Thermal and lithospheric structure of the Chilean-Pampean flat-slab from gravity and magnetic data

CHAPTER

Thermal and lithospheric structure of the ChileanPampean flat-slab from gravity and magnetic data

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Marcos A. Sánchez⁎,‡, Héctor P.A. García⁎,‡, Gemma Acosta⁎,‡, Guido M. Gianni⁎,‡, Marcelo A. Gonzalez⁎,‡, Juan P. Ariza⁎,‡, Myriam P. Martinez⁎,‡, Andrés Folguera† Seismological Geophysical Institute Ing. Volponi (IGSV), FCEFyN, National University of San Juan, San Juan, Argentina⁎ Department of Geological Sciences, National Scientific and Technical Research Council (CONICET), IDEAN—Institute of Andean Studies "Don Pablo Groeber", FCEN, University of Buenos Aires, Buenos Aires, Argentina† National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina‡

1 ­Introduction Flat-slab settings around the world have been related to low heat flow regimes based on seismic, borehole, and magnetic data (e.g., Ziagos et al., 1985; Henry and Pollack, 1988; Currie et al., 2002; Gutscher, 2002; Shapiro and Ritzwoller, 2004; Manea et al., 2005, 2016; Marot et al., 2014; Collo et al., 2017; Li et al., 2017). Low heat flow regimes in these settings find an explanation as related to the expelling of the mantle wedge and consequent upper plate cooling during flat-slab development. However, the particular subduction configuration in these settings has been explained through different hypotheses: (a) the subduction of anomalously buoyant (thickened) oceanic crust (Cloos, 1993; Gutscher et al., 1999); (b) the fast overriding of the continent over young subducted oceanic lithosphere (van Hunen et al., 2002); (c) the hydrodynamic suction force due to thick continental roots that couples the subducting slab with the overriding plate (Manea et al., 2012); (d) a mantle plume-subduction zone interaction, where a buoyant plume dynamically sustains the subducted slab (Murphy et al., 1998; Gianni et al., 2017). While some flat subduction settings around the world seem to be related to one of these single processes, others seem to be related to multiple factors, showing some complexity in their underlying mechanisms. In particular, the Chilean-Pampean flat subduction zone, the second largest subhorizontal subduction zone on Earth, developed in the Southern Andes between 26° and 33° S (Fig. 1), is an example usually associated with a single control—the subduction of an aseismic ridge—although some authors have suggested that the mechanism that confers anomalous floatability to the subducted oceanic crust could be more complex (e.g., Hu and Liu, 2016). In this contribution, we focus on the Chilean-Pampean flat-slab, analyzing the thermal and lithospheric structure, which remains to date poorly assessed. Main characteristics of this setting are the absence of arc volcanism in the last ~4–2 Ma (Gutscher, 2002), the increase of intraplate seismicity with respect to neighbor regions (Barazangi and Isacks, 1976; Jordan et al., 1983; Alvarado et al., 2009; Gans et al., 2011), the widespread basement foreland deformation (Ramos et al., 2002), and dynamic subsidence at the leading edge of the flat segment (Dávila and Lithgow-Bertelloni, 2015). Andean Tectonics. https://doi.org/10.1016/B978-0-12-816009-1.00005-8 © 2019 Elsevier Inc. All rights reserved.

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Chapter 17  Structure of the Chilean-Pampean flat-slab

FIG. 1 Red dotted box shows the study area over a digital elevation model ETOPO1 (Amante and Eakins, 2009). Contours of the subducted Nazca slab from the Slab1.0 model from Hayes et al. (2012) shown as dashed black lines at 100-km intervals, with 0 km at the trench. A-A′ indicates the section modeled on Figs 8-10.

