Crustal destruction processes at continental margins: a case study of forearc subduction erosion and crustal delamination in the trench to back-arc transect across the Central Andean Puna plateau.

KAY, S.M.; COIRA, B. Y C., MPODOZIS

Tennessee, USA.

Conferencia; 17th International Basement Tectonics Association Conference.; 2004

The active Andean margin is the type locality for studying creation and destruction of continental crust and mantle lithosphere along a convergent continental margin.  The most important of these two process along the Central Andean margin could be destruction  Evidence for two destruction processes, loss of continental crust and lithosphere by forearc subduction erosion  and foundering of dense over -thickened crust and lithosphere (delamination), can be seen in a transect across the southern Puna/Altiplano plateau at ~ 25° to 27°S latitude.  Models for crustal growth need to balance crustal destruction and creation, as well as to take into account modulations in the rates of these processes through time. Important information on crustal destruction processes comes from Neogene arc and backarc magmatic rocks between 25.5°S and 27°S whose variable distribution and chemistry reflect eastward migration of the frontal arc, moderate changes in the angle of the subducting slab, and delamination of over thickened crust (Coira et al. 1993; Kay et al. 1994, 1999; Kay 2003).  The magmatic pattern in this transect is distinct from those to the north and south where the angle of the subducting slab has been inferred to steepen and shallow respectively in the last 16 Ma (see Kay et al. 1999).  The Neogene magmatic evolution between 25.5°S and 27°S begins with ~ 26 to 20 Ma large dacitic ignimbrite complexes in the arc and small dacitic porphyries in the backarc.  This pattern changes as these centers extinguish and smaller ~ 19 to 16 Ma mafic to dacitic stratovolcanoes extend into the backarc near 26°S.  Subsequently, widespread ~ 15 to 8 Ma centers consisting of small to very large mafic andesitic to dacitic stratovolcanic/dome complexes and minor ignimbrites extend up to 300 km east of the arc front which is > 20 km further east than at 20 Ma.  More dramatic changes occur between ~ 7 and 3 Ma as the arc front is displaced further eastward, coincident with widespread backarc mafic magmas with both intraplate and backarc chemical signatures and dacitic ignimbrites.  Some andesitic to rhyodacitic magmas occurring in both the arc and the backarc region have extreme “adakitic” signatures (La/Yb = 30 to 100) indicating that they equilibrated with garnet-bearing granulitic or eclogitic mafic crust.  The magmatic history finishes after 3 Ma with the stabilization of the Central Volcanic Zone arc front some  ~ 60 km east of the Early Miocene front, the eruption of the giant ~ 2 Ma backarc Cerro Galan ignimbrite, and bimodal magmatic activity from small mafic backarc monogenetic centers and silicic centers. The observation that the arc magmatic front migrated some  50 to 60 km to the east in 20 Ma requires substantial geometrical changes in the forearc and subarc crust and mantle lithosphere that are best accommodated by removal of the base of the overriding plate by a subduction erosion process.  Chemical evidence for the entry of this crust into the arc and backarc magma source at times of frontal arc migration comes from transient “adakite”-like signatures that appear in arc and backarc magmas at times of arc migration, the absence of these signatures at times of frontal arc stability, and a change from less to more crustal-like Sr, Nd, and O isotopic ratios in primitive mantle-derived basaltic magmas erupted before and after frontal arc migration (Kay 2003). Geophysical evidence for a thinner mantle lithosphere along with a higher average elevation relative to adjoining regions to the north and south (Whitman et al. 1996), a change from a contractional to a mixed stress regime, and a distinctive backarc magmatic history from the rest of the Central Andes led Kay and Kay (1993) and Kay et al. (1994) to argue for delamination of thickened, dense eclogitic crust from beneath the southern Puna. In accord with this suggestion, Yuan et al. (2002) use recent geophysical data to argue that a ~ 55 km thick silicic crust with no mafic root and a < 100 km thick continental lithosphere are present beneath the Puna at 25°S.   Radiometric ages for Puna mafic lavas suggest that delamination had begun by ~ 7 to 6 Ma as the frontal magmatic arc migrated eastward between ~7 and 4 Ma and was over by the time of  the Cerro Galan ignimbrite at ~2.2 Ma  (Kay et al. 1999).    Magmatic evidence for the existence of a thicker crust and mantle lithosphere in the southern Puna before ~ 6 Ma comes from trace element chemistry of Late Miocene dacites that require a basaltic composition eclogitic residue and pre Late Miocene magmatic styles and chemistry that show no evidence for a pre-existing thin lithosphere beneath this region. Open questions regarding the driving forces for the destruction processes affecting the continental lithosphere of this region are the role of the southward migrating Juan Fernandez ridge on the subducting Nazca plate and changes in plate convergence parameters.  Reconstructions of the subducting Juan Fernandez ridge (Yáñez et al 2001) show that it was passing beneath the southern Puna at ~ 12 to 7 Ma as the magmatic arc broadened and was gone by the time that delamination occurred. Thinning of the underlying continental lithosphere by the passing of the subducting ridge could have facilitated the sinking of a dense crustal root into the underlying mantle.  Another potential external influence is the effect of  circum-Pacific plate convergence vectors on the timing and rate of the subduction erosion process as postulated times of maximum frontal arc migration coincide with compressional periods that have been long recognized all along the Andean margin.