Structure and tectonic evolution of the Andes of Río Negro (41°30

Structure and tectonic evolution of the Andes of Río
Negro (41°30´ - 42°S): An Early Pliocene extensional
reactivation controlling retroarc-volcanism
Jonathan E. Tobal*, Darío Orts, Emiliano Renda, Andrés Folguera, Víctor A. Ramos.
Laboratorio de Tectónica Andina. Instituto de Estudio Andinos Don Pablo Groeber (IDEAN). Universidad de Buenos Aires.
*[email protected]
Abstract. The Andean sector between 41º and 43º S, from
Bariloche to Esquel cities, has been uplifted in two main
contractional episodes: The first occurred in pre-Oligocene
times and a second and more important in the Late
Miocene. These events are now exposed in the fold and
thrust belt as areas of upper crustal deformation from the
Andes to the foothills, fossilized by younger less deformed
strata. In this work, a new Pliocene extensional episode
that affected differentially this sector of the Patagonian
Andes is postulated based on the evidence of normal faults
and synrift packages in the Pliocene strata.
Key words: North Patagonian
Pliocene, extensional tectonics.
Andes,
equivalent to the Mallín Ahogado Formation, is covered by
the Collón-Cura Formation (15-11 Ma). Both units are
exposed between the two volcanic belts and constitute the
infill of the Ñirihuau foreland basin (Giacosa y Heredia,
1999; Ramos et al., 2011; Orts et al., 2012).
orogenesis,
1 Introduction
The North Patagonian Andes (NPA) constitutes one of the
geologically less known morphotectonic domains of
northwestern Patagonia (Figure 1). Main constructional
stages are barely defined because of a weak stratigraphic
framework and scarce structural mapping. A geologic
framework of the NPA and the adjacent foothills area
between 41º and 42ºS can be summarized as constituted by
broad exposures of plutonic to volcaniclastic Mesozoic
rocks, with subordinate Cenozoic units. Two plutonic belts
are exposed at both sides of the El Bolsón valley located at
the main Andean front, namely: the Cordilleran Patagonian
Batholith (CPB; Giacosa y Heredia, 1999) on the NPA and
the Subcordilleran Patagonian Batholith (SPB; Gordon and
Ort, 1993) on the foothills. An Early to Middle Jurassic
intra-arc volcaniclastic sequence is distributed in a
continuous belt on the NPA and the eastern side of the El
Bolsón valley. The Early Miocene Mallín Ahogado
Formation (Diez y Zubia, 1981; Encinas et al., 2011) is
well represented along this depression and constitutes the
westernmost remnants of the Ñirihuau basin, associated
with a foreland foredeep (Giacosa y Heredia, 1999; Orts et
al., 2012). Further east, two Paleogene volcanicsubvolcanic belts are exposed: an inner El Maitén belt
(Western Oligocene belt, WOB) and an outer Pilcaniyeu
belt (Eastern Paleocene-Eocene belt, EPEB) (Rapela et al.,
1988). The Early Miocene Ñirihuau Formation, partly
Figure 1. Tectonic setting of the North Patagonian Andes and
main morphostructural units. The black box indicates the study
area and the yellow box shows the location of the figure 2. Red
triangles indicate the volcanoes belonging to the Present arc front
and continuous lines indicate the depth of the subduction zone.
2 The Esquel fold and thrust belt
2.1 Main regional structures
A key feature of the Patagonian Andes between 41º and
43º S is the fold and thrust belt that develops throughout
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3 Improving the tectonic evolution model for
the North Patagonian Andes
the NPA and the adjacent foothills (Ramos and Cortés,
1984; Giacosa y Heredia, 1999; 2000; 2004). This
deformational N- to NNW-trending belt has a bend
approximately at the latitude of Lago Mascardi; its width
varies from 100 km in the south (42ºS) to 55 km north of
this inflexion (41ºS), where the different structures become
tighter and the belt gets narrower (Giacosa y Heredia,
2004). The inner/western sector of the fold and thrust belt,
bounded to the east by the east-verging Ventana-Catedral
fault (Giacosa y Heredia, 2000), is characterized by the
predominance of thrusting (Ramos y Cortés, 1984)
affecting mainly Paleozoic and Mesozoic rocks (Giacosa y
Heredia, 2004). The Ventana-Catedral thrust constitutes
the tectonic contact between the pre-Tertiary basement and
Oligocene volcanic rocks of the Ventana Formation
(Giacosa y Heredia, 2004). The El Bolsón valley, another
key feature of this sector, was interpreted as the superficial
expression of a triangular zone developed between two
thrust systems of opposite vergence: the western BolsónTronador thrust and the eastern Piltriquitrón and Serrucho
back-thrust system; this last structural system may join the
Ventana-Catedral at depth (Giacosa y Heredia, 2000;
2004). Both structures were interpreted as being part of a
previous extensional fault system, active during the Early
Jurassic, which would have controlled the distribution of
the volcaniclastic sequences west of it, controlled in turn
by inherited Paleozoic basement structures.
