Seismic and non-seismic regions of the Andean backarc crust above the flat-to-inclined slab subduction of Argentina (30-33°S) from ambient noise and regional seismic source analyses

ALVARADO, P.; AMMIRATI, J.B.; ARAUJO, M.; VENERDINI, A.; LINKIMER, L.; WARD, K.; BECK, S.

San Jose

Conferencia; 2nd IASPEI Regional Assembly Latin - American and Caribbean Seismological Commision - LACSC; 2016

In the last two decades, seismic monitoring in Argentina has increased observations in the flat-slab subduction region between 30°S-33°S. This region has caused the most damaging earthquakes in Argentina in the past century and seems to provide an efficient mechanism for mountain building along a more than 600 km horizontal east-west orientation. Continuously recorded data from permanent seismic stations from INPRES, UNSJ (Argentina) and global networks (CTBTO, GSN), as well as temporary IRIS-PASSCAL broadband deployments cover fourmain morphostructural geological units and accreted Paleozoic terranes. We use: 1) Ambient noise (AN)showing Rayleigh wave tomographic images at different levels of the continental crust, which clearly define regions of low seismic velocities associated with basins surrounding others of high seismic velocities beneath the Precordillera and Sierras Pampeanas(SP) outcrops. 2) A joint inversion of receiver functions and AN-Rayleigh wave dispersion curve showing that adding a small percentage (15%) of AN-surface wave information greatly improves the 1-D lithosphere seismic velocity structure beneath each seismic station. 3) Relocations and focal mechanisms of 514 crustal earthquakes. From west to east, our results indicate Moho depths of approximately 70 km beneath the main Andean cordillera, 66 km in the thin-skinned Precordillera, 47 km in the thick-skinned western SP and 41 km in the eastern SP. Mid-crustal discontinuities in seismic velocities at about 21-km and 35-km depths correlate well with suggested decollements and compositional changes at depth for more mafic lithologies. The lower crust exhibits similar properties to those of the uppermost mantle likely related to partial eclogitization beneath the western terranes. The most seismically active Precordilleran and SP region, shows mainly reverse fault focal mechanisms. No earthquakes are observed beneath the Bermejo basin or in the lower crust. In summary, combining different types of seismic analysis allows us to significantly improve our understanding of the crust.