THE STRUCTURAL STYLE OF FOOTWALL SHORTCUTS ALONG THE EASTERN FOOTHILLS OF THE COLOMBIAN EASTERN CORDILLERA. DIFFERENCES WITH OTHER INVERSION RELATED STRUCTURES

Abstract

For the first time we show geological evidence of unambiguosly documented footwall shortcuts adjacent to the trace of inverted master nomal faults, in the Eastern Cordillera of Colombia. The Eastern Cordillera is an orogen whose width and location are traced by a Mesozoic graben. However, few structures related with the graben have been documented up to the date. In this study we propose the Ariari-Guatiquía region as a type location for a unique observation of footwall shortcuts. The master normal faults in the Ariari-Guatiquia region, and documented in this manuscript, were active during the Lower Cretaceous, partially inverted during the Andean orogenesis (since the Oligocene at least) and active still nowadays. In the hangingwall basins of those master normal faults, like the Servitá fault, all the Cretaceous syn-rift sequence has been deposited and maximum paleo temperatures in the lowermost Cretaceous rocks are higher than those for the Zircon FT partial annealing zone (~250°C; 23,15 K). In contraction, the inverted master normal faults are high angle basement involved features that generated the main topographic contrast and exposing Lower Cretaceous units or older. In contrast, in the adjacent footwall shortcuts only part of the syn-rift Lower Cretaceous sequence was deposited or more commonly was not deposited at all. Maximum paletemperatures reached by the basal Cretaceous units exposed in the hanging wall blocks of the footwall shortcuts are always less than those of the Zircon FT partial annealing zone (~250°C; 23,15 K). Finally we use AFT data to document that the footwall shortcuts originated during the Late Miocene and later as shallowly dipping faults generating low elevation hanging wall areas. All the described features are present in the Ariari-Guatiquia region. However, northwards and along strike in the Eastern foothills there is a lot of partially analogue scenarios with respect to those described in the Ariari-Guatiquia region. Therefore we deduce that a similar structural segmentation should be present along the entire Eastern foothills of the Colombian Eastern Cordillera. Based on that we propose plausible candidates for master inverted normal faults and footwall shorcuts in other areas of the Eastern foothills.