The Mantle Transition Zone (MTZ) beneath South America is investigated with stacking of Receiver Functions (RFs) of converted P-to-S phases from the velocity discontinuities at 410 km (d410) and 660 km (d660). A total of 785 seismic stations provided 22,235 high quality RFs, allowing 1,717 MTZ thickness measurements in circular bins of 2⁰ radius. An apparent MTZ structure is derived using the IASP91 reference model, where the d410 mean depth is 409.1 km and the d660 is 661.5 km, and the mean MTZ thickness is 251.8 km. This model presents coherent depressions and uplifts of the discontinuities matching well known velocity anomalies. After experimenting with eleven tomography models, a velocity correction adopting the SAW642ANb model with a 50 km top layer of the JOINT model was found to be the best approach towards a true depth model. This model wields a mean d410 at 413.2 km and the d660 at 662.8 km, whereas the MTZ thickness is 249.6 km. The correlation of depths and MTZ thickness variations supports previously determined Clapeyron Slopes (γ) of 2 MPa/K for the d410 and -3 MPa/K for the d660, reconciling γ with seismic observations. The results are fateful to the tectonic structure of South America, where colder-than-normal MTZ anomalies are found along the Andes, while hotter-than-normal anomalies are found along the Atlantic coast. The latter observed MTZ characteristics spatially correspond well with rift related structures and the former to locations where subducted slab has been inferred. The main inference from these observations is that tectonic processes play a major role in the control of thermal and chemical proprieties of the MTZ --Abstract, page iii
