MAGMATIC AND STRUCTURAL CONSTRAINTS ON THE TECTONIC EVOLUTION AND UPLIFT OF THE PUNA PLATEAU OF THE CENTRAL ANDES

Neogene magmatic and structural patterns reflect the tectonic evolution of the Puna plateau. The Puna (~ 23° to 27°S) differs from the Altiplano to the north in that the subducting Nazca plate begins to shallow south of 24°S; a gap in intermediate depth Wadati-Benioff seismicity occurs at 25° to 27°S; late Neogene strike-slip and extensional faults are common; small discontinuous, diachronous basins occur; chains of NW-trending magmatic centers cross the plateau; and seismic, topographic and gravity data support a thin lithosphere. Between ~22º and 25ºS, Neogene magmatic centers record the steepening of the Nazca plate after an episode of pre-late Miocene shallow subduction. Voluminous ~10-6 Ma ignimbrites related to giant calderas reflect crustal and mantle melting, and possibly crustal delamination as the mantle wedge thickened beneath the steepening slab. Pliocene ignimbrites and Pleistocene arc rocks in the west record the final steepening. Further south (~24º to 28ºS), an early Neogene magmatic gap is absent, 15-8 Ma andesitic to dacitic stratovolcanoes and dome complexes with subduction-related chemical signatures occur, and post-7 Ma mafic lavas from small centers along faults erupted contemporaneously with ignimbrites that culminated in the ~2 Ma Galán eruption. Pre-late Pliocene lavas have arc chemical trend while < 3 Ma lavas can have intraplate tendencies. Pliocene dacites equilibrated with eclogitic mafic crust. This pattern reflects subdued Miocene shallowing of the Nazca plate as the Juan Fernandez ridge passed beneath the region. Subsequent Pliocene delamination of eclogitic lower crust and mantle lithosphere was facilitated by a thickening mantle wedge that later generated the < 3 Ma intraplate lavas. Limited crustal shortening in the southern Puna is consistent with crustal flow facilitating crustal thickening and uplift during a ~8-4 Ma peak of thrusting and magmatism to the north and shallowing of the flat-slab to the south.