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Earth and Planetary Science Letters 245 (2006) 289 – 301 www.elsevier.com/locate/epsl Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina Lara S. Wagner ⁎, Susan Beck, George Zandt, Mihai N. Ducea Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, AZ 85721, USA Received 22 July 2005; received in revised form 2 February 2006; accepted 6 February 2006 Available online 19 April 2006 Editor: K. Farley Abstract Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with “normal,” steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low Vp / Vs ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this “transition zone”, the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high Vp / Vs ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high Vp / Vs ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low Vp, high Vs, and low Vp / Vs signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2 Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab. © 2006 Elsevier B.V. All rights reserved. Keywords: tomography; flat-slab; harzburgite; Chile; Argentina 1. Introduction Beneath central Chile and western Argentina, the subducting Nazca slab has an unusual geometry. Between ∼29°S and 33°S, the oceanic plate descends to 100 km depth and then flattens, traveling horizontally for ⁎ Corresponding author. Fax: +1 520 621 2672. E-mail address: [email protected] (L.S. Wagner). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.02.014 hundreds of kilometers before resuming its descent into the mantle (Fig. 1). This segment of flat slab subduction is spatially correlated with the cessation of arc volcanism and the inland basement cored uplifts of the Sierras Pampeanas. Jordan and Allmendinger [1] and others have suggested that the central Chilean flat slab and the Sierras Pampeanas are a modern analogue of a period of flat subduction of the Farallon plate beneath the western United States during Laramide deformation. While this comparison has its limitations [2], some insights into the 290 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 Fig. 1. Map of study area. Diamonds show the locations of the CHARGE seismic stations used in the tomographic inversion and stars show the locations of regional array stations reporting to the ISC that are also included in the inversion. Triangles show the locations of active volcanos. Contour lines show the depth to the top of the subducted Nazca plate according to Cahill and Isacks [4]. Grey shaded lines show the projected location of the subducted Juan Fernandez Ridge at specified times (0, 10, and 12Ma). correlation between flat or shallow subduction and inland basement cored uplifts can still be made. In order to study the mantle above this unusual slab geometry, we performed an inversion of P, S, and S–P seismic travel times from regional events recorded at stations from the Chile Argentina Geophysical Experiment (CHARGE). The experiment consisted of 22 broadband seismic stations deployed for 18months beginning in late 2000. This data was complemented by data collected from regional arrays that report data to the International Seismic Center (ISC). The additional data improves resolution, but does not change any of the major results from the inversions using only CHARGE stations. The tomographic results and the locations of the CHARGE stations are shown in Fig. 2. Further information on this inversion and associated resolution tests can be found in Wagner, et al. [3]. Above the flat slab and below the crust in central Chile and Argentina we found a sliver of material with low Pwave velocities, high S-wave velocities, and anomalously low Vp /Vs ratios (anomaly A, Figs. 2 and 3) [3]. The northern and eastern boundaries of this anomaly are poorly constrained, but the anomaly appears to be continuous across the top of the flattened portion of the slab. South of the flat slab, the downgoing plate is either bent or torn to accommodate the change in dip from horizontal to nearly 30°. There is no direct evidence for a tear in this area, and previous authors have modeled this transition as a steep bend (though the lack of deep seismicity in this area makes the determination of exact structure impossible) (e.g., Cahill and Isacks [4]). The southern edge of the low P, high S, low Vp /Vs anomaly appears to lie directly along the edge of this bend (Fig. 3). As soon as the slab is no longer completely horizontal to the south, the low Vp /Vs anomaly is no longer visible directly above the slab. In its place, above this “transition zone” from flat to normally dipping slab, we find high P-wave velocities and generally high Swave velocities and Vp /Vs ratios (anomaly B, Figs. 2 and 3). Above the westernmost portion of this high velocity anomaly at 65km depth, in the corner between the east verging dip and the south verging dip of the Nazca plate, lies a small region with very low P- and S-wave velocities L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 291 Fig. 2. Tomography results for Vp, Vs, and Vp / Vs at 65 and 85km depth from Wagner et al. [3]. Green stars show CHARGE seismic station