The thermal state of the Chilean-Pampean flat-slab was initially studied by Ruiz and Introcaso (2004) through Curie point depths determinations obtaining values of deep crust heat flow between 43 and 80 mW/m2 for the Precordillera and Western Sierras Pampeanas ranges (Fig. 1). In a recent study, Idárraga-García and Vargas (2018), using the EMAGv2 global magnetic model (Maus et  al., 2009), computed a deep crust heat flow map for South America. However, the map presents a low

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resolution for the flat-lab region, with values lower than 45 mW/m2. On a global scale, Hamza et al. (2008) determined consistent values ranging from 80 mW/m2 to 60 mW/m2 for the flat-slab region from development in harmonics based on a collection of surface data. More recently, Collo et al. (2017) used a compilation of temperature data from oil wells making a heat flow map for the whole flat-slab area, obtaining results that range from 20 to 70 mW/m2. However, a more recent work based on highresolution aeromagnetic data by Sánchez et al. (2018) showed a more complex thermal structure for the Chilean-Pampean flat slab than previously determined. Different seismological studies have allowed evaluating the lithospheric structure in the Chilean-Pampean flat-slab region (e.g., Pesicek et  al., 2012; Ammirati et  al., 2016; Portner et  al., 2017). Thus, Ammirati et  al. (2016) proposed a partially eclogitized lower crust in the central orogenic sector over the Chilean-Pampean flat-slab, based on receiver function analysis along a longitudinal profile. In a more regional study, maps from a seismic tomography obtained by Pesicek et al. (2012) indicate the presence of an attenuated area in the subducted oceanic lithosphere. More recently, Portner et al. (2017) based on a seismic tomography survey determined a low-velocity anomaly beneath the flat-slab region. These authors interpreted this feature as the result of the drag of the Juan Fernández mantle plume below the subducted Nazca plate. In this work, we integrate the aforementioned information with the aim of constructing a structural and thermal lithospheric model for the Chilean-Pampean flat-slab region that could conceal these different observations. To analyze the lithospheric thermal structure, we used the high-resolution GECO gravity geopotential model and an extensive compilation of magnetic data. Additionally, we considered the Moho depth from regional seismological studies (Assumpção et al., 2013), local studies of a Receiver Function analysis (Ammirati et al., 2016) and the oceanic slab speeds heterogeneities (Portner et al., 2017; Pesicek et al., 2012).

2 ­Tectonic setting The Chilean-Pampean flat slab links to the subduction of the Nazca plate beneath the South America plate (Barazangi and Isacks, 1976; Barazangi and Isacks, 1976; Cahill and Isacks, 1992; Gutscher et al., 2000). The change from shallow to flat subduction started at ~19 Myr (Jones et al., 2014, 2015, 2016), achieving a full flat-slab geometry at around ~7–6 Myr when the arc expanded toward the Precordillera and Sierras Pampeanas regions (Ramos et al., 2002; Gans et al., 2011; Gutscher et  al., 2000; Kay et  al., 1991; Kay and Mpodozis, 2002). The eastward expansion of the Neogene magmatism associated with the establishment of an arc gap between 27° and 33°S occurred coincidentally with the development of a broken foreland zone in the latest Miocene to Pliocene-Quaternary (Allmendinger et al., 1990; Cahill and Isacks, 1992; Jordan et al., 1983; Kay et al., 1988, 1991; Kay and Abbruzzi, 1996; Pilger, 1981; Smalley and Isacks, 1987; Stauder, 1973). The geometry of the Chilean-Pampean flat slab has been defined mainly from geophysical methods showing that the Nazca plate lays subhorizontal for ~400 km, beneath the highest Andes region to the east and plunging again into the asthenospheric mantle with “normal” angle (~from 30° to 35° to the east) (Fig. 1) (Barazangi and Isacks, 1976; Tassara et al., 2006; Alvarado et al., 2007; Anderson et al., 2007; Gilbert et al., 2006; Gimenez et al., 2009; Gans et al., 2011; Marot et al., 2014).

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Chapter 17  Structure of the Chilean-Pampean flat-slab

3 ­Methodology 3.1  ­Magnetic database In this study, we compiled an extensive magnetic database adding flight and terrestrial data to our previous dataset (Sánchez et al., 2018) from (i) a terrestrial dataset, which comprises 1035 magnetic stations from the Geophysical Seismological Institute of the National University of San Juan—Argentina (IGSV), and (ii) five aerial databases available of the total magnetic field (Blocks 4, 9, 17, 18, and 26 by their commercial nomination), obtained from the Argentinean Mining Geological Survey (SEGEMAR), where the companies of acquisition corrected the daily magnetic variation (Dobrin, 1976). We calculated the magnetic anomaly by subtracting the effect of the main magnetic field obtained from the International Geomagnetic Reference Field (IGRF) model, from each block at the acquisition date (Blakely, 1995). Finally, we built an anomaly map by using all the described databases and following the methodology proposed by Cheesman et al. (1998) and Ruiz et al. (2011) (see Fig. 2). Then, we calculated a high-resolution grid with a cell size of 250 × 250 m, using the Minimum Curvature method (Briggs, 1974). This chosen cell size combines the wavelengths of the aerial and terrestrial anomalies in a homogeneous grid.

