T68 Evolution of the Midcontinent Rift: A Window into Proterozoic Environments, a Repository of Minerals, and a Lesson in Rifting II

Sunday, 4 November 2018
1:30 PM - 5:30 PM
Room 242

KEWEENAW FAULT GEOMETRY AND KINEMATICS NORTH OF PORTAGE LAKE, MICHIGAN

Paper number: T68-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

The Keweenaw Fault (KF) is perhaps the most significant fault of the Midcontinent Rift System based on its length of 325 km, postulated slip of 9 km, and thrusting of Cu-bearing Portage Lake Volcanics (PLV, 1.1 Ga) over younger Jacobsville Sandstone (JS). A long-held view is that it began as a steep, rift-bounding, normal fault during crustal extension and later was inverted during crustal shortening. This scenario would produce a fault dipping > 45° NW with high cut-off angles across PLV layers. However, published maps and cross-sections show the KF dipping < 45⁰ NW (often < 30°) for much of its length, with PLV layers sub-parallel to its surface. Prior work also documents layer-parallel slip within the PLV. These observations suggest a thrust system detached along PLV layers. To better define the nature of the KF, we have begun a structural analysis project along the Keweenaw Peninsula, combining published maps and core data with new mapping and fault slip measurements to build balanced cross-sections.

Two regional cross-sections were constructed ~25 km apart across the KF and lesser faults in its hanging wall. Over this stretch the KF changes strike from N35°E to N55°E going SW to NE, often dips less than 25°NW and, together with HW faults, bounds PLV layers with similar to counter-regional dips. Cross-section work suggests that observed geometry was produced by a SE-propagating thrust system with a basal detachment 15-20 km beneath Lake Superior. Farther northeast, previous maps show the KF as a single sinuous trace that curves clockwise even more to about S70°E. However, our new mapping of the fault’s eastern extent reveals instead a series of left-stepping, ESE-trending, fault segments. Slip indicators show mostly dextral strike slip with lineations plunging ≤ 30°. Segment connections are not yet well defined, but tilt directions and dip reversals in PLV strata indicate that areas between segments have shortened. Outcrops of the JS-PLV unconformity here, unlike to the SW where these units are in fault contact, and regional constraints suggest that vertical separation across the KF decreases from ~3000 m near Portage Lake to ~300 m at the fault’s eastern extent. We therefore interpret the KF thrust system to lose displacement northeastward, ultimately terminating in a series of left-stepping splays with dextral strike slip.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-321079

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