locations. Red triangles indicate active volcanos. Dashed lines are contours for slab depth from Cahill and Isacks [4]. Boxes show areas used in forward modeling. A: low P, high S, low Vp / Vs area; B: high P, high S, high Vp / Vs area; C: low P, low S, high Vp / Vs bulls-eye. Black line indicates the location of the north–south cross-section in Fig. 3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) and relatively high Vp /Vs ratios (anomaly C, Figs. 2 and 3). While this area is just south of the actual flat slab, it is still characterized by a lack of surface volcanism. Normal arc volcanism resumes just to the south of this low Vp, low Vs, high Vp /Vs bulls-eye (Fig. 2). Interpreting these anomalies would ideally help decipher the compositions, states, and temperatures that are likely to give rise to the observed seismic velocities, and also help explain how they have been produced. In this paper, we discuss a range of possibilities, explore their tectonic implications, and propose that the most likely explanation involves the presence of ancient continental lithosphere and a complicated subduction history. 2. Tectonic history While there is ongoing debate about many of the details of the regional geology in our study area, a general tectonic history seems to be agreed upon. This area of Chile and Argentina is a conglomeration of accreted terranes with generally north south trending boundaries [5] (Fig. 4). The first terrane to accrete onto the western margin of Gondwana was the Pampian terrane after the complete subduction of the Puncoviscan ocean under the Rio de la Plata craton in the Early Cambrian [6]. By the Early Ordovician, subduction of the Iapetus Ocean began beneath the Pampian terrane, giving rise to Famatinian arc volcanism. There has some debate whether the Famatina terrane was accreted or was simply a product of this volcanism [6,7], but most studies indicate that Famatinian volcanism has continental (rather than island) arc signatures (e.g., Saavedra et al. [8]). By the Late Ordovician, the subduction of the Iapetus Ocean was complete, resulting in the accretion of the Cuyania/Precodilleran terrane. This terrane is believed to have originated from Laurentia, with numerous data showing 292 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 Fig. 3. North–south cross-section of the tomography results. Colors for Vp and Vs are in percent deviation. Contours for Vp are every 0.2km/s. Contours for Vs are every 0.1 km/s. Vp / Vs colors show absolute Vp / Vs values. Ovals indicate the locations of the anomalies indicated in map view in Fig. 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Grenvillian ages for basement rocks, both from the Precordillera and from the Pie de Palo range [5,9–14]. The final terrane to accrete was the Chilenia in the Devonian. Little is known about the origin of the Chilenia terrane, as most of its original basement rocks have been affected by extensive volcanism and metamorphism. It is also unclear whether the subduction that resulted in the accretion of Chilenia was west-verging or east-verging [5,7], but more recent works [7,12] seem to prefer the former. This would imply that no subduction has occurred beneath the Cuyania/Precordilleran terrane at least since the Ordovician. The locations of the terrane boundaries are also somewhat uncertain. Perhaps the least controversial boundary lies between the Chilenia and Cuyania terranes, along the Calingasta–Iglesia valley west of the Precordillera [5] (Fig. 4). The boundary between the Cuyania terrane and the Pampian terrane is now believed to lie along the SSE trending Sierras de Valle Fertil [7,15], though some sort of ancient suture is visible for a short distance along the western edge of the Pie de Palo range extending SSW towards the Precordillera. Originally believed to be the boundary between the Precordillera and the Pampian terrane, this suture is now suggested to be evidence of Middle Proterozoic subduction that resulted in the accretion of the Pie de Palo range to the Precordillera prior to rifting from Laurentia [16,17]. These terrane boundaries, between Chilenia and Cuyania, and Cuyania and Pampea, are the most relevant to this paper, as the tomography results do not sample far enough east to study the boundary between the Pampean terrane and the Rio de la Plata craton. Following these accretionary events, the Gondwanian cycle (Early Carboniferous to Early Cretaceous) was characterized by extension and relative stability within the continent [18]. Subduction of the proto-Pacific plate beneath the western margin of Gondwana resulted in the formation of cordilleran type batholiths during the Carboniferous [19]. This era ended with the rifting of Gondwana and the opening of the Atlantic, which heralded the beginning of the Andean cycle (Early Cretaceous to present). This cycle was characterized by several periods L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 293 Fig. 4. Geological features in the study area. The white diamonds show CHARGE seismic stations. Red triangles show active volcanos. Purple lines indicate terrane boundaries. Shaded areas