GEOINDICATORS ASSOCIATED WITH NEOTECTONIC FAULTING IN SIERRAS
PAMPEANAS, CENTRAL ARGENTINA. A SYNTESIS OF KEY EVIDENCES
Armando Carlos Massabie
Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales,
Departamento de Ciencias Geológicas, Pabellón II, C1428EHA, Buenos Aires, Argentina;
armando@gl.fcen.uba.ar
INTRODUCTION
Neogene deformation due to Andean Orogeny has caused block faulting in the Sierras
Pampeanas area, fracturing a Precambrian – Paleozoic crystalline basement. These dislocations
produced Quaternary landscape changes which are mainly expressed in clear and good preserved
fluvial path modifications in sedimentary cover.
The study of alterations of fluvial channel pattern associated with Quaternary tectonics has
special neotectonic value in this central area of Argentina as a basic method to gain more precise
temporal restrain of recent movements.
Major difficulties for a more precise dating of neotectonic faulting in the area of Sierras
Pampeanas is a direct consequence of the random distribution of Neogene continental sediments
located in different isolated basins which are mainly lacking of good bioestratigraphic control
for comprehensive correlation.
Structural and tectonics studies, in progress since more than a decade ago in the area has
revelled the presence of several places with good exposures of neotectonic movements due to
Andean Orogeny, which are put in evidence by fault contacts of Precambrian basement in
hanging wall blocks over Late Pleistocene-Holocene sediments in the footwall block.
First steps in the study of Quaternary faulting dating in Sierras Pampeanas start with the
pioneer paper of Schlagintweit (1954) which reported a Quaternary faulting in Potrero de Garay
area of Córdoba supported by stratigraphic and morphostructural considerations. Afterthen other
papers about Quaternary faulting were presented based on similar geologic constraints (Lencinas
y Timonieri, 1968; Massabie, 1976, 1987; Massabie and Szlafsztein, 1991; Kraemer et al., 1993;
Massabie et al.,1998, 1999 and 2001).
At present there is a good and broad background about the regional importance of
Quaternary faulting in Sierras Pampeanas of central Argentina. Initial studies have been
concentrated in Sierras Pampeanas Orientales from Córdoba, San Luis and Santiago del Estero
Provinces but now they have been extended to more interior lands of Sierras Pampeanas
Noroccidentales in La Rioja Province (Massabie et al., 1998).
The eastern border of Sierras Pampeanas, Sierras Pampeanas Orientales, has a N-S length
of near 500 km and a wideness and a height over sea level of almost 130 km and 3 kilometres. In
spite of its location about 700 km far from the east Pacific subduction zone of Nazca plate, in
central Argentina, intraplate seismicity is important in this mountains.
There is a set of key geologic features associated with this neotectonic faulting that are
considered proper geoindicators of its presence and recent activity.
Key features have been subdivided in two main groups. The first one, regional or local,
includes landscape and geomorphic evolutions tectonically controlled by neotectonic faulting. In
the second group there are included local structure of fault zones and specific rock fault suites.
Few places with neotectonic faulting in the area show a complete set of all key features
recognised, but the presence of some of them (regional or local) induces positive searching for
the others.
Geoindicators from neotectonic faulting in Sierras Pampeanas are here presented as
evidences of recent movements of direct tectonic source.
LOCATION AND TECTONICS
Localities with neotectonic faulting considered include Sierras Pampeanas Geologic
Province of central Argentina extended mainly over Córdoba, Santiago del Estero, San Luis,
Tucumán, Catamarca, San Juan and La Rioja states, but observations presented here are
restricted to Córdoba, San Luis and La Rioja states.
As it was mentioned, the area is far from west Andean orogenic border of South American
plate which concentrates the highest seismic activity in Argentina. In fact it is part of the interior
lands of the eastern foreland of Andean Mountains Chain, but it includes a rather important
seismicity of low to medium intensity with some destructive events (Ej.: Sampacho,
Córdoba,1934; San Francisco, San Luis,1936) in Sierras Pampeanas Orientales. Neogene
deformation of this extended area and its continuation in Holocene and present times is indicated
by widespread seismicity. Its tectonic activity is considered a consequence of Nazca plate flat
slab mode subduction between Lat. 28º S and 33º S (Barazangi & Isacks, 1976; Smalley et al.,
1993).
