Alicia Favetto - Academia.edu (original) (raw)
Papers by Alicia Favetto
Geophysical Journal International, 1998
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Municipal and Industrial Waste Disposal, 2012
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Geologica Acta, 2019
The present research explores the Bañitos-Gollete geothermal field located in the Frontal Andes C... more The present research explores the Bañitos-Gollete geothermal field located in the Frontal Andes Cordillera over the Pampean flat-slab. We carried out an audiomagnetotelluric survey in order to define the underground geoelectrical structure and to understand the link between the geothermal fluid flow path and the main geological structures. 2-D audiomagnetotelluric models suggest that the deep-rooted N-S fault system controls the geothermal flow path. We propose a conductive heat-driven system, taking into consideration the geologic setting and the supposed low geothermal gradient of this tectonic environment. The mature Na-Cl waters from Gollete and an estimated reservoir temperature of ~140ºC are consistent with this conceptual model. Further investigations are required to assess the geothermal potential of the study area, and the present work likely represents only the first but necessary step in the exploration process.
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The Sierras Pampeanas of Argentina lie between about 26°S and 34°S. They are a region of active t... more The Sierras Pampeanas of Argentina lie between about 26°S and 34°S. They are a region of active thickskinned deformation between stable cratonic South America and the Andean Cordillera and are located east of both a quiescent Andean volcanic arc and the thin-skinned thrust belt of the Eastern Cordillera. These ranges largely consist of Precambrian to Lower Paleozoic granitic and metamorphic basement blocks which have been differentially tilted and uplifted along broad, high-angle, N-S trending thick-skinned thrust systems. They represent the Neogene eastward progression of Andean deformation into cratonic South America. The Sierras Pampeanas coincide with a region of flat subduction of the Nazca Plate (see Fig. 1). The flat portion of the subducted slab is widest in the south and progressively narrows to the north. It has been suggested that the slab dip at the northern end of the Sierras Pampeanas is evolving from a past flat configuration to more normal dip and that this is respon...
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Revista de la …, 2009
RESUMEN Un importante problema asociado a los rellenos sanitarios es la formación de lixiviado y ... more RESUMEN Un importante problema asociado a los rellenos sanitarios es la formación de lixiviado y el riesgo de contaminación del agua subterránea. La conductividad eléctrica del lixiviado es más alta que la del agua natural, consecuentemente el contraste en esta ...
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Journal of Applied Geophysics, 2013
ABSTRACT Despite its reduced penetration depth, audiomagnetotelluric (AMT) studies can be used to... more ABSTRACT Despite its reduced penetration depth, audiomagnetotelluric (AMT) studies can be used to determine a broad range of features related to little studied geothermal fields. This technique requires a stepwise interpretation of results taking into consideration diverse information (e.g. topographic, hydrological, geological and/or structural data) to constrain the characteristics of the study area. In this work, an AMT study was performed at the hot springs in the northern segment of the La Candelaria Range in order to characterize the area at depth. Geometric aspects of the shallow subsurface were determined based on the dimensional and distortion analysis of the impedance tensors. Also, the correlation between structural features and regional strikes allowed us to define two geoelectric domains, useful to determine the controls on fluid circulation. The subsurface resistivity distribution was determined through 1D and 2D models. The patterns of the 1D models were compared with the morpho-structure of the range. Shallow and deep conductive zones were defined and a possible shallow geothermal system scheme proposed. A strong correlation was found between the AMT results and the geological framework of the region, showing the relevance of using AMT in geothermal areas during the early stages of subsurface prospecting.
