Magmatism and the Shallowing of the Chilean Flatslab in the Central Andes

The magmatic history of the flatslab region between the Central and Southern Andean volcanic zones reflects shallowing of the slab, lithospheric thinning, narrowing of the asthenospheric wedge, crustal thickening and forearc removal by subduction erosion. Newly revised contours on the northern margin of the modern flatslab (Mulcahy et al. 2014) show the flattest part extends from ~28° to 33°S and is bounded by Pleistocene volcanic activity. An eastward broadening of the magmatic arc began after 18 Ma as westward drift of South America accelerated, but the most distinctive retroarc magmatism occurred after near normal subduction of the southward drifting Juan Fernandez Ridge began at ~11 Ma and ended as magmatism ceased in the Pampean ranges, ~ 700 km east of the trench at ~4.7 Ma.

Recent seismic work in the retroarc area indicate a ~60 km thick crust under the Precordillera fold-thrust belt with transitions at ~20 and ~40 km that are considered to be the top of crystalline basement and an eclogitic facies transition. Chemical constraints from ~15-7 Ma magmatic rocks suggest eclogization is related to crustal thickening over the shallowing slab in accord with field relations for major thrusting in the region by ~8-7 Ma. High Ba/Th ratios in <9 Ma volcanic rocks are interpreted to reflect phengite breakdown in the mantle wedge with the fluids facilitating eclogization of the lower crust. Evidence for mantle melt contributions in the magmas up until ~7 Ma comes from more primitive isotopic values in 1088-1251 Ma amphibolite and granulite facies xenoliths (eNd = 0 to -3; ⁸⁷Sr/⁸⁶Sr =704-0.710) than in Miocene volcanic rocks (eNd = 0-1.7; 0.70325-0.70345; zircon eHf ~ 0).

From ~8 to 3 Ma, the active volcanic arc front near 28°S and 33°S was translated ~ 40-50 km eastward in a suspected response to forearc removal by subduction erosion. Given the position of the arc and distance to the trench, the same amount of forearc was likely removed in the intervening flatslab region. Trapping of subducted forearc material in the mantle wedge could help to explain a low Vp/Vs ratios (1.65-1.72, Wagner et al. 2008 ) in the mantle wedge above the flat-slab as a low Vp could result from orthopyroxene formation by reaction of silicic material with the mantle wedge and a high Vs can be explained by the current slab being too cool to hydrate the mantle wedge.