FLOW FABRIC DETERMINATION OF TWO MESOPROTEROZOIC MIDCONTINENT RIFT DIKE SWARMS, NORTHEASTERN MINNESOTA
A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science
by
Elizabeth May Fein
May, 2009
Thesis written by
Elizabeth May Fein
B.A., Oberlin College, 2003 M.S., Kent State University, 2009
Approved by
______, Advisor Daniel Holm
______, Chair, Department of Geology Daniel Holm
______, Dean, College of Arts and Sciences Timothy Moerland
ii DEPARTMENT OF GEOLOGY THESIS APPROVAL FORM
This thesis, entitled Flow fabric determination of two Mesoproterozoic midcontinent rift dike swarms, northeastern Minnesota has been submitted by
Elizabeth Fein in partial fulfillment of the requirements for the Master of Science in Geology. The undersigned member’s of the student’s thesis committee have read this thesis and indicated their approval or disapproval of the same.
Approval Date Disapproval Date
______Daniel Holm Daniel Holm
______Donald Palmer Donald Palmer
______David Schneider David Schneider
iii Table of Contents
List of Figures…………………………………………………………………………....v
List of Tables…………………………………………………………………………….vi
Acknowledgements…………………………………………………………………….vii
Abstract ………………………………………………………………………………….1
1. Introduction
Role of dikes in continental rifting……………………………………………..3
Midcontinent Rift System tectonic setting…………………….………………6
Previous work on Midcontinent Rift System dike swarms…………………16
Research focus and hypotheses……..………………………………………21
2. Magnetic Fabric Study
Technique background………………………………………………………..24
Methodology…………………………………...……………………………….32
Data Analysis…………………………………………………………………..42
Results………………………………………………………………………….47
Interpretations………………………………………………………………….58
Conclusions…………………………………………………………………….67
References Cited……………………………………………………………………....72
Appendix...……………………………………………………………………………...78
iv List of Figures
1. Tectonic rift models.…………………………………………………………………4
2. Global tectonic context for the Midcontinent Rift System.………………………7
3. Midcontinent Rift System map.………………………………...…………………..8
4. Mesoproterozoic magnetostratigraphy.………………………………………….12
5. Midcontinent Rift System evolution in the Lake Superior region.……………..14
6. Sample site location maps.……………………………………………………….19
7. The AMS ellipsoid and models of magma flow in dikes….……………………26
8. Carlton County dike swarm description.………………………………………...33
9. Duluth dike swarm description.…………………………………………………..35
10. Steps in the process of obtaining AMS data…………………………………..38
11. Normal vs. inverse AMS fabrics as functions of magnetic grain size……….45
12. Low field and high field magnetic susceptibility plots…………………………49
13. Directional AMS results, 13 Carlton County swarm dikes……………………51
14. Directional AMS results, 13 Duluth swarm dikes.……………………………..52
15. Site averaged AMS stereoplots………………………………………..………..55
16. Carlton County dike swarm magnetic lineation maps………………..……….56
17. Duluth dike swarm magnetic lineation maps…………………………………..57
18. AMS fabrics and dike properties plots………………………………………….62
19. Tectonic interpretation model of an evolving rift environment……………….65
v List of Tables
1. Dike swarm comparison……………………………...... 17
2. Sampled dike locations……………………………………………………………79
3. Field data……………………………………………………………………………80
4. Site-averaged AMS tensor parameters……………………………………….....81
5. Site-averaged AMS anisotropy factors…………………………………………..82
6. Site-averaged AMS principal directions…………………………………………83
7. Rotated site-averaged AMS principal directions………………………………..84
8. High-field magnetic hysteresis data………..…………………………………….85
vi Acknowledgements
Daniel Holm provided consistent and thoughtful guidance and a model of continuous improvement to strive for throughout every stage of this research, from proposal conception through manuscript writing and everything in between.
Eric Ferré shared his extensive expertise as well as his rock magnetic laboratory facilities at Southern Illinois University, where Justin Skord performed the high field tests. France Belley and Kevin Butak were also readily helpful in the lab.
Donald Palmer and David Schneider supplied significant and constructive comments during the research proposal and defense stages of this work.
Stephanie Maes kindly entertained my questions and offered valuable advice.
Audrey and Gordey Madson and Astrid Holm made field work luxurious by generously providing accommodations and home cooked meals in Duluth, MN.
Susanna Fein and David Raybin provided love and financial support, without which this work would never have been possible. My parents also lent their combined editorial eye to the manuscript itself.
Invaluable support for the completion of this work was also given by: Yonathan Admassu, Cristina Robins, David Waugh, Merida Keatts, Karen Smith, Kate Harper, Edith Fein, Carolyn Fein, Carolyn Turner Schneiderman, Carolyn Tustin- Gregory, Debbie Sellers, and Ray Leone.
