Colloquium Recap: Nathan Anderson

The Bloody Dick gneiss of Montana....

The processes occurring during the Paleoproterozoic (2.5-1.4 Ga) along boundaries of different crustal blocks in North America are not completely understood, a problem that Nathan Anderson is tackling with his master’s thesis.  The occurrence of maybe the most interesting-named gneiss, the Bloody Dick gneiss, and the Maiden Peak gneiss in SW Montana allow for the study of the evolution of the western margin of North America, as they are near the Wyoming craton, the Great Falls tectonic zone, and the Farmington zone. 

Creating the first detailed complete map of the Bloody Dick gneiss, Nathan collected samples to analyze in the lab.  Analyses of zircons in the gneiss, along with field observations showed that the Bloody Dick gneiss consists of multiple gneiss units that were originally igneous rocks emplaced at ~2.47 Ga, and then metamorphosed at ~1.76 Ga. 

Broad scale paleomap of the region. Nathan's map is located at the pink dot... go read his paper or thesis to see the final result.

This would be consistent with previous models suggesting northward-dipping subduction, and the Medicine Hat block being thrust over the Wyoming craton; however, juvenile hafnium values in the zircons suggests the rocks were derived from the mantle shortly before they crystalized.  This would be consistent with a northward dipping subduction model, although if the direction of subduction was instead to the south, that model would fit the new results. 

Nathan uses the results of his thesis to expand previous models, and suggests a new model having a flip in subduction direction in the Paleoproterozoic, similar to modern day New Zealand.

Recap by Ryan Goldsby

Laurentia!: Talk Summary

Last week’s colloquium was a shared effort given by ISU’s very own Dr. Dave Pearson and Dr. Paul Link.  The talk was titled: Tectonics and Sedimentation at the western Laurentian rift margin in the northern Rockies. 

Laurentia is a large continental craton that forms the core of the North American continent.  The western margin of this craton formed approximately 600 million years ago, during the breakup of the supercontinent Rodinia.

Dave gave the first portion of the talk, titled: The Lemhi Arch of east-central Idaho: a stranded fault block within the western Laurentian rift margin. Dave discussed his recent research with Connor Hansen (MS, 2015) on the Lemhi Range in east-central Idaho.  The Lemhi Arch is a northwest trending topographic high that ran parallel to the southwest margin of the Belt basin through central Idaho ~540 million years ago. It is recognized in the current rock record by the appearance of middle Ordovician rocks lying unconformably on Mesoproterozoic rocks, but how did this topographic high form?  Dave and Connor identified an inverted normal fault associated with the rifting of Rodinia with over 7 km of stratigraphic offset.  This discovery is very interesting as it offers potential a mechanism for the uplift of the Lemhi arch.

From Hansen (2015)

 

This discovery also prompts the question, why do we see a change in the geometry of Rodinian rifting north of the Snake River Plain?  Dave proposes that extension related to the deposition of the Belt basin (~1350 million years ago) resulted in mafic transitional crust forming beneath the Belt basin.  This mafic crust is much stronger than its felsic counterpart and could cause the change in Rodinian rift orientation seen through central Idaho. 

Paul gave the second portion of the talk, titled:  Distinctive detrital zircon populations in Paleozoic strata of Idaho.  Zircons are durable minerals formed in most felsic igneous and metamorphic rocks.  Zircons can also be weathered, transported, and deposited as detrital grains in sedimentary rocks.  Even though many sandstones are composed primarily of quartz, it is the zircon that contains the most information!  Uranium-lead geochronology allows geoscientists to date the formation of these grains and learn about the provenance of sedimentary units.  Paul has spent a large portion of his career using this method to explore the stratigraphy of the Rocky Mountains.

Events you may be familiar with such as Challis volcanism, Idaho batholith intrusion, and even older events like the Grenville orogeny can all be identified in the detrital zircon signatures of sands and sandstones throughout Idaho.  The Grenville orogeny took place on the east coast during the Mesoproterozoic, forming 1.0 to 1.2 billion-year-old zircons. These grains were transported all the way to the west across the continent and can be found in sedimentary rocks.  However, this Grenville signature disappears in the Cambrian.  This change in provenance may be the result of the uplift of the Transcontinental arch, which cut the western margin off from the eastern supply of sediment.

From Linde et al. (2014)

Paul and Nick Krohe (MS, 2016) recently found a significant Grenville-aged zircon population in the Ordovician Ramshorn slate unit of the structurally complex Clayton quadrangle, located near Stanley, Idaho.  This signature is only seen in one other Ordovician unit in western N. America, the lower Vinini formation of the Roberts Mountain Allochthon. 

From Krohe (2016)

The discovery of Grenville-aged zircons in the Ordovician of central Idaho raises some questions.  Was there a western Grenville source? Or a northern one?  How is the Ramshorn associated with the Lower Vinini?  Was there is a breach of the Transcontinental Arch?  Southward tectonic transport possibly associated with the Antler orogeny?

Dr. Link and Dr. Pearson will be tackling these and other questions in upcoming research projects.  If you had any doubt that ISU geologists are conducting interesting research right here in Idaho, please consider that doubt resolved!