indicate major geologic features. Black contour lines show the depth to the subducted Nazca plate according to Cahill and Isacks [4]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) of compression, including the most recent beginning in the Tertiary [19]. Development of the current unusual slab geometry in this area began during the Miocene. Based on the timing of the eastward progression and eventual cessation of arc volcanism and the arrival of the subducting Juan Fernandez ridge track to these latitudes, flattening in central Chile is likely to have begun between 20 and 18Ma, with the most dramatic eastward progression occurring between 10 and 2Ma [11,20]. Most volcanism ended between 6 and 4.8Ma, with only a small amount of volcanism from the Sierra de San Luis occurring later around 1.9Ma [21,22] indicating the slab's current configuration has likely been stable for at least the past ∼2–5Ma. 3. Compositions, states, and temperatures Determining the composition, state, and temperature of a material with specific seismic characteristics can be difficult, but some constraints can be made. Accurate analysis requires a good understanding of error bounds on both the tomographic results and the thermodynamic and experimental results that link petrology and temperature to elastic properties. In order to investigate the error bounds of the tomographic results, we have employed a forward modeling approach to test the range of P and S velocities that can explain the travel times for the three anomalies discussed here. This method involves creating a 3-D velocity model through which travel times are calculated for rays used in the tomographic inversion. Event locations are fixed at the final locations determined during the tomography. We then identify an area of roughly the size and shape of the anomaly of interest, and vary the velocities within that anomaly, keeping the rest of the model constant. Travel times and errors between observed and predicted travel times are calculated for each velocity model and are plotted in Figs. 5, 6, 7 and 8. For the low Vp / Vs region (anomaly A) above the flat slab, we created a 40km thick layer in a rectangular shape above the flat slab (see Fig. 2), using the starting model from the tomography for the rest of the volume. Results of this modeling are shown together with possible compositions (to be discussed later) in Figs. 5 and 6. We find that S-wave velocities in this area range from 4.6– 294 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 Fig. 5. Results for anomalies A, B, and C (white stars), including innermost contours from forward modeling results (see Figs. 6, 7, and 8). Endmember peridotite and other compositions plotted are from Hacker and Abers [25], except where explicitly specified. Size indicates temperature, and color indicates composition. 4.7km/s and the Vp / Vs ratios range from 1.64–1.74, with the best fit located at Vs = 4.65km/s and Vp / Vs = 1.70. Decreasing the thickness of this layer results in higher S velocities and lower Vp / Vs ratios, which would make the result more unusual, and the interpretation even more difficult. Modeling the anomalies further to the south is more complicated because rays sampling these regions are more likely to traverse other anomalies either before or after crossing the feature of interest. In order to account for this, we use the final tomography results as a starting model, and then vary the anomaly of interest within that model. The results for the high velocity anomaly (anomaly B) and the low velocity bulls-eye (anomaly C) are shown in Figs. 7 and 8, respectively. For the high velocity anomaly above the transition zone, we find that the Swave velocities are likely between 4.6 and 4.7km/s, and that the Vp / Vs ratios are most likely between 1.72 and 1.84 with the best fit at 4.65km/s and 1.775 for Vs and Vp / Vs, respectively. For the low velocity bulls-eye, we find S-wave velocities between 4.0 and 4.2km/s, and Vp / Vs ratios between 1.72 and 1.80. The best fitting result for this anomaly was 4.1km/s for Vs and 1.76 for Vp / Vs. Errors on the petrologic and thermodynamic end are difficult to quantify. Discrepancies exist between studies [23,24] (Figs. 5, 6 and 7), and in general, error bars are not available. We present mostly results from Hacker and Abers [25], except where other results are available (Figs. 5, 6 and 7). While a definitive answer to the question of composition in each of these regions is difficult, some constraints from Figs. 5, 6, 7 and 8 can be made. We begin with a discussion of anomalies B and C because their explanations are more straightforward. To study the high velocity anomaly B in the slab dip transition zone, we assumed an average pressure of 3GPa and an average temperature of 500°C (since this material has likely been cooling during the past 2Ma of flat slab subduction). We L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 295 Fig. 6. Results of the forward modeling for the low Vp, high Vs, low Vp / Vs anomaly A. The white star indicates the lowest residual. The yellow symbols show velocities from Hacker and Abers [25] for specified