This thesis is dedicated to the memory of Ooky, who was with me at the start, and to Maudey, who is with me now.
Funding for this project was provided by student research grants from the: Geological Society of America; Institute on Lake Superior Geology; Sigma Xi; Kent State University Graduate Student Senate; and Kent State University Department of Geology (School of Hard Rocks Mineral Resources Award).
vii Summary
Dike structures represent evidence of planar conduits along which magma is transferred via flow in the upper crust. This study documents regional-scale igneous flow patterns in two Midcontinent Rift System (MRS) mafic dike swarms by measuring anisotropy of magnetic susceptibility (AMS) fabrics as a proxy for magmatic fabrics. The 1100 Ma MRS stretches about 2,300 km across the central North American continent and comprises one of the thickest packages of igneous and sedimentary rocks in the world. The Carlton County (CC) and Duluth dike swarms, located in and around Duluth, MN, are proximal but distinct in strike pattern, age, and chemical composition. The subparallel, reverse polarity (older)
CC dikes intrude Paleoproterozoic metagreywackes, whereas the more irregularly striking, normal polarity (younger) Duluth dikes intrude MRS volcanic rocks. The dikes in both swarms appear massive and lack visible flow structures, making traditional, macroscopic fabric measurement impossible. AMS, defined in this study by the preferred orientations of magnetic mineral grains, provides for sensitive delineation of fabrics in apparently isotropic rocks. Using a Kappabridge
KLY4-S susceptibility bridge at field intensity 300 A/m, a significant measure of magnetic fabrics was achieved for 32 oriented block samples from 26 dikes (13 from each swarm). The bulk magnetic susceptibility (Km) for all 530 cubes (an average of ~17 per sample) yielded a mean value of 3.0×10-2 SI volume,
1
2
meaning that the magnetic signal is robust and likely dominated by ferromagnetic phases. The mean corrected degree of magnetic anisotropy (Pj) for both swarms is 1.036 ± 0.006 with a range of 1.002-1.142. A plot of Pj versus Km also suggests that ferromagnetic phases control the AMS signal in all samples.
The principal directions cluster well at most sample sites. Site-averaged directional results for the CC dikes indicate mostly normal AMS fabrics with subvertical to steeply inclined magnetic lineations that cluster relatively consistently. In contrast, site-averaged directional results for the Duluth dikes are more complex and indicate mostly inverse AMS fabrics, with the interpretable normal sites preserving oblique to the SW magnetic lineations. The inverse AMS fabrics in the Duluth swarm rocks may be a result of the influence of tiny single- domain magnetite grains or may be attributable to the influence of a more complex stress-state during emplacement within the rift axis.
The AMS data indicate vertical regional magma flow in the off-axis CC dike swarm and more complex subvertical to oblique SW to NE regional flow for the on-axis Duluth swarm. Vertical dike emplacement is predicted above a proposed 500-km-radius plume head, consistent with the CC swarm results.
However, local variations in stress state may have led the on-axis Duluth swarm to deviate from this model. This study provides evidence that a single long-lived regional magma source potentially fed both sets of intrusions as the MRS evolved through time via vertical to oblique migration of rift melts from depth. Introduction
Role of dikes in continental rifting
Continental rifting leads to lithospheric thinning and, subsequently, to
decompression and partial melting of underlying hot, ultramafic asthenosphere to produce basaltic magma (Fig. 1A). During extension, vertical thinning of the
crust via tectonic processes such as normal faulting is partially offset by vertical
thickening associated with the addition of crustal mass via magmatic processes.
Vertical magmatic thickening occurs both at the top of the crust through subaerial
eruption of igneous material and, often simultaneously, within the lower crust
through emplacement of batholiths and stocks. Continental rift settings are thus ideally suited for studying the interplay between deeper crustal plutonism and upper crustal volcanism.
