Colin Sturrock
The North America Magmatic Gap (NAMG) defines the period between 1.49-1.61 Ga without significant zircon populations found across the continent (Van Schmus et al. 1993). Thus, detrital zircons in this age range are interpreted to be exotic to North America, coming instead from Baltica, Caledonia, Australia, etc. (Copron and Nelson 2009). 1.5 Ga zircons also have implications for the assembly of the North American Cordillera. In western North America, 1.5 Ga zircons are largely confined to “exotic terranes” that accreted on at various times. Any erosion from these terranes leading to deposition of 1.5 Ga zircon bearing sediments onto North America would then give a minimum age at which a particular exotic terrane accreted on.
This idea has the potential to help resolve timing issues with Cordilleran assembly outlined in Hildebrand (2009, 2013). If there are 1.5 Ga zircon-bearing sediments on what Hildebrand views is cratonic North America, their stratigraphic age would give the latest time Rubia could have collided with North America. To be in agreement with Hildebrand, this age must be later than 124 Ma. However, the absence of such zircons, as is to be expected based on their “exotic-ness”, wouldn’t provide any new information.
Surveys of detrital zircon dates in North American sediments have predictably produced close to zero significant populations of 1.5 Ga zircons (Gehrels and Ross 1998; Gehrels and Pecha 2014; Yonkee et al. 2014). One way to refine the search is to identify exotic terranes that do contain 1.5 Ga zircons, which includes the Belt-Purcell, the Chase Formation of the Okanagan, and possibly the Yukon-Tanana (Copron and Nelson 2009; Link et al. 2007). Since the Belt-Purcell borders the proposed suture between allochthonous and autochthonous North America in the collision model of Hildebrand (2013), this seems like a reasonable starting point. By this reasoning, areas bordering the easternmost extent of allochthonous Rubian thrusts are the most likely to become imparted with 1.5 Ga zircons. This includes Alberta, Montana, Wyoming, eastern Utah, and the topmost part of the Arizona Nevada border.
Unfortunately, many of these sediments are Mesozoic in age and are thus not well-equipped to discern whether exotic terranes existed to the west or not pre-124 Ma. Of all the detrital zircon studies surveyed, only one location might be qualified to provide evidence that 1.5 Ga zircons were making it over into North America before 124 Ma. The 171 Ma strata of the Sawtooth Formation and Swift Formation in the Ellis Group in central Montana contained two very small humps at ~1500 Ma (Fuentes et al. 2009), though these are similar in amplitude to other broad peaks at these older dates and more than likely are just noise.
Ultimately, this test was unsuccessful at either supporting or refuting the model of Hildebrand (2013). Broadly, the lack of any 1.5 Ga zircons in cratonic North America is consistent with most views on North American geology, so the fact that nothing was found is unsurprising.
References:
Fuentes, F., DeCelles, P. G., & Gehrels, G. E. (2009). Jurassic onset of foreland basin deposition in northwestern Montana, USA: Implications for along-strike synchroneity of Cordilleran orogenic activity. Geology, 37(4), 379-382.
Gehrels, G. E., & Ross, G. M. (1998). Detrital zircon geochronology of Neoproterozoic to Permian miogeoclinal strata in British Columbia and Alberta. Canadian Journal of Earth Sciences, 35(12), 1380-1401.
Gehrels, G., & Pecha, M. (2014). Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America. Geosphere, 10(1), 49-65.
Link, P. K., Fanning, C. M., Lund, K. I., & Aleinikoff, J. N. (2006). Detrital-zircon populations and provenance of Mesoproterozoic strata of east-central Idaho, USA: Correlation with the Belt Supergroup of southwest Montana. SPECIAL PUBLICATION-SEPM, 86, 101.
Van Schmus, W. R., Bickford, M. E., Anderson, J. L., Bender, E. E., Anderson, R. R., Bauer, P. W., & Gilbert, M. C. (1993). Transcontinental Proterozoic provinces. The Geology of North America, 2, 171-334.
Yonkee, W. A., Dehler, C. D., Link, P. K., Balgord, E. A., Keeley, J. A., Hayes, D. S., & Johnston, S. M. (2014). Tectono-stratigraphic framework of Neoproterozoic to Cambrian strata, west-central US: Protracted rifting, glaciation, and evolution of the North American Cordilleran margin. Earth-Science Reviews, 136, 59-95.