SCOPE
The aim of this contribution is to show characteristic features, geomorphic, structural,
hydrogeologic an seismic, associated with well sustained neotectonic faulting in Sierras
Pampeanas of central Argentina (Figure 1).
This faulting is related with recent movements from Late Pleistocene to Late Holocene age
which at present have recent radiocarbon dating constraints as young as 2,560 b.p. and 1,300 b.p.
from Sierras Pampeanas Orientales (Massabie et al., 2001; Costa & Vita Finzi, 1996).
GEOINDICATORS ASSOCIATED WITH NEOTECTONIC FAULTING
In a way from a more comprehensive to local analysis of the theme it is useful to consider
that Andean Orogeny, mainly Tertiary to Quaternary up to present times, exert a hard
morphostructural control. Therefor Quaternary landscape evolution was taking as a first target
and special attention was paid on foothills areas of Sierras Pampeanas.
From these observations it is summarised a first set of geomorphic geoindicators of
neotectonic faulting recognised in Sierras Pampeanas.
1. Low height partially eroded fault scarps, in the foothills of a master hill which is sculpted
either on basement rocks or Quaternary sediments.
2. Identification of local and regional changes of fluvial patterns due to neotectonic faulting.
3. Disruption, transitory lake formation and river and valley anomalies (Figure 1).
4. Asymmetric erosion of fluvial terrace levels of Quaternary age by elevation and tilting of
hidden or partially hidden basement blocks controlled by recent faulting.
5. Aligned marshy areas (may be transitory) or springs of phreatic groundwater along a
neotectonic fault trace in fluvial terraces laying over tilted fault blocks.
Figure 1. Eastern Sierras Pampeanas. Location of fault segments with neotectonic reactivation
and the Late Holocene Quilpo River neotectonic damming up. Adapted from Massabie et al.
(2000, 2001)
Figure 2. Neotectonic Nono Fault. Paleozoic Achala Granite in the hanging wall overrides
Brochero Formation (Late Tertiary) and Las Rabonas Formation of Late Pleistocene age. View
towards the west of the right bank of Los Sauces River. See Figure 1 for location.
In a more detailed scale, based on structural local observations, it could be grouped another
set of fault zone geoindicators of neotectonic faulting (Figs. 2 and 3).
1. Fault contact of basement rocks (Precambrian to Lower Paleozoic) over Quaternary
continental sediments, mainly fluvial.
2. Lithologies involved: igneous and metamorphic rocks are override on fanglomerates and
fluvial terrace deposits.
3. Structural style: NW, N and NE strike overthrusts of low to medium dip angle. In some
cases they are upthrusts. The faults have dip slip an also strike slip components.
4. Fault breccias of 1 to 5m thick are formed in the basement hanging wall block, and dark
red or green colour gauge and clay are placed in the contact with Quaternary sediments of
the footwall block. In some places where neotectonic faulting cuts only basement rocks,
cataclasites and microbreccias were formed in the fault zone.
5. Presence of parallel clivaje in the lower clay fault subzone (0.20 to 1.5 m thick) of the
fault zone.
Figure 3. Neotectonic Potrero de Garay Fault. Brecciated Precambrian metamorphic rocks
override Late Pleistocene fluvial deposits of the foot wall block in the north east border of Los
Molinos Lake. View towards de west. See Figure 1 for location.
CONCLUSIONS
Sierras Pampeanas of central Argentina shows widespread seismicity and some destructive
earthquakes which are linked to neotectonic faulting.
The study of alterations of fluvial pattern and Quaternary landscape evolution has an
important neotectonic value in this central area of Argentina as a method to gain more precise
temporal constraints of recent surface movements.
Geoindicators of recent surface movements are here divided in two main groups:
geomorphic geoindicators and fault zone geoindicators.
Geomorphic geoindicators are recognised in both regional (landscape) and local (outcrop)
scales of observation, but fault zone geoindicators are identified in local scales (outcrop and hand
samples).
Geoindicators associated with neotectonic faulting in Sierras Pampeanas are considered
evidences of Late Quaternary to recent surface movements of direct tectonic source.
AKNOWLEDGEMENTS
The author wish to thank Department of Geology, Faculty of Sciences of Buenos Aires
University and Ex048 UBACYT project for support.
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