During continental rifting, the maximum principal stress is vertical (due to
the force of gravity), and the minimum horizontal principal stress is approximately
perpendicular to the rift axis; this horizontal stress is tensile, leading to
emplacement of swarms of subvertical dikes oriented parallel to the rift axis (van
der Pluijm & Marshak, 2004). These scars in the earth’s upper crust can record
the history and character of the tectonic evolution and of the magmatic processes
which operate during a rifting event (Green et al., 1987). Structurally, subvertical
dikes are the planar conduits that connect magma bodies deeper in the crust with
3 4
A C NW Midcontinent Rift SE
Archean Crust Archean Proterozoic Crust NW Midcontinent Rift SE >2.7 Ga <1.9 Ga Basaltic Archean Crust Proterozoic Crust NFZ Extrusives >2.7 Ga Archean <1.9 Ga Lithosphere Sheeted Dike Complex NFZ Lithosphere Mantle Plume Gabbroic Intrusives Depleted 100km Mantle Plume Asthenosphere Depleted Asthenosphere 100km B Vertical Injection Model Horizontal Injection Model
Crust
Lithosphere Localized melting zone Large melting zone
Magma storage Dike plane Flood Basalts
Figure 1. Tectonic rifting models. (A) Active continental rifting model for litho- spheric extension depicting mantle and crustal structure under western Lake Superior during Mesoproterozoic rifting. A mantle plume thermal anomaly initi- ates decompression melting which produces voluminous tholeiitic basalts. Dike emplacement occurs perpendicular to extension direction (here NW-SE) in the upper crust. The plume head spreads laterally beneath the continental litho- sphere, creating an ~800 km diameter rift zone. Rift development occurs within the Archean crust and not along the younger Niagra Fault Zone (NFZ) to the southeast (after Nicholson & Shirey, 1990). (B) Constrasting active continental rifting dike emplacement models of mass transfer from magma chambers in the mid to lower crust to surficial flood basalts. Two end-member models: vertical dike injection from sill-like magma chambers and horizontal dike injection from long-lived plutonic complexes. The thin black arrows within the dashed dike planes represent dike emplacment directions (after Callot et al., 2001). (C) Oceanic rifting dike emplacement model where lateral, along-axis magma flow in dikes may be an important crustal construction mechanism (after Ryan, 1994).
5
their volcanic equivalents and thus can provide records of mass transfer through the crust. The commonly held view of the role of dikes in the process of intracratonic rifting involves vertical transfer of mass as hot, relatively less dense magma from magma chambers at depth moves upward through extant country rock, solidifying into dike structures and feeding extrusive volcanics. Simply put, vertical mass transfer via buoyancy forces is the large-scale function of dikes.
The existence of subvertical magmatic fabrics in some rift dike swarms supports the interpretation that dike formation involves the vertical transfer of mass.
However, recent refined datasets from both oceanic and continental rift environments (e.g., Craddock et al., 2008) suggest a more complex process of dike formation, which involves an important lateral component to mass transfer that produces subhorizontal magmatic fabrics in some dikes (Fig. 1B).
Horizontal propagation of magma in subvertical dikes may be a predominant mechanism of crustal construction in Mid Ocean Ridge settings
(Fig. 1C), where formation of sheeted dike complexes accommodate most of the separation of tectonic plates (Ryan, 1994; Buck et al., 2006). Horizontal dike propagation has been recognized in active subaerial volcanoes of the Mid-
Atlantic Ridge, notably during modern eruptions between 1975 and 1984 at
Krafla, Iceland, where it has been recorded at lateral distances of up to 70 km from the volcanic center (Staudigel et al., 1992; Varga et al., 1998; Buck et al.,
2006; Craddock et al., 2008).
6
In a similar way, detailed regional-scale igneous fabric studies are
beginning to reveal both subtle complexities and fundamental patterns of dike
formation during continental rifting. Evidence for lateral magma flow in dike
swarms carries important implications for understanding the mechanisms of
crustal construction in both continental and oceanic rifts, as well as for predicting
the eruptive behavior of volcanoes (i.e., Poland et al., 2004).
This study deals solely with active, as opposed to passive, continental
rifting processes. Active continental rifting results from lithospheric extension
above a hot mantle plume, which produces narrow rift zones such as the
contemporary East African Rift System (EARS). In contrast, passive continental rifting accommodates extensional plate boundary forces via formation of wide rift zones such as the modern Basin and Range province.
