THE PICURIS OROGENY PART 1: A CONTINENT SCALE CONVERGENT MARGIN

We present a comprehensive tectonic model for North America between 1.53 and 1.35 Ga, during the emplacement of voluminous ferroan granites, that shows that convergent margin tectonism produced the spatial and temporal distribution of igneous rocks on the continent during this time period. This Mesoproterozoic event is one of the most significant crustal recycling and modification events to have affected Laurentia. Workers have debated the extent to which convergence, extension, and hot spot volcanism each shaped the record of Mesoproterozoic magmatism in Laurentia. Our model shows that a continuous convergent margin extending from eastern Canada to the southwestern US can explain spatial, temporal, and structural patterns in Mesoproterozoic igneous rocks.

This tectonic model is based on time slice maps showing the spatial distribution of igneous rocks. We have built a database of ~650 igneous zircon U-Pb crystallization ages from the literature, using both preexisting regional age compilations and individual sources that have not been aggregated previously. The dataset spans the southwest, the midcontinent, and the northeast. We have plotted the location of these samples on a map of North America, and partitioned the data into time intervals based on geographically distinct distributions of the ages.

Our continent-scale model reveals new patterns in the age data. We demonstrate that rocks such as the ferroan granites, which workers have considered enigmatic because they do not exhibit clear age progressions, do indeed show distinct geographic and temporal patterns. Furthermore, these patterns are compatible with a long-lived convergent margin along the southern margin of Laurentia.

This tectonic model can account for geologic features of the Mesoproterozoic that have previously been considered anomalous. Critically, this model links the Pinware orogeny, which has been documented in eastern Canada, with the Picuris orogeny, which has been identified in the southwest US. The model ties together tectonic features of the northeast, the midcontinent, and the southwest, and is also the first to provide mechanisms for generation of the Eastern and Southern Granite Rhyolite provinces. This tectonic model can now be used to test hypotheses about the growth of North America through geologic time.