2009 Folguera et al Payenia

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Journal of Volcanology and Geothermal Research 186 (2009) 169–185
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Retroarc volcanism in the northern San Rafael Block (34°–35°30′S), southern Central
Andes: Occurrence, age, and tectonic setting
Andrés Folguera a,⁎, José A. Naranjo b, Yuji Orihashi c, Hirochika Sumino d, Keisuke Nagao d,
Edmundo Polanco b, Victor A. Ramos a
a
Laboratorio de Tectónica Andina, FCEyN, Universidad de Buenos Aires — CONICET, Argentina
Servicio Nacional de Geológía y Minería, Casilla 10465, Santiago, Chile
Earthquake Research Institute, the University of Tokyo, Bunkyo, Tokyo 113-0032, Japan
d
Laboratory for Earthquake Chemistry, Graduate School of Science, the University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
b
c
a r t i c l e
i n f o
Article history:
Received 16 October 2008
Accepted 30 June 2009
Available online 7 July 2009
Keywords:
retroarc basalts
back arc extension
Mendoza
Payenia
K–Ar dating
volcanoes
a b s t r a c t
One of the major retroarc volcanic provinces in the southern Central Andes (34° and 37°S) is developed in the
Andean foothills of the San Rafael region between the orogenic front and foreland basement uplifts of Late
Miocene age. Here we present the first comprehensive geochronological study of the Quaternary volcanism,
previously dated mainly on the basis of stratigraphy. The new unspiked K–Ar radiometric and two
radiocarbon determinations encompass many volcanic centers, most of them monogenetic and of basaltic
composition exposed between 34° and 35°30′S. The data constrains the basaltic volcanism to between
~ 1.8 Ma and the Holocene. The spatiotemporal distribution of the ages indicates that eruption in the retroarc
was episodic with some distinct patterns. The orogenic front of the San Rafael Block is associated with 1.8–
0.7 Ma volcanic eruptions, while the Malargüe fold and thrust belt front in the Andean foothills is related to
younger eruptions produced at 0.1–0.01 Ma. Both areas are associated with Late Cenozoic normal faults that
dismembered an uplifted a Late Miocene peneplain as indicated by younger over older fault-relationships
between Paleozoic rocks and Tertiary strata. This linkage indicates a major relationship between Pleistocene–
Holocene retroarc eruptions of the basaltic centers, and extensional collapse of the foreland region, that
shows a migration of the last volcanic activity towards the trench.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Jurassic to Neogene magmatism along the western South American
margin is the direct consequence of subduction of oceanic lithosphere.
While arc magmatism has been associated with a single phenomenon
related to the dehydration of the subducted oceanic crust at depth,
volcanism at retroarc positions (Fig. 1) has been explained by different
processes that encompass from development of asthenospheric windows, back-arc extension, eastward arc migration due to shallowing of
the subducted lithosphere and lower lithosphere overheating due to slow
plate displacements (see discussion in Kay et al., 1999, 2005, 2006, 2007;
Risse et al., 2008). The largest—less than 5 Ma retroarc volcanic plateau in
the entire Southern Andes—corresponds to the Payenia volcanic field
(Fig. 1; 34°30′–38°S) (Muñoz and Stern, 1988; Stern, 1989) that covers
the Andean Late Miocene orogenic front. This has been explained as
⁎ Corresponding author.
E-mail addresses: [email protected], [email protected]
(A. Folguera), [email protected] (J.A. Naranjo), [email protected]
(Y. Orihashi), [email protected] (H. Sumino),
[email protected] (K. Nagao).
0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2009.06.012
related to strong asthenospheric influx due to the steepening of the
subducted Nazca plate after a cycle of shallow subduction in the area (Kay
et al., 2006). Recently, seismic tomographies showed abnormal “heated”
sublithosphere beneath this volcanic province that supports the previous
hypothesis (Gilbert et al., 2006). Poor radiometric covering has not
allowed to reconstruct accurately eruptive evolution of the area, as well
as associated Quaternary tectonism.
Compositional variations and changes in volcanic and structural
style through time along the Present south Andean arc (Fig. 1), as well
as their related causes, have been discussed in numerous works (see
Jordan et al., 1983; Kay et al., 2005, among others). Regional studies
have shown the segmented nature of the volcanic arc from 2° N to
55° S, where around 200 stratovolcanoes and 10 potentially active
calderas are present (Stern, 2004; Stern et al., 2007). This segmentation is a direct consequence of many variable tectonic factors along the
western active margin of the South American plate, such as age of the
subducted oceanic floor and thickness of the Andean crust, that
determine distinctive geochemical patterns and consequent eruptive
mechanisms and type of volcanic rocks. These segments also show
remarkable variations regarding general ages of main volcanic
provinces and life-span of associated individual centers.
170
A. Folguera et al. / Journal of Volcanology and Geothermal Research 186 (2009) 169–185
Fig. 1. Southern Andean tectonic setting and Cenozoic retroarc plateau basalts in Patagonia. The Payenia plateau basalts constitute the largest retroarc volcanic province generated in
the last 5 Ma in the entire Southern Central and Patagonian Andes (taken from Ramos et al., 1982; Kay et al., 2006, 2007).
In this context, the Southern Volcanic Zone (Fig. 1) (SVZ, 33°–46° S) is
of special interest due to the occurrence of most of the active volcanoes
along the margin, and because of their relation to highly populated areas
on both slopes of the Andes. The northernmost section of this segment
around 33°S is characterized by a west to east arc to retroarc zoning
describing four discrete areas where eruptive styles, magmatic composition and volcanic types were highly variable during the Miocene to
Holocene time-interval: (1) the Maipo and its associated Diamante
caldera, Palomo, Tinguiririca, and Planchón volcanoes are the biggest
volcanic centers in this sector, and form the arc front located on the Main
Andes next to the continental divide (Fig. 2); (2) major volcanic centers
such as the Overo, Guanaquero and Sosneado volcanoes on the eastern
side of the Main Andes, although smaller than the ones located at the arc
front, defining the maximum heights of the eastern slope of the Andes
(Fig. 3); (3) Immediately to the east, over the orogenic front a series of
monogenetic basaltic fields named the Hoyada, Lagunita, Loma Negra and
Hoyo Colorado (Fig. 3); (4) further to the east, emplaced around the San
Rafael Block (Figs. 2–4), a basement block uplifted in the foreland area.
In this paper we focus on the last two groups, describing their age
and morphology, and finally their structural control. We present the first
unspiked K–Ar data set of the region to temporally define this retroarc
province, hosted in the northern San Rafael Block (34°–35°15′S), east of
the Main Andes (Figs. 2 and 3). Then, we present an evolutionary model
for the progression of Quaternary deformation in the area and related
volcanism.
2. Previous work in the region
Several workers have studied partial aspects regarding the retroarc
associations that are present at the San Rafael Block and in the eastern
Andean foothills. Since the ´70 these studies have intended to
interpret these mafic fields from a tectonic point of view using very
limited radiometric tools, as well as geochemical analyses. Valencio
et al. (1970) performed the first temporal determinations, using the
K–Ar method and paleomagnetic analyses over Pleistocene volcanic
sequences south of the latitude of the present study. Then Toubes and
Spikermann (1979) obtained K–Ar ages in Pliocene to Pleistocene
volcanic successions through the San Rafael Block, and found the
oldest ages for these retroarc associations. Araña Saavedra et al.
(1984) studied these retroarc volcanics between 34° and 37°S
determining an alkaline signature and a magmatic source enriched
in K, Al and Ti contents. These authors discussed their potential
linkage to the pre-Pliocene calc-alkaline volcanics outcropping in the
same area.
