SPACE/TIME EVOLUTION OF MAGMATISM IN THE KLAMATH MOUNTAINS PROVINCE (KMP), CA/OR: IMPLICATIONS FOR CORDILLERAN ARC MAGMA PERIODICITY

We present a compilation of over 350 zircon U-Pb crystallization ages from igneous intrusions in the KMP and integrate these data with new calculations of the area and volume of plutons to quantify the space/time magmatic evolution of the KMP. A surge occurred ~400–430 Ma, followed by more widespread magmatism in the Jurassic and Early Cretaceous, peaking at ~155–175 Ma and ~135–150 Ma. Magmatic flare-ups in the KMP generally last 15–20 myr.

The aerial exposure of the main lithotectonic belts and plutons was measured at regional and larger scale in ArcMap. The oldest lithic belt in the KMP is the Mesozoic and Paleozoic eastern Klamath belt, which is ~7,126 km², and was intruded by ~507 km² of plutonic material, covering 7%. The Paleozoic Central Metamorphic belt of ~1,320 km² accreted during a magmatic lull. The western Paleozoic and Triassic belt (actually Mid Jur. to Paleozoic) added ~15,233 km² to the KMP, making its area at least 23,679 km² by 168 Ma. Then ~2,661 km² of igneous material intruded the KMP (11% of the province). The western Jurassic belt’s accretion at ~155 Ma added ~6,127 km² and brought the KMP’s total area to ~29,806 km². Through the Late Jurassic and Early Cretaceous, ~2,166 km² of igneous rock intruded the KMP, covering 5%. The entire area of exposed plutonic rock in the KMP is ~4,765 km² (~15% of KMP). The total initial extent of each belt and the plutons is now obscured due to erosion and the under-thrusting of successive terranes during Jurassic-Cretaceous orogenesis. Thus, these areal values are likely minima.

Based on these data we draw the following conclusions: 1) intrusive material was added in discrete batches; 2) the locus of plutonism migrated NNE during the Jurassic; 3) the Early Cretaceous magmatism defines a NNW trend along the entire length of the KMP. An initial volume estimate for the well-studied 156–159 Ma Wooley Creek batholith is ≥ 960 km³ indicating a volume addition rate of >320 km³/my. Ongoing volumetric calculations from geochemical mass balances may indicate a much greater volume. We will calculate volumes for the entire KMP based on structural cross section restorations, gravity data, geochemical mass balance modeling, outcrop patterns, and exposed relief. Integration of this spatial and temporal data leads to calculations of magma addition rates, which will be normalized to the host belt area.