Date of Award

2021

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 DeGraff

Advisor 2

Chad Deering

Committee Member 1

Jeremy Shannon

Abstract

The Keweenaw fault is perhaps the most significant fault associated with the Midcontinent Rift System of the north-central United States. The fault, parallel to the south edge of the rift, has been interpreted as a rift-bounding normal fault that was inverted by subsequent compressional events, thrusting copper-bearing Portage Lake Volcanics (~ 1.1 Ga) over younger Jacobsville Sandstone. Geologic maps and cross sections published in the 1950s by the U. S. Geological Survey depict the fault with a well-defined, single, sinuous trace that is locally associated with smaller branch faults. Cross-sections from that time generally show a simple listric stratal geometry for hanging-wall lava flows and variable dip of footwall siliciclastic layers, but with little to no fold definition.

New field mapping between Mohawk (MI) and Bête Gris Bay indicates that the Keweenaw fault here is better characterized as a fault system consisting of: 1) segments striking east-northeast with steep northerly dip, 2) segments striking east-southeast also with steep northerly dip, and 3) segments striking north-northeast with moderate-to-shallow westerly dip. The members of these three fault sets define a multistranded Keweenaw fault system and several large fault-bounded blocks of Portage Lake Volcanics. Mapping also revealed folding in hanging-wall Portage Lake Volcanics and has defined fold geometry in footwall Jacobsville Sandstone. Fold axes are generally subparallel to adjacent faults and therefore are probably related to fault movement. Along Bruneau Creek, multiple anticlines and synclines in the footwall of a Set 3 fault and a faulted anticline in the hanging-wall indicate significant shortening across a west-dipping thrust fault. Along the east tributary of Snake Creek, a single syncline in the footwall of a Set 2 fault resulted from tilting of Jacobsville strata to vertical by north-side-up movement of a steeply dipping, right-lateral, strike-slip fault, with minor shortening across the fault.

The pattern and relationships of faults and folds in the area are consistent with a fault system dominated by dextral shear rather than by reverse movement as in the currently accepted model. Kinematic slip indicators measured on 55 small faults demonstrate mostly right-lateral strike slip and lesser north-side-up reverse slip, with a 2.5:1 ratio of strike-to-dip slip. Inversion of fault-slip data shows that maximum shortening during faulting was along an azimuth of 285°-105°, suggesting a WNW-ESE maximum paleostress direction that is nearly parallel to expected Grenville or Appalachian orogenic forces.

The new mapping and structural analyses in this area have revealed a multistranded Keweenaw fault system that is transpressional in nature, dominated by dextral strike slip, and has lesser reverse slip with north side up. Fault-bounded blocks having ENE-oriented long dimensions generally moved eastward along set 1 and 2 faults, while thrusting Portage Lake Volcanics over Jacobsville Sandstone along set 3 faults. The estimated WNW-ESE maximum shortening direction associated with fault movement strongly suggests that the Grenville Orogeny was primarily responsible for movement of the Keweenaw fault system, with possible reactivation occurring during the Appalachian Orogeny.

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