Dynamic topography control on Patagonian relief evolution as inferred from low-temperature thermochronology
Benjamin Guillaume *1,2 , Cécile Gautheron 3, Thibaud Simon-Labric 4, Joseph Martinod 5, Martin Roddaz 5, Eric Douville 6, Rodrigo Riquelme 7 1 Géosciences Rennes, Université de Rennes1, Campus de Beaulieu, Rennes, France 2 LPG Nantes, Université de Nantes, Nantes, France 3 Interactions et Dynamique des Environnements de Surface, Université Paris-Sud, Orsay, France 4 ISTerre, Université Grenoble 1, Grenoble, France 5 Géosciences Environnement Toulouse, Université de Toulouse, Toulouse, France 6 LSCE/IPSL, Gif-sur-Yvette, France 7 Depto. Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile
*Contact email: [email protected]
Abstract. We combine low-temperature thermochronology topography may also be variable in time and space, and apatite (U-Th)/He data (AHe) and semi-analytical modeling exert an important control on landscape evolution, but this of dynamic topography to investigate the role of slab issue has been poorly addressed so far. window and climate on cooling/heating history and relief evolution of the Patagonian Cordillera. In particular, we Patagonian is one of the few regions on Earth where a slab discuss a new thermochronological dataset consisting in 22 samples divided into four elevation transects. Sampling window is currently developing. The arrival at trench of sites were chosen at the same distance from the trench the Chile Ridge separating the Nazca and Antarctic plates (250-300 km), on the leeward eastern side of the orogen, at the latitude of 54°S ca. 16 Ma ago and the westward for latitudes ranging between 45°S and 48°S to detect a motion of South America led to the intermittent migration potential northward migration of the thermal signal toward the north of the associated triple junction and the associated with the northward migration of the slab window. progressive enlargement of the Patagonian slab window, We show that history of heating and cooling for this region which is clearly identified on tomographic images as a of the southern Andes compares well with the time- low seismic velocity anomaly in the upper mantle. The evolution of slab window. In particular, a phase of heating is contribution of slab-window-related dynamic topography recorded at 10-6 Ma to the south and at ≤5 Ma to the north, synchronous with the opening of the slab window at these in the topographic evolution of the Patagonian Cordillera latitudes, followed by a phase of rapid cooling and has generally not been considered mainly because local denudation, with values as high as 0.5 mm/yr. We also flexural and isostatic adjustments due to tectonics and show that present-day latitudinal topographic variations erosion obscure the dynamic topography signal. cannot be explained by climate alone but require an additional support by dynamic topography. 2 Methodology
Keywords: subduction, slab window, Patagonian Cordillera, apatite (U-Th)/He thermochronology, dynamic In this study, we combine low-temperature topography. thermochronology apatite (U- Th)/He data and semi- analytical modeling of dynamic topography to investigate the role of slab window and climate on cooling/heating 1 Introduction history and relief evolution in the Patagonian Cordillera. In particular, we obtained a new thermochronological The formation and evolution of relief in subduction- dataset consisting in 22 samples divided into four related orogens result from a variety of processes acting at elevation transects (Figure 1). Sampling sites were chosen different scales of time and space. The interplay between at the same distance from the trench (250-300 km), on the tectonics and erosion (river incision, glacial erosion) is leeward eastern side of the orogen, within the easternmost generally the principal contributor to the relief occurrences of the Patagonian batholith. Latitudes of the development. However, Earth’s surface topography is also sampling sites range between ~45°S and 48°S to detect a shaped by mantle convection, the latter generally potential northward migration of the thermal signal producing a low amplitude, long-wavelength deflection of associated with the northward migration of the slab the surface as a response to the distribution of density window. Samples were collected when possible every 200 anomalies in the mantle. For regions where mantle m of elevation along transects. dynamics may change rapidly, e.g. in subduction zones where slab windows form, the signal of dynamic We also performed semi-analytical models of dynamic 311 topography based on the Skoteslets approximation, using additional mechanism. In addition, for regions located the code developed by Husson (2006). To take into between 51°S and 53°S, Patagonian Cordillera exhibits account the formation and progressive enlargement of slab anomalously low elevations whereas Thomson et al. window we reconstructed its lateral extension on the basis expect small glacial erosion rates South of 46°S. of the convergence velocities between Nazca/South America and Antarctic/South America plates presented by Interestingly, dynamic topography models show that the Breitsprecher and Thorkelson (2009) that we used as a sector of the Andes lying between 46°S and 49°S proxy for the subduction velocity of the Nazca and experienced high dynamic uplift for the 6-0 Ma period, Antarctic slabs, respectively. Dynamic topography has following the opening of the slab window at these been calculated each 1 Myr time-step for the last 16 Myr. latitudes (Figure 2). In addition, the timing of maximum dynamically-induced uplift follows the same trend as youngest AFT and AHe ages between 49°S and 46°S. By 3 Results and discussion locally creating additional relief, slab window may enhance erosion processes that would result in younger Low temperature thermochronometers along the central AFT and AHe ages toward the north. We thus suggest that Patagonian Andes reveal a heating phase around ~10-6 Ma part of the present-day high topography just south of the in the southern part and younger than 5 Ma in the northern triple junction latitude is dynamically supported, in part, caused by a late Neogene burial or a basal heating response to slab window formation. For the sector located phase. Burial can be discarded because there are neither south of 51°S, dynamic topography models show a structural data to support burial beneath a Miocene thrust dynamic subsidence of the overriding plate associated to fault, nor evidence to support burial beneath a several km- the subduction of the Antarctic slab starting at ~5 Ma and thick (~3-3.5 km) Miocene sedimentary basin (see producing a minimum deflection of ~150 m at 52°S, Haschke et al., 2006) for the southern sector (~47.5°S), or which would contribute to the low elevations associated ~1.5 km thick dacite and/or sediments on top of the dacitic with old AFT and AHe ages found there (Figure 2). volume in the northern part (45.5°S) (Ramos, 1989; Ramos and Kay, 1992). Instead, we ascribe this increase in temperature to basal heating of the Central Patagonian Acknowledgements lithosphere by asthenospheric flow through the slab window (Thorkelson, 1996; Russo et al., 2010; Guillaume This research was supported by the French ANR GiSeLE et al., 2010). project, the ANR-06-JCJC-0079 project granted to C. Gautheron for the analytical part of this work and the An episode of fast cooling and denudation is recorded by “Reliefs de la Terre” INSU program for fieldwork. We low-temperature thermochronometers (AFT and AHe) thank Manuel Ignacio Muñoz Cordero for his help during during the late Miocene across the entire Patagonian the 2009 sampling campaign and IRD for material Andes (Thomson et al., 2001; Haschke et al., 2006; facilities provided during the field campaign. Rosella Thomson et al., 2010; Guillaume et al., in review). Pinna is thanked for the apatite picking selection and the However, it is not synchronous at the scale of the orogen U-Th chemistry preparation. We thank L. Husson for as can be deduced from AFT and AHe minimum ages that providing us with the Stokeslets code and for helpful decrease from ~10 Ma to ~1 Ma between 51°S and 45°S discussions. (Thomson et al., 2010).
