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Some Values of Effective Elastic Thickness Determined for the South American Plate

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Some Values of Effective Elastic Thickness Determined for the South American Plate Some values of Effective Elastic Thickness determined for the South American Plate M.S.M. Mantovani* S.R.C. de Freitas** W. Shukowsky *Instituto Astronomico e Geofisico University of Sao Paulo, Rua do Matao 1226, Sao Paulo, S.P., 05508-900 Brasil; e- mails: [email protected], [email protected] **Centro Politecnico, Universidade Federal do Parana. In the last decades, the effective elastic loads are compensated exclusively at the Moho. thickness (Te) has been considered an important McNutt (1983) applied the isostatic response function parameter for the study of the lithosphere rheology. (admittance) to an E-W cross section at 41.5°N of From the mechanical point of view, Te is associated to northern California. the flexural rigidity (D) of a plate by Forsyth (1985) introduced the coherence D_!_ function alternatively to the admittance for a 2-layer T = [12(1- d) - P,being (E) the Young Modulus elastic plate. In this approach, subsurface loads e E correspond to the relief on the Moho and surface loads and (v) the Poisson ratio. A physical measure of the are compensated by deflecting the Moho. For D::;: 0 rigidity corresponds to the amount of load that an ideal coherence will approach 1.0 at wavelengths long plate can support without perceivable flexure compared to the characteristic flexural wavelength and (bending). In the flexure model loads are partially will approach 0.0 at wavelenght short enough for loads supported by elastic stresses within the lithospheric to be supported by stresses within the plate. The plate overlying a weak and fluid asthenosphere, while transition from coherent to incoherent topography and in the fully isostatic compensation model occurs gravity will give a direct indication of the flexural directly beneath the topography by thickening a rigidity of the plate. Bechtel et al. (1987) extended this constant density (Airy, 1855) or by lateral changes in formulation to a 3-layer plate to study the density of the crust (Pratt, 1855). The flexure model compensation mechanism at the Kenya Rift. assumption of surface loads compensation by a A 3-layer model was also applied by Zuber et don ward flexure with crustal thickening, corresponds al. (1989) to different tectonic provinces of Australia, to a regional version of Airy's model. giving for the whole continent values of the effective A commonly used way for measuring the elastic thickness (Te) varying within 17 km and 134 rigidity of a plate is to compare the wavelenght pattern km. Although Zuber et al. (1989) concluded that for between the topography (latu sensu) and gravity fields. the Australian continent the relationship between the It means that when loads are totally supported by the size of a region and the elastic thickness is only plate (D ~ 00), no isostatic compensation will occur apparent and not an artifact of the coherence method, and the measured gravity field will differ from that Ebinger et al. (1989) showed that for East Africa and calculated for the fully compensated topography Afar Plateaus three sub-regions were too small to (isostatic equilibrium). Conversely, when the plate is observe the transition from high to low coherency deflectible (weak) an isostatic compensation will occur, providing lower bounds on the rigidity, and that large and the measured gravity field will approach to that gaps in gravity data coverage bias the estimate of predicted by Airy's model with null rigidity (D=O), elastic plate thickness to stronger values. being the degree of compensation for the topography Taking into account the poor gravity coverage load the ratio of the lithosphere deflection (w) to the in the South American plate, its size, and the restricted isostatic equivalent [w(D) / w(D ~ 00)]. area of some tectonic units, an alternative method to Operationally, the comparison between the predicted evaluate Te based on the elastic response to gravity (calculated from topography) and the observed gravity tides is here proposed. Tidal gravity residuals obtained field is performed in the frequency domain. from ocean loading effect corrected amplitude gravity Different approaches have been employed by factor (~M2)' related to the main Lunar wave, were several authors. Lewis & Dorman (1970) used a calculated for qualified stations of ICET-TWTGP in transfer function to determined the gravity component Australia, presenting a correlation between heat flow of the United States, taking into account the isostatic and effective elastic thickness. The obtained gravity response [R(k)] exclusively due to the surface residuals were compared to the Te values of Zuber et topography; R(k) being the ratio between the 2- al. (1989) for at least 6 tectonic units, and a linear dimensional Fourier Transform of the Bouguer correlation function was determined. Rigidity values anomaly (B(k)) and that of the Topography (H(k)): for South America, were calculated from the tidal [R(k) = B(k) / H(k)]. In their example, surface deviatoric gravity response in agreement to PREM - 877 Preliminary Reference Earth Model (Freitas & Condie, K.C., 1982. Plate Tectonics and Crustal Mantovani, 1995). The effective elastic thickness was Evolution. Pergamon, USA., 2nd edition, 310 pp. calculated for 11 Southamerican tectonic units of Freitas, S.R.C. de & Mantovani, M.S.M., 1995. Condie (1992) which comprise at least one qualified Observed tidal phase-lag and upper mantle anelasticity tidal record within the selected criteria (Table 1). in South American Plate. An. Acad. bras. Cienc., 67: To test the validity of this approach for the 321-326. South American Plate, the effective elastic thickness of Lewis, B.T.R. & Dorman, L.M., 1970. Experimental .a cratonic area, which comprises the southernmost part Isostasy. 2. An Isostatic model for the USA, derived of Brazil and Uruguay, was determined. by the from gravity and topographic data. J. Geophys. Res., coherence method (Mantovani et al., 1995). The 75(17): 3367-3386. conventionally obtained value of Te = 95 kID. compared Mantovani, M.S.M., Shukowsky, W., Hallinan, S. E., to Te = 90 Ian from the tidal observation corroborates 1995. Analise da espessura elastica efetiva no segmento the proposed assumption. litosferico Rio de La Plata - Dom Feliciano. An. Acad. bras. Cienc., 67(2): 200-220. REFERENCES Pratt, J.H., 1855. On the attraction of the Himalaya Mountains and of the elevated regions beyond them. Airy, G.B., 1855. On the computation of the effect of Philos. Trans. R. Soc., 145: 53-100. the attraction of mountain masses as disturbing the Zuber, M.T., Bechtel, T.D., Forsyth, D.W., 1989. apparent astronomical latitude of stations of geodetic Effective elastic thicknesses of the lithosphere and surveys. Philos. Trans. R. Soc., 145: 101-104 .: mechanism of isostatic compensation in Australia. J. Betchel, T.D., Forsyth, D.W., Sharpton, V.L., Grieve, Geophys. Res., 94(B7): 9353-9367. R.A.F., 1987. Mechanism of isostatic compensation in the vicinity ofthe East African rift, Kenya. Geophys. J. R. astr. Soc., 90:445-465 Table 1. Effective elastic thickness for the South American Plate, calculated from each tectonic unit which contains at least one tidal station qualified within the established selection criteria. Province Stations # Average Te (Ian) Andean Cordillera (North) 7201; 7202; 65 7205;7253 Andean Cordillera 7408; 7412; 80 (Central) 7500;7815 Andean Cordillera (South) 7814;7816 105 Andean Cordillera 7817 35 (Patagonia) Amazon Basin (West) 7411 95 S-A Platform 7310; 7506; 85 (Tapaj6s Province) S-A Platform 7805; 7810; 85 (Southern Province) 7812;7819 S-A Platform 7313 45 (Parnaiba Province) South Atlantic Shield 7311;7317 80 (NE cont. margin) South Atlantic Shield 7303; 7305; 60 (Mantiqueira Province) 7314 South Atlantic Shield 7895 90 (Rio de LaPlata craton) 878.
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