Midcontinent Rift System tectonic setting
The Mesoproterozoic Midcontinent Rift System (MRS) developed in the
Laurentian supercontinent approximately 1.1 Ga, west of the Grenville orogenic
front (Fig. 2), and broke through lithosphere composed of numerous accreted
Archean terranes of the Superior Province and overlying Paleoproterozoic
continental margin prism rocks, including the Animikie Group sedimentary
sequence (Thomas & Tesky, 1994; Fig. 3A). Like its modern analog, the EARS
(Fig. 3B), the MRS initiated due to an immense hot mantle plume (Ojakangas et 7
A 2.21 Ga Nipissing Sills, Senneterre Dikes (Palmer et al., 2007) B 2.20 Ga Kikkertavak Dike Swarm & 1.64 Ga Kokkorvik Sills (Cadman et al., 1992) C 2.06 Ga Kenora-Kabetogama Dikes (Craddock et al., 2008) D 1.877 Ga Little Presque Isle Dikes (Craddock et al., 2007) E 1.267 Ga Mackenzie Dike Swarm (Ernst & Baragar, 1992) F 1.235 Ga Sudbury Dike Swarm (Ernst, 1994) G 1.108 Ga Logan Sills E (Middleton et al., 2004) B H 1.096 Ga Sonju Lake Intrusion (Maes et al., 2007) 1.1 Ga MRS Dike Swarms * (this study) A G CH F 1.1 D MRS* Mafic Dikes and Sills
Normal Faults OAXACA GRENVILLE Mafic Volcanic rocks, Anorthosite, Granite Sedimentary and Volcanic Rocks
Grenville Orogen Pre-1.3 Ga Crust
Mafic dike or basin age (Ma) 1.3-1.0 Ga
Figure 2. Global tectonic context for the Midcontinent Rift System (MRS). Dashed circle represents an estiamte of the extent of the MRS-initiating hot mantle plume head, with an approxiamately 500 km radius (Jim Miller, pers. comm.; Campbell & Griffiths, 1990; Griffiths & Campbell, 1990). Plate reconstruction map of the Grenville Orogen (in green), including the locations of recent magmatic flow studies employing the AMS technique in Pre- cambrian intrusions (emplaced from ~2.2 to 1.1 Ga) on the North American midcontinent. Parallel lines within Grenvillian collisonal provinces repre- sent fabric trends produced by compression. Coincident midcontinental extention is marked by formation of a giant radiating dike swarm (E) and normal faults (after Karlstrom et al., 2001). 8
98º 91º A 0 200 km 2.7 Ga 48º
Duluth 2.2 Ga 2.1-1.8 Ga 1.9
1.47
44º 1.85 Ga
Midcontinent Rift Grenville Igneous Rocks Front Sediments
B
20˚
0˚
20˚
Figure 3. Mesoproterozoic Midcontinent Rift System (MRS) map. (A) The MRS crosscuts terranes that are 3.6-1.6 Ga. Star indicates study area location. MRS rocks are exposed only around Lake Superior. The buried extent is defined by a strong positive gravity anomaly (after Nicholson & Shirey, 1990; Seifert and Olmsted, 2004; Holm et al., 2007). (B) As a hot-mantle-plume-initiated rift, the MRS is analogous to the modern East African Rift System. Note similar triple junction rift geometry (after Dott & Prothero, 1994).
9
al., 2001). Where intracratonic rifting is driven by a mantle plume, additional heat
from the plume drives increased partial melting during decompression of the
asthenosphere, producing enormous volumes of hot, low-viscosity magma that
erupt as high-volume flood basalts (van der Pluijm & Marshak, 2004). The rift
zone developed via concomitant large scale subsidence and emplacement of
large volumes of mafic, mantle-derived magma (Green et al., 1987). The MRS
comprises one of the thickest packages of igneous and sedimentary rocks in the
world with up to an estimated 2.0×106 km3 of extrusive igneous material, mainly
tholeiitic flood basalts, and a comparable modeled volume of intrusive material
below the rift (Ojakangas et al., 2001). Deep seismic reflection profiles provide
an estimate of the immense volume of igneous rock associated with the MRS,
which is at least 15-30 km thick beneath Lake Superior (Hutchinson et al., 1990).
Although exposures of MRS igneous rocks occur only around Lake
Superior, the MRS extends approximately 2,300 km in the subsurface of the
central North American continent, as delineated by a strong positive (and flanking
negative) gravity anomaly (Green et al., 1987; Hutchinson et al., 1990). The
MRS exhibits a triple junction geometry, with three rift arms converging in the
Lake Superior region. One rift arm extends southwestward in the subsurface to
northeast Kansas, a second reaches southeastward, at least as far as northwest
Ohio, and a third, more recently recognized rift arm, projects northward into
Canada (Green et al., 1987; Middleton et al., 2004).
10
The MRS was active within an area on the earth’s surface approximately
1,000 km in diameter (Figs. 1A and 3A). This horizontal extent is similar to the
size of modern hot spot thermal anomalies that result from lateral spread of a
mantle plume head beneath the continental lithosphere. Also, most MRS
volcanic rocks and interflow sand deposits are subaerial rather than submarine,
consistent with rifting above a thermally uplifted region (Hutchinson et al., 1990;
Nicholson & Shirey, 1990). That the vast melt volume associated with the MRS
was emplaced rapidly (most in a narrow interval of 3-5 m.y.) also supports a
mantle plume origin for MRS extension (Vervoort et al., 2007). Gravity data
indicate crustal thinning in the rift center of 25-30 km and a mantle potential
temperature of 1500-1570°C. Gravity data also suggest magmatic underplating
of the lower crust along the MRS from observed mass anomalies in the
subsurface and identify large-scale, rift-bounding deep crustal faults (Behrendt et
al., 1988; Hutchinson et al., 1990; Thomas & Tesky, 1994).