3.2 ­Curie point-depth estimation Terrestrial magnetism originates mainly from the Earth core and crust, with the mantle not considered as a significant generator of the geomagnetic field due to the demagnetization of ferromagnetic minerals above 600°C (temperature Curie—Hinze et al., 2013, Blakely, 1995). The global average geothermal gradient is about 25°C/km near the surface (Alfe et al., 2003; Vlaar et al., 1994), a value that if extrapolated linearly at depth implies that within the earth crust, the Curie temperature of ferromagnetic rocks is exceeded. The depth at which this process takes place is known as Curie point depth, or Curie depth, from which no sources of magnetic field within the lithosphere below that depth exist. This means that if the petrophysical properties of the rocks present in the crust were known, the depth of the Curie point depth could be estimated (Ross et al., 2006). Different minerals have different Curie temperatures, a fact that would require alternative analysis of the Curie point to determine the thickness of the magnetic crust. However, Frost and Shive (1986) showed that the lower part of the crust is magnetite-rich, being the only significant magnetic source. Also, the temperature of the magnetite increases with pressure at a rate of 1.8°C/kbar. Hence, these authors suggest that deeper rocks lose their magnetization at about 600°C. The conventional methods used to estimate the depths of magnetic sources are based on statistical analysis of the magnetic anomaly in a frequency domain by using the power spectrum. The magnetized crust is assumed as a horizontal semi-infinite plate to simplify the calculation, whose top and bottom are at depths of Zt, and Zb, respectively. If its magnetization is uniformly random and its sources are not correlated, then the Power Spectrum of the magnetic anomalies can be related to its characteristic depths through the following equation (Blakely, 1995), 1− e ·

− k ( Zb − Zt )

ln Φ ∆T ( k ) = ln A − 2 Z t k + 2 ln, ´

(1)

where ΦT is the averaged Power Spectrum of the magnetic anomaly, k is the module of the wave vector, and A is a constant with no physical significance. This assumption for the estimation is valid

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FIG. 2 Magnetic anomaly compiled from terrestrial aerial databases, used to calculate a heat flow map on the Chilean-Pampean flat subduction zone. Black dots indicate the used land magnetic stations. Dashed white line represents the aerial database.

because it contemplates all the ferromagnetic crust materials and no magnetized bodies below the magnetized slab. So, the Curie point depth can be linked to the bottom of this magnetized slab, Zb. For the estimation of the Curie point depth in the study area, we have applied a method developed by Soler (2015) derived from the ones proposed by Ross et al. (2006) and Ravat et al. (2007). The method developed by Soler (2015) uses the Levenberg-Marquardt least square fitting algorithm for nonlinear problems in order to adjust the last equation on the previously radially averaged power spectrum of the magnetic anomalies. This method allowed estimating several Curie point depths through an interactive

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Chapter 17  Structure of the Chilean-Pampean flat-slab

selection of square subregions (100 × 100 km) of the magnetic anomaly grid. Then, by using the aerial and land magnetic database explained here, we obtained several estimates of the Curie depth. These specific values were regularized to obtain the continuous distribution shown in Fig. 3 (Briggs, 1974). Then, we compared the obtained Curie depths with the seismic tomography of Pesicek et al. (2012) that shows an attenuated area in the subducted oceanic plate in the flat-slab region, to observe if these changes in the lithospheric thermal structure had an effect in the calculated heat flow map.

FIG. 3 Curie point depth obtained from the Program CuDePy (Soler, 2015). Black dots indicate the location from which we calculated the CDP. Dashed black lines are the contours depth in kilometers of the subducted Nazca plate (Hayes et al., 2012).

