Temporal histories of Cordilleran continental arcs: Testing models for magmatic episodicity

Abstract

Magmatic activity in continental arcs is known to vary in a non-steady statemanner, the mechanisms for which are a matter of ongoing discussion. Ofparticular importance is the question to what extent episodic magmatism incontinental arcs is governed by external factors (e.g., plate motions) and internal factors (e.g., feedback processes in the upper plate). In order to test existing models for magmatic episodicity, which are mostly based on temporally and spatially limited records, this study uses large datasets of geochronological, geochemical, and plate kinematic data to document the Paleozoic to Mesozoic development of the Cordilleran orogen in eight transects from British Colombia to Patagonia. The temporal distribution of U/Pb bedrock and detrital zircon ages, used as a proxy for timing of magmatic accretion, shows that some minima and maxima are nearly synchronous for thousands of km along the arc. Some age patterns are characterized by a periodicity of 50?80 Ma, suggesting a cyclic controlling mechanism. Other magmatic lulls or flare-ups find no equivalent in adjacent sectors, indicating that either discrete events or variable lag times also may be important in governing magmatic activity in continental arcs. Magma composition in Mexico, the Peninsular Ranges, and the Sierra Nevada varies episodically and proportionally with the temporal record of arc activity. During flare-up events, there is an increase in Sm/Yb, indicating deeper melting, and a decrease in εNdi, suggesting a higher degree of crustal assimilation. Geochemical scatter also increases during the initiation of flare-up events. Plate kinematic data provide a means of evaluating mantle heat input. The correlation between plate convergence rate and magmatic accretion varies for each sector, suggesting that different flare-ups or lulls likely reflect variable combinations of processes.