171
Fig. 2. Main morphotectonic units in the northern part of the Southern Andes and Payenia volcanic zone. Numbers indicate thickness in meters of Late Miocene accumulations related
to the Río Grande foreland basin (Yrigoyen, 1994) that was covered by retroarc volcanic rocks corresponding to the Mendoza Basaltic Volcanic Field. Structure was compiled from
Polanski (1954, 1963, 1964), Desanti, (1956), González Díaz (1964, 1972a,b,c, 1979), Holmberg (1964, 1973), Fidalgo (1973), Núñez (1976 a, b, 1979), Delpino and Bermúdez (1985),
Cortés (2000).
Bermúdez and Delpino (1989) studied several aspects regarding
the volcanic associations cropping out between 35° and 37°S. First,
they recognized mesosiliceous volcanic rocks forming part of the
basement of the Pliocene to Pleistocene–Holocene mafic associations
more than 500 km away from the oceanic trench, similarly to Araña
Saavedra et al. (1984). Second, they related mafic widespread and
voluminous volcanic eruptions to an extensional retroarc setting,
interpreting them as a product of combined arc and intraplate sources
characterized by low melting percentages from the mantle. These
authors compared this volcano–tectonic setting with others throughout the world where either the crust extends behind the arc front or
the crust collapses in an intraplate setting controlling the eruption of
tholeiitic to alkaline series, such as the Northern Island of New
Zealand, the Japan Sea, Korea, eastern China, Basin and Range province
in the western United States, and Río Grande rift in New Mexico.
In a regional analyses performed between 34° and 39°S, Muñoz et al.
(1989) recognized a series of N to NW-trending volcanic chains east of the
Late Pleistocene to Holocene arc front emplaced in the low lands of the
Main Andes. Those form part of a Pliocene to Lower Pleistocene arc very
well developed between 37° and 39° S, which indicates a strong
westward shifting of the arc front in the last 2 Ma. Those Pliocene to
Lower Pleistocene centers are forming andesitic and basandesitic
stratovolcanoes and rhyolitic and dacitic calderas. Both Late Pleistocene
to Holocene associations at the arc front, and the Pliocene to Lower
Pleistocene volcanoes in the western retroarc area are subalkaline (Fig. 2).
However, further to the east alkaline assemblages dominate
eventually over the eastern sector of the fold and thrust belt with a
few exceptions such as the Tromen volcano in the Chos Malal fold and
thrust belt around 36° S (see Kay et al., 2006). Muñoz et al. (1989) also
proposed that most of these alkaline associations were hosted in
172
Fig. 3. Radiometric ages obtained in the present study from retroarc monogenetic basaltic cones. Note two groups differentiated by age: one older of Early Pleistocene located at the San Rafael Block on the Andean orogenic front, and another
younger at the eastern Malargüe fold and thrust belt of Late Pleistocene–Holocene (see details of the dated samples in Table 1).
173
Fig. 4. Morphotectonic map of the northern San Rafael Block and adjacent main cordillera, where the Mendoza Basaltic Volcanic Field is displayed in relation to neotectonic activity in
the area. Note a general northwest structural trend associated with the retroarc eruptions as detected by Polanski (1963) and Cortés and Sruoga (1998). Structure is locally based on
Bastías et al. (1993), Lucero (2002) and Costa et al. (2004).
graben-like structures, which allowed to infer an extensional generalized tectonic setting between 34° and 39°S latitudes. Pliocene to
Lower Pleistocene arc-related rocks east of the Present arc front
(Fig. 2) have higher K2O ratios and higher amounts of incompatible
alkaline and light rare earth elements than the Late Pleistocene to
Holocene ones. 87Sr/86Sr isotopic ratios for the arc-related eastern
series are between 0.7038 and 0.7042 independently of the SiO2
content, with similar ranges than the arc front, inferring a common
source in the arc zone for the Pliocene to Holocene lapse.
Composition of the arc association is sensitive to fractional
crystallization which leads to a progressive increase in K2O, light
rare earth, and incompatible element contents. Other proposed
process that constrained compositional variations through time is a
decrease in the degree of partial melting for the youngest associations.
South of 39°S Muñoz et al. (1989) recognized a clear difference, not
distinguishable north of this latitude, between arc, and retroarcalkaline associations. Progressive increase in the age of the subducted
oceanic floor to the north and subduction of the Valdivia fracture zone
at 39°S (Fig. 1) were invoked as the main mechanisms controlling
those major changes at this latitude.
Bermúdez et al. (1993) discriminated retroarc volcanic associations located between 35° and 37°S in two separate volcanic fields,
Llancanelo (10,700 km2) and Payún Matrú (5200 km2), that constitute
the Payenia volcanic province (Fig. 1). Both developed from Pliocene
to Holocene times with four peaks of intensity nucleated in 3.6 Ma
corresponding to the 5.1–2.6 eruptive stage, 1.7 Ma, corresponding to
the 2–1.5 Ma interval, 450 ka, corresponding to the 650–100 ka
interval, and finally a fourth category in the Holocene, mainly based on
morphological criteria. The first three stages are based on radiometric
and paleomagnetic studies (Valencio et al., 1970; Mendía and
Valencio, 1987; Muñoz et al., 1987; Linares and González, 1990)
complemented by lithostratigraphic and morphological analyses
(Bermúdez and Delpino, 1989). Bermúdez et al. (1993) characterized
on physical, petrological and geochemical grounds those timecategories comparing them with the arc front at the same latitudes
represented by the Tatara–San Pedro complex and the Planchón
volcano (Fig. 2) (see Dungan et al., 2001).
In relation to contemporary-to-volcanism structure, Cisneros and
Bastías (1993) studied neotectonic deformations at the eastern border
of the San Rafael Block that affected retroarc monogenetic basaltic
fields, detecting an important NNW-trending alignment, where Las
Malvinas fault zone was recognized (Fig. 4) (Cisneros et al., 1989). This
area was associated with the Villa Atuel-Las Malvinas earthquake in
May 30, 1861 that destroyed Las Malvinas and Villa Atuel villages
(Fig. 4), whose effects were interpreted as liquefaction in saturated
sand soils. Other important earthquakes developed in this area
correspond to San Carlos, (August 29, 1861), immediately to the
north of the study area, and General Alvear (October 4, 1913) (Fig. 4).
They also recognized other fault systems located in the opposite west
border of the San Rafael Block as related to neotectonic deformations,
particularly in the Carrizalito range (Fig. 4), where morphotectonic
parameters indicated left lateral transtensional faults. These Quaternary faults were recognized as exerting a strong control in the eruption
of Neogene and Quaternary volcanic chains aligned along NNW and
174
NW trends, located mainly at the eastern border of the San Rafael
Block. Recurrence in the activity during Quaternary times is also
inferred by younger faulting affecting lava flows that had been
emplaced in transtensional structures. Theoretical estimations suggest the occurrence of M 5.14 and maximum accelerations in the order
of 88.96 cm/seg2 for Colonia Las Malvinas and 57.68 cm/seg2 for Villa
Atuel (Fig. 4).
Cortés and Sruoga (1998) identified a structural control for
monogenetic basaltic cones erupted at the Andean foothills east of
the Carrizalito range (34°–34°30′S) between the Papagayos and
Diamante rivers (Fig. 4). They have interpreted their links to
Pleistocene and Holocene faults with extensional components.
Naranjo et al. (1999a) characterized compositionally a monogenetic
basaltic–andesitic field in the eastern fold and thrust belt at the Salado
river (around 35°30′S), 70 km east of the arc front, known as the
Infiernillo volcanic field (Fig. 4). These centers show a strong extensional
control as depicted by Dajczgewand (in press), who described a normal
fault with basaltic feeders along the Salado river. Their chemistry is also
significantly different from the arc front at these latitudes when
compared with the Planchón center (Naranjo et al., 1999b), which is
characterized by a higher differentiated source potentially connected to
a contrasting compressive regime at the western Andes.