Geochemical data from a wide range of MRS tholeiitic basalts throughout
the Lake Superior basin depict remarkably consistent enrichment in Nd and Pb
isotopic compositions, suggesting a single mantle plume source (Nicholson &
Shirey, 1990; Nicholson et al., 1997). As the plume’s thermal strength decreased, the younger basalts show signs of a more depleted Nd isotopic signature. A recent trace element study of North Shore Hypabyssal Group dikes and sills finds that the dikes of the Duluth dike swarm have positive εNd values, a
11
primitive geochemistry, and small negative Nb and Ta anomalies (Seifert &
Olmsted, 2004). These findings indicate the presence of a mantle plume melt
source with some input from partial melting of continental lithosphere during
emplacement.
Temporally, MRS-associated magmatism proceeded for about 23 m.y.,
between about 1109 and 1086 Ma (Nicholson et al., 1992; Cannon & Nicholson,
1996; Ojakangas et al., 2001; Middleton et al., 2004). U-Pb zircon dates of MRS
igneous rocks have provided detailed age relationships for the progression of
MRS evolution and identified an early stage of magmatism from 1109-1106 Ma, a
latent amagmatic stage from 1106-1100 Ma, and a main stage of magmatism
from 1100-1094 Ma (Vervoort et al., 2007). Combined with igneous rock age
relationships, the magnetostratigraphy recorded in MRS rocks exposed on the
north shore of Lake Superior demonstrates that MRS magmatism spanned two
periods of normal magnetic polarity and two periods of reverse magnetic polarity
(Middleton et al., 2004; Fig. 4).
The magnetic polarity reversal relevant to this study occurred at 1097±1
Ma, as constrained by U-Pb dating and field relationships (Marshall & Lidiak,
1996). The main stage of MRS magmatism (1100-1094 Ma) occurred during a
period of reversed magnetic polarity and spanned a reversal of the earth’s magnetic polarity, making relatively older rift-related igneous rocks in the study area magnetically reversed and relatively younger rift-related igneous rocks 12
Age Northern Nipigon- (Ma) Polarity Minnesota Thunder Bay Coldwell
Mitchipicoten Volc. 1090
N D Portage Pigeon R. Volc. Duluth Intrusions Complex 1100 R CC North Shore-Osler N Volcanic Group
R Coldwell Logan Sills Complex 1110 Sibley Group
Paleoproterozoic Animikie Group
Archean Basement
Figure 4. Mesoproterozoic magnetostratigraphy as recorded by the igneous rocks on the north shore of Lake Superior. Red ovals indicate possible age ranges for emplacement of the predominantly reverse polarity Carlton County dike swarm (CC) and normal polarity Duluth (D) dike swarms in northern Minne- sota (after Middleton et al., 2004).
13
magnetically normal (Green et al., 1987; Ojakangas et al., 2001). The regional
igneous sequence includes stratigraphically lower, reversely polarized rocks that
yield U-Pb dates of 1109-1098 Ma and stratigraphically higher, normally
polarized rocks that yield U-Pb dates of 1096-1086 Ma. The high-precision
analytic results require a magnetic reversal at 1097±1 Ma (Marshall & Lidiak,
1996).
Orientations of tensional dike features record upper crustal strain
associated with rift zone development and therefore indicate the direction of minimum stress. The orientation of individual dikes in a swarm reflects the stress field at the time of intrusion, and the minimum horizontal principal stress direction
(the extension direction) is generally assumed to be perpendicular to the mean strike direction of a dike swarm as a whole (Poland et al., 2004). Dikes also
often make use of preexisting weaknesses in the country rock. In the Lake
Superior region, during rifting time interval 1 (Fig. 5), two reversely polarized dike swarms, the Logan and Baraga-Marquette, record initial crustal extension oriented NW-SE to N-S. During time interval 2, as rifting progresses, three more reversely polarized dike swarms, the Carlton County (this study), Grand Portage, and Pukaskwa, record continued crustal extension at differing orientations (NW-
SE, N-S, and now NE-SW) as the rift arms open. During time interval 3, subsequent to the magnetic polarity reversal at 1097±1 Ma, two normally polarized dike swarms, the Duluth (this study) and Pigeon River, record E-W and 14
Time 1
Time 2
Time 3
N
0 100 km
Midcontinent Rift Igneous Rocks Sediments Dike Swarm Dike Swarm, this study
Figure #.5. Midcontinent Rift System Evolutionevolution in the Lake Superior region.Region. ((TimeTime 1) Reverse polarity Baraga-MarquetteLogan (L) and Baraga-Marquette (BM) and Logan (BM) (L) dike swarms record N-SNW-SE to N toW N-S-SE crustal extension at inception of rifting. ((TimeTime 2) Reverse polarity Carlton County (CC), Grand Portage (GP), and Pukaskwa (P) dike swarms record NNW-SE,W-SE, N-S, and NE-SW extension as rift arms open. ((TimeTime 3) Subsequent to magnetic polarity reversal at 1097±1 Ga,Ma, normal polarity CCCarlton dikes,County Duluth dikes, (D), Duluth and (D), Pigeon and RiverPigeon (PR) River dike (PR) swarms dike swarmsrecord E-W record to NE-WW-SE to extensionNW-SE extension (after Green (after et Green al., 1987 et al., and 1987; Seifert Seifert and Olmsted,& Olmsted, 2004). 2004).