To explain latitudinal variations in the elevation of the References Cordillera, Thomson et al. (2010) argue for a climatically driven pattern of erosion that would result from a Breitsprecher, K.; Thorkelson, D.J. 2009. Neogene kinematic history latitudinal difference in long-term glacial erosion of Nazca–Antarctic–Phoenix slab windows beneath Patagonia efficiency. But surface processes only cannot account for and the Antarctic Peninsula. Tectonophysics 464: 10-20.
the present-day trench-parallel topography of the Guillaume, B.; Moroni, M.; Funiciello, F.; Martinod, J.; Faccenna, C. Patagonian Cordillera. Between 48°S and 46°S, on the 2010. Mantle flow and dynamic topography associated with slab eastern flank of the orogen, youngest AHe and AFT ages window opening: Insights from laboratory models. Tectonics decrease toward the north to ages as low as 2-3 Ma. If this 496: 83-98. trend was related to an increase in the long-term glacial erosion efficiency, the topography should match this Guillaume, B.; Gautheron, C.; Simon-Labric, T.; Martinod, J.; Roddaz, M.; Douville, E. In review. Dynamic topography control tendency with a progressive decrease in the mean and on Patagonian relief evolution as inferred from low-temperature maximum elevations toward the north. Instead, mean and thermochronology. EPSL. maximum elevations along this transect of the Andes are similar to those south of 48°S, and the effective elevation Haschke, M.; Sobel, E.R.; Blisniuk, P.; Strecker, M.R.; Warkus, F. decrease occurs quite abruptly at 46°S latitude. 2006. Continental response to active ridge subduction. Geophys. Topography for these regions has to be supported by an Res. Lett. 33: L15315.
312 Husson, L. 2006. Dynamic topography above retreating subduction Thomson, S.N.; Hervé, F.; Stöckhert, B. 2001. Mesozoic-Cenozoic zones. Geology 34(9): 741-744. denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes. Tectonics 20(5): Ramos, V.A. 1989. Foothills structure in the Northern Magellanes 693-711. Basin. Argentina. AAPG Bull. 73: 887-903. Thomson, S.N.; Brandon, M.T.; Tomkin, J.H.; Reiners, P.W.; Ramos, V.A.; Kay, S.M. 1992. Southern Patagonian plateau basalts Vásquez, C.; Wilson, N.J., 2010. Glaciation as a destructive and and deformation: Backarc testimony of ridge collisions. constructive control on mountain building. Nature 467: 313-317. Tectonophysics 205: 261-282. Thorkelson, D.J. 1996. Subduction of diverging plates and the Russo, R.M.; VanDecar, J.C.; Comte, D.; Mocanu, V.I.; Gallego, A.; principles of slab window formation. Tectonophysics 255: 47-63. Murdie, R.E. 2010. Subduction of the Chile ridge: Upper mantle structure and flow. GSA Today 20(9): 4-10.
Figure 1. AHe age-elevation profiles for the sampled locations. (A) Mean AHe ages associated to their age dispersion (gray triangles) and minimal AHe ages (black triangles underlined by a m symbol) are reported for the northern zone. (B) Mean AHe ages associated to their age dispersion for the DES (black squares) and LL (gray squares) elevation profiles. (C) Mean AHe ages associated to their age dispersion (black circles) and central AFT ages (gray diamonds; Haschke et al., 2006) for rocks from the Cerro Barrancos. Pink boxes underline the period during which samples have been located above slab window, since ~8 Ma for the southern zone and ~4 Ma for the central sector.
Figure 2. Synthetic 3D sketch showing the response of the South American plate to the formation of the Patagonian slab window between 51°S and 47°S. (A) During the 10-6 Ma time interval, the subduction of the southernmost Chile ridge segment produces the opening of a slab window inducing the heating of the overriding lithosphere with associated plutonism and backarc magmatism, and dynamic uplift associated with mantle flow. To the north, the Nazca slab is still subducting. (B) During the last 6 Ma, the northernmost segment of the Chile Ridge also entered the trench producing the opening of a slab window at these latitudes and a shift toward the north of the dynamic uplift. To the south, the incipient subduction of the Antarctic slab should instead promote dynamic subsidence. The late Miocene to present-day period is also marked by glaciations in the entire region that participated in shaping the Patagonian landscape.
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