TECTONIC STRESS REGIME RECORDED IN ZIRCON TH/U

We investigate the relationship between tectonic stress regime and zircon geochemistry by comparing zircon Th/U ratios of published igneous zircon U-Pb ages in the North American Cordillera to their synmagmatic stress regime. In the North American Cordillera, zircons formed in melts associated with extensional magmatism contain variable Th/U (0.3 to >3.5), including significant zircon populations with Th/U>1.0. Zircons from compressional magmatism exhibit low variability and low Th/U (<0.8). Higher temperature, more-fractionated, short-duration, bimodal magmatism in extensional magmatic systems favors highly variable and elevated Th/U in zircon. Lower temperature, long-lived, granitoid compressional magmatism is more conducive to low Th/U zircon crystallization. Therefore, Th/U in zircon may be associated with tectonic stress regimes that favor distinct magmatic conditions.

To test the hypothesized correlation, we evaluate zircon U-Pb age and Th/U ratios to the North American and southern Gondwanan zircon record. North American detrital zircon (<2.0 Ga; n=30,587) contain large populations of zircon with elevated Th/U (>1.0) that are temporally associated with (1) final phases of orogenesis, and (2) large-scale extension in Laurentia. Low Th/U ratios correspond to overall compressional processes (terrane accretion, orogenesis). Zircon U-Pb ages and Th/U results from southern Gondwawnan volcaniclastic and igneous suites display a time transgressive trend of extensional magmatism that suggests an unzippering of the Gondwanan margin beginning in Australia at 340 Ma and spreading to South America by 240 Ma. Along strike migration of extension may correlate to slab roll back along the eastern Gondwana (Australia and Antarctica) margin that culminated in Permo-Triassic slab break-off beneath western Gondwana (South America).