In defense of magmatic diapirs

Diapirism as a crustal magma ascent mechanism has been recently criticized. We contend that this reflects an overly simplistic view that diapirs must resemble modeled hot-Stokes diapirs and the perception that magma ascent in dikes is a more problem-free mechanism for the construction of plutons. We describe four Cordilleran plutons that have characteristics much more comp atible with diapirs than dike-fed chambers. These plutons were emplaced at depths ranging from similar to 10 to 30 km and record different parts of di apiric ascent paths. Most ascended during complex visco-elastic flow of hos t rock during regional deformation, have narrow structural aureoles indicat ing power-law behavior of host rock, and were constructed of multiple batch es of magma, attributes enabling them to ascend greater distances than sing le hot-Stokes diapirs. Some features of these plutons are not typically att ributed to diapirs, and thus we introduce the term visco-elastic diapir for bodies consisting of one or more batches of magma rising together, with le ngth to width ratios < 100, surrounded by host rock deforming by brittle an d ductile processes, and for which ascent is driven by buoyancy plus region al stress. We conclude that diapirism remains a valid magma ascent mechanis m for the crust. (C) 1999 Elsevier Science Ltd. All rights reserved.