Reappraisal of the effective elastic thickness for the sub-Andes using 3-D finite element flexural modelling, gravity and geological constraints

Abstract

SUMMARY Estimates of effective elastic thickness (Te) for the western portion of the South American Plate using, independently, forward flexural modelling and coherence analysis, suggest different thermomechanical properties for the same continental lithosphere. We present a review of these Te estimates and carry out a critical reappraisal using a common methodology of 3‐D finite element method to solve a differential equation for the bending of a thin elastic plate. The finite element flexural model incorporates lateral variations of Te and the Andes topography as the load. Three Te maps for the entire Andes were analysed: Stewart & Watts (1997), Tassara et al. (2007) and Pérez‐Gussinyéet al. (2007). The predicted flexural deformation obtained for each Te map was compared with the depth to the base of the foreland basin sequence. Likewise, the gravity effect of flexurally induced crust–mantle deformation was compared with the observed Bouguer gravity. Te estimates using forward flexural modelling by Stewart & Watts (1997) better predict the geological and gravity data for most of the Andean system, particularly in the Central Andes, where Te ranges from greater than 70 km in the sub‐Andes to less than 15 km under the Andes Cordillera. The misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the Marañon basin in Peru and the Bermejo basin in Argentina, regardless of the assumed Te map, may be due to a dynamic topography component associated with the shallow subduction of the Nazca Plate beneath the Andes at these latitudes.

Crustal thickness map of Brazil: Data compilation and main features

“…Besides giving information on its crustal evolution, isostatic compensation, and intraplate stresses (e.g. Lithgow-Bertelloni and Guynn, 2004;Sacek and Ussami, 2009), it is essential for modelling wave-propagation in global and regional seismic studies and for developing surface corrections to investigate the upper mantle (e.g. Mooney and Kaban, 2010).…”

Upper mantle viscosity and dynamic subsidence of curved continental margins

“…Therefore, the flexural rigidity of the lithosphere also influences the magnitude of the differential subsidence, but this is only significant when the effective elastic thickness T e is large (T e 415-20 km) because the viscous lithosphere naturally filters out short-wavelength stresses applied to its base 17 . In addition, the increase in flexural rigidity does not necessarily smooth differential subsidence, but it may eventually amplify differential subsidence due to the curved geometry of the margin 18,12 .…”

Open-source modular solutions for flexural isostasy: gFlex v1.0

“…Many take advantage of the linear nature of the flexure equation for constant elastic thickness to superimpose analytical solutions of point loads (in the spatial domain) or sinusoidal loads (in the wavenumber domain) in order to produce the flexural response to an arbitrary load (Comer, 1983;Royden and Karner, 1984). Other models produce numerical solutions to the thin plate flexure equation by solving the local derivatives in plate displacement with numerical (mostly finite difference) methods (e.g., Bodine et al, 1981;van Wees et al, 1994;Stewart and Watts, 1997;Pelletier, 2004;Govers et al, 2009;Sacek et al, 2009;Wickert, 2012;Braun et al, 2013). Models in this latter category allow for variations in the elastic thickness of the plate, a factor of growing importance as variations in elastic thickness through space and time are increasingly recognized, measured, and computed (e.g., Watts and Zhong, 2000;Watts, 2001;Van der Lee, 2002;Flück, 2003;Pérez-Gussinyé and Watts, 2005;Tassara et al, 2007;Pérez-Gussinyé et al, 2007Tesauro et al, 2009;Kirby andSwain, 2009, 2011;Lowry and Pérez-Gussinyé, 2011;Tesauro et al, 2012bTesauro et al, , a, 2013Braun et al, 2013;Kirby, 2014).…”