In relation to physical volcanology, south of the San Rafael Block
and north of Río Colorado (Fig. 2), 40 monogenetic volcanic centers of
eastern volcanic field were studied by Bertotto et al. (2006), who
described variations between Strombolian and Hawaiian activity for
the formation of successions of weakly welded lapilli and bomb beds
and agglutinated spatter beds. Part of these eruptions comprises one
of the longest pahoehoe inflated basaltic lava flows in the Payenia
volcanic province (Pasquaré et al., 2008). Recently, Risso et al. (2008)
described the Llancanelo volcanic field south of the Malargüe town
with similar characteristics (Fig. 2).
3. Tectonic setting
The analyzed volcanic field is within the Andean Southern Volcanic
Zone (SVZ) (Fig. 1) as defined by López Escobar et al. (1977, and
contemporary works cited herein) and popularized in subsequent works
(Stern, 2004, among others). On the tectonic point of view it is part of the
southern Central Andes (27°–38°S) that are formed by two broad
segments differentiated by structural styles and degree of shortening
absorbed in the Neogene. The northern limit of the study area roughly
coincides with the southern end of the Pampean flat slab subduction
segment of the Nazca plate beneath the South American plate (27°–33°S;
Barazangi and Isacks, 1976). This boundary is the northern end of a
normal subduction segment developed to the south of 33°S (Jordan et al.,
1983; Ramos et al., 2002). Morphotectonic changes along strike are
gradual and strongly influenced by pre-existing heterogeneities previous
to the Andean deformation, such as rift systems and thick sedimentary
prisms represented by the Meso-Cenozoic basins.
The Andes, south of the present flat slab zone, between 34° and
35°30′S are formed by two distinct mountain systems separated by a
Neogene foreland basin: the Main Andes encompassing the arc and
western retroarc areas, and the San Rafael Block in a foreland position
(Figs. 2 and 4). The Main Andes constitutes the drainage divide
between the Pacific and Atlantic oceans that has been shaped from 19
to 17 Ma with the tectonic inversion of the Eocene–Late Oligocene
Abanico Basin (Godoy et al., 1999; Charrier et al., 2002, 2005). This
basin fill is presently exhumed at the western side of the Andes. The
eastern Andean sector has been constructed by the stacking of Late
Triassic half-grabens and locally by thin-skinned structures that
deformed late Mesozoic and Cenozoic successions from 15 to 8 Ma
(Giambiagi et al., 2008). Andean uplift at these latitudes was recorded
in the Río Grande Basin, a more than 2500 m thick foreland basin
developed between 34° and 37°S that has been partially cannibalized
because of the uplift of the San Rafael Block to the east, during the
onset of a shallow subduction zone from 34°30′ to 37°S (Fig. 2)
(Ramos and Folguera, 2005; Kay et al., 2006). Land mammal bearing
synorogenic sequences were exposed in the Middle to Late Miocene,
which constrained the last phase of orogenic uplift at these latitudes
(Soria, 1983). At this time, Middle to Late Miocene arc-derived
volcanic rocks were emplaced over the eastern flank of the Main
Andes as well as on the San Rafael Block, more than 500 km away from
the trench (Giambiagi et al., 2005, 2008; Kay et al., 2006). Widespread
intraplate volcanic rocks were erupted mainly around the San Rafael
Block and Río Grande Basin during Pliocene to Pleistocene times
(Bermúdez et al., 1993), while small amounts were concentrated in
the eastern Malargüe fold and thrust belt as described by Cortés and
Sruoga (1998) and Saavedra (in press) (Fig. 2). Those are uncomformably covering the Late Miocene compressive structures and are
mainly hosted in extensional troughs that are displacing previous
structures. Main Pliocene to Pleistocene troughs in the region are the
Llancanelo Basin (Fig. 2), located in the Río Grande foreland basin
south of the study area, where monogenetic basalts are forming an
almost continuous volcanic field covering folded synorogenic and
modern piedmont deposits. Another important trough, the Nihuil
Basin, has a series of NW extensional faults which are affecting the
backlimb of the San Rafael Block producing a series of half-grabens
where basaltic volcanic fields were erupted (Figs. 2 and 4).
During the last million years, minor transpressional deformation
affected the eastern front of the San Rafael Block (Bastías et al., 1993;
Costa et al., 2004, 2006; Lucero, 2002) deforming Pleistocene volcanic
sequences erupted during the aforementioned phase of extension in
the area. These deformations in the orogenic front are associated with
important evidence of crustal seismicity.
4. Mendoza retroarc volcanic field
The extensive Mendoza Basaltic Volcanic Field comprises more
than 400 (Bermúdez et al., 1993), or even around 800 (Risso et al.,
2008) monogenetic small volcanoes in addition to a number of
stratovolcanoes and shield volcanoes, distributed in an area of 10,000
to 20,000 km2, between 34° and 38°S (Fig. 2). In this paper we focus
the attention on the northern part of this area, around the San Rafael
tectonic block, from 34° to 35°10′S (Figs. 3 and 4).
Monogenetic volcanoes and composite stratocones within the
Mendoza Basaltic Volcanic Field are clustered, but they also constitute
linear chains that follow tectonic structures or are distributed on the
flanks of large shield volcanoes such as Payun Matrú volcano to the
south of the area.
Due to the low erosion rates in a fairly dry climate, relative dating
based on their geomorphology is difficult. Thus, we have employed
unspiked K–Ar geochronological techniques to describe the activity and
longevity (Fig. 3 and Table 1), as well as related neotectonic setting of
this volcanic field. At this stage, the main physical features of the
Mendoza Basaltic Volcanic Field include a number of individual vents,
their distribution and relationship to modern faults. In the Table 2, it is
summarized some individual physical characteristics and longevity, of
the northern part of the Mendoza Basaltic Volcanic Field. This can be
considered as a large but low density volcanic field compared for
example with the Springerville volcanic field, Arizona, USA (Condit and
Connor, 1996). The study area has approximately 84 vents formed over
the last 1.7 Ma, although co-genetic origin for multiple vents could also
have occurred. Thus, cases of alignment of different cones might be
considered to represent single volcanic events.
In terms of spatial distribution and relative chronology among
vents, short local alignments show no shift in their locus, but largescale shifts have been observed from east to west along parallel
structures trending NW. Vent clusters consisting of a 1 to 10 individual
vents also show this general pattern within the northern Mendoza
Basaltic Volcanic Field. It can be estimated that the distribution of
clusters can be directly correlated with the distribution of main faults.
175
Table 1
Analytical values of the unspiked K–Ar ages.
Sample no.