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3.3 ­Heat flow determination Eq. (2) combines heat flow (q), Curie Temperature (θc), and Curie point depth (Dc), considering no internal heat sources between the Earth surface and the Curie point depth (Tanaka et al., 1999; Ruiz and Introcaso, 2004). (2)

q = kθ c / Dc

This equation allows making indirect calculations of the crustal heat flow through a good coverage of magnetic data, from the determined Curie point depth. The value of q calculated is mainly related to the heat conduction from the temperature at the lower crust. For the study area, the average geothermal gradient recently obtained by Collo et  al. (2017) is dT/dz = 31°C/km. From Fig. 3, the calculated average Curie point depth for this zone is Dc~45 km, and so, from Eq. (2), θc = 1395°C (1668°K). Springer and Förster (1998), Ruiz and Introcaso (2004), and others used different values for the coefficient of thermal conductivity (k) over the Central Andes region. We assumed k~2.5 W−1 m−1°K−1 for this estimation due to the existence of volcanic rocks and sediments, which have a different thermal behavior. The latter is in correspondence with the study of Li et al. (2017) who proposed a similar value for k in continental zones using the Earth Magnetic Anomaly Grid of 2-arc-minute resolution (Maus et al., 2009; EMAG2, http://geomag.org/). Regarding the map obtained, it is noteworthy that previous seismic investigations have found anomalies of seismic wave velocities in the Precordillera and Western Pampean Sierras area (Pesicek et al. 2012). This correspondence is represented in Fig. 4. Different authors have proposed heat flow values for the study area (SHF) (e.g., Collo et al., 2017; Sánchez et al., 2018). From these, we use a mean q value of ~70 mW/m2. Then, by assuming a mean regional Curie point depth Dc = 45 km and a mean heat flow qm~70 mW/m2, we have obtained k θc ≅ 3150 Wm−1 from Eq. (2). Fig. 5 shows a heat flow map obtained from the Curie point depth, considering qc = 3150 Wm−1.

3.4 ­Gravity database We obtained the Bouguer anomaly from the GECO model (n = 2190), to find the longest wavelength contributions to the gravity field (related to the mantle-crust discontinuity or Gravity Moho), (Gilardoni et al., 2016; http://icgem.gfz-potsdam.de/ICGEM/ICGEM.html). This model combines terrestrial, marine, and satellite data from GOCE and EGM2008 models, being data available in a grid of 181 × 241 nodes equispaced every 0.05° (Förste et al., 2014). Then, we inverted the gravity Moho depth from the gravity anomaly (AB) using a Python code developed by Soler (2015) that makes use of open-source libraries Scipy, Matplotlib, and Faitando a Terra (Jones et al., 2001; Hunter, 2007; Uieda et al., 2014). We obtained this inversion by adding the normal crustal thickness t to the inverted deflection winv, which was calculated by using the Parker-Oldenburg algorithm (Parker, 1973; Oldenburg, 1974). This was computed by an iterative method through Eq. (3), which starts with an arbitrary deflection (w0) and iterates until the desired error is reached. F ( wi ) = −

F ( AB ) e kt

2π G ( ρ m − ρ c )

N

+∑ n=2

( −1)

n

k n −1

n!

(

F win−1

)

(3)

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Chapter 17  Structure of the Chilean-Pampean flat-slab

FIG. 4 Comparison between Curie depth determinations and low-velocity zones from tomographic data from Pesicek et al. (2012). Blue line corresponds from 200 to 280 km depth, while yellow line from 160 to 220 km depth. Note the correlation between the shallower Curie points and low-velocity zone. Dashed white lines are the contours depth of the subducted Nazca plate (Hayes et al., 2012).

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FIG. 5 Heat flow map obtained by inversion of the Curie depth determinations.

The previous equation consists in calculating the deflection (wi) from the one obtained in the previous iteration wi − 1, where F is the Fourier transform, AB the Bouguer anomaly, t the normal crustal ­thickness, G the Universal gravitational constant, ρm the upper mantle density, ρc the lower crust density, N maximum summation order (in this work the expected error was reached with order equal to 18), n index of summation, and k the wave-vector module.

