Abstract

Quantitative analysis of tectonic subsidence in Cambrian and Ordovician platform carbonates and associated strata exposed in the Spring Mountains (Nevada) and the Nopah, Funeral, and Inyo Ranges (California) indicates that subsidence associated with this segment of the early Paleozoic passive continental margin is exponential in form, consistent with thermal contraction of the lithosphere following extension. As in other parts of the North American Cordillera, continental separation in the southern Great Basin appears to have taken place between 590 and 545 Ma. These results are not sensitive to uncertainties in stratigraphic thickness, biostratigraphic age control, or paleobathymetry. Uncertainties in the Cambrian time scale lead to predictable variations in the inferred time of onset of thermal subsidence, but they have no effect on the inferred stratigraphic position of the rift to post-rift transition. A younger age for the base of the Middle Cambrian results in a younger inferred age of onset of thermal subsidence accompanied by greater rates of subsidence during the Cambrian, whereas a significantly older estimate of the onset of thermal subsidence can be obtained only if the base of the Middle Cambrian is substantially older than 540 Ma, a possibility that is inconsistent with available data.

Results of the subsidence analysis are particularly significant because this is one of the few regions along the length of the North American Cordillera where they can be compared directly to the geologic evidence for syn-rift and post-rift deposition. Basement-involved faulting associated with the Amargosa basin ("aulacogen") ceased during deposition of the Noonday Dolomite, which is thought to be older than 700-680 Ma on the basis of stromatolites of late Riphean affinity. The overlying Johnnie Formation contains supposed Vendian stromatolites (younger than 700-680 Ma). If it is assumed that our results indicate the timing of the final rift to post-rift transition, then either the ages inferred from stromatolites are incorrect or the lithosphere was thinned regionally after deposition of the Noonday. The latter possibility is supported by limited geologic evidence for extension in latest Proterozoic and Early Cambrian time. The lack of appreciable physical evidence for crustal extension after deposition of the Noonday, however, may imply that (1) a uniform extension model for lithospheric thinning is inappropriate for this part of the margin or that (2) some or all of the localities studied are continentward of the hinge zone, and that the observed subsidence is exaggerated by flexural loading in a deeper basin to the west.

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