Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling

Abstract

SUMMARY We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000‐2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P -t oS-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north‐south high Cordillera with higher P-wave velocity and thickness in the northern segment (north of 33 ◦ S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33 ◦ S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/Vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31 ◦ S and 32 ◦ S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30 ◦ S and 34 ◦ S.