Electrical Resistivity Imaging Of Lithosphere Structure In The North American Midcontinent

North American midcontinent that centered by the Great Lakes contains both the areas of the United States and Canada.Tectonically,this region is a typical Precambrian unit consisting mainly of Archean Superior Craton,Proterozoic accretionary zone,and the Midcontinent rift(MCR).It is a natural laboratory for studying the formation and evolution of the ancient continents as no major post-Grenville tectonic events affected this region.The MCR,one of the prominent Precambrian geologic features of central North America,is a failed rift formed within Laurentia at 1.1 Ga.It contained enormous volumes of igneous rocks outcropping near Lake Superior and was explained by the upwelling and decompression melting of the Keweenaw mantle plume or by the anomalously hot or fertile upper mantle.Three-arm model of continental rifting associated with mantle plume suggested that commonly two arms become a plate boundary while the third rift becomes a failed arm.Apart from the two well-known arms,Nipigon Embayment was suggested as the third failed arm of MCR,which is still debated.To understand the structure and evolution of western Superior Province and its possible relationship to MCR,we construct the first three-dimensional(3D)electrical resistivity structure beneath the North American midcontinet using both the USArray and Lithoprobe MT data.Our model shows a prominent highly conductive system that originates from the asthenosphere and extends upward to the Moho or lower crust,with a horizontal length of at least 600 km.This feature extends approximately south-north within the Archean western Superior craton.Extensive studies of this highly conductive anomaly system suggested that the sulfides are the most likely causes of the observed anomaly.As reported by the latest electrical laboratory measurement of iron sulfides,a few volumes%of sulfides are needed to explain our results.However,the sulfide content in the mantle is typically low(0.06%)and still not sufficient in the volcanic field that may enrich the sulfide such as the xenoliths from Sierra Nevada,California containing sulfides up to 0.4%.However,stronger tectonic events such as the plume may enrich the sulfide to a content explaining the above high conductivity.Therefore,the elongated west-north high conductivity near western Lake Superior at a depth of about 200 km was interpreted as the footprint of the Keweenaw plume that impinged at the base of the lithosphere and this plume is responsible for our observed highly conductive anomalies system.Incompatible elements like C,H,and S can only be locally enriched in the lithospheric mantle due to the presence of thick stable lithospheric roots of the Superior Craton,which restricts the extensive modification of the mantle,and this locally modified mantle as observed in our model may enrich sufficient sulfide to explain our conductivity anomalies.Based on the geometry and spatial location of this high-conductivity system,we proposed that this system may represent the third failed arm of MCR.In addition,we considered that the western upwelling melts were blocked by the subducted slab and moved along it.In contrast,the eastern melts were obstructed by the vertical high resistive body and underplated beneath Moho.