PALEOMAGNETISM OF MID-CONTINENTAL RIFT SYSTEM ROCKS FROM THE THUNDER BAY AREA (ONTARIO, CANADA): NEW DATA AND NEW QUESTIONS

The Mid-Continent Rift System (MCRS) in the Lake Superior region (North America) is represented by a series of extrusive and intrusive rocks emplaced in the late Mesoproterozoic from ~1150 Ma to ~1080 Ma. We present the results of a paleomagnetic study of several MCRS rock sequences exposed in the vicinity of Thunder Bay (Ontario, Canada). Units studied include the Nipigon and Logan sills (dated at 1107-1114 Ma and ~1115 Ma, respectively), ~1100 Ma Crystal Lake Gabbro and three different sets of dykes: the NE–trending Pigeon River dykes (presently dated at 1078±3 Ma and 1141±20 Ma), the NW-trending ~1109 Ma Cloud River dykes, and the NE trending ~1109 Ma Mt. Mollie dyke. Paleomagnetic directions were measured from a set of 220 independently oriented samples using detailed thermal and alternating field demagnetization preceded by initial low-temperature magnetic cleaning. After removal of a soft viscous overprint, most samples revealed a one-component characteristic remanent magnetization (ChRM). Baked contact tests have been performed to test the primary nature of ChRM. Our rock magnetic analyses including magnetic hysteresis and thermomagnetic behavior suggest pseudosingle-domain low-Ti titanomagnetite as the principal magnetic carrier in studied rocks. Most of the directions show the typical Keweenawan asymmetry of the normal (N) or reversed (R) paleomagnetic component. However, some of our results (such as the N polarity of the Cloud River dyke) are inconsistent with the existing age data and/or the accepted geomagnetic polarity history of the MCRS. We will critically address the observed inconsistencies between the paleomagnetic and geochronological data. Our study contributes to further refinement of the MCRS magnetostratigraphy and the late Mesoproterozoic apparent polar wander path for North America.