Abstract

Kinematic data from cogenetic dikes, magmatic foliation, and high-temperature solid-state deformation observed in 1.4 Ga plutons and their aureoles in the southwestern United States show a synmagmatic strain regime involving regional contraction. We propose that these plutons were preferentially emplaced along Paleoproterozoic shear zones and province boundaries. Thermal softening of the middle crust (8-12 km) during intrusion allowed regional strain and pluton-enhanced metamorphism to be recorded as much as 1500 km inboard of the southern margin of Laurentia. We abandon the term "anorogenic" and instead view these plutons as recording compressional or transpressional plate-margin tectonism from 1.5 to 1.3 Ga along southern Laurentia. Further study of 1.4 Ga plutons may also offer insights into middle-crustal processes that occur in broad zones of transpressional deformation.

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