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! 

Upcoming projects!

We're excited to report that ISU Geosciences will be leading the volcanology science team for BASALT (Biologic Analog Science Associated with Lava Terrains), a large, multi-institutional project led by NASA and funded by the PSTAR program. We will be working on the lavas of Craters of the Moon National Monument and Preserve (just up the road in Idaho) and Hawaii for the next 4 years, studying how heterogeneities in rock influence microbial habitat and how to actually carry out similar research on Mars.

Martian landscape, as seen by Pathfinder. Fortunately, we get a few more trees in Idaho. Image from http://mars.nasa.gov/MPF/index1.html

In the long-term, this project will support the search for extraterrestrial life. Before we spend large amounts of time and money searching for various forms of life on Mars, we need to do what we can on Earth to understand what we're looking for and where to look. Our two field sites are rough analogues for different periods in Martian history, with Idaho representing modern Mars and Hawaii standing in for a wetter past. We've already seen enough of Mars to know that we're not looking for the little green Martians from classic science fiction or anything else on that size/scale. Instead, we're interested in very tiny life forms, such as microbes. These little guys are much harder to find than our cinematic aliens, so we need to figure out where to look for them and how to go about it in a low-cost, low-stakes situation here on Earth before we start hunting them on Mars. While ISU researchers guide science work on the lava flows, engineers and astronauts will be working side-by-side with us to figure out how to turn our fieldwork into reasonable tasks off-planet. For my part, I'm super excited at the idea of having rovers carry heavy field gear for me as they practice navigating uneven terrain on the lava flows.

If there are aliens like these on Mars, then they're creepily good at hide-and-seek. We're working on the science and exploration plans that will help us to find evidence of microbes, either dead or alive. Image from http://pixar.wikia.com/Aliens

This project will support graduate and undergraduate students at ISU over the next 4 years, as well as bring researchers from all over the US to SE Idaho. We get to do good science, train the next generation of scientists, and get a small economic boost in our area -- this is 100% win!

The project officially kicks off this summer. We'll try to post updates throughout the project, including photos of us working in the field. (Hint: the engineers will probably look pretty normal, but the geologists will look like we've been living off the land, hiding in the lava tubes, for a little too long. It's how we roll.)

Where are the geothermal resources in Southeast Idaho?

Figure 1. A summary of the geothermal systems in the Great Basin. The study area is focused in the NE area of the basin, in SE Idaho. From McCurry and Welhan (2012). 

Lots of magmatic heat resides below the surface around the Snake River Plain region in Idaho, as evidenced by the Yellowstone hot spot and regional volcanism. However, SE Idaho seems to lack obvious signs of thermal activity at the surface (Figure 1). The presence of the hot spot and other volcanics in the area should provide a reasonably good source for geothermal energy. If there is at least some magma body residing in the shallow crust to produce geothermal resources, then we expect to see some type of response at the surface (think hot springs like at Yellowstone). However, the expression of these geothermal resources at the surface in SE Idaho is not as strong as expected.

Three hypotheses are presented in this paper to explain this phenomenon. The first hypothesis states that there are no easily accessible magmatic heat sources in the area. This may be due to a lack of any magmas near the earth’s surface and instead are located too deep within the earth for us to access or detect. Also, it could be that any magmas that were once close to the surface had already erupted, preventing us from using them as a heat source today. Hypothesis 1 is unlikely because geotechnical seismic work indicates a significant magma storage exists in the mid- to upper-crust. This indicates that there is at least some magmatic fluid in the “shallow” crust.

The second hypothesis is that there is physically accessible magmatic heat but the amount of heat available is relatively low. This could be due to a low permeability layer (or in other words, a rock layer that prevents heat or fluids from travelling through it), preventing us from sensing the heat at the surface. Hypothesis 2 is also unlikely because previous work has demonstrated that the H2O content in the magma was 2-6%, which is comparable to other magma systems in the Basin and Range, and indicates the magma is not dry.

Figure 2. A conceptual model for the China Hat dome field and Blackfoot Reservoir rift zone. Modified from Autenrieth et al. (2011). This figure illustrates the movement of magma through faults toward the NE, away from the source. Original paper details the abbreviations. From McCurry and Welhan (2012). 

The third hypothesis states that there are geothermal systems in the area, but we don’t see them as well at the surface because the heat is reduced or diverted away. For example, a large, shallow water aquifer below the surface could absorb some of the heat that migrates toward the surface. Also, there may be fractures below the surface that allow the heat to migrate along the fracture paths away from the original magma source. Such a scenario may produce heat signs somewhere else in the area. Hypothesis 3 is favored due to the presence of a large groundwater system in the area that could dilute or divert thermal responses from deeper high-temperature magmatic fluids. Additionally, the study area contains west-dipping faults in the subsurface, allowing for magmatic fluids to travel away from its source (Figure 2).

Recent volcanic fields (less than 2.6 Million years old) in SE Idaho point towards a significant storage of magma and heat energy in the upper crust between 2 and 15 km deep. This region may be a strong candidate for future hydrothermal exploration work. However, the presence of a broad aquifer in the subsurface poses challenges to studying this type of resource where migration of magmatic heat is involved.

Paper: McCurry, M., and Welhan, J. (2012)Do Magmatic-Related Geothermal Energy Resources Exist in Southeast Idaho? GRC Transactions V36, p699-707.

http://www.researchgate.net/publication/235654596_Do_Magmatic-Related_Geothermal_Energy_ResourcesExist_in_Southeast_Idaho/file/79e41512510a13e1c4.pdf