Abstract

Spatial, temporal, and chemical data may be used to calculate paleodip angles for paleosubduction zones, assuming the position of the paleotrench is reasonably well known. In turn, paleoconvergence rates may be calculated according to the equation developed by Luyendyk (1970), that relates dip angle to convergence rate. Data from Cretaceous-Tertiary magmatic rocks indicate paleoconvergence rates of 7 to 8 cm/yr for the period 130 to 85 m.y. ago. Convergence rates markedly increased to an average of about 14 cm/yr for the period 85 to 45 m.y. Decreased convergence rates are indicated from 45 to 20 m.y. Paleoconvergence rates determined from magmatic rocks are generally in agreement with those determined independently by other workers from paleomagnetic data on ocean-floor rocks. The anomalously high rates of convergence determined from the igneous-rock data may be due to subduction of younger, more buoyant oceanc crust after 65 m.y. B.P., although recent observations from independent data suggest that Eocene convergence may have been a high 20 to 26 cm/yr. Paleomagnetically, the period of constant convergence, constant dip subduction from 130 to 85 m.y. ago correlates almost exactly with the long Cretaceous normal polarity interval as suggested on the new Lowrie-Alvarez 1981 geomagnetic time scale. In contrast, the post-85-m.y. period of variable convergence and variable dip subduction correlates with polarity events of short duration and frequent reversal. Tectonically, the period of constant convergence rates from 130 to 85 m.y. ago correlates with Sevier orogeny in Nevada and Utah. The period of increased convergence from 85 to 45 m.y. ago correlates with classic Laramide orogeny in Wyoming and Colorado. The period of decreased convergence from 45 to about 20 m.y. ago correlates with a poorly understood and incompletely defined mid-Tertiary orogenic event.

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