compositions at 500°C. Green circles are for pure forsterite, blue circles are for pure enstatite, also from Hacker and Abers [25]. Stars indicate velocities from Ji and Wang [23] for a range of compositions between pure enstatite and pure forsterite. All values are calculated for 3GPa at specified temperatures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) follow the Hacker, et al. [24] approach of 4 possible starting compositions, MORB, lherzolite, depleted lherzolite, and harzburgite. All of these compositions are assumed to be hydrated, but the percent hydration depends on the phases present at a given pressure/ temperature condition. At 500°C and 3GPa, these compositions would manifest themselves as zoisite eclogite (see Table 3 in Hacker et al. [24] for exact composition), chlorite serpentinite wehrlite (I) (chl16at g2ol57cpx26), chlorite serpentinite wehrlite (II) (ol89at g2chl5cpx3), and serpentine chlorite dunite (ol94at g2chl4), respectively. Increasing the temperature 100°C would not change these compositions, and most of these reactions are relatively pressure insensitive. Thermodynamic calculations from the spreadsheet associated with Hacker and Abers [25] provide the velocities for these compositions shown in yellow in Fig. 7. These results indicate that the high velocity anomaly could be modeled by any cool hydrated peridotite composition, with the marginally best fit going to depleted lherzolite. We repeat this process for the low velocity bulls-eye at 65km depth (anomaly C). We assume a pressure of 2GPa and a temperature of 350°C, which would result in jadeite–lawsonite blueschist (see Table 3 in Hacker, et al. [24] for exact composition), serpentinite (I) (atg4chl20cpx33br43), serpentinite (II) (atg7br80chl11cpx5), and serpentinite chlorite brucite (atg7br87chl6) for MORB, lherzolite, depleted lherzolite, and harzburgite, respectively. The velocities for these compositions are shown in red in Fig. 8. The previously calculated velocities for the 296 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 Fig. 7. Results of the forward modeling for the high Vp, high Vs, high Vp / Vs anomaly B. The white star indicates the lowest residual. The yellow symbols show velocities from Hacker and Abers [25] for specified compositions at 500 °C. Green circles are for pure forsterite, blue circles are for pure enstatite, also from Hacker and Abers [25]. Stars indicate velocities from Ji and Wang [23] for a range of compositions between pure enstatite and pure forsterite. All values are calculated for 3 GPa at specified temperatures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) phases at 500°C and 3GPa are also plotted in Fig. 8, in yellow as in Fig. 7. In this case, the notably best fit goes to the MORB composition, with the depleted end-members having distinctly lower S-wave velocities than observed in the tomographic results. The higher pressure and temperature results do not match the observed velocity, reaffirming the assertion that this material must be especially cold. The low P velocities, high S velocities, and low Vp / Vs ratios above the flat slab in anomaly A, on the other hand, cannot be modeled using these hydrated compositions (Figs. 5 and 6). Instead, we look at the velocities of pure end-member compositions (Fig. 5) to determine what the range of possible compositions might be for these high shear velocities and low Vp / Vs ratios. Looking at the trends shown in Figs. 5 and 6, we infer that the material above the flat slab is probably cold (because S-wave velocities increase and Vp / Vs ratios decrease with decreasing temperature), dry (because the addition of even a small amount of water dramatically increases the Vp / Vs ratio), and depleted (because of the low S velocity and high Vp / Vs ratio of clinopyroxene and the iron endmembers of all three peridotite compositions). Recent work by Rossi and Abers [26] indicates a similar low Vp / Vs (1.6–1.7) structure above the subducting plate beneath Alaska. They argue that this Vp / Vs ratio L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 297 Fig. 8. Results of the forward modeling for the low Vp, low Vs, high Vp / Vs anomaly C. The white star indicates the lowest residual. The red and yellow symbols show velocities from Hacker and Abers [25] for specified compositions. Red shows results at 350 °C and 2GPa, and yellow shows results at 500°C and 3GPa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) is too low to be explained by any peridotitic composition (based on Hacker and Abers [25]), and suggest that the only mineralogy that has a Vp / Vs ratio b 1.7 at mantle conditions is quartz. They propose that this addition of silica could be achieved by metasomatism, or by the subduction of felsic material. For reference, we plot the three phases of quartz that might be found at upper mantle conditions in Fig. 5. Alpha and β quartz are stable at lower pressures (below 2–3GPa), with β quartz being the high temperature phase. Coesite is stable at higher pressures (between 2–3GPa, depending on temperature) [27]. Only α quartz has a Vp / Vs ratio significantly below 1.7 at 3GPa, but