THE ELECTRICAL SIGNATURE OF SUTURES IN THE CONTIGUOUS UNITED STATES

Extensive empirical evidence shows that suture zones are often associated with electrical conductivity anomalies imaged by magnetotelluric (MT) data. These high conductivity (>10^-1 S/m) anomalies often appear as steeply dipping planes that cut through the crustal column and can extend for hundreds or even thousands of kilometers along strike. The cause of high conductivity values has been attributed to graphite- and/or sulfide-bearing metasedimentary units trapped within ancient subduction zones. We present three-dimensional electrical conductivity images from the contiguous United States that display conductivity anomalies associated with key sutures across southern Laurentia. For example, sutures associated with the ~1.85 Ga Trans-Hudson Orogeny appear as high-conductivity belts that extend for hundreds of kilometers along the margins of the constituent crustal domains. Notably, conductivity anomalies are also lacking along inferred terrane boundaries that prevailing models for Laurentian Precambrian tectonics propose to extend through the U.S. Midcontinent. For example, there are no conductive lineaments associated with the boundary between ~1.8-1.7 Ga and ~1.7-1.6 Ga crustal provinces in southern Laurentia. As the lack of a conductivity anomaly in association with a suture has clear implications for the characteristics of the structure in question, we discuss the importance of these “missing” suture-bound conductivity anomalies for models of Precambrian tectonics in southern Laurentia.