Abstract

Magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar (25–30 km depth). In western North America, undeformed Late Cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern California to southeastern Alaska; this belt has been interpreted as consisting of accreted terranes. Because the present crustal thickness within this belt is about 30 km, the plutons imply a crustal thickness at the time of emplacement of 55–60 km, comparable to that in the central Andes. The plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 °C/km and may be closer to 12 °C/km. Such cool, thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western North America. The implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift; it is on the order of 1 mm/yr. The rapid uplift of the mobile belt by and large did not affect the craton.

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