the low Vp / Vs ratios are accompanied by very low S-wave velocities. While Rossi and Abers do not specify absolute S-wave velocities in Alaska, we find high S-wave velocities associated with our low Vp / Vs ratios, limiting the amount of α quartz that can be present in our mantle material if the absolute velocities are to be accounted for. It is also possible that at depths closer to the flat slab, α quartz may be transitioning to coesite [27], which has a Vp / Vs ratio and S-wave velocity comparable to harzburgitic compositions such as forsterite and enstatite, making its presence hard to discriminate. Given the elastic properties of mineral compositions provided by Hacker and Abers [25], between 20% and 30% pure α quartz mixed with pure forsterite would match our observed velocities. However, there are many potential issues with how to introduce pure α quartz into a harzburgitic, cold, depleted mantle composition. Subducted continental crustal material would also introduce Fe, Ca and K rich compositions, which tend to have higher Vp / Vs ratios. More importantly, any crustal material would bring significant quantities of water into the mantle. At lower temperatures, this water would likely react with the peridotites in the mantle to form serpentinites (which have high Vp / Vs ratios), and at higher temperatures, this water would likely induce melting. It is unclear how it would be possible to isolate 298 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 large quantities of α quartz out of crustal material before or during subduction in order to lower the mantle Vp / Vs to match the observations of this study and that of Rossi and Abers. Whether or not α quartz is present in any significant quantity in our low Vp / Vs layer, at least 70% of this material must still be cold, dry, and depleted, as explained earlier. Looking at the seismic velocities presented in Hacker and Abers [25] (Fig. 6), it appears, assuming quartz is not present, that the material responsible for the low Vp / Vs anomaly would have to be dominated by pure enstatite in order to approach the observed low Vp / Vs ratios. However, work by Ji and Wang, [23] done at room temperature (0°C) and 3 GPa indicates that the Vp / Vs ratios for both enstatite and forsterite may be lower than reported by Hacker and Abers [25], suggesting that any harzburgitic mixture of forsterite and enstatite could produce the observed seismic anomalies. This finding is consistent with the work of Lee [28] who showed that Mg# (defined as 100 * Mg / Mg + Fe) is directly proportional to Vs, and inversely proportional to Vp / Vs. He finds little correlation between Mg# and Vp, although Vp does appear related to olivine mode. Lee [28] points out that while both temperature and composition could be responsible for an observed change in Vs, Vp / Vs is far less sensitive to temperature than it is to composition, making it an ideal indicator for mantle composition. Low Vp / Vs ratios, together with high Vs, would indicate depleted, pyroxene rich, Mg rich, mantle [28]. 4. Tectonic implications The low temperatures required by both the observed velocity structures and the absence of volcanism indicate that normal asthenospheric corner flow is not currently present in any of these areas, probably because this flow has been pinched out by slab flattening [29]. Since the cessation of volcanism is believed to indicate the timing of slab flattening, the material in this area has probably been trapped there at least since volcanism ceased some 2–5Ma. Once trapped, the material is continuously cooled by the subducted cold oceanic lithosphere. Normally at these depths, the downgoing oceanic crust would be dehydrating, releasing water into the overlying asthenosphere, producing melt. In the completely flat portion of the slab below the low Vp / Vs ratios, the slab is probably not currently dehydrating, nor is it likely to have dehydrated since volcanism ended, or we would see evidence of water in the seismic velocities of the overlying material. The only way to remove water from a hydrated mineralogy (in the absence of melt) is to increase the temperature, but because the temperatures above the slab must be decreasing, any water released by the slab would stay in the overlying rocks in the form of serpentinite and manifest itself in the tomographic results as zones with higher Vp / Vs ratios. While a non-dehydrating slab may help explain the cold and dry character of the trapped mantle material, it does not explain the depleted signature of this material. The seismic anomaly above the flat slab suggests that this material contains little if any clinopyroxene, and virtually none of the iron end-members of olivine and orthopyroxene. Most likely, this zone is composed of some mixture of enstatite and forsterite, though the exact composition is uncertain. Such a high level of depletion suggests that the material may be ancient continental lithosphere or depleted oceanic lithosphere (residual oceanic mantle column). Kopylova, et al., [30] found that the Archaen cratonic lithosphere beneath the Canadian Slave province is characterized in some