Crustal thickening during Proterozoic metamorphism and deformation in New Mexico

Abstract

Proterozoic rocks in northern and central New Mexico underwent simultaneous metamorphism and deformation, tentatively dated at 1410 Ma. Structural relationships record a minimum of 20%–30% shortening during the latter part of deformation, and chemical zoning in garnet and plagioclase indicates a 20% increase in depth of burial during the same interval. Locally, deformation thickened the upper continental crust by at least 20%. This crustal thickening was distributed over a broad area and caused rocks across 75 000 km 2 to recrystallize at peak metamorphic conditions near 525 °C and 4 kbar. The metamorphic terrane cooled isobarically, at rates less than 5 °C/m.y. Such slow cooling may be normal at middle crustal depths. Preservation of the regionally uniform peak metamorphic conditions reflects an unusual tectonic history: heat from the metamorphic event outlasted deformation, and the terrane was not subjected to rapid uplift following its thermal peak.