40
38
Ar/36Ar
Ar rad
K
(wt.%)
10
− 0.26 ± 0.36
0.08 ± 0.38
0.02 ± 0.58
0.15 ± 0.30
0.29 ± 0.21
0.51 ± 0.47
−8
3
cm STP/g
(40Ar/36Ar) initiala
Fractionated Ar assumed
Age
(Ma)
Air fraction
(%)
294.7 ± 1.5
295.5 ± 1.5
295.9 ± 1.7
294.3 ± 1.3
293.6 ± 1.1
293.3 ± 1.7
−0.049 ± 0.068
0.015 ± 0.076
0.005 ± 0.116
0.034 ± 0.066
0.069 ± 0.050
0.10 ± 0.10
100.4
99.90
99.98
99.78
99.48
99.39
a) Río Salado Group
1
Las Hoyadas
2
Las Hoyadas
3
Las Hoyadas
4
Lagunilla1
5
Hoyada
6
Lagunilla 2
090499-1C
090499-2
090499-3A
111199-2
111199-5
111199-6
1.35 ± 0.04
1.29 ± 0.04
1.28 ± 0.04
1.18 ± 0.04
1.09 ± 0.03
1.20 ± 0.04
b) Papagayos Group
7
Los Leones W
8
Pozo
9
Pozo
10
Pozo
081199-1
081199-8A
081199-9
081199-10
1.09 ± 0.03
0.80 ± 0.02
0.99 ± 0.03
0.98 ± 0.03
2.56 ± 0.15
0.796 ± 0.048
0.408 ± 0.039
0.350 ± 0.039
0.18835 ± 0.00045
0.18824 ± 0.00046
0.18832 ± 0.00040
0.18805 ± 0.00059
0.607 ± 0.039
0.257 ± 0.017
0.106 ± 0.011
0.092 ± 0.011
95.88
96.70
98.68
98.75
c) Los Tolditos Group
11
Chato
12
Rodeo
13
Rodeo
091199-8
091199-9
091199-10B
1.16 ± 0.03
1.10 ± 0.03
1.04 ± 0.03
2.83 ± 0.15
3.17 ± 0.17
2.77 ± 0.15
0.18873 ± 0.00049
0.18866 ± 0.00051
0.18800 ± 0.00054
0.631 ± 0.039
0.739 ± 0.045
0.684 ± 0.042
89.93
86.54
91.45
d) Diamante Volcano Group
14
Diamante
15
Diamante
16
Diamante
17
Diamante Basement
18
Diamante
19
Diamante
20
Diamante
21
Diamante
22
Diamante
091199-13
091199-14
091199-15B
091199-16
091199-17
101199-10A
101199-10B
101199-12
101199-13
0.70 ± 0.02
1.26 ± 0.04
1.51 ± 0.05
1.49 ± 0.04
1.14 ± 0.03
0.86 ± 0.03
1.06 ± 0.03
0.98 ± 0.03
1.79 ± 0.05
1.349 ± 0.084
0.268 ± 0.021
0.478 ± 0.028
42.8 ± 2.2
2.24 ± 0.12
1.616 ± 0.086
4.77 ± 0.26
0.950 ± 0.060
0.375 ± 0.039
0.18785 ± 0.00044
0.18752 ± 0.00047
0.18791 ± 0.00044
0.18801 ± 0.00058
0.18793 ± 0.00077
0.18785 ± 0.00064
0.18793 ± 0.00056
0.18851 ± 0.00047
0.18783 ± 0.00056
0.495 ± 0.034
0.0546 ± 0.0046
0.0817 ± 0.0053
7.38 ± 0.43
0.505 ± 0.031
0.484 ± 0.030
1.164 ± 0.073
0.251 ± 0.018
0.0540 ± 0.0059
97.42
98.72
96.66
39.35
95.19
94.48
77.05
97.05
99.12
e) Medio Group
23
Ao Hondo NW
24
Ao Hondo
25
Loma del Medio
101199-2
101199-4
101199-6A
1.09 ± 0.03
1.08 ± 0.03
1.12 ± 0.03
1.89 ± 0.10
1.82 ± 0.10
0.590 ± 0.049
0.18769 ± 0.00049
0.18800 ± 0.00040
0.18829 ± 0.00045
0.449 ± 0.028
0.434 ± 0.027
0.136 ± 0.012
91.33
93.06
98.63
f) Las Malvinas Group
26
Negro
27
El Puntudo
28
Guadal
29
Puntano
30
Solo
111202-1A
111202-2
111202-3D
111202-4
111202-5
0.92 ± 0.03
0.93 ± 0.03
0.78 ± 0.02
0.83 ± 0.02
0.27 ± 0.01
2.87 ± 0.15
3.35 ± 0.18
2.45 ± 0.14
5.74 ± 0.30
0.801 ± 0.049
0.18842 ± 0.00043
0.18781 ± 0.00045
0.18773 ± 0.00068
0.18823 ± 0.00050
0.18781 ± 0.00047
0.801 ± 0.049
0.932 ± 0.056
0.805 ± 0.051
1.78 ± 0.11
0.750 ± 0.051
84.20
86.20
85.93
93.53
97.66
g) Guadal Volcano
31
La Carbonilla
32
La Carbonilla
121202-1A
121202-1B
1.30 ± 0.04
1.34 ± 0.04
2.67 ± 0.14
3.26 ± 0.17
0.18843 ± 0.00047
0.18823 ± 0.00050
0.530 ± 0.033
0.629 ± 0.038
95.76
95.49
h) Aisol Volcano
33
Nihuil
121202-2
0.88 ± 0.03
5.02 ± 0.26
0.18849 ± 0.00048
1.474 ± 0.089
84.25
40
0.18758 ± 0.00049
0.18785 ± 0.00047
0.18797 ± 0.00054
0.18746 ± 0.00040
0.18723 ± 0.00035
0.18714 ± 0.00052
36
( Ar/ Ar) initial = 296. 0 is assumed. Error: 1s.
a 40
( Ar/36Ar) initial was estimated from the measured 38Ar/36Ar ratio, which was fractionated from the atmospheric value of 0. 1880.
Moreover, in some cases a strong correlation has been recognized
between structural trends and vent alignments which may indicate
contemporaneous cone building associated with single episodes of
dike injection (Fig. 4).
In the northern part of the Mendoza Basaltic Volcanic Field,
Diamante volcano composite cone (Figs. 3 and 4) is the unique case
where magma supply has been sufficient to maintain a thermal
anomaly around a central vent. In the monogenetic cones magma
supply rates have been so low that new ascending magma batches
have found their own fault conduits to the surface, with no
opportunity to accumulate at shallow crustal magma chambers.
4.1. Eruptive centers in the northern part of the Mendoza retroarc
volcanic field
We have distinguished ~84 volcanic centers in the northern part of
the Mendoza Basaltic Volcanic Field (Fig. 3, Tables 1 and 2), where
general morphological descriptions were available (Desanti, 1956;
González Díaz, 1972a; Bermúdez et al., 1993). Scoria cone is the most
common type and few examples of tuff rings and tuff cones also occur.
These commonly occur in groups, although isolated cones have been
distinguished. More complex structures such as Diamante stratovolcano and Negro small shield volcano are exceptional cases, as well as
lava flows with open craters as Hoyo Colorado, Hoyada and Lagunillas
among others (Naranjo et al., 1999a).
Cinder cones are truncated, conic or horseshoe-shaped, with
typical bowl-shaped craters in the younger examples, but no crater at
the top of the older ones. Elongate cones are scarce even in those built
above fissures, where more complex aligned vent systems occur. The
deposits of scoria cones typically consist of bombs, scoriaceous lapilli
and minor ash. Spatter cones and scoria-agglutinate cones consisted
largely of welded lava spatter are also common.
Conspicuous examples of maars are Pozo and Arroyo Hondo NW
volcanoes. The former consists of a NW oriented twin explosion craters
176
Table 2
Main morphologic characteristics of the retroarc vents in the study area.