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Geochemistry, Geophysics, Geosystems, 2013
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Journal of South American Earth Sciences, 2011
The South American Guaraní Aquifer System covers the entire Parana basin and part of the Chaco-Pa... more The South American Guaraní Aquifer System covers the entire Parana basin and part of the Chaco-Parana basin. This system is one of the most important groundwater reservoirs; it is shared by four neighboring countries covering an area larger than one million square kilometers.The geological units closely related to the Guaraní Aquifer are the Piramboia and Botucatu Formations that consist of
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Geophysics, 2009
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Journal of Applied Geophysics, 2002
... the dead band. We tested this for each site by predicting the apparent resistivities in the d... more ... the dead band. We tested this for each site by predicting the apparent resistivities in the dead band from the phase and resistivities outside the dead band using the rhoplus code of Parker and Booker (1996). This is rigorous ...
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Tectonophysics, 2008
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Along the Andean Cordillera, the variations in tectonic style are associated with changes in the ... more Along the Andean Cordillera, the variations in tectonic style are associated with changes in the geometry of the subducted Nazca Plate. In Western Argentian, San Juan Province straddles the region of thin skin overthrusting called the Pre-Corillera in the west and the region of thick-skinned tectonics known as Sierras Pampeanas (SP) in the east. Directly under San Juan the subducted plate is horizontal. Steeply-dipping thrust faulting associated with the Pie de Palo block of the SP has been associated with devastating earthquakes. The subducted plate is also seismically active west of the Pie de Palo, but has been very quiet to the east. Very little is clearly understood about the reasons for the east-west change in tectonic style and whether it is directly related to processes in the underlying slab or possible intervening asthenosphere. In Nov. and Dec, 2000, we collected a roughly east-west magnetotelluric (MT) profile across these structures. Preliminary 2D inversion of these data show a major electrically conductive feature that dips westward under the Pie de Palo. It appears to extend through the Moho and flatten above the subducted slab. This suggests the presence of asthenosphere between the lithosphere and the slab, significant lateral variation of this asthenosphere and possible control of major active structures in the crust by this lateral variation in the mantle
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When the Nazca flat-slab rolls over and plunges into the transition zone under Argentina, it appe... more When the Nazca flat-slab rolls over and plunges into the transition zone under Argentina, it appears to separate an electrically resistive transition zone to the west from an electrically conductive transition zone to the east. The simplest explanation for this is that the water content of the transition zone is much lower to the west than the east. The low conductivity to the west can be explained if anhydrous upper mantle mantle is being carried down into the transition zone by slab motion. The much higher conductivity to the east is beneath the Rio de la Plata Craton whose root almost certainly inhibits vertical motion east of the slab. Thus water injected by the descending slab is likely to accumulate in the transition zone. This idea was first presented in a Nature paper in 2004. Since then, we have collected more magnetotelluric data to the south where the slab dip is normal, but voluminous back-arc basaltic volcanism occurs and in the region where the slab is said to be flexing continuously between the two geometries. A goal of this work is to test whether the slab has a similar relation to transition zone conductivity along strike. The new data, originally collected along linear profiles perpendicular to the expected strike of the slab in the mantle clearly indicated that 2-D interpretation would be problematic. Indeed, analysis of new data in the flexure region using 2-D methods reveals a narrow, roughly east-west, near vertical resistive structure extending down to the top of a conductive transition zone. A possible, but controversial interpretation of this structure is that it is the signature of a slab tear rather than the widely-accepted continuous flexure geometry. If a tear is indeed correct, then there is an opportunity to test how the slab is influencing the transition zone conductivity and by inference the water content by looking at the southern edge of the plunging 'flat- slab' as it enters the transition zone. Since the original data were collected we have been building an array of sites in both regions to permit fully 3-D analysis. Arrays of sufficient scale to undertake 3-D inversion now exist and preliminary results will be presented. Whatever the outcome of our current work, we want to stress the importance of probing the electrical conductivity of the transition zone in regions where identifiable vertical motions can influence the water content.
