The northern Patagonia Somuncura plateau basalts: a product of slab‐induced, shallow asthenospheric upwelling?

Abstract

The Meseta de Somuncura forms the largest basaltic plateau (20 000 km2) of southern Argentina (extra‐Andean domain). Most of these tholeiitic to alkaline rocks were extruded at ˜ 25 Ma (late Oligocene). The absence of rifting–thinning processes, plume activity, or slab‐window phenomena leaves only one major possibility for the generation of Somuncura: asthenospheric (‘OIB‐like’) corner flow leading to a transient thermal anomaly above the subducting plate. It is suggested herein that the intake of hot asthenosphere was forced into a favourable topography (concave‐up) of the subducting plate, when a major plate reorganization event (Farallon to Nazca) was taking place in late Oligocene to early Miocene time. The fast and vigorous intake of asthenosphere would have been induced by slab roll‐back, leading to decoupling of the subducting plate. The Somuncura volcanic episode can be regarded as a marker of the passage from the extremely oblique subduction of Farallon, to the birth of the Nazca plate and roughly perpendicular convergence between South America and Nazca.

Influence of the substrate on maar–diatreme volcanoes — An example of a mixed setting from the Pali Aike volcanic field, Argentina

“…No "subduction" signature is detectable in the geochemical composition of the erupted products, leading to slab-window models for the PAVF volcanism (e.g., D'Orazio et al, 2000;Wang et al, 2008), in common with other volcanic fields further north (e.g., Ramos and Kay, 1992). However, some workers now prefer a slab rollback model for back-arc volcanism in this region (e.g., Muñoz et al, 2000;de Ignacio et al, 2001). Mazzarini and D'Orazio (2003) analyzed PAVF lineaments on a satellite image and found that 77% of these lineaments fall into four main families: N-S (azimuth range 350°-010°), NE-SW (021°-060°), E-W (080°-100°) and NW-SE (110°-150°).…”

The backarc mantle lithosphere in Patagonia, South America

“…Kay et al (1993a) suggest they are related to a transient hot spot that affected a mantle whose composition was modified by previous (Eocene) subduction events. De Ignacio et al (2001) propose their genetic link with asthenospheric corner flow leading to a transient thermal anomaly above the subduction plane.…”

Mantle wedge deformation by subducting and rotating slab and its possible implication

“…They found that the age distribution of the Somuncura plateau volcanic rocks concentrates between ∼27 to ∼20 Ma and the older rocks distribute closer to the volcanic front than the younger ones (see below). De Ignacio et al (2001) pointed out that this period almost coincides with a rapid rotation of the Fallaron-Nazca plates. Such a rapid rotation of plates may severely deform the subCopyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”