Volcano
Morphology
Elevation
(H, m)
Diam (apron included)
(km)
La Carbonilla
La Carbonilla SE
Los Leones
Los Leones W
Sepultura
Del Medio
Gaspar
Zorro
Guadaloso
Pozo
Cone crater 120 m diameter
No crater cone
No crater cone
No crater cone
No crater cone and SE explosion crater
No crater cone
No crater cone
No crater cone
No crater cone
NW oriented twin explosion craters-the
western-youngest
Explosion crater ring tuff 20 m deep
Pyroclastic cone with crater
Pyroclastic cone with crater
1 km diameter crater, 0.6 km nested cone
with a crater of 35 × 0.40 composed cone
Pyroclastic cone with lava flows
Pyroclastic cone
Pyroclastic cone
Pyroclastic cone
40
80
160
b 15
30 + 10 deep
65
75
40
65
Nested dome 50 m height–0.65
diameter–58 m and 75 deep
35
185
176
220
1.2
3.5
1.75 × 1
0.4
0.5–0.5
0.5
0.75
0.9
2
1.3 (2)
Twin tuff ring craters
0.60
1
0.9
1.3
Twin tuff ring crater
0.2 km crater
0.25 km crater, b12 km lava flow
Multi-crater and nested cone
200
80
100
121
Deformed crater
350 m flat crater
250 m flat crater
0.2 km crater
Explosion crater
Uncratered cone
Uncratered cone
Pyroclastic cone
Asymmetric cone
Cone
10 m
45
50 m
35 m
40
50
1.3
1
0.85
1 × 0,7
0.4
0,4
0.4
0.35
0.70 × 0.50
2 × 1.6
2.3
Cone
Cone
Cone
Eroded cone
40
45
110
108
Stratovolcano
830
1.5
2.3
1.5
1.5
0.8
6.2
Stratocone
140
3×2
Ao Hondo NW
Ao Hondo
2449
Loma del Medio
2247
Bs Blancas 1
Bs Blancas 2
Bs Blancas 3
Bs Blancas 4
Bs Blancas 5
Bs Blancas 6
Bs Blancas 7
Bs Blancas 8
Chato
Rodeo
1784
del Medio
La Chilena
Las Bolas 1
Las Bolas 2
Diamantito (parasitic cone)
Diamante
Diamante Basement
Morado
Chico
Chato W
Negro WW
La Leña W1
La Leña W2
La Leña W3
La Leña
El Nihuil NW
Nihuil
El Nihuil NE
Nihuil S
Nihuil SW 1
Nihuil SW 2
Nihuil SW 3
Negro E
Negro
El Puntudo
Solo
Guadal
Morado N
Morado S
Aguirre
El Chenque N
El Chenque
El Chenque S
Aguirre W
Aguirre SW
Chihuido N
Ancho
Ancho S
Guadal E
Stratocone
Extrusive dome
Coulee
Pyroclastic cone
Torta dome coulee
Torta dome coulee
Cone
Eroded cone
Deeply eroded cone
Lava field
Deeply eroded cone
Cone
Cone
210
300
260
70
60
120
200
50
270
100
210
20
20
Eroded cone
Small shield volcanoes
Eroded shield
Cone
Double crater cone
Eroded cone
Elongated cone
Deeply eroded double cone
5–6 cone cluster
Elongated cone
Undetermined lava
Eroded cone
Eroded cone
Small cones aligned
Elongated cone
Deeply eroded cone cluster
Compound cone
185
275
240
21
290
190
220
190
b 70
60
70
90
100
55
305
265
215
4
3.7 × 2.3
2.9 × 1.8
1.3
2.9 × 2.2
2 × 1.25
2.9 × 2.3
3
6
4×2
6.7
2.5
2
1
4.9
10 × 7.5
4 × 1.7
0.5 × 0.3
3.2
2.4
2.7 × 1
2.3
1.45 total
2.5 × 0.8
3
1
1.1
1.5 × 0.4
5.1 × 3
3.5
5
Mesa
Small stratovolcano cone and lavas
232
5.2 × 3
La Parva
Punón Trehue
Eroded cone
Partually collapsed? Cone
130
440
2.5
6
Emission-centre
Tuff ring crater 6 m deep
Uncratered cone 1.25 km lava flow
Open crater 0.24 km
Uncratered cone
Uncratered cone
0.17 m diameter crater, open to N
Uncratered cone
Uncratered cone, 1 km NE lava flow
Uncratered cone, NE lava flow
0.7 km diameter crater, 3 km lava flows,
15–20 m thick, parasitic cones
0.25 km crater, 2.6 km lava flows,
25 m thick
Uncratered stratocone
Torta dome
Torta dome
Uncratered cone
Flowing from W
Flowing from W
Eroded crater cone
Eroded crater cone
Eroded crater cone
Eroded emission centre
Eroded crater cone
Uncratered cone
Uncratered cone
Uncratered cone
Uncratered summit
Not determinable
Uncratered cone
0.23 m craters
0.24 km eroded crater
Eroded summit craters
b 0.1 km craters
No evidence
Eroded crater
Eroded crater
2 summit crters 0.65 km, 100 m deep
Not determinable
Multiple emission centres and crater
150 × 250 and eroded
Partially eroded summit crater and
2.75 km lavas
Eroded summit emission crentre
Irregualar 0.3 × 0.2 crater
177
Table 2 (continued)
Volcano
Morphology
Elevation
(H, m)
Diam (apron included)
(km)
Emission-centre
Chato 667
Los Mojados
Los Embanques
Cinder cone
Eroded cone
Deeply eroded cone cluster
45
95
140
1.3
1.7
2.7 × 2
de los Chanchos
Castrino
Mal Barco
Hoyada
Deeply eroded cone
Deeply eroded cone
Lava flow
Open crater and lava flow
115
80
120
50
2.8
2.3
0.9 (7.5 km long lava)
0.5
Lagunilla
Open crater and lava flow
50
0.6
Lagunilla 1
Lagunilla 2
Flat lava cone–coalescence craters
Double lava cone
100
36
0.65
0.65 × 0.46
Loma Negra
Open crater lava cone and flow
40
0.6
Mesillas
Pyroclastic cone and lava flow
125
1.2
Laguna Blanca
Hoyo Colorado
Flat lava? Cone
Lava field
70
40
1.4
0.6
0.25 km diameter semi-circular crater
0.2 km diameter semi-circular crater
Multiple emission centres and
destroyed crater
Eroded summit emission centre
Eroded summit emission centre
Uncratered dome
0.27 diameter and 50 m deep crater,
2.8 km long lava flow
0.34 diameter and 50 m deep crater,
2.4 km long lava flow
70 m deep 0.43 × 0.3 km double crater
Doubles crater NW–SE 0.12–0.16 diam
(16–40 mdeep, respectively)
30 m deep SE open crater,
1.6 km long lava
0.35 km diam open S crater;
2.3 km long lava
0.3 km diam 5 m deep crater
2.2 to 4 km long and b 45 m thick lavas
already interpreted as maars by Cortés and Sruoga (1998). A 650 m
diameter and 50 m high dome is nested within the western and
youngest crater. The eastern maar is clearly excavated in the piedmont
substrate, where ignimbrite deposits dated in 0.450 Ma (Stern et al.,
1984) are exposed about 10 m under the present surface (Fig. 5A–E).
Their deposits are composed of juvenile pyroclastic (lapilli and bombs)
and accidental clasts derived from the subvolcanic basement including
gravels from the piedmont, where magma–water interaction occurred
in the shallow water-table (Fig. 5D).
Negro volcano consists of a small 10 × 7.5 km uncratered shield
volcano with a 275 m high summit (Fig. 6). On the other hand,
Diamante is a 6.2 km diameter stratovolcano, with a 0.7 km diameter
summit (830 m high) crater, 3 km lobed blocky lava flows of 15–20 m
thick and a couple parasitic cones. Other pyroclastic cones are aligned
and fault controlled (Fig. 7).
Four examples of lapilli and ash dispersion lobes from different vents
are conspicuously distinguishable on the piedmont surface, demonstrating the Pleistocene arid prevailing climatic conditions of the area. In fact,
western Pozo tuff ring explosion generated a 20 km long by 6 km wide
dispersion lobe directed to the east (Fig. 5D), which clearly overly the
dispersion lobe of Arroyo Hondo cone, a 33 km long by 10 km wide lobe
dispersed to the northeast. The same direction took the tephra dispersion
of the easternmost vent of Loma del Medio, producing a 20 by 10 km lobe.