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To understand geologic processes that shaped western North America in the early Cenozoic, it is u... more To understand geologic processes that shaped western North America in the early Cenozoic, it is useful to look at southern South America, where similar processes such as flat-slab subduction and intraplate basaltic volcanism are active today. Payún Matrú is a very large shield volcano east of the Andes at 36.5 S 69.5 W. It is the largest feature of the “Payunia” basalt province that covers about 15,000 sq km. The most recent activity may have been about 1,000 years ago. Lava chemistry has evolved with time. Despite being only 150 km east of the axis of the Andean Southern Volcanic Zone, the younger lavas are essentially OIB. We have collected 38 long period magnetolluric (MT) sites in an array that extends beyond the Payunia basalts to investigate the source of magma. MT impedance tensor data (including vertical to horizontal magnetic field transfer functions) have been inverted for smoothest log resistivity using a 3D non-linear conjugate gradient (NLCG) algorithm. The model includes the Atlantic and Pacific oceans, which both significantly affect the data. The image shows the isosurface at 30 Ohm-m together with the locations of sites (inverted triangles), earthquakes deeper than 50 km (filled circles) and volcanoes with geological recent activity (normal triangles). The result is that Payún Matrú (the large triangle) lies at the northern end of a conductive finger at the top of the mantle. This finger has a pimple that rises into the upper crust just west of the caldera. The finger appears to originate in anomalously conductive mantle deeper than 150 km that extends south and to the east (away from the Andes). If this conductor is due to fully interconnected basalt partial melt, the region inside this isosurface has more than 3% partial melt. This structure appears to rule out such processes as crustal delamination or a vertical plume for this volcanic province. It also suggests that the lithosphere east of the asthenospheric wedge under the Andes is quite thin. Perhaps not much more than the continental crust. The anomalous mantle to the south and east could be a consequence of the early Miocene Somun Cura “hot spot” volcanism about 500 km south-southeast of Payún Matrú and the conductive finger could be related to the subsequent motion of South America relative to the hot spot frame of reference.
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Near 31.5°S, the subducted Nazca slab beneath Chile and western Argentina levels out near 100 km ... more Near 31.5°S, the subducted Nazca slab beneath Chile and western Argentina levels out near 100 km depth and does not roll over to plunge steeply into the mantle for several hundred km to the east. This flat slab prevents formation of an asthenospheric wedge under the Andes and consequently there are no active volcanoes. To the south this slab returns to a dip of ~30°, an asthenospheric wedge forms and there are active volcanoes south of 33.3°S in the Andean Southern Volcanic Zone (SVZ). The prevailing view is that the Nazca slab is warped continuously between its flat and dipping segments. Over the last decade, we have collected 83 magnetotelluric (MT) sites that form an array in Argentina from 60° - 70°W and 31.5° - 34°S. 18 sites extend this coverage along a profile near 31.5°S past where the flat slab plunges into the lower mantle. These data were initially collected as linear profiles. Eventually it became clear that the structure at mantle depth was 3D. The site geometry was thus expanded to its current 2D array. The final sites in this array were collected in late 2009. While each year’s data set was originally internally consistent, preparation of the complete data set required recalculation of the impedance tensor at each site so that all data are compatible and comparable. MT impedance tensor data (including vertical to horizontal magnetic field transfer functions) are being inverted for smoothest log conductivity using a 3D non-linear conjugate gradient (NLCG) algorithm. Initial results of this 3D study will be presented. Several conclusions have already emerged from 2D interpretations of subsets of the data. There is an asthenospheric wedge east of the flat slab. Its top at 100 km is probably at the base of the lithosphere through which it has not penetrated, presumably because of compressive stresses. This wedge is horizontally thin and is bounded to the east by the root of the Rio de la Plata Craton and to the west by the plunging slab. It extends to at least the base of the craton root and perhaps all the way to the transition zone below 400 km. This structure suggests that the flatslab rolls over where it does because it runs into the mechanically strong craton root. Conductive mantle, presumably the asthenospheric wedge associated with the SVZ, is clearly detected. It appears to extend at least 350 km east of the SVZ. In the east, the mantle structure north of this wedge is highly resistive to considerable depth. In the west however, there is an east-west conductive zone within this resistive mantle that coincides with the maximum flexure of the warped slab. Determining the extent of this structure should help constrain whether this is evidence for a torn rather than continuous slab or whether it is some result of the high strain within the warp. In fact, a revised 2D structure along a North-South line through Encon shows a conductor that appears to be coincident with the slab as it dips down slightly to the south along the profile. The model along a different profile along the 200 km isodepth of the slab shows no evidence of slab-related structure.
