A quantitative procedure has been developed for calculating tectonic subsidence in fully lithified strata and has been applied to stratigraphic sections in the early Paleozoic miogeocline of the southern Canadian Rocky Mountains. The results indicate that tectonic subsidence along the inner edge of the miogeocline was controlled mainly by thermal contraction of heated lithosphere. Comparison of a palinspastically restored cross section of the inner part of the miogeocline with a cross section constructed from a two-dimensional stretching model suggests that thinned continental crust was present beneath the inner miogeocline. These results support the passive-margin model that has been proposed for the miogeocline. The extensive transgression onto the craton east of the miogeocline in Cambrian time, however, cannot be explained by subsidence processes operating within a passive margin, and the transgression could be evidence for a eustatic rise of sea level.

The form of the tectonic subsidence curves strongly implies that cooling of the heated lithosphere, which was initiated at the time of breakup, could not have begun earlier than the latest Precambrian or earliest Cambrian (555 Ma to 600 Ma). Ages of 800 Ma to 900 Ma that have been assumed previously for rifting in the miogeocline are too old to have led directly to continental breakup. Scattered occurrences of mafic volcanics interlayered with arkosic sediments have been reported in the latest Precambrian to earliest Cambrian Hamill Group exposed in the middle to outer part of the miogeocline. These deposits may record the phase of rifting that immediately preceded formation of the proto-Pacific margin in the southern Canadian Rockies.

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