniería en Minas, Universidad de Santiago de Chile (USACH). Av. Libertador Bernardo O`Higgins 3363. Santiago, Chile

3 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Avda. Rivadavia 1917, CP C1033AAJ, Ciudad de Buenos Aires, Argentina. 4 Laboratorio de Modelado Geológico (LaMoGe), Instituto de Estudios Andinos Don Pablo Groeber. Universidad de Buenos Aires, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina. 5 Laboratorio de Tectónica Andina, Instituto de Estudios Andinos Don Pablo Groeber, Universidad de Buenos Aires. Departamento de Ciencias Geológicas, Buenos Aires. Argentina. *Contact email: pajara@ing.uchile.cl Abstract. We performed a series of analogous experiments with different geometric configuration that permit us to simulate differential deformation applied in an inverted basin. These models, allow us to discern between the resulting geometries according to the stress and the degree of deformation analyzed. These results will be useful to recognize the processes that may occur during the evolution of natural cases of inverted basins to generate the resulting geometry, both in plan and profile. Keywords: analogous experiments, inverted basin,

differential deformation. 1 Introduction As is well known, the developments of rift basins is associated with an extensional regime and are prone to reactivate previous anisotropies or lithospheric weakness. The evolution along the entire basin may be heterogeneous and segmentation producing sub-basins is common. These sub-basins can display sedimentary and structural differences. Furthermore, the subsequent inversion of these extensional systems can produce a reactivation of preexisting normal faults (Cooper and Williams, 1989) and different trending contractional structures could coexist. Previous analogue models have been performed to study the tectonic inversion and compare the resulted geometries depending on the inherited characteristics of the extensional system (McClay, 1995; Amilibia et al., 2005; Yagupsky et al., 2008). In this contribution, we performed a series of models using combinations of differential or homogeneous extension within a region and its subsequent inversion, which could be affected by homogeneous or differential compression. Five analogue experiments were carried out to reproduce the simplest scenarios to discriminate the resulting geometries, both in cross section and in plan view, and then link them to the different processes to which the deposits were subjected.

2 Experimental setting The experiments were carried out in the LaMoGe (Laboratorio de Modelado Geológico), Universidad de Buenos Aires, Argentina. All of them were built-up in a 70cm x 50cm x 3cm deformation rig. Layers of well sorted dry quartz sand with well rounded grains smaller than 500 µm were used, stained in order to generate different colors for the subsequent observation of the layers in profile. A thin basal layer of silicone, selected to simulate stretching at the base of the brittle upper crust, was prepared according to the necessary shape and disposition for each experiment. The movement, during both the extensional and compressional phases, was done by a stepper motor, anchored to a mobile backstop wall properly prepared to be used with homogeneous or differential movements (Fig. 1); this wall was attached to a basal plate that only moves during extensional experiences, to generate extensional movement below the silicon layer. The resulting differential deformation was progressive and performed by pivot rotation of the backstop wall. This axis support that generates the pivot rotation, can be released to generate homogeneous movement, which allowed the combinations necessary to compare the results according to the processes generated: i) extensional differential deformation, ii) compressional differential deformation, iii) differential extension and homogeneous compression, iv) homogeneous extension and differential compression, and finally, v) extension and subsequently compression, both differentials.

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Figure 1. Experimental setting: BW: backstop wall, MBP: mobile basal plate, M: motor. Numbers represents the possible movements applied: 1) Differential compressional movement generated from the pivot, 2) Differential extensional movement generated from the pivot, 3) homogeneous compression, 4) homogeneous extension. 3 Results The first experiment corresponds to an i) extensional differential deformation (Fig. 2). In this case, the shape of the basal silicone fit the moving plate trace (triangular). To compare and define the control of the silicone borders on the resulting geometries, we performed the same experience with a square basal silicone shape. During extensional deformation, the first stage, in all cases, showed the formation of two grabens, separated by a horst (Fig. 2A). These grabens begin synchronously with the formation of the outermost faults of the basin, and its orientation was parallel to the moving backstop wall. As the extensional displacement progressed, the basin deepened and new faults defined a series of grabens within the larger one (Fig. 2B and C). Most of the faults do not change their original strike (parallel to the maximum normal stress at the time of its formation), even though the backstop wall is changing its orientation. Because of this, most of the structures show, in plan view, an oblique orientation to the main external faults in the basin (which rotated to generate the final width of the basin). The same structural configuration was produced using either square or triangular shape silicone, which gives the idea that silicone shape does not control the final stage of the larger basin configuration, despite the amount of extension applied. In the differential compression experiment (ii), folds were generated from the innermost area (near the backstop wall) to the outermost area as the deformation progressed. Because of the differential compression applied, compressive structures were concentrated in the area of maximum shortening. Folding and thrusting was generated above and outside the silicone basal layer.