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Chapter 17  Structure of the Chilean-Pampean flat-slab

The first term of Eq. (3) amplifies the high frequencies, so the algorithm convergence is achieved using a low-pass filter (Oldenburg, 1974; Braitenberg and Zadro, 1999; Gomez-Ortiz and Agarwal, 2005). The Hamming filter was applied to each iteration following Soler (2015) 1   k   1 + cos  π   k < kcut Hamming filter =  2   kcut    k ≥ kcut 0 

(4)

Considering a cut-off wavenumber (kcut⁓ 0.0538) equivalent to a wavelength of λ ⁓ 116 km, which according to Eq. (4) proposed by Featherstone (1997) z=

Rλ

( 360 − λ )

(5)

corresponds to mass anomalies at a depth of ⁓18 km, where R is the mean Earth radius. Since our objective is to determine the upper mantle-lower crust discontinuity (gravity Moho) depth, we eliminated equivalent frequencies of the masses located on the upper crust at shallower depths than ~18 km (García et al., 2018). We used a normal crustal thickness of 35 km to estimate the gravity Moho (Assumpção et al., 2013). Then, we performed a set of inversion models by using values of density contrasts between the upper mantle and the lower crust (ρm − ρlc) in the order of 0.3, 0.35, 0.40 g/cm3, being a range of values typically used in gravity fields to estimate the mantle-crust discontinuity depths (e.g., Prezzi et al., 2009; Uieda and Barbosa, 2017; García et al., 2018). With the aim to select the most optimal density contrast, these gravity inversions were compared with the Moho reported by Assumpção et al. (2013) (see right column in Fig. 6). We observed a significant quantitative difference over the flat-slab zone, independent of the density contrast used, implying a shallower gravity Moho than the seismic Moho. This difference between a Moho obtained from the inversion of gravity data and those obtained from seismic velocities implies the need of incorporating some complexities to the former models. As a first approximation, we observed a straight correlation between this Moho difference (between gravity and seismological Mohos) and the low seismic-velocity zones from tomographic studies by Pesicek et al. (2012), particularly for the western Sierras Pampeanas (see Fig. 6A and B). In addition, this difference roughly coincides with high heat flow values determined from magnetic data in this work and with an eclogitized lower crust zone suggested by Ammirati et al. (2016), along a longitudinal profile (see Fig. 7A and B). To overcome this uncertainty, we tested three 2D models of gravity inversion (profile A-A′ traced on Fig. 1) following different possibilities and complexities introduced in recent seismic studies (Pesicek et al., 2012; Ammirati et al., 2016; Portner et al., 2017). With the aim of using as a constraint a seismological Moho depth in the 2D gravity models, we carried out our models along the receiver functions profile of Ammirati et al. (2016). In order to introduce a sector with eclogitization of the oceanic crust in our model (Peacock, 1996; Peacock and Wang, 1999; Gutscher et al., 2000), we used a density of ρ = 3.45 g/cm3. Additionally, we took into account a progressive increase in density through the Nazca plate at depth (Prezzi et al., 2009; Uieda and Barbosa, 2017; García et al., 2018). Model 1 considers the results obtained by Ammirati et al. (2016) using receptor functions, introducing a partially eclogitized lower continental crust under the higher topographies (Fig. 8). Although the model showed some similitudes between the observed and calculated Bouguer anomalies, it does not reach acceptable amplitude fit in the zone where the model considers a higher density in the lower continental lithosphere and oceanic lithosphere.

FIG. 6 (A, C, E) Calculated gravity Moho depth using density contrast for the crust-mantle transition of Δ = 0.3, 0.35, and 0.45 g/cm3 , respectively. Although the geometry of the Moho is similar for the three cases, the figures on the right (B, D, and F) numerically show that the model that presents the minimum difference with the seismological model is the one calculated with a density contrast for the lower crust mantle of Δρ = 0.4 g/cm3.

498 Chapter 17  Structure of the Chilean-Pampean flat-slab

FIG. 7 (A) Comparison between heat flow determinations and low-velocity zones from tomographic data from Pesicek et al. (2012) and gravity Moho depths (in white lines) for a lower crust-mantle density contrast of Δρ = 0.4 g/cm3. (B) Comparison between the gravity and seismic Moho differences and low-velocity zones from tomographic data from Pesicek et al. (2012).

