Assessment of recent tectonic activity on the NW Iberian Atlantic Margin by means of geomorphic indices and field studies of the Lower MiA-o River terraces

Until recently, NW Iberia was assumed to be tectonically quiescent (stable). However, a combination of tectono-morphological analyses demonstrates that neotectonic movements do occur, which are in agreement with recent findings by other workers. We use a DEM-based tectono-geomorphic approach in combination with field work to assess the presence and nature of neotectonics in the Lower MiA-o catchment in NW Iberia. The area is characterized by clearly developed lineaments, coinciding with Paleozoic and Mesozoic faults and fractures and remnants of a peneplain stepping down towards the coast. Longitudinal river profiles show knick points where lithological control can be excluded. Valley shapes show that some valleys have recently been rejuvenated. Systematic asymmetry tendencies of river catchments indicate tilting of fault-bounded blocks. Finally, at several locations faulted terrace deposits of the MiA-o River have been found. The types of motions that can clearly be evidenced by our analyses are normal faulting and tilting of fault-bounded blocks. Little clear evidence exists for strike-slip faulting, whereas reverse faulting has not been found in this study. A striking morphological feature of the study area is the presence of small fault-bounded basins, along the course of the river MiA-o. The tectonic movements take place along inherited, Paleozoic and Mesozoic faults, which apparently act as weakness zones. The reactivation of these faults under the present-day stress-conditions is probably the reason for the complicated pattern and style of present-day tectonic motions that we find. The small basins are particularly difficult to explain. They could result from the interaction of partially, obliquely reactivated strike-slip faults (step-over) and normal faults.