Figure 2. A) Plan view of early stage of differential extension. B) Final stage of differential extension and C) profiles pictures for different extension rates at final stage B). The other three experiments correspond to the simulation of an extensional regime with a superimposed compressive deformation: iii) differential extension and homogeneous compression, iv) homogeneous extension and differential compression, and, v) extension and subsequently compression, both differentials. The remarkable difference in all three cases is the orientation of structures formed during the extensional regime and the control that this previous faulting exerted during compressive deformation. In all cases, some normal faults were preserved, new compressive structures were generated, and many grabens were preserved within large amplitude folds. In the experiment with differential extension and homogeneous compression (iii), first thrusts and folds were generated in the innermost area near the backstop wall; as the shortening increase, a second fold was produced in the external sector. In the final stage (Fig. 3A), the two principal folds are converging (approaching each other), preserving some of the original geometry of the basin (Fig. 3). The cross sections show that the external fold and the deformation in the extended area (highlight with a red arrow in Fig. 3A), is controlled by inherited structures, located in the basal silicone region.

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Figure 3. Plan view of final stages of: A) differential extension and homogeneous compression, B) homogeneous extension and differential compression, and finally, C) extension and subsequently compression, both differentials. During iv) homogeneous extension and differential compression, unlike the previous case, the basin is closed like a pair of scissors through the two main folds (Fig. 3B), with axis in the zone of higher shortening rate. External structures are parallel to the basin and internal thrust faults are gradually turning to a more diagonal direction, parallel

to the shortening direction vector. Finally, with v) differential extension and compression, the two principal folds formed, closed basin like a scissors, but unlike the previous case (experiment iv), with an axis located in the lower displacement zone (Fig. 3C). As deformation proceeds, the two main folds converge, and new structures, like backthrusts, are generated progressively. 3 Discussion This work constitutes a first approach to explain the differences in the degree of deformation analyzed by differential deformation, and the associated different structural characteristics observed in an inverted basin. The series of analogous experiments performed, allow us to discern between the resulting geometries according to the stress and geometric configuration of each case. These results will be useful to recognize the processes that may occur during the evolution of natural cases of inverted basins to generate the resulting geometry, both in plan and profile. Acknowledgements Funding for this project was provided by “Ayuda para estadìas cortas de investigación”. Vicerrectoria de asuntos académicos. Departamento de postgrado y postitulo de la Universidad de Chile. We acknowledge funding by PICT Agencia de Promoción Científica y PIP Consejo nacional de Investigaciones Cientificas (CONICET), Laboratorio de Modelaminento Analogico del Departamento de Geología de la Universidad de Chile and LaMoGe (UBA). We also thank the valuable ideas of T. Nalpas (Université de Rennes) to generate the experimental device. References Amilibia, A., McClay, K.R, Sàbat, F., Muñoz, J.A., Roca, E., 2005.

Geologica Acta: an international earth science journal. Universidad de Barcelona. España.

Cooper, M.A, and Williams , G.D., 1989. Inversion Tectonics. Geological Society, London. Special Publication Nª44.

McClay, K.R., 1995. The geometries and kinematics of inverted fault systems: A review of analogue model studies. From Buchanan, J. G. and Buchanan, P.G. (eds), 1995. Basin Inversion, Geological Society Special Publication Nª88.

Yagupsky, D., Cristallini, E., Fantín, J., Zamora, G., Bottesi, G., Varadè, R., 2008. Oblique half-graben inversion of the Mesozoic Neuquén Rift in the Malargüe Fold and Thrust Belt, Mendoza, Argentina: New insights from analogue models. Journal of Structural Geology (30) 839-853.

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