The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean 206Pb/238U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change.

The Data folder contains three files: MP_Pmag_Summary.xlsx: This excel spreadsheet has all of the site mean data used in the study. There are four sheets within the spreadsheet (1) the latex formatted text that was used to generated the table in the GSA data repository; (2) an interactive worksheet where Fisher means can be calculated from the flow means; (3) a sheet with preliminary formatting for the MagIC database and (4) data in the format that was exported for the Mamainse_Point_data.csv file. Mamainse_Point_data.csv: This csv file contains the flow (i.e. site) mean paleomagnetic data as well as the locations of the flows stratigraphically and geographically (lat, long is in WGS84). These data are imported and analyzed in the IPython notebook. NSVGsubset_VGP.txt, SCV_VGP.txt, SI_upperthird_VGP.txt: Text files used for comparitive analysis in the IPython notebook. See paper text for more details. The Code folder contains an IPython notebook that contains statistical data analysis and plotting associated with paleomagnetic data in the the manuscript. In addition to the IPython notebook, the folder contains the pmag.py, pmagplotlib.py, and pmagpyrc.py libraries of PmagPy version 2.206 as well as a library IPmag.py that modifies some of the interactive programs of PmagPy for use in the IPython environment in addition to providing some new functions. This folder also contains a PDF version of notebook to be viewed outside of the IPython generated using nbconvert. Note that the IPython notebook can also be viewed online without an IPython installation at this link where it is rendered better than in the PDF output: http://nbviewer.ipython.org/github/Swanson-Hysell/2014_Sw...