Date of Award

2023

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Geology (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

James M. DeGraff

Committee Member 1

Aleksey V. Smirnov

Committee Member 2

Donald J. Lafreniere

Abstract

The Keweenaw fault is perhaps the most significant geologic structure on the Keweenaw Peninsula, with an estimated 7-11 km of reverse slip juxtaposing volcanic strata of the ~1.1 Ga Portage Lake Volcanics above ~1.0 Ga Jacobsville Sandstone. The fault has been interpreted as a rift-bounding normal fault later inverted by compressional pulses of the Grenville Orogeny or, more recently, as part of a detached thrust fault system unrelated to an earlier normal fault. The fault is shown on published maps as a single continuous fault trace whose sinuosity implies multiple fault segments and complex slip dynamics. Mapping along Bête Grise Bay in 2017-2018 revealed that the Keweenaw fault at the east-most exposure on land is better characterized as a network of interconnected, left-stepping fault segments with easterly strike and exhibiting a 2:1 ratio of dextral strike-slip to reverse-slip. Mapping west from Bête Grise Bay to Mohawk, MI in 2019-2020 as part of the USGS EDMAP program focused on constraining the geometry and slip kinematics of this adjacent portion of the Keweenaw fault system.

This thesis project focused on the eastern half of the 2019-2020 project area and mapped the Keweenaw fault system between Bête Grise Bay and Gratiot Lake. The new mapping and structural measurements have produced an updated bedrock geology map and cross-sections that better constrain the three-dimensional geometry of the fault system and reveal folds and fault-bounded blocks in the fault’s footwall. Three directional fault sets were identified and interpreted in the study area: 1) ENE-striking faults with steep northerly dips; 2) ESE-striking faults with steep northerly dips; and 3) NNE-striking faults with shallow to moderate westerly dips, connecting Set 2 faults to Set 1 faults. Inferred layer-parallel detachment faulting along Portage Lake Volcanic sedimentary units is modeled in cross-sections and constrained by locally dense surface control and by subsurface diamond drill hole logs. Analyses of fault slip measurements indicates an overall strike-to-dip slip ratio of 1.7:1 and tectonic shortening in a direction 083°-263°. The strike-to-dip slip ratio of each directional fault set is inferred to vary as a function of its strike relative to the shortening direction.

Fault-bounded blocks were transported eastward along Set 1 and Set 2 strike-slip faults, which movement was probably facilitated by layer-parallel detachments and Set 3 thrusts that ramp up towards the east on the block’s leading edge. The three fault sets with a combination of dextral strike slip and reverse dip slip define a multistranded transpressional network of fault segments. These findings are consistent with the other studies correlating far-field compressive pulses of the Grenville Orogeny to Keweenaw fault slip events.

Creative Commons License

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

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