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Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentin... more Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have ...
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S. Mendoza Province, south of the Nazca flat slab, has widespread recent basaltic volcanism, but ... more S. Mendoza Province, south of the Nazca flat slab, has widespread recent basaltic volcanism, but no historic activity. Over the last 1 MY, the basalt has evolved from having a slab signature to OIB. In early 2005, we collected 18 MT sites from 67 to 70°W along 36.7°S, near the large caldera Payún Matrú. Significantly 3D data cause difficulty identifying regional strike and performing minimum structure 2D inversions. Use of phase tensors and induction vectors allow identification of possible regional strikes. Various starting models for the minimum structure inversion allow comparison of effect of different regional strikes on the resulting electrical conductivity models. Preliminary analysis of a piecewise-2D region at the center of the profile suggests the west end of the profile to contain lower crustal to upper mantle conductivity which decreases and deepens to the east.
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Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentin... more Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have essentially no arc signature. Kay, et al. (2006) and Folguera, et al. (2006) argue that this back-arc igneous province is the result of extension due to trench roll-back following steepening of a flat slab that existed in the middle to late Miocene. Magnetotelluric data collected in 2005 at 18 sites along an east-west profile from 70W to 67W have been used to probe the source of the Payún Matrú basalts. These data imply significantly 3D structure. However, preliminary analysis of an arguably 2D region at the center of the profile allows tentative identification of a conductive mantle plume surfacing at Payún Matrú that rises from below 200 km depth. This suggests that Payún Matrú Volcanic Field is sampling mantle deeper than 200 km, perhaps just above the subducted Nazca slab or perhaps from the mantle transition near where the Nazca slab penetrates into it at 400 km. The 2008 fieldwork extends the earlier profile to a spatial array extending from Laguna Llancanelo north of Payún Matrú to beyond the Cortaderas Lineament that bounds the basaltic province to the south. These 19 new sites consist of 5 to 10 days of 4 Hz horizontal electric and 3-component magnetic field time- series. Data processing in the field suggests that the deep crust or upper mantle has northwest-southeast striking structure increases in conductivity to the southwest of Payún Matrú. This underlies shallow structure with north-south strike between Payún Matrú and the Colorado River. This complexity explains our initial difficulty with 2D interpretation. We will display a preliminary 3D interpretation of these new data.
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Nature, 2004
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Journal of Geophysical Research, 2007
A magnetotelluric (MT) profile and conventional seismic reflection lines are used to characterize... more A magnetotelluric (MT) profile and conventional seismic reflection lines are used to characterize the sedimentary basin across the Andean foothills in the Tucumán Plain. This basin is a depocenter surrounded by crystalline basement and filled with a several-kilometer-thick sedimentary sequence. Because of the high contrast in electrical resistivity between the basin sedimentary rocks and the crystalline basement, this interface dominates the MT response and masks details of the structure within the sedimentary units. This paper presents a new interpretation of the MT data incorporating structure constraints obtained from a network of seismic lines. Our results indicate that a significantly improved characterization of the conductive horizons within the basin is obtained when resistivity model discontinuities are introduced at the seismic reflectors corresponding to the most important formation boundaries. Improved resistivities of the main formations, Cretaceous-Miocene and Miocene-Pliocene, were inferred. The comparison of constrained and unconstrained models constitutes an instructive case study of how controlled MT inversion can improve structure imaging. It also shows that the application of constraints in the inversion process is a worthy option to find a geologically supported model, especially if data misfit shows a marked nonrandom distribution.
