CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT. The Fuegian thrust-fold belt (TFB) forms the thin-skinned outer wedge of 26 the Andes in Tierra del Fuego. Using subsurface and outcrop data from two areas 27 (Western and Eastern) of the TFB front in Argentina we aimed to verify and characterize 28 the apparent structural variations along its strike. Both areas reveal pro- and retro29 vergent fault-related folds detached at similar horizons, with a youngest early to 30 middle(?) Miocene deformation age. However, the Western Area has gentle, large31 wavelength folds whereas the Eastern Area is characterized by a very tight structural 32 geometry, with closer fold geometries. This difference manifests itself in the shortening 33 of analogous structures: below 5.5% in the west as against ~22% in the Eastern Area. 34 Our findings verify structural style variations along the strike and suggest that the 35 Eastern Area endured higher strain. We evaluate two possible causes of this strain 36 gradient, assuming homogeneous regional shortening: (i) lateral rheological variations at 37 the base of the thrust wedge, namely the occurrence of more competent beds which 38 would have restrained the propagation of the detachment toward the east; and (ii) the 39 effect of strong buttressing in the eastern TFB exerted by the Río Chico arch basement 40 promontory during deformation. Published results, together with our current subsurface 41 and outcrop data, rule out significant rheological gradients in a preferred direction along 42 the TFB. On the other hand, we present evidence of the nucleation of frontal thrusts 43 above basement steps at the Río Chico arch western margin, which comprise local 44 buttresses. We speculate that this buttressing was mantained along the TFB front and is 45 enhanced toward the east, where forward TFB propagation was hindered due to the 46 southern projection of the Río Chico promontory. This would explain the higher strain 47 and more complex structural style in the Eastern Area.