15
continued NW-SE crustal extension. Carlton County dike swarm emplacement
persisted during time interval 3, as evidenced by the presence of some normal
polarity dikes in that swarm (Green et al., 1987; Reichhoff, 1987).
The MRS exists in its present form because it is an aborted rift that failed
to develop into an ocean due to compressional tectonic forces exerted by the
contemporaneous continent-continent collision of the Grenville province to the
east (Fig. 2). The Grenville orogen is a laterally extensive feature that was active
between 1.24 and 1.06 Ga (Hutchinson et al., 1990) along both the eastern and
southern boundaries of the continent. This change in the MRS tectonic setting
from extensional to compressional was accommodated by reactivation of existing
rift-bounding normal faults with compressional reverse motion (Cannon et al.,
1991; Cannon, 1994; Soofi & King, 2002). The tectonic history of the MRS
suggests an ascendancy of compressional, collisional tectonic forces driven by
lithospheric plate motions over hot-spot-driven extensional forces.
In this context of a far-field compressional stress regime, buoyant hot
mantle plume material exerts a vertical force that bends the lithosphere to its
breaking point, allowing the plume material to rise to depths where
decompression melting occurs, producing voluminous flood basalts. Plume- initiated rifting is observed in the modern-day EARS. If the MRS had been
initiated by a similar hot mantle plume below continental lithosphere in a far-field
extensional stress regime, the continental lithosphere may have succumbed to
16
the combined extensional forces and rifted apart into an ocean basin, as in the contemporary Red Sea and Gulf of Aden. Crustal evolution above a plume is controlled by widespread underplating and ductile deformation of the warm crust, processes that also hinder seafloor spreading (Zeyen et al., 1997).
There is no recognized hot spot track associated with the MRS mantle
plume (Nicholson & Shirey, 1990). The upward pressure of hot, low-density
mantle plume material could elevate a cold, old lithospheric block, like the thick
continental lithosphere geophysically observed below the Superior Province
without this force creating enough horizontal bending stresses to break the
craton or leave lasting evidence of its relative motion beneath the lithospheric
plate (Nicholson and Shirey, 1990). For example, the EARS plume was able to
uplift but not rift the Tanzanian craton (Zeyen et al., 1997).
Previous work on Midcontinent Rift System dike swarms
This current study focuses on dikes intruded at shallow crustal levels
during MRS extension in northeastern Minnesota. Reichhoff (1987) differentiated
the dikes found in the vicinity of Duluth, Minnesota into two swarms, the Carlton
County and Duluth dike swarms, based on the rocks’ field characteristics
(geographic locations, magnetic polarities, and strike patterns in map-view) and
chemical compositions (Table 1). The Carlton County and Duluth dike swarms
each record about 1-2% dilation of the upper crust (Green et al., 1987; 17
Carlton County Dike Swarm Duluth Dike Swarm Polarity Reverse Normal Relative age Older Younger Strike N 30° E Variable, generally N-S Dip Steep to the W to vertical Steep to the W to vertical Outcrops Thompson Dam to Ely's Peak Ely's Peak NE to Lakewood Overlap each other Esko Quadrangle: St. Louis River, Jay Cooke State Park, Ely's Peak Widths < 1 m to 68 m, most < 5 m < 1 m to 50 m, most < 4 m Composition Basalt to basaltic andesite Basalt to basaltic andesite Mineralogy Olivine, plagioclase, augite, Olivine, plagioclase, augite, magnetite, ilmenite; uncommon magnetite, ilmenite; generally more plag & olivine phenocrysts olivine than Carlton County Most abundant Quartz tholeiite Olivine tholeiite rock type (based on rock geochemistry) Texture Aphanitic to phaneritic with Aphanitic to phaneritic with intergranular, subophitic, ophitic, & intergranular, subophitic, & ophitic, quench textures common textures common Alteration Little, some alteration features Slight-moderate, fresh olivine present common Country rock 2.1 Ga Thompson Formation, 1.1 Ga North Shore Volcanic Group deformed 1.8 Ga and basal MRS lava flows, Duluth Complex, & large sequence Ely's Peak basalts differentiated sills Joints 1 margin parallel set & 1 margin Blocky jointing common, columnar perpendicular columnar set joints rarer than in Carlton County Weathering Reddish color; relatively resistant to Reddish or greenish color; less erosion resistant to erosion than Carlton County but more than felsic NSVG Textural flow Possible SW magma transport, Variable flow direction, analysis results "at best a tenuous conclusion" "no conclusion can be made as to the general flow direction" Results based on 3 samples, 2 with good orientation 5 samples, 2 with good orientation developed developed showing opposite flow directions Flow analysis 1, 2A (small dike), 2B, 5, 8B 30B, 37, 41, 46, 55 samples Discarded due to 9, 18, 65, 68 29, 31, 35, 52, 57 no obvious plag orientation
Table 1. Dike swarm comparison (after Reichhoff, 1987)
18
Reichhoff, 1987).
