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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geology

Collaborative Specialization

Planetary Science and Exploration

Supervisor

Osinski, Gordon R.

Abstract

The Canadian High Arctic contains two impact structures created by hypervelocity impact events in carbonate-rich target rocks. The remote locations of the Tunnunik and Haughton impact structures means that there are aspects of these impact structures which have yet to be fully investigated. This study characterizes the range of impact-generated dykes exposed from both impact structures which include lithic breccias, impact melt-bearing breccias, and impact melt rocks. Breccias may include silicate impact glass fragments and evidence for carbonate melt. Impact melt rocks from the Haughton impact structure contain the rare terrestrial mineral moissanite. This is only the third reported occurrence of moissanite associated with an impact structure and the first to observe its presence in situ. Inclusions and variation of polytypes in moissanite provide information regarding high temperatures present during crater formation. The carbonate-rich rocks that form these impact structures contain well-developed shatter cones as evidence of shock metamorphism. As a shock classification system does not currently exist for carbonates, the effect of shock on the crystal structure of calcite and dolomite is examined using X-ray diffraction to better understand the extent of strain in both these minerals. Previous studies of shocked carbonates from terrestrial impact structures is limited and the goal here is to assign numerical values to indicate strain and thereby better quantify and compare shock in carbonates among impact structures.

The parallel studies of impact-generated dykes and shock at the Tunnunik and Haughton impact structures allow for the comparison of two impact structures with similar diameters, 28-km for Tunnunik and 23-km for Haughton, in different states of preservation. The deeply eroded Tunnunik impact structure and well-preserved Haughton impact structure provide insights into complex crater formation in carbonate rich rocks that would otherwise not be available by only studying one site. Results from this pair of impact sites has expanded the knowledge of carbonate-rich impact structures and will help future investigations of other known carbonate-rich impact sites and ones yet to be discovered.

Summary for Lay Audience

Impact craters form when a large projectile, typically a fragment from an asteroid or comet, survives its transit through Earth’s atmosphere and strikes a solid rocky surface. The resulting crater may be tens of metres to several hundred kilometres in diameter, depending on the size and speed of the projectile. Examining the rocks affected and generated by impact events allow the impact process to be better understood.

This study focuses on two remote impact sites in the Canadian High Arctic, the Tunnunik impact structure and Haughton impact structure, that formed in carbonate rocks consisting mainly of limestone and dolostone. Rocks affected by the shock created during the impact often display shatter cones near the centre of the impact structures which appear as small fractures or striation to the unaided eye. A technique called X-ray diffraction uses X-rays to investigate the crystal structure of calcite and dolomite, the primary minerals in the carbonate rocks that form the impact structures. Shock effects increase strain within the crystal structure of these minerals and the strain values derived from the X-ray diffraction analyses are compared among samples collected from different locations in each impact structure.

The rocks generated by the impact event examined in this study include impact breccias and impact melt rocks found in impact-generated dykes. Breccias consist of fragments from one or more different types of carbonate rock and are held together by finer fragments that are too small to see without higher magnification. Breccias may also include small silicate glass fragments or melted carbonate clasts. Impact melt rocks consist of fine-grained recrystallized calcite, clasts from the limestone rocks adjacent to the dykes, and crystals of a rare mineral called moissanite. Moissanite is rare due to very specific conditions required for it to form and these conditions help identify temperatures reached in the impact melt rocks when they were generated.

Comparing the results from the Tunnunik and Haughton impact structures has provided insights into their formation and expanded the knowledge of carbonate-rich impact structures.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Available for download on Thursday, December 31, 2020

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