Scientific Drilling ICDP Publications

The central Andean plateau offers an excellent natural laboratory to study mantle flow along an active continental margin as well as the link between plateau uplift and lithospheric delamination. The region between 25°S to 28°S, known as the southern Puna plateau, is characterized by a number of anomalous features possibly indicative of delamination. A total of 43 US and 30 German broadband three component seismic stations were deployed across the southern Puna plateau for approximately two years. The region of study has the advantage of deep and intermediate depth seismicity beneath the array that can be used to constrain the depth distribution of seismic anisotropy in the upper mantle. Teleseismic shear wave splitting results show a transition from east-west fast directions in the east of the array to east-west and north-south in the middle of the array, beneath Galan, and to predominantly north-south in the west part of the array. Azimuthal analysis of local events shows that the events coming from the North of the array split predominantely in the west-south north-east direction. The events coming from the south show splitting into the south-east north-west. Events coming from the West and East show predominant slab parallel splitting. Furthermore, a comparison of the teleseismic and local splitting lag times would suggest the presence of a significant amount of inter- or sub-slab anisotropy. Surface wave measurements indicate the presence of a high velocity block beneath Galan, a very large ignimbrite volcanic center, at depths between 190km (0.007 Hz) and 150 km (0.009 Hz). This can be interpreted as a delaminated block that has resulted in widespread crustal melting. At those same depths there are two high velocity zones, south east and north west of vicuña pampa. At 105 km (0.0125 Hz) we start to see the slab which seems to be deeping to the south. A low velocity zone further east could be responsible for the flatness of the slab at 26°S. At shallower depths, 66 km (0.02Hz), two prominent low velocity zones appear beneath Galan and to the east of Vicuña Pampa. These can be interpreted as post-delamination asthenospheric upwelling. Overall, the mantle beneath the region seems to be isotropic in the western part of the array but anisotropy increases further east. At crustal depths the seismic anisotropy appears to have south-west – north-east fast directions.

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Calixto Mory, F. J; Sandvol, E. A.; Mahlburg Kay, S.; Comte, D.; Alvarado, P. M.; Heit, B.; Yuan, X. (2011): Seismic Anisotropy beneath the southern Puna Plateau. AGU 2011 Fall Meeting (San Francisco 2011).