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Geophysical Journal International, 1998
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Municipal and Industrial Waste Disposal, 2012
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Geologica Acta, 2019
The present research explores the Bañitos-Gollete geothermal field located in the Frontal Andes C... more The present research explores the Bañitos-Gollete geothermal field located in the Frontal Andes Cordillera over the Pampean flat-slab. We carried out an audiomagnetotelluric survey in order to define the underground geoelectrical structure and to understand the link between the geothermal fluid flow path and the main geological structures. 2-D audiomagnetotelluric models suggest that the deep-rooted N-S fault system controls the geothermal flow path. We propose a conductive heat-driven system, taking into consideration the geologic setting and the supposed low geothermal gradient of this tectonic environment. The mature Na-Cl waters from Gollete and an estimated reservoir temperature of ~140ºC are consistent with this conceptual model. Further investigations are required to assess the geothermal potential of the study area, and the present work likely represents only the first but necessary step in the exploration process.
Bookmarks Related papers MentionsView impact
The Sierras Pampeanas of Argentina lie between about 26°S and 34°S. They are a region of active t... more The Sierras Pampeanas of Argentina lie between about 26°S and 34°S. They are a region of active thickskinned deformation between stable cratonic South America and the Andean Cordillera and are located east of both a quiescent Andean volcanic arc and the thin-skinned thrust belt of the Eastern Cordillera. These ranges largely consist of Precambrian to Lower Paleozoic granitic and metamorphic basement blocks which have been differentially tilted and uplifted along broad, high-angle, N-S trending thick-skinned thrust systems. They represent the Neogene eastward progression of Andean deformation into cratonic South America. The Sierras Pampeanas coincide with a region of flat subduction of the Nazca Plate (see Fig. 1). The flat portion of the subducted slab is widest in the south and progressively narrows to the north. It has been suggested that the slab dip at the northern end of the Sierras Pampeanas is evolving from a past flat configuration to more normal dip and that this is respon...
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Revista de la …, 2009
RESUMEN Un importante problema asociado a los rellenos sanitarios es la formación de lixiviado y ... more RESUMEN Un importante problema asociado a los rellenos sanitarios es la formación de lixiviado y el riesgo de contaminación del agua subterránea. La conductividad eléctrica del lixiviado es más alta que la del agua natural, consecuentemente el contraste en esta ...
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Journal of Applied Geophysics, 2013
ABSTRACT Despite its reduced penetration depth, audiomagnetotelluric (AMT) studies can be used to... more ABSTRACT Despite its reduced penetration depth, audiomagnetotelluric (AMT) studies can be used to determine a broad range of features related to little studied geothermal fields. This technique requires a stepwise interpretation of results taking into consideration diverse information (e.g. topographic, hydrological, geological and/or structural data) to constrain the characteristics of the study area. In this work, an AMT study was performed at the hot springs in the northern segment of the La Candelaria Range in order to characterize the area at depth. Geometric aspects of the shallow subsurface were determined based on the dimensional and distortion analysis of the impedance tensors. Also, the correlation between structural features and regional strikes allowed us to define two geoelectric domains, useful to determine the controls on fluid circulation. The subsurface resistivity distribution was determined through 1D and 2D models. The patterns of the 1D models were compared with the morpho-structure of the range. Shallow and deep conductive zones were defined and a possible shallow geothermal system scheme proposed. A strong correlation was found between the AMT results and the geological framework of the region, showing the relevance of using AMT in geothermal areas during the early stages of subsurface prospecting.
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Geochemistry, Geophysics, Geosystems, 2013
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Journal of South American Earth Sciences, 2011
The South American Guaraní Aquifer System covers the entire Parana basin and part of the Chaco-Pa... more The South American Guaraní Aquifer System covers the entire Parana basin and part of the Chaco-Parana basin. This system is one of the most important groundwater reservoirs; it is shared by four neighboring countries covering an area larger than one million square kilometers.The geological units closely related to the Guaraní Aquifer are the Piramboia and Botucatu Formations that consist of
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Geophysics, 2009
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Journal of Applied Geophysics, 2002
... the dead band. We tested this for each site by predicting the apparent resistivities in the d... more ... the dead band. We tested this for each site by predicting the apparent resistivities in the dead band from the phase and resistivities outside the dead band using the rhoplus code of Parker and Booker (1996). This is rigorous ...
