DOI: 10.6078/d17g6j
title: Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

description: 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
 &gt;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.
description: 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...