Geometry of the Deep Andean Subduction Zone Beneath Chile and Argentina from New Finite-Frequency Teleseismic P-wave Tomography

Abstract:

The Nazca plate subducts beneath Chile and Argentina between ~26° S and 40° S with a variable slab dip. Between 29° S and 32°S, the Chilean flat slab extends with a near horizontal slab dip for several hundred km before steepening. South of the Chilean flat slab, the slab dips normally beneath an active volcanic arc. This region is characterized by a lack of deep (>200 km depth) seismicity that restricts our knowledge of the subduction zone of the southern Andes. To investigate the geometry of the subducting Nazca slab, we invert teleseismic P-wave arrival times from stations covering a large portion of central Chile and Argentina for a tomographic model of the P-wave velocity structure using a finite-frequency method. We use data from ~350 stations that are part of 9 different temporary seismic networks deployed at various times since 1999 and 4 permanent networks operating as far back as 1994. The use of >30,000 arrival times from these seismic networks provides high-resolution imaging of the subduction zone from ~100 km depth down through the mantle transition zone.

In map view, the slab is visible as a high velocity feature with a cuspate shape pointed eastward near 32°S. The slab anomaly is much stronger to the north of the cusp than south. Between about 240 to 365 km depth, the point of the cusp appears to be interrupted by a localized low velocity feature. Further testing is required to determine whether the slab is torn here or merely thinned. Significantly, the high velocity feature south of the cusp continues downward through the transition zone. Despite the lack of seismicity our results clearly show the slab south of 32°S is present to depths of at least ~700 km. In cross-section views where the slab is well imaged, the slab penetrates through the entire transition zone to ~800 km depth, beyond which resolution is poor. Several cross-sections of the slab south of the cusp appear to show flattening of the slab in the uppermost lower mantle. Other strong non-slab velocity anomalies are present but require further testing of their resolution.