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Tectonophysics, 2008
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Along the Andean Cordillera, the variations in tectonic style are associated with changes in the ... more Along the Andean Cordillera, the variations in tectonic style are associated with changes in the geometry of the subducted Nazca Plate. In Western Argentian, San Juan Province straddles the region of thin skin overthrusting called the Pre-Corillera in the west and the region of thick-skinned tectonics known as Sierras Pampeanas (SP) in the east. Directly under San Juan the subducted plate is horizontal. Steeply-dipping thrust faulting associated with the Pie de Palo block of the SP has been associated with devastating earthquakes. The subducted plate is also seismically active west of the Pie de Palo, but has been very quiet to the east. Very little is clearly understood about the reasons for the east-west change in tectonic style and whether it is directly related to processes in the underlying slab or possible intervening asthenosphere. In Nov. and Dec, 2000, we collected a roughly east-west magnetotelluric (MT) profile across these structures. Preliminary 2D inversion of these data show a major electrically conductive feature that dips westward under the Pie de Palo. It appears to extend through the Moho and flatten above the subducted slab. This suggests the presence of asthenosphere between the lithosphere and the slab, significant lateral variation of this asthenosphere and possible control of major active structures in the crust by this lateral variation in the mantle
Bookmarks Related papers MentionsView impact
When the Nazca flat-slab rolls over and plunges into the transition zone under Argentina, it appe... more When the Nazca flat-slab rolls over and plunges into the transition zone under Argentina, it appears to separate an electrically resistive transition zone to the west from an electrically conductive transition zone to the east. The simplest explanation for this is that the water content of the transition zone is much lower to the west than the east. The low conductivity to the west can be explained if anhydrous upper mantle mantle is being carried down into the transition zone by slab motion. The much higher conductivity to the east is beneath the Rio de la Plata Craton whose root almost certainly inhibits vertical motion east of the slab. Thus water injected by the descending slab is likely to accumulate in the transition zone. This idea was first presented in a Nature paper in 2004. Since then, we have collected more magnetotelluric data to the south where the slab dip is normal, but voluminous back-arc basaltic volcanism occurs and in the region where the slab is said to be flexing continuously between the two geometries. A goal of this work is to test whether the slab has a similar relation to transition zone conductivity along strike. The new data, originally collected along linear profiles perpendicular to the expected strike of the slab in the mantle clearly indicated that 2-D interpretation would be problematic. Indeed, analysis of new data in the flexure region using 2-D methods reveals a narrow, roughly east-west, near vertical resistive structure extending down to the top of a conductive transition zone. A possible, but controversial interpretation of this structure is that it is the signature of a slab tear rather than the widely-accepted continuous flexure geometry. If a tear is indeed correct, then there is an opportunity to test how the slab is influencing the transition zone conductivity and by inference the water content by looking at the southern edge of the plunging 'flat- slab' as it enters the transition zone. Since the original data were collected we have been building an array of sites in both regions to permit fully 3-D analysis. Arrays of sufficient scale to undertake 3-D inversion now exist and preliminary results will be presented. Whatever the outcome of our current work, we want to stress the importance of probing the electrical conductivity of the transition zone in regions where identifiable vertical motions can influence the water content.