Finally, one of the last summit eruptions of Diamante volcano originated
an elongated ash dispersion lobe of 46 km long and a maximum of 15 km
wide, also directed to the northeast. They consist of veneer deposits of
lapilli size of dense juvenile scorias and accidental lithics.
whole rock sample (0.3–0.6 g) was fused at 1700 °C and evaporated gas
was purified and analyzed. Ar isotope analyses were made on a relatively
small amount of sample Ar gas (b2×10− 7 cm3 STP). If the amount of
Ar gas extracted from the sample exceeded this limit, the amount of
Ar gas was reduced using the purification line. Errors on 40Ar
sensitivity and 40Ar/36Ar ratio are estimated to be 5% and 0.2%,
respectively, based on repeated measurements of the atmospheric
standard containing 1.5 × 10− 7 cm3 STP of 40Ar. K concentration was
determined for an aliquot of the crushed and sieved whole rock
fraction used for Ar analysis by the X-ray fluorescence (XRF) method
(Phillips PW 2400) at Earthquake Research Institute, the University of
Tokyo. Details of procedures applied for dating are described in Nagao
et al. (1991) and Orihashi et al. (2004).
Geochronological results and analytical values are indicated in
Table 1. The errors shown in the Table 1 are 1σ of single analysis of each
sample, including statistical errors associated with ion collection of Ar
isotopes and errors in blank correction (less than 1% of the sample gases)
and in the sensitivity and discrimination factors of the mass spectrometer. Most 38Ar/36Ar ratios for the samples were in agreement with
the modern atmospheric value of 0.1880 within the range of analytical
error by 2σ. Six samples have either lower 38Ar/36Ar ratio (0.18723;
#11199-5) than the atmospheric value beyond the range of the analytical error or negative values (#090488-1C, 2 and 3A, #111199-2 and 6)
in K–Ar age calculated by the conventional method. In these cases, the
mass fractionation effect was corrected using the measured 38Ar/36Ar
ratios of these samples and then K–Ar ages were recalculated.
6. Discussion
5. K–Ar dates and analytical procedure
6.1. Structure of the San Rafael Block
The first thirty three K–Ar ages for the Mendoza Basaltic Volcanic
Field were determined using the unspiked sensitivity method, in which
the radiogenic 40Ar concentration is determined by a direct comparison
between the 40Ar/36Ar ratio and 40Ar signal intensity of the samples and
those of volumetrically calibrated amount of atmospheric Ar at the same
condition of the mass spectrometer. The technique can precisely date
younger rocks than 0.1 Ma since it permits measurement of small
amounts of radiogenic 40Ar and determines the isotopic composition of
the initial Ar in the sample by measuring 38Ar/36Ar without assuming
that the 40Ar/36Ar ratio in sample is equal to the modern atmospheric
value of 296 (e.g., Nagao et al., 1991; Matsumoto and Kobayashi, 1995;
Orihashi et al., 2004; Scaillet and Guillou, 2004).
Ar analyses were performed using a noble gas mass spectrometer MSIII (modified-VG5400) in the Laboratory for Earthquake Chemistry,
Graduate School of Science, University of Tokyo. The crushed and sieved
The San Rafael Block is an east-verging asymmetric basement block
(Figs. 2, 4 and 7). Its eastern steeper flank is associated with a series of
high angle faults that uplift Late Triassic clastic sequences over their
basement constituted by Paleozoic sequences highly deformed in Early
Permian times (Figs. 6 and 8) (González Díaz, 1964).
These faults are mainly the result of inverted normal faults that
constituted the western edge of the Triassic Alvear Basin located to the
east and buried beneath thick piles of Tertiary synorogenic sequences
(Figs. 2 and 8). The Alvear Basin constitutes the southern end of a
series of extensional troughs of Late Triassic age that were partially
incorporated in the fold and thrust belt (Ramos and Kay, 1991) (Figs. 7
and 8). The structural sections of the Andean orogen, where vergence
is mainly controlled by the polarity of previous normal faults, are
defined by thick-skinned deformation, such as in the San Rafael Block
178
and the Malargüe fold and thrust belt at the same latitudes (Fig. 8)
(Kozlowski et al., 1993; Giambiagi et al., 2008). Therefore, the
northern half of the San Rafael Block (34°–35°30′S) is characterized
by a marked NW trend defined by the strike of Triassic extensional
deformation and the Andean inverted thrusts that have exhumed a
Paleozoic peneplain which defines its morphology (Figs. 4 and 8). This
foreland system joins at its northern edge the Main Andes front
through the incorporation of the Río Grande foreland basin in the
179
Fig. 6. Eastern neotectonic front of the San Rafael Block. Late Triassic half-grabens are defining the eastern edge of this system controlling the emplacement of Early–Middle
Pleistocene monogenetic basaltic fields. (A) Vertical displacements in Pleistocene lavas associated with reverse faulting at the eastern San Rafael Block. (B) 3D digital elevation model
superimposed to TM and interpretation of basement structure, vertically exaggerated X4. Note the easternmost inverted halfgraben producing an eastward facing scarp affecting
Pleistocene Cerro Negro lavas dated in 0.801 ± 0.049 Ma (see Fig. 3 for location).
tectonic wedge constituting a series of low hills (Yrigoyen, 1993),
while to the south the two systems are highly differentiated by the
maximum longitudinal development of the Río Grande Basin in
between (Figs. 2 and 3).
Contrastingly, the western edge of the San Rafael Block is defined
by a series of normal faults that are affecting up to Middle–Late
Miocene strata of the Aisol Formation. The northwest-trending Valle
Grande and Carrizalito faults have down-thrown blocks to the west,
and can be interpreted as normal faults with minor transtensional
components (Fig. 7). Both have recorded normal displacements
affecting previously folded Paleozoic sequences. Their displacements
together with other minor faults in the area produce the gradual
sinking of the Paleozoic peneplain and exhumed Neogene sequences
beneath alluvial fan deposits that are flanking the San Rafael Block
(Figs. 7 and 8). Most of these faults are spatially associated with
Pleistocene monogenetic centers in the area (Fig. 7). Their vents are
usually aligned through fault scarps, and occasionally lava emissions
are faulted as revealed by satellite images and radar topography
(Fig. 7). However, stronger indicators of post-Pleistocene deformation
are present at the eastern edge of the San Rafael Block, where lavas
associated with monogenetic activity are vertically and laterally
displaced and even folded with an east-vergence (Fig. 6) (Costa
et al., 2006).
Structure associated with Pleistocene eruptions is particularly
revealed at the northernmost half of the San Rafael Block, where
minor isolated patches of basaltic cover allow visualizing the relation
with their basement in comparison with the southern sector.
6.2. Structural control on basaltic eruptions in the northern San Rafael
Block
The northern part of the San Rafael Block has widespread
neotectonic activity (Polanski, 1963) and has been affected by
extensional deformation after the deposition and uplift of the
synorogenic sequences in Late Miocene times. However, Pleistocene
volcanic sequences have also been affected by those fault systems
indicating a much younger tectonic activity (Figs. 6 and 7). Four NWtrending fault systems can be individualized in relation to Quaternary
eruptions in the area, that partially complement those described even
further north by Cortés and Sruoga (1998). The Carrizalito fault
(Fig. 7) defines the western border of the San Rafael Block at 35°S
(González Díaz, 1964). This structure is well developed at surface
defining a tectonic contact between Paleozoic basement and Cenozoic
strata, but it is neither associated with young indicators of tectonic
activity, nor important volumes of erupted volcanic material. On the
contrary, the eastern fault systems of the San Rafael Block show robust
morphological evidence of more recent activity deforming Pleistocene
lava flows (Figs. 6 and 7). The Valle Grande fault is a conspicuous west
facing scarp determining a normal relationship between Permian
rocks to the east and Late Miocene sediments to the west and controls
Fig. 5. (A) View to the east of El Pozo volcanic field (see Fig. 3 for location) corresponding to the youngest volcanic centers in the area with ages between 0.1 and 0.6 Ma. Note the
proximal facies of the pyroclastics plume ejected to the east from the El Pozo volcanic center and the northwest-trending lineament that controlled the emplacement of the other
cones (3D perspective). (B) Panoramic view to the southwest of the western El Pozo maar, unspiked concordant ages of 0.106 ± 0.011 and 0.092 ± 0.011 Ma were obtained for the
nested lava dome (Table 1). (C) Panoramic view to the northeast of the eastern El Pozo maar. The excavated substrate clearly shows the ignimbrite interbedded in the piedmont.
