Magnetotelluric studies of the transition from normal subduction to flat slab subduction in Southern Peru

The Andean subduction zone extends along the entire western margin of South America and is characterized by regions with both normal and flat slab subduction. The causes of flat slab subduction, and the effects of changes in slab angle on magmatic processes are not well understood. The magnetotelluric (MT) method has proven a useful tool in understanding the distribution of partial melt and aqueous fluids in subduction zones. In the case of the Central Andes, MT studies in Northern Chile and Bolivia have shown significant along strike variations in crustal and mantle resistivity structure, implying changes in the distribution of partial melts. To date no regional scale MT studies have taken place in Peru, where a well-defined transition occurs from normal subduction to a flat slab.

In 2017-2019 the University of Alberta and INGEMMET collected two regional MT transects in Southern Peru. The first profile (P1) extended from the Pacific Ocean near Arequipa across the Altiplano near Juliaca and northeast towards the Amazon Basin. The second profile (P2) extended northeast from Nazca in the region where the slab is flattening. On each profile broadband MT data were collected every 4-5 km with 1-2 days of recording at each station. Both 2-D and 3-D inversions were applied to the data and similar resistivity models were obtained. The 3-D inversion model revealed high resistivity zones in the forearc on both profiles where Pre-Cambrian basement is present. East of the volcanic arc, an extensive region of low resistivity was imaged on P1 in the lower crust of the South American Plate and this feature can be interpreted as a zone of partial melt and aqueous fluids. This feature was connected to the surface by two dipping low resistivity layers. The first rose from the lower crust to the northeast and the second rose to the southwest, where it reached the surface some 50 km northeast of the modern volcanic arc. On P2 these dipping features were present but they had a higher resistivity, implying a lower fluid content than on P1. The volcanic arc in the region of P2 has been extinct for several million years and the reduced melt content reflects the cessation of magmatism.