Abstract

Professor R. M. Shackleton commented that a central feature of the authors’ interpretation is that the Peruvian Batholith owes its linearity to the presence of a deep-fault or basement lineament, up which the magmas rose. While it is clear that at the present level of erosion the granites are associated with, and are often limited by, important faults, it seems that these faults are probably the effect and not the cause of the linear distribution of the granites. The thickening of the crust under the Andes and the recent rapid uplift must be attributed to large accessions of material from the mantle, since crustal shortening during Mesozoic and Tertiary times was clearly trivial and could not account for the thickening. Additions from the mantle are to be expected as a result of melting from a downgoing slab. A linear array of plutonic intrusions parallel to a trench is a general feature of subduction zones and is surely the result of melting at a critical depth rather than ascent along deep faults for which there is no seismic evidence. Uplift pressures exerted by the rising magma would cause faulting in the upper crust, although closer to the surface such pressures would not be in evidence. The fact that granites of different ages form separate linear arrays is further evidence against deep fault control since different faults would be needed to explain each array. The trench-arc gap is normally sharply defined. Therefore, the trench-side limit of plutons or volcanoes is itself sharply

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