Both swarms outcrop within six 7.5 minute quadrangles (Fig. 6A).
Subvertical dikes of the reversely polarized, older, Carlton County dike swarm
are subparallel in map-view and strike on average about N30°E (Fig. 6B). The
Carlton County swarm outcrops from the Thompson Dam area in Cloquet,
Minnesota east to Ely’s Peak and includes at least 43 dikes which stretch a
minimum of 55 km laterally (determined from magnetic data). Subvertical dikes of the normally polarized, younger, Duluth dike swarm strike more irregularly, with an average approximate N-S strike. The Duluth swarm can be found from
Ely’s peak northeast through Duluth to Lakewood, Minnesota and includes at least 44 dikes. Beyond this outcrop area, the lateral extent of the Duluth swarm is unknown. The swarms overlap where both reversely and normally polarized dikes intrude the Ely’s Peak basalts in the Esko Quadrangle on Ely’s peak and in
Jay Cooke State Park along the St. Louis River (Green et al., 1987; Reichhoff,
1987). Both dike swarms are basalt to basaltic andesite in composition.
However, the Carlton County swarm is predominantly quartz tholeiite, whereas the Duluth swarm is predominantly olivine tholeiite (Reichhoff, 1987).
The Carlton County swarm intruded Paleoproterozoic metasedimentary
rocks and reversely polarized, basal MRS Ely’s Peak basaltic lavas. The
Thompson Formation consists of a thick sequence of laterally continuous
interbedded metagreywacke, slaty greywacke, and slate. Thompson Formation 19
A
N
Major North Shore Fold Volcanic Group B C lava flows Axes 22 Duluth Complex 196969
intrusive rocks 6666 70702170 18 722072
Undivided 6565 22 6161 intrusive rocks 23
11 Pleistocene
75 glacial deposits 2475 25 20
Thompson 8787 Formation 26 6 7 12 4 1 8 Dikes Interflow sand
72 2 1772
80 74 9 8585 74 5 3 15 14 13 10 N N 16 Lake Superior 23 1.5 km 1.5 km
Figure 6. Sample site location maps. (A) The Carlton County and Duluth dike swarms crop out in these six 7.5 minute quadrangles in the vicinity of Duluth, Minnesota. This study avoids the central region where the two swarms overlap (after Reichhoff, 1987). A total of 13 dikes are sampled in each swarm (numbers in red circles). (B) Carlton County dike swarm sample sites. Dikes are defined by aeromagnetic data (purple lines). Major fold axes in Thompson Formation are located (black traces) (after Wright et al., 1970; Kiburg & Morey, 1977; Clark, 1985). (C) Duluth dike swarm sample sites. Representative volcanic flow con- tact orientations are located (after Boerboom et al., 2002; Green & Miller, 2008).
20
sedimentary rocks were deposited beginning about 2.1 Ga in the Animikie Basin
and were deformed during the Penokean Orogeny around 1.8 Ga into open, upright, subhorizontal, symmetric or asymmetric, E-W trending folds with axial
planar cleavage. The axial planar cleavage mainly strikes E-W (within 5-10°)
with dips ranging from 80°N to 80°S (Clark, 1985).
The Duluth Dike swarm intruded MRS-generated, normally polarized
North Shore Volcanic Group (NSVG) lava flows, Duluth Complex intrusive rocks,
and large differentiated sills. The NSVG is a 7-10 km thick sequence of lavas
erupted in a broad basin during MRS extension, which dips gently, about 20° east, towards Lake Superior. In the NSVG lava flows, olivine tholeiite and olivine basalt predominate with minor felsic flows and thin interflow sedimentary rocks.
The oldest NSVG lava flow is the reversely polarized Ely’s Peak basalt separated from the rest of the normally polarized NSVG by normally polarized Duluth
Complex igneous intrusions (Clark, 1985; Reichhoff, 1987; Miller & Chandler,
1997). The 1.1 Ga Duluth Complex is a laterally extensive composite gabbroic complex, encompassing about 4700 km2, that intruded rift-related volcanics and
older Proterozoic and Archean country rocks during MRS extension (Ojakangas
et al., 2001). Beneath NSVG subaerial lavas, the Duluth Complex formed
simultaneously through numerous and complex magma emplacement events,
and it contains multiple layered intrusions (Nicholson et al., 1992; Cannon &
Nicholson, 1996).
