INGEOSUR   20376
INSTITUTO GEOLOGICO DEL SUR
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Kinematic evolution of the Malargüe fold-and-thrust belt at the Río Diamante (34º40 S.L.), Southern Andes of Argentina.
Autor/es:
TURIENZO MARTÍN Y DIMIERI LUIS
Lugar:
Colorado Springs, USA
Reunión:
Taller; Integrated Solid Earth Sciences Summer School; 2008
Institución organizadora:
Integrated Solid Earth Sciences Group, Collorado College
Resumen:
The Malargüe fold-and-thrust belt is a thick-skinned belt in
central-western Argentina
developed during the Andean orogeny, related to the subduction between the
oceanic Nazca and continental South American plates. The basement is mainly composed
of pre-jurassic igneous rocks that are covered by Mesozoic marine and
continental sediments of the Neuquén basin. Neogene volcanic activity and
sedimentation contemporaneously with the mountain building allow us to infer
the relative ages of major structures. Based on detailed field mapping, seismic
and well information, three balanced structural cross-sections were made
resulting an average shortening of 14.2 km (24%) for basement and cover rocks (Turienzo,
2008). Dating of pre and post tectonic volcanic and subvolcanic rocks (Baldauf,
1992; Giambiagi et al., 2005) allow us to postulate that the structures were
significantly active from around 14.8 Ma to 2 Ma, which suggests a rate of
shortening equal to 1.1 mm/year. Regionally in the studied area, two basement
uplifts occur surrounding a central region where thin-skinned deformation
prevails. At the west of this central region, three large basement wedges are observed
related to thrust faults developed during Andean compression which propagate
into the sedimentary cover along favourable horizons (commonly gypsum). These
wedges transfer shortening to the cover rocks producing the thin-skinned
deformation, which suggest a close spatial and temporal relationship between
basement and cover deformation. In the thin-skinned region, the abundance of
shales and salt horizons at the western sector facilitate the formation of
tight folding while the more competent units placed at the eastern zone are
deformed into duplex and imbricate thrusts. A more detailed analysis between
the structures and the tertiary igneous rocks show that the deformation in the
west sector was completed at 10.8 Ma, the oldest age of horizontal basaltic
lavas (Giambiagi et al., 2005) covering the western structures. In the central
thin-skinned region, the fact that the Sosneado thrust does not cut the Laguna
Amarga intrusive (10.5 Ma, Baldauf et al., 1992) supports the hypothesis that
cover deformation is related to the western basement structures and both were
completely developed at that time. It represents an important middle Miocene compressive
event (~2.5 mm/year) since 10
km of strata were shortened in approximately 4 Ma. After
a high structural relief was formed faulting progressed toward the foreland in
agreement with the critical wedge theory. In the eastern region, lack of
detachment levels in the cover rocks made difficult the development of basement
wedges and instead important backthrusting occurred. Several subvolcanic
andesitic bodies (10.5 to 5.5 Ma) are placed over the backthrusting zone and
thus their intrusion could be controlled by these antithetic faults (Turienzo,
2008). Further compression makes the main fault, named the Carrizalito fault, to
propagate cutting toward the surface overthrusting pre-jurassic rocks onto
tertiary synorogenic sediments. This deformation took place in the north and
central areas before basalts overflow it (2 Ma) while in the southern area basement
structures remain in the subsurface. Supercritical wedge geometry in the southern
area as observed in the cross-section could be responsible of the actual
seismicity registered there. A shortening of 4.2 km between 10.8 and 2 Ma
(rate of 0.4 mm/year) suggests that Andean contraction becomes slow during
upper Miocene-Pliocene times.