areas by low Pwave velocities, high S-wave velocities, and low Vp / Vs ratios, based on both calculated and measured elastic properties of local mantle xenoliths. They suggest that low Vp, high Vs, and low Vp / Vs are the expected elastic properties of any highly depleted cratonic lithosphere. This is consistent with the work of Lee [28] who finds increased Mg# and corresponding increased Vs and decreased Vp / Vs in cratonic peridotite xenoliths. Seismic studies of cratons are less conclusive on this issue. Many look at only S-wave velocities, using surface wave tomography (e.g., Morelli and Danesi [31] in Antarctica, Fishwick, et al. [32] in Australia, Feng, et al. [33] in South America/Brazil, and Nettles and Dziewonski [34] in North America). These studies show increases in S-wave velocities beneath these cratons, but the Vp / Vs ratios are unknown. Because temperature can mimic the effect of composition on Vs, Vp / Vs is an important discriminant in areas where the thermal gradient is uncertain. The one paper with results for both Vp and Vs is by Fouch, et al. [35] who investigate the lithosphere beneath the Kaapvaal craton using relative teleseismic travel times. They find increases in both P and S velocities, but cannot explicitly calculate Vp / Vs due to the relative nature of the data. The study of Niu et al. [36] infers relative Vp / Vs ratios beneath the Kaapvaal craton by looking at S–P times, and finds evidence for comparatively low Vp / Vs in the mantle. The low Vp, high Vs, and low Vp / Vs above the flat slab in central Chile and Argentina correlate well with the values predicted by both Kopylova, et al. [30] and Lee [28] for Archaen cratonic lithosphere. The origin of the depleted lithosphere cannot be uniquely determined. It may be oceanic or continental in origin, though because of difficulties explaining how L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 Fig. 9. Vp / Vs results at 85km depth from the tomographic inversion in Wagner et al. [3] overlain by the projected current location of the Juan Fernandez Ridge (grey line), and the location of the terrane boundaries (red lines). Green stars indicate CHARGE seismic station. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) oceanic lithosphere could become associated with the overriding plate, we are currently hypothesizing this material is more likely to be continental. There are a number of terrane boundaries transecting this study area, all of which trend roughly north south (Fig. 4). These terranes are believed to have accreted by the Devonian, though the direction of subduction remains unclear in some cases. If the observed anomalies are indicative of original lithosphere, we might expect to see evidence of these terrane boundaries in the tomography, especially in areas under which subduction has occurred. We see no strong evidence of east–west changes in the velocity anomalies (Fig. 9), which may indicate that the terrane boundaries at the surface do not accurately represent these terrane boundaries at depth, or that all of the terranes were underlain by similar cratonic lithosphere. Because the amount of recent volcanism above this area is insufficient to create a new depleted lithosphere, ancient lithosphere is the most likely explanation for the observed anomalies. Nd isotopic data available on Miocene–Quaternary mafic volcanic rocks from the area [37,38] are consistent with the presence of relatively old (500Ma) continental mantle lithosphere beneath the studied area; however, the interpretation of the isotopic data is non-unique (e.g., the volcanic rocks may have been modified by crustal contamination). The only terrane in the area whose origin is constrained is the Cu- 299 yania terrane, which may have been part of Laurentia until the Ordovician. It is possible, then, that the observed seismic anomalies above the flat slab are indicating the presence of ancient Laurentian lithosphere. We recognize that Laurentian lithosphere would likely be of Grenvillian age (1.1Ga), which is significantly younger than the age of the Archean cratons mentioned earlier. However, it is uncertain whether there would be a significant difference in seismic velocities between Archean cratons and Grenville-age cratons. Evidence from Raleigh wave velocities indicates the presence of a high S-wave velocity Grenvillian keel beneath the eastern United States [39] which could be consistent with our observed seismic velocities. Interpreting the velocity anomalies above the transition zone is less complicated. Unlike the completely flat portion of the Nazca plate, the trapped mantle above this area (anomaly B) need not be depleted, and the slab in the transition zone may still be dehydrating. The subduction zone compositions proposed by Hacker, et al. [24] all contain between 4% and 15% water with the exception of zoisite eclogite, the MORB composition at 3GPa and 500°C. The simplest explanation for the high velocity anomaly is a body of cooled asthenosphere of uncertain composition. The fate of any ancient lithosphere that may have been present in this area cannot be uniquely determined. The low velocity bulls-eye above the high velocity anomaly (anomaly C) can be explained as a small region of extracted cooled melt with a MORB composition. It is interesting that this anomaly lies directly beneath the Cortaderas ophiolite complex [12]. These Precambrian, Ordovician, and Silurian mafic rocks have generally EMORB affinities, but it is difficult to explain how our comparatively large anomaly at 60km depth might be associated with the sliver of ophiolites observed at the surface. We discuss our preferred explanation for how these compositions may have evolved in the following section. 