IBIGEO   22622
INSTITUTO DE BIO Y GEOCIENCIAS DEL NOA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Paleo-topography construction on the southern border of the Puna plateau
Autor/es:
MONTERO LÓPEZ, MARÍA CAROLINA; STRECKER, MANFRED
Lugar:
Neuquen, Argentina.
Reunión:
Congreso; XVIII Congreso Geológico Argentino; 2011
Resumen:
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The
Puna-Altiplano region, located in the Southern Central Andes between 13° and
27° south latitude constitute a non-collisional orogenic plateau (e.g., Isacks
1988, Allmendinger et al. 1997) characterized by high topography, low internal
relief, and pronounced relief contrasts compared to adjacent areas. This
particular architecture makes the Andean plateau a good example for studying
the influence of tectonism on the long-term behavior of climate (Alonso et al.
2006, Strecker et al. 2007). In addition, due to the arid climate, relief
conditions and excellent outcrops, particularly the southern rim of the plateau
is well suited to evaluate the structural and topographic evolution of this
region. We have selected Las Papas area (27°00S 67° 45W), located in the
border between the Southern Puna and Sierras Pampeanas, in order to study the
spatiotemporal frame of the topographic construction.
We have
carried out geological mapping and structural analysis as well as sampling of volcanic
and basement rocks for geochronological and thermochronologic analysis in the
study area. Along the Las Papas valley schist, amphibolites and gneisses of the
Neoproterozoic-Lower Paleozoic basement are outcropping, with some Ordovician plutonic
intrusions. This basement is overly by ignimbrite deposits of Upper Miocene
(Las Papas Ignimbrite, Montero López 2009) and Quaternary age (Cerro Blanco
Volcanic Complex, Seggiaro et al. 2000, Arnosio et al. 2005, Montero López
2009), and sedimentary deposits of Pliocene and Quaternary ages (Punaschotter
Formation, Penck 1920). Las Papas Ignimbrite comprises pinkish-white to pale purple
in color, welded deposits with characteristic columnar jointing and fiamme
texture. A white, slightly-welded ignimbrite deposit that belongs to the
youngest volcanic event of Cerro Blanco Volcanic Complex crop out at the
intersection of Las Papas and quebrada Blanca valleys. The Punaschotter deposits
in this area consist of proximal alluvial fan conglomerates (Schoenbohm and
Strecker 2009), including clasts derived from the igneous-metamorphic basement,
Ordovician and Neogene volcanic rocks. To the west of the Las Papas valley a
N-S oriented fault carried the metamorphic basement over this ignimbrite and the
Neogene-Quaternary sedimentary deposits, accommodating an E-W shortening
(Montero López 2009, Schoenbohm and Strecker 2009). An E-W oriented fault to
the north of the Las Papas valley is affecting the Las Papas Ignimbrite and the
metamorphic basement, accommodating a nearly N-S extension (Fiambalá Fault,
Schoenbohm and Strecker 2009, Montero López 2009).
Previous
studies in the northern section of the Bolsón de Fiambalá (Carrapa et al. 2006),
immediately to the south of the study area, have proposed that the uplift of
the ranges flanking the southern margin of the Puna has occurred by about 6 Ma.
However, we suppose an older age for the construction of the paleotopography inferred
by the presence of the thick welded Las Papas Ignimbrite filling a portion of
the gorge, which has been dated to ca. 9 Ma. This
is a crucial observation, because these pyroclastic deposits fill a paleorelief
and provide an important minimum constrain on the timing of the existence of
paleorelief along the southern Puna rim. In order to further refine this
hypothesis and place more constraints on the development of paleorelief, we collected
7 samples along an altitude transect in the Las Papas valley, located between 2100
and 3200 ma.s.l., from the Neoproterozoic-Lower Paleozoic basement. Apatite
grains were separated to analyses them by (U-Th)/He methodology which is used
to constrain the history of exhumation and relief development at the southern
margin of the Puna plateau. To date, 40Ar/39Ar
geochronologic and (U-Th)/He thermochronological analyses are being processed
and the results help us to provide a minimum constraint on the timing of
surface uplift (which must have preceded valley incision) at the plateau
margin.
In summary, thermochronologic studies have been
performed in the surrounding regions of the Las Papas area, but these were done
using the apatite fission-track (AFT) methodology, which is useful for
determining exhumation history, but is not sufficiently sensitive to record the
near-surface thermal perturbations induced by valley incision. However, the low
closure temperatures of the (U-Th)/He system in apatite makes it particularly
sensitive to near-surface cooling and thermal perturbations (Braun et al.
2006). Because isothermal surfaces close to the surface tend to follow the form
of the topography (Braun 2002), apatite (U-Th)/He cooling ages should reflect
the rapid cooling induced by downward movement of the closure isotherm in
response to deep valley incision (Schildgen et al. 2007). We apply the
(U-Th)/He methodology in apatites to resolve the differences in ages that have
been suggested for the development paleorelief based on AFT exhumation data
(e.g., 6 Ma, Carrapa et al. 2006) and our data based on infilling volcanic
units of Upper Miocene age