Space/time evolution of magmatism in the Klamath Mountains Province, CA/OR: Implications for Cordilleran arc magma periodicity
The Klamath Mountains Province (KMP) preserves a history of Mesozoic accretion and subduction that occurred along the western North American margin during the formation of the North American Cordillera. Arc magmatism in the KMP was episodic, and it is becoming clear that most arcs have episodic magmatism. In this report are presented a compilation of over 300 zircon U-Pb crystallization ages from igneous intrusions in the KMP, as well as K-Ar and Ar-Ar dates when U-Pb geochronology data are unavailable, and these age data are integrated with calculations of the area and volume of plutons to quantify the space/time magmatic evolution of the KMP and calculate magma addition rates. These spatial data include measurements of the area of major igneous intrusions in the province as well as the area of host terranes. A new method was developed to calculate a minimum volume for intrusions using digital elevation models, and more in depth estimates of the volumes of individual plutons are provided based on subsurface projections of their shapes.
The KMP consists of a succession of accreted terranes from early in the Paleozoic to the Early Cretaceous. Therefore, the area of the terrane increased over time, and for this research the areas of terranes were measured to normalize magma addition rates to the present area of the terrane at the time of each magmatic event. The total initial extent of each terrane is now obscured due to thrusting and erosion; therefore, these values are minima. The KMP has a total surface area of about 29,807 km2, and about 4,980 km2 or 17% of the exposed area is underlain by plutonic rocks. The KMP experienced three magmatic flare-ups at ~400–430 Ma, ~154–174 Ma, and ~136–150 Ma. The Jurassic episode (~154-174 Ma) was the largest, with at least 25,289 km3of magma added at a magma addition rate of 0.052 km3/myr/km2 and exposed over 3,005 km2.
This analysis also finds two interesting trends in the distribution of plutonic activity: 1) the locus of plutonism migrated NNE during the Jurassic; and 2) the Early Cretaceous plutonism defines a NNW trend along the entire length of the KMP. The Middle Jurassic arc of California was active in the KMP, and the Middle Jurassic episode in the KMP reflects that seen in the western Sierra Nevada. However, the large Cretaceous event that built the Sierra Nevada batholith is not represented in the KMP, as magmatism was finished in the KMP by at least 130 Ma. The magmatism in the KMP may have continued into the Late Cretaceous if it had not been displaced westward early in the Cretaceous.
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