AT THE CORE OF THE MOUNTAINS: A SYNOPSIS OF PROTEROZOIC EVOLUTION IN SOUTHWESTERN COLORADO

The Needle Mountains Proterozoic complex is the foundation of the geologic record in southwestern Colorado, and gives a snapshot of continent production and modification on the southern margin of Laurentia from 1800 to 1400 million years ago (Ma). These “old” rocks have also influenced Phanerozoic tectonic and magmatic events, as well as Cenozoic erosion and landscape development.

The basic unit division and sequence of events in the Needle Mountains Proterozoic complex is well established. This complex is an ancient analog to modern tectonic systems where new continental crust is constructed by accretion of volcanic arcs followed by crustal stabilization and thickening with episodic tectonic reactivation and crustal growth during convergent-plate interactions. First-cycle orogenic crust in the Needle Mountains formed from 1800 to 1750 Ma, and was accreted onto the margin of the continent before the emplacement of late-synorogenic granitic plutons between 1735 and 1715 Ma. The period from 1690 to 1500 Ma was amagmatic and dominated by crustal stabilization and erosive recycling punctuated by an episode of thin-skinned deformation and regional greenschist-facies metamorphism. Crustal reactivation caused by regional deformation and metamorphism took place about 1425 Ma and accompanied the emplacement of mantle- and crust-derived magmas.

Important questions about the Proterozoic record in the southwestern Colorado, and the influence of the Precambrian rocks and structures on later geologic events, remain to be explored. Only a small fraction of the Proterozoic complex has been mapped at a scale larger than 1:62,500, and more detailed fieldwork will lend further insight into its history and provide a foundation for all geologic studies. The timing and stratigraphic relationships of Paleoproterozoic sedimentary rocks is poorly constrained, as is the structural and metamorphic history of some Proterozoic units. Major fault systems occur in the Proterozoic rocks but a regional understanding of their distribution and history is lacking. Regional geophysical studies would provide insight into the architecture of the mantle and crust in this region to evaluate the influence of Proterozoic crust on younger tectonic and magmatic events.