Morphogenesis of the Ourense plains (NW of Spain)

Abstract

We present a geomorphological analysis of Ourense Province (NW Spain) characterized\nby: a general narrowing of the fluvial network, highlands with smooth reliefs partially\neroded and lowlands with residual reliefs, several extensive plains of erosion frequently\nlimited by fractures -among which Tertiary grabens are inserted-, some ?Hollow\nSurface?-type morphology, absence of sedimentary deposits outside the grabens, and a\ngeneralized outcrop of the Hercynian Massif substratum. Traditionally, this ?piano?s keyboard\nmorphology? has been interpreted as expression of block tectonics in tensile regimen;\ninstead we suggest the existence of: an isostatic upheaval simultaneous to a sequence\nof tectonic pulses of compressive regimen with activity in favour of transcurrent\nfaults, a General Surface (R600), several plains that present a ?Hollow Surface?-type morphology\n(R1600 R1400 R1000), a generalized alteration that correspond to a same process\nof decomposition associated to fluctuating conditions of redox equilibrium, a erosional\nterraces related principaly to the palaeo-fluvial nets; moreover, we propose the existence\nof two morphoestructural lineament: the first one represented by the Fault of Vila\nReal (NE-SW) -a ramification of the ?Basal Pyrenean Overthrust?-, that would have been\nactive at an early moment of the tectonic sequence with a left transcurrent fault,\nsecondly the lineament represented by the Fault of Maceda (NNW-SSE) that would be\nrelated to the ?Fault System NW-SE? and would have produced a right transcurrent fault\nduring a late tectonic pulse.