Creating a 7.5′ geological map series of the Phantom Lake Quadrangle in the Medicine Bow Mountains of southeastern Wyoming

State Map Project, Wyoming State Geological Survey (Summer 2021 – Present)

Rachel Toner, Kelsey Kehoe, and Daniel Colwell

The Snowy Range of the Medicine Bow Mountains is a popular destination for locals and visitors to explore on foot, skis, snowmobile, or off-road vehicles, depending on the season. But, the area has also seen a boom of prospectors looking for gold and other resources in the past, and it has a rich geologic history dating back to the Late Archean, more than 2.5 billion years ago. The USGS and WSGS together have taken a greater interest in mapping this lesser explored mountain range at a small scale to look for mineralization associated with the Cheyenne Belt suture zone and to better understand the structural and geologic history. Phantom Lake sits immediately west of Medicine Bow Peak at the Archean core of an anticline around which these mountains were built, and is just six miles northwest of the Cheyenne Belt. This shear zone, which cuts across the southeastern corner of the state, is the boundary where an island arc collided with the ancient rocks of the Wyoming craton around 1.7 billion years ago. Complex geological structures and widespread glacial deposits make mapping the geology of this area both challenging and exciting; the only thing I can ever be 100% sure to find out here are breathtaking views!

Exhumation at the Southwest Indian Ridge: Constraints on temperature and depth of water ingress during detachment faulting

Master’s Thesis Research, University of Wyoming (2019 – Present)

Daniel J. Colwell1, Michael J. Cheadle1, Barbara E. John1, Susan M. Swapp1, Daniele Brunelli2, Henry J.B. Dick3, and Benjamin M. Urann1,3
1University of Wyoming, 2University of Modena and Reggio Emilia, 3Woods Hole Oceanographic Institution

Despite mid-ocean ridges (MORs) generating ~60% of the Earth’s crust, their inaccessibility dictates that they remain one of the least understood tectonic features on Earth. Ultra-slow spreading ridges, an end-member of the plate spreading process, comprise 36% of the global MOR network(Dick et al., 2003) and are especially poorly understood because of their remote location (in the Arctic and Indian Oceans) and relative lack of exploration. The purpose of this study is to understand the nature of faulting, seawater ingress, and metamorphism at ultra-slow spreading mid-ocean ridges, using new bathymetric data and dredge samples collected in 2019 from the Southwest Indian Ridge (see below).

The Southwest Indian Ridge (SWIR):

  • Has one of the slowest spreading rates in the world at ~14 mm/yr (Cannat et al., 2008) (See map to the right for Effective Spreading Rates (ESRs)),
  • Spreads primarily by normal faulting instead of magmatic accretion (Dick et al., 2003),
  • Has an elastic lithosphere ~11-17 km thick in its slowest-spreading segments (Schlindwein and Schmid, 2016; Grevemeyer et al., 2019).

Cruse TN365 to the SWIR ~45°E in Spring 2019 recovered 19 fresh samples of plastically-deformed mantle peridotite rocks (see photomicrograph on left) from near the ridge axis. The ductile deformation of these rocks must have occurred below the brittle-ductile transition, implying that they were then transported >11-17 km to the surface. However, no single fault along this segment has slipped far enough to exhume these samples from that depth, raising the question: How are peridotite mylonites exhumed from >11-17 km depth at ultra-slow spreading ridges?

Combined analysis of rock deformation conditions and seafloor bathymetry are the key to solving this problem. Microstructural (electron backscatter diffraction) and chemical analyses (energy dispersive x-ray spectroscopy) on a scanning electron microscope (SEM) enable determination of temperature, differential stresses, water content, and the dominant deformation mechanisms recorded in the peridotite mylonite rocks. Combined with measurements of fault orientation and offset from seafloor bathymetry, we created models (see example on left) that show how cross-cutting normal faults exhume mantle rock from >11-17 km depth to the seafloor and allow water percolation as deep as ~15 km at the Southwest Indian Ridge.