FIG. 8

3 ­ Methodology

Model 1. (A) Gravity inversion model obtained by considering as constraints the Moho geometry and a partially eclogitized lower crust found through receiver function analyses (Ammirati et al., 2016). (B) Crustal model proposed by Ammirati et al., (2016). (C) Velocity sections obtained from receiver function (modified from Ammirati et al., 2016).

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Chapter 17  Structure of the Chilean-Pampean flat-slab

FIG. 9 Model 2. (A) Gravity inversion model obtained by considering as constraints the Moho geometry from Ammirati et al. (2016). This model incorporates a thinned Nazca Plate at depth presumably associated with a mantle upwelling proposed by Pesicek et al. (2012) (B and C).

Model 2 includes an attenuated/thinned zone in the subducted Nazca plate beneath the western Sierras Pampeanas region observed in the seismic tomography model of Pesicek et al. (2012). Although it is unlikely that a literal hole in the plate could exist with an underlying asthenospheric anomaly, a notable Vp/Vs anomaly exists in previous models (Pesicek et al., 2012). Then, Model 2 (Fig. 9) contemplates a lateral change in density in the lower part of the Nazca plate emulating a thinned lithospheric mantle. Again, as in the previous case, the adjustment between the model and data introducing this local anomaly is not satisfactory. Model 3 is based on the tomographic model of Portner et al. (2017) combining teleseismic primary and depth phase arrivals with local arrivals of aftershocks from the Maule earthquake (M8.8 Maule 2010)

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FIG. 10 Model 3. (A) Gravity inversion model obtained by considering as constraints the partially eclogitized continental lower crust from Ammirati et al. (2016) and a low-velocity zone below the Chilean-Pampean flatslab following Portner et al. (2017) model that gets the best fit to the Bouguer anomaly. (B) Cross-section of P wave velocity perturbations of the seismic tomography obtained by Portner et al. (2017).

(Fig. 10). This model highlights the presence of a low-density (velocity) zone below the subducted Nazca plate that Portner et al. (2017) interpret as related to the entrainment of an asthenospheric anomaly, potentially related to the Juan Fernandez Hot Spot (JFHS) beneath the flat-slab segment as a transient plume. We tested the existence of this low-velocity zone assigning an anomalously low density (ρ = 3.25 g/cm3) compatible with plume-mantle material. Additionally, we introduced an eclogitized lower crust (Ammirati

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Chapter 17  Structure of the Chilean-Pampean flat-slab

et al., 2016) previously considered in Model 1. Also, we contemplate a lateral variation in lithospheric thickness along the subducted plate reflected by a “plate hole” (Pesicek et al., 2012) considered in Model 2. Thus, this model combines features solved in different seismological studies for the flat-slab region. Notably, this model reproduces one of the best fits to the observed Bouguer anomalies.