5. Proposed regional history We propose the following progression for the formation of these seismic velocity anomalies in the upper mantle. Originally, the mantle directly below the crust in the area currently characterized by a cessation of arc volcanism was made of a relatively uniform ancient depleted lithosphere. As the flattening of the Nazca plate progressed eastward between 10 and 2Ma, a large volume of asthenosphere was displaced from the area east of the flattening slab. This asthenosphere would most likely escape to the south, because the transition to normal dip to the north is much more gradual, and would accommodate much less material. We propose that the displaced 300 L.S. Wagner et al. / Earth and Planetary Science Letters 245 (2006) 289–301 asthenosphere flowed around the southeastern corner of the flat slab, below the ancient depleted lithosphere, and into the corner now characterized by the low P, low S, high Vp / Vs bulls-eye (anomaly C). There, it would join with the normal asthenospheric corner flow responsible for arc volcanism, and would eventually be moved out of the area along the top of the normally subducting plate. During this initial displacement phase, as the slab was still in the process of flattening, the movement of this additional material, added to the pre-existing corner flow, contributed to a delamination of the ancient lithosphere up to the base of the crust beneath the main cordillera. The level of this delamination appears to become shallower west of San Juan, as the depleted lithosphere appears to be intact east of San Juan at 85km depth. As flattening progressed, the slab beneath what is now the transition zone became shallow enough to prevent new asthenosphere from entering. The asthenosphere above this part of the slab became trapped and started to cool. Remnant hydration and melt collected and pooled directly below the crust in the corner, resulting in the low velocity bulls-eye. Flow patterns of the asthenosphere's movement around the southeastern corner of the flat slab became frozen into the mantle fabric, and are visible today in the form of shear wave splitting directions [40]. As flattening ceased to progress further east, additional asthenosphere was no longer displaced to the south, and normal corner flow continued south of the transition zone, resulting in the stable magmatic arc that is still active today. The final result is preserved lithosphere above the horizontal portion of the flat slab, injected frozen asthenosphere in the transition zone, pooled frozen melt in the corner between the south dipping and east dipping portions of the slab, and normal asthenospheric corner flow to the south below the modern active arc. 6. Conclusions Evaluation of errors on both the seismic tomographic results and the petrologic and thermodynamic constraints leads us to the following major conclusions: 1) Forward modeling tests of seismic velocity structures in the region of the central Chile/Argentina flat slab confirm the tomographic results of Wagner, et al. [3], finding low Vp, high Vs, and low Vp / Vs (anomaly A) above the horizontal portion of the flat slab, high Vp, high Vs, and high Vp / Vs (anomaly B) in the transition zone immediately to the south of this portion, and low Vp, low Vs, and high Vp / Vs (anomaly C) in the corner between the south and east dipping portions of the downgoing Nazca plate. 2) The seismic signature of low Vp, high Vs, and low Vp / Vs (anomaly A) found above the horizontal portion of the central Chilean flat slab is consistent with the highly depleted, cold, Mg rich compositions found in cratonic lithosphere. While the precise composition and origin of the material cannot be uniquely determined, it is possible that this material observed here is ancient Laurentian lithosphere, though the possibility remains that this lithosphere is oceanic. 3) The high velocity anomaly (anomaly B) to the south of the flattest portion of the slab is most likely cooled asthenosphere that was displaced in advance of the eastward progression of the flattening slab between 10 and 2Ma, and eventually trapped by the increased shallowing of the slab in the transition zone. 4) The low velocity, high Vp / Vs anomaly (anomaly C) at 65km depth in the corner between the east and south dipping portions of the Nazca plate is probably caused by a frozen pool of extracted melt with a MORB-like composition. Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. EAR9811870 at the University of Arizona. 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