Boletín de Ciencias de la Tierra

The Irra Basin is a small pull-apart basin at the south end of North Cauca basin located between the Cauca and Romeral fault system, near their northern convergence in the Colombian Andes. The Irra Basin developed in the Late Tertiary time and records slip reversals on the Romeral fault zone. The basin is believed to have developed above a right step-over on the Romeral system during a rightlateral slip episode. It was subsequently deformed during a younger episode of left-lateral slip inducing the closure of the basin during. To analyze sediment dispersal fl ow patterns of into the Irra Basin, we used anisotropy of magnetic susceptibility as a proxy for sediment fabric. It was found that the magnetic ‘fabric’ impressed in these young terrestrial clastics is strongly correlated with the deformation (i.e. the structure), as well as with the sedimentary fabric. Therefore, the paleomagnetic remanence may record the identifi ed rotational movements. In general, the sediments preserved a widespread overprint of low coercivity, with both normal and reversed polarities, superimposed on a high coercivity component also with both normal and reversed polarities. The tilt-test shows that the low coercivity component correspond to a post-folding magnetization. The declinations of the characteristic direction and of its secondary overprint are consistent with approximately 30º and 20º net clockwise rotations, respectively, although clearly rotations of opposite senses have occurred here according to the structural evidence. The younger overprint is consistent with a late episode of right-lateral slip. Present-day microseismic activity on the Romeral Fault Zone supports left-lateral slip. Collectively, the structural, magnetic and microseismicity data are consistent with four regimes of opposed lateral slip on the Romeral fault zone in this region during the Late Tertiary. Alternations of lateral slip are indicated, with right-lateral slip domination due to long-term northeastward convergence of the Nazca plate towards South America plate.