Colwell, D.J., Cheadle, M.J., John, B.E., Swapp, S.M., Brunelli, D., Dick, H.J.B., and Urann, B.M., 2021, Exhumation of mantle at the Southwest Indian Ridge: Constraints on temperature and depth of water ingress during detachment faulting: Abstract 102-9 presented at 2021 Fall Meeting, GSA Connects, Portland, Oregon, 10-13 October. doi:10.1130/abs/2021AM-370438

3D-modeling and magnetic (AMS) analysis of Mesozoic diabase dikes in North Carolina

Undergraduate Honors Thesis Research, University of North Carolina at Chapel Hill (2018 – 2019)

Daniel J. Colwell1, Allen F. Glazner1, William J. McCarthy2
1University of North Carolina at Chapel Hill, 2University of St Andrews

Diabase dikes that formed during Triassic-Jurassic breakup of Pangea are well-exposed in quarries in eastern North Carolina. In a quarry north of Rocky Mount, NC, steeply dipping, north-striking dikes up to 10 m thick cut ~300 Ma granite of the Rolesville batholith. We created 3D maps of dike outcrops and conducted anisotropy of magnetic susceptibility (AMS) analyses to determine the direction of magma flow.

At the end of the Triassic period, basaltic lavas flowed over many parts of eastern North America, western Africa, South America, and Europe as Pangea broke up and the Atlantic Ocean formed. This event, which produced the Central Atlantic Magmatic Province (CAMP), ravaged ecosystems and likely contributed to the end Triassic extinction (Blackburn et al., 2013). Where did these lavas come from, and how did they travel through the crust?

The dikes through which these magmas flowed
crop out extensively in eastern North Carolina, and are well exposed in quarries across the Piedmont . Here we present 3D reconstructions of these excavated dikes and estimates of the magma flow direction from magnetic analysis.

Left: Distribution of CAMP magmatism during the breakup of Pangea (Marzoli et al. 1999). Inset from Ragland et al. (1983) shows dike locations in the southeastern US. Blue star marks the Nash County Quarry in NC.

Excavation at the Nash County Quarry exposes dikes on each bench level, revealing changes in the structure and interconnectivity of the dikes both laterally and vertically. Continued excavation of the dikes allowed us to create a collection of 3D models of the same dikes across time, and we used these models to visualize more of the dikes’ 3D structure. At the outcrop scale, each 3-10 m thick dike is composed of multiple smaller dikelets 20-100 cm wide.

We conducted aerial surveys with ground control points and used Pix4D Mapper and Cloud software to construct high-resolution, georeferenced 3D models of each set of dike exposures. 3D polylines were traced along the contacts between the dike and granite wall rock, and these were combined with a 3D model of the whole quarry in ESRI ArcScene to visualize, measure, and reconstruct dike planes across the excavated area (left).

Magma flow tends to foliate or lineate crystals along the direction of flow as they form, resulting in a subtle mineral alignment in the fully crystallized rock. We used anisotropy of magnetic susceptibility (AMS) analysis on oriented diabase samples to measure the changing magnetic foliation orientation across the width of a dike. The figure on the right (Porreca et al., 2015) shows how these foliation measurements can be used to estimate the direction of magma flow.

Magnetic foliation planes collected across multiple dike outcrops at the Nash County Quarry are plotted on the stereonet to the right. The foliation planes most frequently strike northeast or are imbricated on either side of the dike plane (yellow). This geometric relationship indicates horizontal magma flow, but more extensive sampling and AMS analysis is needed to constrain an azimuth of magma flow.

Ernst and Baragar (1992) conducted a much broader AMS study of the MacKenzie dike swarm in northwestern Canada, which radiates outward from a focal point on Victoria island, similar to the CAMP dikes. Magma in the MacKenzie dikes flowed vertically near Victoria Island, and more horizontally as it traveled away from the source. AMS results from the dikes in North Carolina suggest a similar pattern of outward horizontal flow from the paleo-center of this magmatic province.

Colwell, D.J., Glazner, A.F., and McCarthy, W.J., 2019, 3D-modeling and magnetic (AMS) analysis of Mesozoic diabase dikes in North Carolina: Abstract 11-46 presented at 2019 Meeting, GSA Southeastern Section, Charleston, South Carolina, 28-29 March. doi:10.1130/abs/2019SE-327844

Colwell, D.J., Glazner, A.F., and McCarthy, W.J., 2019, 3D-modeling and magnetic (AMS) analysis of Mesozoic diabase dikes in North Carolina: Poster presented at 2019 Anadarko Student Research Symposium, Department of Geological Sciences, University of North Carolina at Chapel Hill, 12 April.

Colwell, D.J., 2019, 3D-modeling and magnetic (AMS) analysis of Mesozoic diabase dikes in North Carolina [B.S. Honors thesis]: University of North Carolina at Chapel Hill, 27 p.