Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic AR‐ages for Blueschists in the Franciscan Complex of California

Modeling of heat flow through a wedge‐shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of Ar‐isotopic ages and metamorphic progressions across blueschist belts. If convergence rates are maintained at rates of a few centimeters per year or more, material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where Ar‐loss by diffusion and recrystallization effectively ceases. Maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider. Because of this difference in thermal history, K‐Ar and 40 Ar/ 39 Ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth. Application of this thermal modeling to the Franciscan subduction complex of northern California indicates that the coherent blueschists in the Eastern Belt (typically with 115 to 125 m.y. Ar‐isotopic ages) and the high‐grade blueschist blocks in the mud matrix melanges of the western portion of the Central Belt (typically with 140 to 150 m.y. Ar‐isotopic ages) could both have formed during a single, prolonged period of continuous convergence.