Previous works established two main contractional pulses
(Giacosa y Heredia, 2004; Tobal et al., 2012) that took
place through the Andean domain: an older event that led
to the development of a thin-skinned fold and thrust belt
that deformed the Early Jurassic Huemul Formation and a
younger one that led to the development of a thick-skinned
fold and thrust belt represented by the main thrusts and
back-thrust systems described above (Tobal et al., 2012).
This last event would be at least reactivated in the UpperMiocene (Giacosa y Heredia, 2004; Tobal et al., 2012)
based on the U/Pb age of foreland deposits accumulated in
the El Bolsón depocenter (Encinas et al., 2011 and Encinas
et al., this Congress). Part of the contractional structures
could have taken place even in Late Cretaceous as zircon
fission track data suggest to the southwest (Thomson,
2002). The tectonic emplacement of the CPB by a series of
out of sequence thrusts could be part of this stage. In Early
Pliocene times, main contractional structures were affected
by an important relaxation. The out of sequence thrust that
exhumed the Patagonian Batholith seems to have acted in
the south as an extensional master fault provoking the
collapse of the Andes to the west and controlling the
emplacement of ignimbritic-lava packages at Cerro Silvia
depocenter. To the north the Early Pliocene Cerro Bastión
depocenter seems to have been generated in a similar way.
The extensional relaxation of an Early Jurassic half-graben
in this area had controlled the emplacement of voluminous
intraarc products during Mesozoic times (Tobal et al., in
preparation). Thus, Early Pliocene extensional phenomena
extensively described to the north in the Andes of Neuquén
and Mendoza (Lara y Folguera, 2006; Miranda et al.,
2006) seems to be an important process at this part of the
Andes, explaining volcanism and morphology of the
Northern Patagonian Andes.
2.2 Structure of the Cordon del Hielo Azul (North
Patagonian Andes)
Further west, on the main Patagonian Andes at the Cordón
del Hielo Azul, a series of thrusts affects the Huemul
Formation (volcanic rocks of the Early to Middle Jurassic
intra-arc volcaniclastic sequence; Diez y Zubia, 1981)
(Tobal et al., 2012). In the southern section of the Cordón
del Hielo Azul, two basement faults of opposite vergence
overly the CPB over the Huemul Formation: a back-thrust
(Laguna de Lali) to the east and an out-of-sequence thrust
(OST) to the west. This Mesozoic volcanic unit interposed
between these two systems is tightly deformed by a series
of emergent and blind thrusts interpreted as a cannibalized
thin-skinned fold and thrust belt (Tobal et al., 2012).
To the north, in the Cerro Hielo Azul, the trace of the OST
converges with the Hielo Azul fault (Giacosa y Heredia,
2004) which juxtaposes a sequence composed of
ignimbrites of the Cerro Silvia (previously wrongly
assigned to the Ventana Formation) over the Huemul
Formation (Figure 2). Tobal et al. (2012) interpreted the
Hielo Azul fault as a normal fault, pointing out that these
volcanics were emplaced in a small and restricted
extensional depocenter hosted in the axial part of the
Patagonian Andes. Moreover, its basal sections describe
typical synrift geometries implying a synextensional
emplacement (Figure 2). At Cerro Bastión, an equivalent
volcanic sequence, a lava flow was dated in 4.6 Ma (Tobal
et al., in preparation) suggesting that these depocenters
developed in the Main Andes are Pliocene in age.
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