Magnetotelluric studies of the Andean subduction zone 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 2017-2018 the University of Alberta and INGEMMET began to collect a series of regional MT transects that will image this transition in slab angle. The first profile extended from the Pacific Ocean near Arequipa across the Altiplano near Juliaca and northeast towards the Amazon Basin. Broadband MT data were collected every 4-5 km with 1-2 days of recording at each station. Inversion of the MT data reveals high resistivity zones in the forearc where Pre-Cambrian basement is present at the surface. A number of mapped faults appear to be associated with changes in resistivity and the MT data are being used to defined the subsurface geometry of the faults. East of the volcanic arc, an extensive region of low resistivity is imaged in the crust and upper mantle. Additional MT profiles are planned to map the resistivity structure in 3-D as the angle of slab subduction changes.