126 ANDES continental shelf reflect the effects of crustal thinning due to fore-arc tectonic erosion The basement of the southern coastal region consists of the high-grade metamorphic rocks of the Precambrian ($1 Ga) Arequipa block To the east, the Western Cordillera is mainly composed of deformed metamorphic rocks and Mesozoic sediments that are cut and covered by Mesozoic to Tertiary magmatic rocks The extensive Middle Cretaceous to Palaeogene Peruvian batholith that extends into the coastal region forms the western part The highest peaks occur in the $8 Ma Cordillera Blanca pluton whose western boundary, the Cordillera Blanca shear zone, has been interpreted as a low-angle detachment fault The Eastern Cordillera is made up of pre-Mesozoic metamorphic rocks along with subordinate Palaeozoic and Tertiary intrusives It hosts the Early Tertiary ‘inner arc’ and was the locus of intense Late Paleocene and Middle Eocene east-verging thrusting The Subandean belt further east consists of Mesozoic and Tertiary sedimentary rocks and Palaeozoic basement cut by Late Miocene to Recent reverse faults that extend almost to the Brazilian border Central Volcanic Zone/Altiplano–Puna Plateau segment ($15 to 28 S) This segment includes the active Central Volcanic Zone arc and the widest part of the high Andean Cordillera that includes the Altiplano–Puna Plateau The whole region is built on a variable age Precambrian and Palaeozoic basement Uplift of the plateau is principally attributed to ductile thickening of the lower crust in response to compressional shortening, and subordinately to magmatic addition Compressional shortening is reflected in the fold–thrust belts of the upper crust Total amounts of shortening are widely debated with values in various transects ranging from 300 to 500 km in the north to 6300 m) are Late Miocene to Pliocene andesitic to dacitic stratovolcanic and giant dacitic ignimbrite complexes Only sequences older than $10 My are significantly deformed The Altiplano section in Bolivia north of $22 S is mostly comprised of a relatively flat sediment-filled basin covering a largely Palaeozoic sedimentary sequence over a Brazilian shield-like Precambrian basement Tertiary volcanic rocks are concentrated in the Western and Eastern Cordilleras where they can host important tin and silver (Potosi) deposits One of the most famous is the silver deposit at Potosi in the Eastern Cordillera Crustal thicknesses under the Altiplano range from $66 to $78 km The Puna section in Argentina to the south is distinctive in being broken into ranges with high peaks and basins and east–west chains of Neogene volcanic rocks (Figure 5) This region also differs in being underlain by a basement that includes Palaeozoic mafic to silicic magmatic sequences formed in complex arc and back-arc settings The greatest average elevation is in the southern Puna where crustal thicknesses appear to be