Remnants of a small lake deposits are exposed at the bottom. (D) Flat-bedded and surge layers exposed in the crater wall of the eastern El Pozo maar. Basaltic juvenile bombs (dated
in 0.257 ± 0.017 Ma) together with angular fragments of broken country rocks and palagonitised lapilli and ash form the constructional pyroclastic part of this maar. (E) Landsat
image showing examples of tephra dispersion lobes from different volcanoes of the Mendoza Basaltic Volcanic Field. The dark colour of the veneer deposits is mainly given by coarse
ash to medium lapilli size dense juvenile scoriaceous pyroclasts.
180
Fig. 7. Northern section of the San Rafael Block where the neotectonic eastern front is associated with monogenetic basaltic eruptions. The K–Ar radiometric ages obtained in this
study are also indicated.
the emplacement of minor monogenetic volcanoes (Fig. 7). El Jilguero
and Cerro Negro faults are associated with east facing scarps that set
the limits of the San Rafael Block through its eastern border (Figs. 6
and 7). These two fault systems are the ones that concentrate the
clearest evidence of young deformation in the area and have
constituted the most important paths for basaltic eruption. Cerro
Negro, Solo, Puntano, and Guadalito, with ages comprised between
0.95 and 0.75 Ma (Figs. 4 and 7 and Tables 1 and 2), are the most
prominent volcanoes resting over the eastern edge of the San Rafael
Block, showing evidence of Pleistocene reactivation of those systems.
Cerro Negro volcano and neighbor basaltic cones are affected by east
facing scarps that displace younger than 1 Ma rocks at the
prolongation of the mountain front (Figs. 6 and 7).
6.3. Structural controls on basaltic–andesitic eruptions in the eastern
Malargüe fold and thrust belt
A series of minor monogenetic cones are located at the eastern
Malargüe fold and thrust belt next to the orogenic front (Figs. 3 and 4)
among which Hoyada, Lagunitas and Puesto Pérez volcanoes are the
most prominent vents (Naranjo et al., 1999a) with ages one order of
magnitude younger than the previously described group (Fig. 3, Table
1). Those radiometric ages are in accordance with a radiocarbon age of
0.0123 ± 0.00016 Ma age from organic sediments of dammed
lacustrine deposits associated with volcanic activity in the area
(Fig. 3). Moreover, those ages are within the range of 0.0065 ± 0.0005
to 0.0060 ± 0.0008 Ma obtained by 3He dating determined from
altered exposed material (Fig. 4; Marchetti et al., 2006).
These centers are controlled by a pattern of north-trending faults
corresponding to Infiernillo fault system (Fig. 4) (Kozlowski et al., 1993;
Dajczgewand, in press) that truncated the Late Miocene contractional
structure and the previous Cenozoic synorogenic deposits.
Further to the east at these latitudes, Saavedra (in press) described
north of the Río Diamante a series of monogenetic cones, as the Cerros
Chato and Negro de las Mesillas, and other basaltic cones, which were
controlled by northeast-trending faults with clear evidence of neotectonic activity (Figs. 3 and 4). The lavas and pyroclastic flows were
separated in early and late Pleistocene groups by Saavedra (in press). The
oldest ones were affected by the orogenic front, while the youngest
cones were associated with neotectonic features that affected Los
Mesones Formation of Middle Pleistocene age. The Cerro Negro and
Cerro Chato were interpreted as younger than 0.450 Ma, because they
are developed above the Los Mesones Formation, first aggradation level
in the foothills. This contains the pyroclastic levels associated with the
Diamante Caldera deposits (Cortés and Sruoga, 1998) dated by fissiontracks in 0.47 Ma and 0.44 Ma of zircon separates by Stern et al. (1984).
This caldera produced the catastrophic emplacement of ~350 km3
of ignimbrites, which were widespread over most of the retroarc
plains of the region (Guerstein, 1990), as well as over the Andean
forearc (Stern et al., 1984). The present Maipo Volcano built within
the Diamante Caldera and had more than seven eruptive stages in
the last ~100,000 years (Sruoga et al., 1998).
6.4. Quaternary tectonic evolution of the northern San Rafael Block
and eastern Malargüe fold and thrust belt, and its relation to
retroarc volcanism
The previous results show that the Mendoza Basaltic Volcanic Field
is a considerably young retroarc volcanic field, less than 1 Ma in most
cases and even younger than 100 ka in the westernmost analyzed area.
181
Fig. 8. Structural cross section at 34°30′S from the eastern Malargüe fold and thrust belt (based on Giambiagi et al., 2008) and the San Rafael Block to the east. Both systems where
uplifted in Late Miocene times, and show extensional deformation during the last 2 Ma. These structures are spatially and temporally associated with retroarc volcanic eruptions.
Attenuated lithosphere geometry beneath the San Rafael block and Payenia volcanic field is taken from Gilbert et al. (2006).
These basaltic rocks were emplaced over old Neogene contractional
structures systematically associated with Pleistocene–Holocene extensionally reactivated faults between 34° and 35°30′S and even further
south, beyond the scope of the present work. Their dispersion is
intimately related to the development of neotectonic activity in the area.
The eruptions of the El Pozo volcanic field with an age of
~ 92,000 years are covering a piedmont aggradation surface in the
western retroarc region (Figs. 3 and 4). The pyroclastic deposits of
~ 450,000 years, associated with the Diamante Caldera, are 10 m
beneath the present surface.
The structures in the eastern San Rafael Block exert a strong
control in the eruption of Pleistocene retroarc monogenetic
volcanoes, through the activity of mainly the Cerro Negro and El
Jilguero fault systems (Figs. 7 and 8). On the other hand, the eastern
Malargüe fold and thrust belt has controlled the eruption of
younger than 100 ka material through the El Infiernillo fault system
(Figs. 4 and 8). These two fault groups associated with retroarc
volcanoes remain spatially individualized with the exception of the
northernmost sector of the study area where they interact with
each other (Fig. 4). There, at the piedmont sector described by
Cortés (2000), a series of very oblique WNW-trending normal faults
of the Carrizalito fault system are joining the Malargüe fold and
thrust belt orogenic front (Fig. 4), where the Diamante volcanic
field is associated with the only polygenetic vent corresponding to
the Diamante stratovolcano.
The extensive Mendoza Basaltic Volcanic Field here described
constitutes part of an extended but low density large Pleistocene–
Holocene basaltic volcanic field developed after the main Andean
orogenic phase of contraction in the retroarc foothills of the Southern
Andes achieved in Late Miocene times. Their vents are linked to
Quaternary extensional relaxation of the orogen at these latitudes.
The unspiked K–Ar technique that can precisely date younger rocks
than 0.1 Ma ages applied in this study produced the first
comprehensive geochronological data set of these volcanic fields.
Then a precise sequence of volcanic events can be determined as well
as the progression of Quaternary deformation in the area to which
volcanism is related. After a cycle of eastward displacement/
expansion of the arc front between 19 and 4 Ma (Fig. 8) a magmatic
retreat can be determined. Extensional collapse and related retroarc
volcanism has started at the eastern border of the San Rafael block at
1.8 Ma, staying at this outer position at some 0.7 Ma (Figs. 8 and 9).
Then a jump in retroarc volcanic activity to the Rio Grande foreland
basin and western San Rafael block is registered that lasted until
0.1 Ma (Figs. 8 and 9). Structural control in the 0.7–0.1 Ma volcanic
stage describes a NW band parallel to the 1.8–0.7 eruptions and
displaced some 40–50 km to the southwest (Fig. 9). Finally, a new
retreat in volcanic activity is registered in the last 0.1 Ma to the
easternmost Malargüe fold and thrust belt (Fig. 9). Westernmost
volcanic field, active in the previous 0.7–0.1 Ma interval, next to the
Malargüe orogenic front, remains active at the time of the youngest
eruptions (Fig. 9).
