Geologic and isotopic constraints on the crustal structure of the northern Great Basin

Geophysical data do not define the western limit of North American continental crust in the northern Great Basin because Phanerozoic deformation, particularly Cenozoic extension, has overprinted original gravity, magnetic, and seismic signatures. Stratigraphic and structural analyses of north-central Nevada, however, allow location of the continental-oceanic crustal boundary on the basis of recognition of continental-slope deposits. The stratigraphically defined location of the continental edge corresponds to the position of an isotopically defined boundary between granitic plutons with initial Sr isotopic ratios less than 0.706 to the west and greater than 0.706 to the east. Three generally north-south-trending subprovinces with different Sr and Nd isotopic character can be defined in the northern Great Basin. The boundary between the western and central subprovinces is defined by the 87 Sr/ 86 Sr initial = 0.706 isopleth and corresponds to the stratigraphically defined western edge of transitional North American continental crust. The boundary between the central and eastern subprovinces is defined by the ϵ Nd = -7 line (Farmer and DePaolo, 1983) in north-eastern Nevada and may be entirely within North American crust. A third isotopically defined boundary, imprecisely located in western Utah, marks the western boundary of the largely undeformed North American craton.