4 ­Results and conclusions The spectral analysis of the aerial magnetic and terrestrial database compiled in this work allowed to build a Curie depth point map and to derive a regional heat flow map with a higher resolution than in previous studies (Figs. 3 and 5) (Ruiz and Introcaso, 2004; Collo et al., 2017; Sánchez et al., 2018). We determined a deeper Curie point depth in the eastern sector of the study area, achieving values between ~30 and 70 km over the northern sector of Precordillera, Northern and Eastern Sierras Pampeanas, and the northernmost extreme of the Frontal Cordillera. These results are locally consistent with the ones obtained by Weidmann et al. (2017) and Collo et al. (2017) over the western Sierras Pampeanas. We found a wide range of depths for the Curie point, with anomalously shallower values between ~8 and 30 km and higher heat flow values (from 30 to 90 mW/m2) following an NNW-SSE trending fringe, over the Main and Frontal cordilleras and the southern sector of the Precordillera and western Sierras Pampeanas. Additionally, we computed from a high-resolution gravity geopotential model (GECO), several gravity inversions to obtain the mantle-crust discontinuity (gravity Moho). These inversions were compared with the Moho depth from the regional seismological survey of Assumpção et al. (2013) to fit the best density constraint. Then, we observe a significant difference between both, for all used density contrast values, reaching up to ~30 km in the Precordillera. These results show that the gravity Moho depth is shallower than the seismological Moho depth over the flat-slab region (Assumpção et  al., 2013; Ammirati et al., 2016). Thus, an intriguing question is: why the obtained gravity Moho is substantially shallower than the Moho determined by seismological surveys? A possible explanation to this could be the existence of a partially eclogitized lower crust, like the one identified by Ammirati et al. (2016) producing an increase in the lower crust density (e.g., Giese et al., 1999; García et al., 2018). In this way, the shallower gravity Moho zone would be a function of the development of partial eclogitization of the lower crust supporting the inferences of Ammirati et al. (2016). However, it is worth noting that a significant proportion of the Chilean-Pampean flat-slab shows a shallower gravity Moho than the seismological Moho from Assumpção et al. (2013) (Fig. 6E). Besides, we note that these shallower gravity Moho inversions roughly correlate with determined higher heat flow values. These heat values are consistent with the analysis of elastic thickness carried out by Sánchez et al. (2018), indicating a zone of low elastic thicknesses with values between 16 and 32 km, spatially correlated with geothermal sources through the Main and Frontal Cordilleras. Also, these values are associated with low-velocity zones below the flat-slab area determined from tomographic data by Pesicek et al. (2012), particularly for the western Sierras Pampeanas region and by Portner et al. (2017) through local tomographic studies. To better understand the determined Moho discrepancies and to analyze possible lithospheric and sublithospheric contributions to them, suggested by recent seismological studies, we built three ­lithospheric-scale 2D gravity models integrating the available data. In these models, we considered the flat-slab geometry, including density variations of the subducted oceanic lithosphere and eclogitization of the subducted material. Then, the models were computed using as constraints the known Moho depth

­References

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obtained from receiver functions and tomography studies (Pesicek et al., 2012; Ammirati et al., 2016; Portner et al., 2017). We observe that the most complex model (Model 3) that includes partial eclogitization of the lower continental lithosphere and the upper oceanic lithosphere, and Nazca plate thinning overlying a low-density asthenospheric medium, presents the best fit of the gravity anomalies. Then, this model validates the Moho depths determined from receiver functions and partially eclogitization of the lower continental crust as suggested by Ammirati et al. (2016). Noteworthy, our results indicate that (i) a more extensive than previously recognized eclogitized area affecting the Andean roots could exist over the flat-slab region; (ii) the need for introducing a thinned oceanic lithosphere (Pesicek et al., 2012) in the flat section of the subducted Nazca plate; and (iii) the presence of low-density (hot) asthenospheric materials below the flat-slab region (Portner et al., 2017). Furthermore, the presence of low-density asthenospheric materials is consistent with the heterogeneous high heat flow obtained in this study and with the area of low effective elastic thickness reported by Sánchez et al. (2018). Therefore, gravity and magnetic analyses presented in this study constitute independent evidence for the existence of an anomalously hot asthenosphere beneath the flat-slab region, potentially coming from the subduction of plume material from the Juan Fernandez hot spot. In this regard, the introduction of a mantle plume and its related buoyant effect into a convergent margin may trigger changes in the slab angle leading to flat subduction, as recognized from ancient (Murphy et al., 1998) and more recent (Gianni et al., 2017) geological examples. This fact adds an additional factor not considered so far among the mechanisms associated with the development of the Chilean-Pampean flat subduction zone. This model opens the need of discussing from numerical constraints the proposed controls on flatslab geometry at the Chilean-Pampean flat subduction zone to eventually include dynamic and additional floatability forces that were tested in previous models as drivers of subducted slab shallowing.

­Acknowledgments We thank Vlad Manea and Gilda Collo for their constructive reviews that allowed us to improve this chapter." We thank Grosso Group Company and the Argentinean Geological Mining Service (SEGEMAR) for supplying the aeromagnetic flight data. 201501-00039-CO (Oriented Research Projects-CONICET--Secretariat of Science and Technology Government of San Juan), 15020150100039CO (Oriented Research Projects-CONICET) and PROJOVI 2018-2019 Project: 80020170300054SJ (National University of San Juan).

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­Further reading Hofmann-Wellenhof, B., Moritz, H., 2006. Physical Geodesy. Springer Science & Business Media. Kane, M.F., 1962. A comprehensive system of terrain corrections using a digital computer. Geophysics 27 (4), 455–462.