Then a retraction in retroarc volcanic activity can be determined for
the last 1.7 Ma between 34° and 35°30′S at the site where the arc
migrated/expanded over the eastern slope of the Andes until 4 Ma. This
retraction in retroarc volcanic activity and associated extensional
mountain collapse are therefore proposed to be linked to the
steepening of the subducted slab after a cycle of shallow subduction
in the area.
Acknowledgements
Field work and logistics of the present study (Andrés Folguera,
Victor A. Ramos, José A. Naranjo) were financially supported by
grant PICT 14144/03 of the Agencia de Investigación Científica and
Tecnológica of Argentina; Fondecyt Project 1960186, Conicyt, Chile
(José A. Naranjo) and Science Research Project from the Ministry
of Education, Culture, Sport, Science and Technology, Japan (no.
13373004) (Yuji Orihashi). The authors kindly acknowledge the
members of Laboratorio de Tectónica Andina for critical comments
and support.
182
183
Appendix A
Major element compositions of the whole rocks of Table 1.
Sample no.
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
(wt.%)
FeOa/MgO
Mg#a
K2O + Na2O
Alkalinityb
a) Río Salado Group
1
Las Hoyadas
2
Las Hoyadas
3
Las Hoyadas
4
Lagunilla1
5
Hoyada
6
Lagunilla 2
090499-1C
090499-2
090499-3A
111199-2
111199-5
111199-6
54.96
54.89
55.61
46.60
53.06
53.63
1.00
0.99
0.97
1.28
1.06
0.99
17.15
17.01
17.04
14.51
17.07
17.50
8.29
8.16
8.08
10.79
8.59
8.28
0.14
0.14
0.14
0.18
0.14
0.13
4.87
5.22
4.92
11.70
6.33
5.51
7.55
7.94
7.23
10.50
8.66
8.31
3.77
3.84
3.87
2.72
3.59
3.59
1.58
1.51
1.54
1.26
1.36
1.48
0.29
0.32
0.33
0.44
0.31
0.34
99.62
100.04
99.73
99.99
100.16
99.76
1.53
1.41
1.48
0.83
1.22
1.35
53.78
55.89
54.68
68.22
59.32
56.86
5.37
5.35
5.43
3.99
4.94
5.08
− 0.61
− 0.52
− 0.78
1.17
− 0.23
− 0.38
b) Papagayos Group
7
Los Leones W
8
Pozo
9
Pozo
10 Pozo
081199-1
081199-8A
081199-9
081199-10
46.76
45.68
45.34
45.80
1.29
1.44
1.53
1.60
16.30
14.67
14.20
14.88
10.08
10.71
10.88
10.85
0.18
0.18
0.18
0.18
8.39
11.05
11.47
10.98
12.00
10.81
11.46
10.93
2.82
2.84
3.02
3.05
1.40
0.93
1.18
1.30
0.45
0.53
0.54
0.52
99.68
98.84
99.80
100.08
1.08
0.87
0.85
0.89
62.23
67.13
67.61
66.72
4.24
3.81
4.21
4.34
1.31
1.14
1.83
1.84
c) Los Tolditos Group
11 Chato
12 Rodeo
13 Rodeo
091199-8
091199-9
091199-10B
44.96
45.73
46.24
1.33
1.23
1.49
15.14
14.30
15.90
11.54
10.94
10.72
0.18
0.18
0.18
10.95
10.23
8.32
11.85
13.73
12.20
2.57
2.14
3.18
1.35
1.03
1.23
0.64
0.68
0.48
100.52
100.20
99.94
0.95
0.96
1.16
65.26
64.93
60.59
3.90
3.17
4.41
1.78
0.71
1.72
d) Diamante Volcano Group
14 Diamante
15 Diamante
16 Diamante
17 Diamante Basement
18 Diamante
19 Diamante
20 Diamante
21 Diamante
22 Diamante
091199-13
091199-14
091199-15B
091199-16
091199-17
101199-10A
101199-10B
101199-12
101199-13
47.29
49.28
54.06
60.64
45.28
46.85
45.54
46.12
52.51
1.57
1.43
1.10
0.65
1.63
1.16
1.43
1.55
1.09
15.10
16.95
18.16
17.83
15.20
14.72
16.00
18.00
19.06
11.61
9.48
8.33
5.53
11.43
10.65
10.57
10.94
8.67
0.17
0.17
0.15
0.17
0.18
0.15
0.18
0.17
0.18
10.01
6.43
3.55
1.47
10.37
10.91
9.37
5.94
2.55
10.00
11.21
8.35
6.20
10.78
11.67
12.23
12.81
8.14
3.33
3.34
3.86
4.29
3.34
2.71
3.15
3.04
4.12
1.03
1.58
2.23
2.61
1.38
1.01
1.30
1.22
2.25
0.42
0.43
0.33
0.24
0.59
0.37
0.51
0.36
0.42
100.53
100.29
100.13
99.63
100.18
100.20
100.28
100.14
98.99
1.04
1.33
2.11
3.39
0.99
0.88
1.02
1.66
3.07
63.04
57.32
45.75
34.45
64.25
66.97
63.68
51.79
36.74
4.34
4.90
6.08
6.92
4.71
3.71
4.44
4.26
6.44
1.36
1.15
0.53
− 1.17
2.41
0.84
2.06
1.65
1.24
e) Medio Group
23 Ao Hondo NW
24 Ao Hondo
25 Loma del Medio
101199-2
101199-4
101199-6A
53.63
46.33
46.65
1.03
1.32
1.38
17.40
15.40
14.75
8.57
10.53
10.47
0.14
0.18
0.17
5.66
10.89
10.79
8.28
10.51
11.18
3.71
3.09
3.04
1.40
1.46
1.39
0.32
0.48
0.42
100.14
100.19
100.24
1.36
0.87
0.87
56.66
67.19
67.10
5.10
4.54
4.41
− 0.28
1.86
1.63
f) Las Malvinas Group
26 Negro
27 El Puntudo
28 Guadal
29 Puntano
30 Solo
111202-1A
111202-2
111202-3D
111202-4
111202-5
47.80
47.07
47.56
48.21
46.34
1.43
1.51
1.41
1.46
1.33
16.20
15.48
15.53
14.55
14.94
9.97
11.19
11.35
11.03
10.70
0.17
0.17
0.18
0.17
0.18
7.30
9.10
9.82
10.41
10.21
10.60
10.98
9.93
9.00
11.67
3.32
3.19
3.36
3.15
3.01
1.60
1.16
1.16
1.10
0.25
0.41
0.17
0.17
0.19
0.05
98.79
100.02
100.47
99.26
98.70
1.23
1.11
1.04
0.95
0.94
59.17
61.69
63.14
65.15
65.39
4.98
4.36
4.50
4.28
3.31
1.51
1.37
1.41
0.74
0.37
g) Guadal Volcano
31 La Carbonilla
32 La Carbonilla
121202-1A
121202-1B
48.32
49.11
1.83
1.40
17.50
16.06
11.35
9.88
0.17
0.17
5.35
7.60
10.52
10.00
3.44
3.51
1.15
1.69
0.36
0.23
99.99
99.65
1.91
1.17
48.25
60.36
4.59
5.22
1.14
1.42
h) Aisol Volcano
33 Nihuil
121202-2
48.20
1.59
15.45
11.85
0.18
8.45
9.88
3.38
1.05
0.31
100.35
1.26
58.53
4.42
1.07
Mg# = 100 Mg/(Mg + Fe2+) calculated with FeO⁎ = 0.9 Fe2O3.
b
Alkalinity = (K2O + Na2O) − 0.37 (SiO2 − 39).
a
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2009 Folguera et al Payenia

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