Bookmarks Related papers MentionsView impact
To understand geologic processes that shaped western North America in the early Cenozoic, it is u... more To understand geologic processes that shaped western North America in the early Cenozoic, it is useful to look at southern South America, where similar processes such as flat-slab subduction and intraplate basaltic volcanism are active today. Payún Matrú is a very large shield volcano east of the Andes at 36.5 S 69.5 W. It is the largest feature of the “Payunia” basalt province that covers about 15,000 sq km. The most recent activity may have been about 1,000 years ago. Lava chemistry has evolved with time. Despite being only 150 km east of the axis of the Andean Southern Volcanic Zone, the younger lavas are essentially OIB. We have collected 38 long period magnetolluric (MT) sites in an array that extends beyond the Payunia basalts to investigate the source of magma. MT impedance tensor data (including vertical to horizontal magnetic field transfer functions) have been inverted for smoothest log resistivity using a 3D non-linear conjugate gradient (NLCG) algorithm. The model includes the Atlantic and Pacific oceans, which both significantly affect the data. The image shows the isosurface at 30 Ohm-m together with the locations of sites (inverted triangles), earthquakes deeper than 50 km (filled circles) and volcanoes with geological recent activity (normal triangles). The result is that Payún Matrú (the large triangle) lies at the northern end of a conductive finger at the top of the mantle. This finger has a pimple that rises into the upper crust just west of the caldera. The finger appears to originate in anomalously conductive mantle deeper than 150 km that extends south and to the east (away from the Andes). If this conductor is due to fully interconnected basalt partial melt, the region inside this isosurface has more than 3% partial melt. This structure appears to rule out such processes as crustal delamination or a vertical plume for this volcanic province. It also suggests that the lithosphere east of the asthenospheric wedge under the Andes is quite thin. Perhaps not much more than the continental crust. The anomalous mantle to the south and east could be a consequence of the early Miocene Somun Cura “hot spot” volcanism about 500 km south-southeast of Payún Matrú and the conductive finger could be related to the subsequent motion of South America relative to the hot spot frame of reference.
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Near 31.5°S, the subducted Nazca slab beneath Chile and western Argentina levels out near 100 km ... more Near 31.5°S, the subducted Nazca slab beneath Chile and western Argentina levels out near 100 km depth and does not roll over to plunge steeply into the mantle for several hundred km to the east. This flat slab prevents formation of an asthenospheric wedge under the Andes and consequently there are no active volcanoes. To the south this slab returns to a dip of ~30°, an asthenospheric wedge forms and there are active volcanoes south of 33.3°S in the Andean Southern Volcanic Zone (SVZ). The prevailing view is that the Nazca slab is warped continuously between its flat and dipping segments. Over the last decade, we have collected 83 magnetotelluric (MT) sites that form an array in Argentina from 60° - 70°W and 31.5° - 34°S. 18 sites extend this coverage along a profile near 31.5°S past where the flat slab plunges into the lower mantle. These data were initially collected as linear profiles. Eventually it became clear that the structure at mantle depth was 3D. The site geometry was thus expanded to its current 2D array. The final sites in this array were collected in late 2009. While each year’s data set was originally internally consistent, preparation of the complete data set required recalculation of the impedance tensor at each site so that all data are compatible and comparable. MT impedance tensor data (including vertical to horizontal magnetic field transfer functions) are being inverted for smoothest log conductivity using a 3D non-linear conjugate gradient (NLCG) algorithm. Initial results of this 3D study will be presented. Several conclusions have already emerged from 2D interpretations of subsets of the data. There is an asthenospheric wedge east of the flat slab. Its top at 100 km is probably at the base of the lithosphere through which it has not penetrated, presumably because of compressive stresses. This wedge is horizontally thin and is bounded to the east by the root of the Rio de la Plata Craton and to the west by the plunging slab. It extends to at least the base of the craton root and perhaps all the way to the transition zone below 400 km. This structure suggests that the flatslab rolls over where it does because it runs into the mechanically strong craton root. Conductive mantle, presumably the asthenospheric wedge associated with the SVZ, is clearly detected. It appears to extend at least 350 km east of the SVZ. In the east, the mantle structure north of this wedge is highly resistive to considerable depth. In the west however, there is an east-west conductive zone within this resistive mantle that coincides with the maximum flexure of the warped slab. Determining the extent of this structure should help constrain whether this is evidence for a torn rather than continuous slab or whether it is some result of the high strain within the warp. In fact, a revised 2D structure along a North-South line through Encon shows a conductor that appears to be coincident with the slab as it dips down slightly to the south along the profile. The model along a different profile along the 200 km isodepth of the slab shows no evidence of slab-related structure.
