Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution

1997 ◽  
Vol 34 (4) ◽  
pp. 476-488 ◽  
Author(s):  
D. W. Davis ◽  
J. C. Green
Keyword(s):  
North American ◽  
Lake Superior ◽  
Flood Basalt ◽  
North American Plate ◽  
Rift System ◽  
Midcontinent Rift ◽  
Subsidence Rate ◽  
The North ◽  
Western Lake ◽  
Midcontinent Rift System

Volcanism in the Midcontinent rift system lasted between 1108 and 1086 Ma. Rates of flood-basalt eruption and subsidence in the western Lake Superior region appear to have been greatest at the beginning of recorded activity (estimated 5 km/Ma subsidence rate at 1108 Ma) and rapidly waned over a period of 1–3 Ma during a magnetically reversed period. The age of the paleomagnetic polarity reversal is now constrained to be between 1105 ± 2 and 1102 ± 2 Ma. A resurgence of intense volcanism began at 1100 ± 2 Ma in the North Shore Volcanic Group and lasted until 1097 ± 2 Ma. This group contains a ca. 7 Ma time gap between magnetically reversed and normal volcanic sequences. A similar disconformity appears to exist in the upper part of the Powder Mill Group. The average subsidence rate during this period was approximately 3.7 km/Ma. Latitude variations measured from paleomagnetism on dated sequences indicate that the North American plate was drifting at a minimum rate of 22 cm/year during the early history of the Midcontinent rift. An abrupt slowdown to approximately 8 cm/year occurred at ca. 1095 Ma. These data support a mantle-plume origin for Midcontinent rift volcanism, with the plume head attached to and drifting with the continental lithosphere. Resurgence of flood-basalt magmatism at 1100 Ma may have been caused by extension of the superheated lithosphere following continental collision within the Grenville Orogen to the east.

Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development

1997 ◽  
Vol 34 (4) ◽  
pp. 504-520 ◽  
Author(s):  
Suzanne W. Nicholson ◽  
Klaus J. Schulz ◽  
Steven B. Shirey ◽  
John C. Green
Keyword(s):  
Mantle Source ◽  
Lake Superior ◽  
Flood Basalt ◽  
Great Depth ◽  
Rift System ◽  
Midcontinent Rift ◽  
Western Lake ◽  
Isotopic Analyses ◽  
Mantle Sources ◽  
Midcontinent Rift System

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source (εNd(1100) value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle (εNd(1100) < 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (εNd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (εNd(1100) = 0 to +3).

Interpretation of seismic reflection and gravity profile data in western Lake Superior

1994 ◽  
Vol 31 (4) ◽  
pp. 652-660 ◽  
Author(s):  
John L. Sexton ◽  
Harvey Henson Jr.
Keyword(s):  
Fault Zone ◽  
Seismic Data ◽  
Seismic Reflection ◽  
Lake Superior ◽  
Rift System ◽  
Isle Royale ◽  
Midcontinent Rift ◽  
Detachment Faults ◽  
Western Lake ◽  
Midcontinent Rift System

The interpretation of 1047 km of seismic reflection data collected in western Lake Superior is presented along with reflection traveltime contour maps and gravity models to understand the overall geometry of the Midcontinent Rift System beneath the lake. The Douglas, Isle Royale, and Keweenaw fault zones, clearly imaged on the seismic profiles, are interpreted to be large offset detachment faults associated with initial rifting. These faults have been reactivated as reverse faults with 3–5 km of throw. The Douglas Fault Zone is not directly connected with the Isle Royale Fault Zone. The seismic data has imaged two large basins filled with more than 22 km of middle Keweenawan pre-Portage Lake and Portage Lake volcanic rocks and up to 8 km of upper Keweenawan Oronto and Bayfield sedimentary rocks. These basins persisted throughout Keweenawan time and are separated by a ridge of Archean rocks and a narrow trough bounded by the Keweenaw Fault Zone to the south. Another fault zone, herein named the Ojibwa fault zone, previously interpreted as the northeastern extension of the Douglas Fault Zone, has been reinterpreted as a reverse fault that closely follows the ridge of Archean rocks. Previous researchers have stated that neighboring segments of the rift display alternating polarity of basins associated with large detachment faults. Accommodation zones have been previously interpreted to exist between rift segments; however, the seismic data do not image a clearly identifiable accommodation zone separating the two basins in western Lake Superior. Thus, the seismic profile may lie directly above the pivot of a scissors-type accommodation fault zone, there is no vertical offset associated with the zone, or the zone does not exist. Seismic data interpretations indicate that application of a simple alternating polarity basin – accommodation zone model is an oversimplification of the complex geological structures associated with the Midcontinent Rift System.

