Geological features of neotectonic deformation in Argentine pampas (intraplate)

BRUNETTO, E.

Oslo

Congreso; In the symposium: STP-01 General contributions to palaeoseismology. 33rd International Geological Congress; 2008

International Union of Geological Sciences (IUGS)

The Pampa region is a great plain characterized by very low slopes (below 1:100) and formed by hundreds of meters thick loose sediments. It is located in an intraplate setting characterized by low tectonic activity. However, a definite neotectonic activity could be demonstrated by applying geomorphological, geological and geophysical methods adapted to this particular vertical and surficial scales. The first results are presented here. That markedly smooth landscape is very sensitive to deformation by reactivation of Cretaceous extensional faults. The pattern of the present subtle faults activity reproduces structures of hemigrabens and accommodation zones originated during the opening of South Atlantic Ocean. Geological proxies lay down the basis for subsequent paleoseismological studies in order to obtain more accurate measurements of movement rate and recurrence period de coseismic displacement. On the other hand, erosion surfaces, paleosols levels in catenary series, paludal and swamp deposits are the key in assessment of vertical deformation during the Quaternary. In North Pampa, a main morphostructure has been defined and named San Guillermo block. Its boundaries have been interpreted as originally normal faults by means seismic data, which underwent recurrent activity during Upper Pleistocene-Holocene. Flooded and pounded environments recorded in geological units, point out sites where topographic inversion has occurred. Vertical displacements are resolved in surface as small and long folds showing asymmetrical profiles. At the eastern side of the block, episodic activity was deduced from multiple knick points (generated by movements of faults) in the longitudinal profiles of small streams, which have a general smooth slope. Such knick points are preserved in loose sediments owing to the very low slopes of the plain: erosion-sedimentation rates are very low. At least three episodes of fault reactivation were discriminated by using intersection of level contours and streams of different hierarchy. The most recent one have provoked visible changes in surficial drainages. The result is a palimpsestic model in which an old parallel streams system oriented toward ENE was followed bya younger one with NE and SE dominant directions. Recent depressions marked by shallow water bodies with rectangular limits are the best insight of changes in the drainage pattern. Whole geomorphological analyses were made in a loess deposit (Tezanos Pinto Formation, Late Pleistocene in age).