Physical and Chemical Variations During Episodic Behavior of Mesozoic Cordilleran Arcs

Abstract:

Models of episodic or cyclic behavior in Mesozoic Cordilleran continental arcs often examine broad patterns based on a single event or cycle. However, our comparison of multiple events/cycles both in single arc segments and in multiple, along-strike segments over ~15,000 km indicate that significant variations occur. For example during the three flare-ups peaking at ~225, ~161, and ~98 Ma in the Sierran segment, magma addition rates (MARs) indicate that the Triassic flare-up produced 3 times and the Cretaceous ~7 times as much magma as the Jurassic and 100 to 1000 times more magma than during lulls. Isostatic modeling combining arc shortening + MARs thus results in small roots/surface elevations in the Jurassic and Triassic and a large root/surface elevation during the Cretaceous. Age-constrained geochemical data in the Mesozoic Sierra Nevada show both general and specific flare-up versus lull trends. General trends during flare-ups include decrease in εNd_i median values, and an increase to higher median values of SiO₂, ⁸⁷Sr/⁸⁶Sr_i, Sr/Y, and (Sm/Yb)_n. However these median values vary dramatically from one flare-up/lull episode to the next. For example the change in ⁸⁷Sr/⁸⁶Sri, Sr/Y, (Sm/Yb)_n during flare-ups and lulls is not as pronounced for the Triassic and Jurassic events compared to the Cretaceous. And the Early Cretaceous displays unusually high median values in Sr/Y and (Sm/Yb)_n. These patterns draw attention to a potential correlation between MARs and geochemical patterns. Although MARs are largely unavailable for other arc segments, along-strike, arc segment comparisons of zircon age spectra indicate flare-up periodicity of ~60-80 Ma over variable spatial scales ranging up to ~6000 km. Variations occur in all of the above geochemical measurements in both general and specific flare-up trends. Our results suggest that driving mechanisms for flare-ups/lulls vary along this Mesozoic arc and that second order effects vary between flare-ups and arc segments.