ACCRETED TERRANES IN NEVADA: THINGS ARE NOT WHAT THEY SEEM

The origins of much of the pre Tertiary crust of northern Nevada are poorly understood. A growing body of evidence supports the hypothesis that the lithosphere in north central Nevada is a composite of several distinct oceanic and continental crustal components. Geologic, geophysical and biostratigraphic evidence all point to an interpretation of terrane accretion from both oceanic and continental sources as the primary mechanism for crustal development in the Western US as far back as the middle of the Paleozoic.

For many years the Rb-Sr .706 line has been used as an approximation of the limit of the lower Paleozoic margin in Nevada. Deep water oceanic facies rocks have also long been recognized outboard of the Paleozoic miogeocline, or carbonate shelf, across the state. However, significant volumes of oceanic-basin derived material crop out inboard of the .706 line in north central Nevada, and have no known continental crustal basement, suggesting inconsistency between isotopic and geologic data.

The Nolan belt is a group of lower Paleozoic rocks exposed across northern Nevada. These rocks have an isotopic signature greater than .706, and contain continental margin-derived sandstones and carbonates at the base of the Paleozoic section; but, they lie outboard of other lower Paleozoic ocean basin derived, deep-water lithology rocks. While rocks of the Nolan belt have both lithologic and isotopic signatures of a continental margin, the details of both the stratigraphic and, especially, the structural history, of these rocks are distinct from those of the true western North American miogeocline in Nevada. The distinct structural and stratigraphic history in this belt, combined with the presence of oceanic-basin derived rocks inboard of these rocks strongly suggest that while the rocks of the Nolan belt in the Osgood Mountains do indeed have a continental isotopic signature, their location during the lower Paleozoic is highly uncertain and they should not be interpreted as representing the margin location at that time. This brings up the interesting question of from whence they have come, and while the answer is presently unknown, general similarities to some of the terranes of the Appalachians, as suggested by others for other terranes of the Cordillera, is a highly feasible interpretation worthy of study.