Low Vp/Vs Ratio in the Archean Crust of the Southern Sao Francisco Craton, SE Brazilian Shield, by Slant Stacking of Receiver Functions. <p> HR: <span class="hr"> 0830h<br></span> AN: <span class="an"> S41D-0108 <a href="../absbook/fm03-S41D.pdf">[PDF]</a><br></span> TI: <span class="ti"> Low Vp/Vs Ratio in the Archean Crust of the Southern Sao Francisco Craton, SE Brazilian Shield, by Slant Stacking of Receiver Functions. <br></span> AU: <span class="au"> * Assumpcao, M <br></span> EM: <span class="em"> marcelo@iag.usp.br<br></span> AF: <span class="af"> Dept. of Geophysics, University of Sao Paulo, Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090 Brazil <br></span> AU: <span class="au"> Bianchi, M <br></span> EM: <span class="em"> mbianchi@iag.usp.br<br></span> AF: <span class="af"> Dept. of Geophysics, University of Sao Paulo, Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090 Brazil <br></span> AU: <span class="au"> Schimmel, M <br></span> EM: <span class="em"> schimmel@ija.csic.es<br></span> AF: <span class="af"> Institude of Earth Sciences Jaume Almera, CSIC, Lluis Sole i Sabaris s/n,, Barcelona, 08028 Spain <br></span> AB: <span class="ab"> The composition of the lower crust in Archean cratons is a matter of debate. Felsic to intermediate granulites (with relatively low Vp and low Vp/Vs ratio) have been proposed by Mooney and Durheim (1991) based on seismic refraction profiles. On the other hand, worldwide estimates of Vp/Vs from receiver functions (Zandt et al., 1995) seem to show high Vp/Vs ratios in Precambrian regions ($>$1.78). It is not yet clear if the Archean crust has any typical value or not (Ni and James, 2003). Determination of Vp/Vs ratios from receiver functions tend to have high uncertainties due to the weak amplitudes of the multiple reflected phases and interference effects after the first Ps conversion, and this difficulty may have prevented the determination of a "typical" value for the Archean crust. A new method is presented here for enhancing the multiple reflected phases (P2ps and Pp2s) in receiver function seismic sections. Slant stacking of receiver functions is carried out in the time x slowness-squared domain (instead of the usual time x distance domain) which allows a better alignment of the converted phases and improves the identification of their arrival times. Phase weighted stacking is also used to remove uncoeherent noise. The picked arrival times can then be used to estimate the average crustal Vp/Vs ratio. Several stations located in the Archean block of the southern Sao Francisco craton and adjacent fold belts yielded Vp/Vs ratios ranging from 1.66 to 1.77 with an average value of about 1.72. This is consistent with the regional average value of 1.71 determined by a Wadati diagram from small local earthquakes. The low Vp/Vs values found for the Sao Francisco craton favour a less mafic composition for the lower crust, more in agreement with the Archean/Proterozoic crustal evolution proposed by Mooney and Durheim (1991). <br></span> DE: <span class="de"> 7203 Body wave propagation<br></span> DE: <span class="de"> 7205 Continental crust (1242)<br></span> SC: <span class="sc"> Seismology [S]<br></span> MN: <span class="mn"> 2003 Fall Meeting<br></span></p></body></html>