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Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentin... more Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have ...
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S. Mendoza Province, south of the Nazca flat slab, has widespread recent basaltic volcanism, but ... more S. Mendoza Province, south of the Nazca flat slab, has widespread recent basaltic volcanism, but no historic activity. Over the last 1 MY, the basalt has evolved from having a slab signature to OIB. In early 2005, we collected 18 MT sites from 67 to 70°W along 36.7°S, near the large caldera Payún Matrú. Significantly 3D data cause difficulty identifying regional strike and performing minimum structure 2D inversions. Use of phase tensors and induction vectors allow identification of possible regional strikes. Various starting models for the minimum structure inversion allow comparison of effect of different regional strikes on the resulting electrical conductivity models. Preliminary analysis of a piecewise-2D region at the center of the profile suggests the west end of the profile to contain lower crustal to upper mantle conductivity which decreases and deepens to the east.
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Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentin... more Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have essentially no arc signature. Kay, et al. (2006) and Folguera, et al. (2006) argue that this back-arc igneous province is the result of extension due to trench roll-back following steepening of a flat slab that existed in the middle to late Miocene. Magnetotelluric data collected in 2005 at 18 sites along an east-west profile from 70W to 67W have been used to probe the source of the Payún Matrú basalts. These data imply significantly 3D structure. However, preliminary analysis of an arguably 2D region at the center of the profile allows tentative identification of a conductive mantle plume surfacing at Payún Matrú that rises from below 200 km depth. This suggests that Payún Matrú Volcanic Field is sampling mantle deeper than 200 km, perhaps just above the subducted Nazca slab or perhaps from the mantle transition near where the Nazca slab penetrates into it at 400 km. The 2008 fieldwork extends the earlier profile to a spatial array extending from Laguna Llancanelo north of Payún Matrú to beyond the Cortaderas Lineament that bounds the basaltic province to the south. These 19 new sites consist of 5 to 10 days of 4 Hz horizontal electric and 3-component magnetic field time- series. Data processing in the field suggests that the deep crust or upper mantle has northwest-southeast striking structure increases in conductivity to the southwest of Payún Matrú. This underlies shallow structure with north-south strike between Payún Matrú and the Colorado River. This complexity explains our initial difficulty with 2D interpretation. We will display a preliminary 3D interpretation of these new data.
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Nature, 2004
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Journal of Geophysical Research, 2007
A magnetotelluric (MT) profile and conventional seismic reflection lines are used to characterize... more A magnetotelluric (MT) profile and conventional seismic reflection lines are used to characterize the sedimentary basin across the Andean foothills in the Tucumán Plain. This basin is a depocenter surrounded by crystalline basement and filled with a several-kilometer-thick sedimentary sequence. Because of the high contrast in electrical resistivity between the basin sedimentary rocks and the crystalline basement, this interface dominates the MT response and masks details of the structure within the sedimentary units. This paper presents a new interpretation of the MT data incorporating structure constraints obtained from a network of seismic lines. Our results indicate that a significantly improved characterization of the conductive horizons within the basin is obtained when resistivity model discontinuities are introduced at the seismic reflectors corresponding to the most important formation boundaries. Improved resistivities of the main formations, Cretaceous-Miocene and Miocene-Pliocene, were inferred. The comparison of constrained and unconstrained models constitutes an instructive case study of how controlled MT inversion can improve structure imaging. It also shows that the application of constraints in the inversion process is a worthy option to find a geologically supported model, especially if data misfit shows a marked nonrandom distribution.
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