A geophysical profile of the southern margin of the midcontinent rift system in western Lake Superior

Tectonics ◽  
1990 ◽  
Vol 9 (2) ◽  
pp. 303-310 ◽  
Author(s):  
William J. Hinze ◽  
Lawrence W. Braile ◽  
Val W. Chandler
Keyword(s):  
Lake Superior ◽  
Rift System ◽  
Midcontinent Rift ◽  
Southern Margin ◽  
Western Lake ◽  
Midcontinent Rift System

Chemostratigraphy of the Neoproterozoic Alona Bay lavas, Ontario

2002 ◽  
Vol 39 (7) ◽  
pp. 1127-1142 ◽  
Author(s):  
James A Walker ◽  
Todd T Gmitro ◽  
Jonathan H Berg
Keyword(s):  
Lake Superior ◽  
Crustal Contamination ◽  
Flood Basalt ◽  
Temporal Variations ◽  
Rift System ◽  
Midcontinent Rift ◽  
Source Contributions ◽  
Midcontinent Rift System ◽  
Basal Portion ◽  
Group Chemical

A basal sequence of flood basalt lavas associated with the Neoproterozoic Midcontinent rift system crops out in Alona Bay along the southeastern shore of Lake Superior in Ontario. The Alona Bay lava succession is about 1200 m thick and lies just north of the well-studied, contemporaneous Mamainse Point Formation. Detailed chemostratigraphy of the Alona Bay lavas suggests they are grossly correlative with the basal portion of the Mamainse Point Formation. For instance, like the basal part of Mamainse Point Formation, the Alona Bay section contains numerous high-MgO lavas and can be subdivided into 4–5 groups with distinct chemical characteristics. Chemical variations within the Alona Bay groups are largely the result of fractional crystallization, likely at moderate pressures. One small group of Alona Bay lavas also carries the compositional imprint of crustal contamination. The remaining inter-group chemical distinctions at Alona Bay are the consequence of temporal changes in partial melting and source character. With time and development of the Midcontinent rift, degrees of melting increased; mean pressures of melting decreased, reducing garnet control; and lithospheric source contributions waned. Similar temporal variations during flood basalt evolution have been documented elsewhere.

Geophysical investigations and crustal structure of the North American Midcontinent Rift system

1992 ◽  
Vol 213 (1-2) ◽  
pp. 17-32 ◽  
Author(s):  
William J. Hinze ◽  
David J. Allen ◽  
Adam J. Fox ◽  
Don Sunwood ◽  
Timothy Woelk ◽  
...  
Keyword(s):  
Crustal Structure ◽  
North American ◽  
Rift System ◽  
Midcontinent Rift ◽  
The North ◽  
Midcontinent Rift System ◽  
Geophysical Investigations

Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example — the 47th Avenue sill

2004 ◽  
Vol 41 (7) ◽  
pp. 829-842 ◽  
Author(s):  
Karl E Seifert ◽  
James F Olmsted
Keyword(s):  
Lake Superior ◽  
Geochemical Data ◽  
Rift System ◽  
Midcontinent Rift ◽  
North Shore ◽  
Volcanic Group ◽  
The North ◽  
Midcontinent Rift System ◽  
Basalt Composition

This study presents geochemical data for several of the numerous small to large dikes and sills, including the 47th Avenue sill, exposed along the shore of Lake Superior in and north of Duluth, Minnesota. These intrusions are late magmatic features of the Proterozoic Midcontinent Rift System and together form the North Shore Hypabyssal Group. The dikes are geochemically distinct from the sills, and, when the two are exposed together, the younger dike intrudes the older sill. Dikes are primitive with Mg# up to 68, have positive εNd values, and are oriented approximately north–south with steep westerly or near vertical dips. The older sills are more evolved, usually have εNd values near or below 0, and have the same gentle easterly dip as the thick sequence of North Shore Volcanic Group flows they intrude. Dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5, with primitive geochemistry and positive εNd values, whereas sill compositions are similar to widespread basalt composition type 4 typical of most North Shore Volcanic Group flows. The 47th Avenue sill in Duluth is an evolved single intrusion North Shore Hypabyssal Group diabase sill with trough banding, sharp lower and upper contacts, and a spectacular fractured and undulating roof zone containing blocks of the overlying ferroandesite flow.

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