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Abstract

Three tectonic terranes, denned on geologic and geophysical evidence, are found in northern Michigan and Wisconsin. Each has its own characteristic gravity and magnetic signature. The northern terrane consists of basalt and sedimentary rocks of the Midcontinent rift system. Both gravity and magnetic maps have high-amplitude positive and negative anomalies in this area, reflecting the wide variation in physical rock properties. Both data sets have a strong northeast-trending fabric, which mimics the orientation of the major tectonic features of the rift.

The central terrane is underlain by rocks of the Penokean fold belt. The north half of this region consists largely of basins of Lower Proterozoic meta-sedimentary rocks, and the south half consists of volcanic rocks, granitoid plutons, arid gneiss domes. The most prominent geophysical feature in the central terrane is a broad, long-wavelength gravity high along the fold belt. Shorter wavelength positive gravity and magnetic anomalies in the north part of the central terrane have a generally east-northeast-trending fabric that corresponds to troughs and basins in Archean rocks that are filled with iron-rich Lower Proterozoic sedimentary rocks. The south half of the central terrane is characterized by a gravity fabric showing numerous steep gradients relative to the north half, scattered orientations of elongate positive and negative anomalies, and a distinct magnetic signature. The south half is separated from the north half by sharp gradients in gravity and magnetic data and a geologically and geophysically mapped fault zone.

The southern terrane is underlain by the Wolf River batholith, which has a prominent north-nprtheast-oriented gravity low above it. Sharp, elliptically shaped magnetic highs and lows correlate with variations in rock types within the batholith. The strong north-northeast geophysical fabric in this region, which correlates with a horth-northeast-trending zone of faulting on the west side of the batholith, distinguishes this terrane from that to the north. East-west-trending fabrics on either side of the gravity low likely reflect orientation of tectonic features associated with the Penokean fold belt.

Geologic features correlate well with anomalies on amplitude-filtered gravity maps. Early Proterozoic basins are within the area of the broad gravity high along the Penokean fold belt, but they are not large enough to account for the gravity high. Gneiss domes and granitic plutons, including the Wolf River batholith, produce gravity lows. Nodes of regional metamor-phism are all within the regional gravity high but within zones of relatively lower gravity within the high.

A two-dimensional gravity model constructed across the broad gravity high suggests that it is caused by uplift of dense lower crustal rocks. But it may also be partly caused by the presence of relatively dense near-surface Archean gneiss beneath it.

Preliminary analysis of a combined gravity and magnetic profile across the Penokean fold belt suggests that geophysical data are compatible with a proposed rift, arc-continent-collision model for the origin of the Penokean fold belt.

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