21
In an attempt to obtain flow direction results, Reichhoff (1987) measured
petrofabrics produced by mineral alignment. Plagioclase phenocrysts in these
rocks, when observed at the outcrop scale, do not display obvious alignment.
Therefore, for a number of dike samples, three mutually perpendicular, oriented
thin sections were petrographically analyzed to determine preferred plagioclase
phenocryst orientations. The MRS dike rocks do not display evidence of having
undergone post-subsolidus strain; indicating that the petrofabrics preserved in
these rocks were produced by primary magmatic flow.
Reichhoff (1987)’s data hinted at the possibility that the dikes in both the
Carlton County and Duluth dike swarms may have been supplied laterally from centrally located magma chambers. Lateral flow was determined from the fact that the best orientation was observed in horizontal thin sections in the majority of the limited number of dikes studied. However, this conclusion was based on petrofabric results from only a few dikes in each swarm that are unconvincing on a broader, dike-swarm scale.
Research focus and hypotheses
The existence of a significant lateral component of mass transfer involved in emplacement of these two dike swarms could indicate along-axis transfer of
mass in the upper crust during active continental rifting. An alternative
hypothesis is that the dike swarms preserve overall subvertical magmatic
22
emplacement fabrics from magma sources located directly below each swarm
(Fig. 1B).
In order to test these hypotheses, the specific aim of this research is to
use anisotropy of magnetic susceptibility (AMS), a tool refined in the twenty years
since Reichhoff (1987)’s work, to obtain magnetic fabric data for rocks from these
two dike swarms. The interpretation of AMS data can provide a reliable measure
of magmatic flow fabrics in rift dikes and thus may be good indicators of flow
direction within these dikes and allow for documentation of regional-scale
igneous flow patterns. AMS has been established as an accepted method for
determining magma flow fabrics ever since a seminal study of Hawaiian dikes by
Knight & Walker (1988) demonstrated that magnetic fabrics determined using the
AMS technique coincide with macroscopic flow features. Determining the
locations of the magma chambers that fed these rift intrusions will augment the
body of knowledge relating to dike flow mechanics and the mechanics of active
continental rifting and crustal evolution. The larger objective of this research is to
increase understanding of magma flow mechanics in planar conduits and to
assess the importance of lateral mass transfer in the process of intracratonic
rifting.
This study will provide answers to the following questions. Do AMS data
indicate significant flow fabrics in the Carlton County and Duluth dike swarms?
Do regional-scale flow patterns exist? Where may the magmatic vent sources
23
have been located for each of these swarms? Were the dikes mainly emplaced vertically or horizontally? What do these regional patterns tell us about dike emplacement processes during intracratonic rifting? How do the inferred magma chamber locations and the mode of dike emplacement relate to the regional tectonic story of Mesoproterozoic midcontinent rifting?
Magnetic Fabric Study
Technique background
Anisotropy of magnetic susceptibility (AMS) analysis is an effective tool for measuring rock fabrics and thus for studying magmatic emplacement histories in undeformed igneous rocks. Volumetric magnetic susceptibility (K) is a dimensionless value (reported in SI units) defined by the ratio of a measured induced magnetization (in amperes per meter) in a rock sample to an applied inducing magnetic field (in amperes per meter). The induced magnetization is not equal in all directions. This spatial anisotropy in the degree to which a rock’s induced magnetism will distort in a given magnetic field is a physical rock property defined by preferred orientations of both crystallographic axes and shape-preferred orientations of individual mineral grains and grain clusters
(Rochette et al., 1992). AMS represents the sum of the magnetic contributions of all of the minerals in a rock (Cañón-Tapia, 2004), although the contributions of ferromagnetic mineral phases, when present, often dominate the AMS signal.
AMS provides for sensitive delineation of rock fabrics in rocks that are macroscopically isotropic, as is often the case with magmatic flow fabrics
(Rochette et al., 1992), even when those rocks have a very low degree of anisotropy of about 1% volume (Cañón-Tapia, 2004). The AMS technique is powerful and can measure the orientations of hundreds of grains in minutes,
24
25
compared to hours of measurement using universal stage microscope or electron
backscatter diffraction approaches (Borradaile & Henry, 1997).
AMS measurement yields both scalar and directional data that define the
shape and orientation of a magnetic susceptibility ellipsoid (Fig. 7A). The AMS
ellipsoid is represented by a symmetric second-order tensor that describes the
length and orientation of three mutually perpendicular principle axes, K1 ≥ K2 ≥
K3. K1 delineates the magnetic lineation orientation, generally assumed to be a
proxy for magma flow direction. K1 and K2 define the magnetic foliation plane,
and K3 is the pole to magnetic foliation (Rochette et al., 1992; Borradaile &
Henry, 1997; Callot & Guichet, 2003).
Dikes are two-dimensional features ideally suited for magma flow studies.
Their single-plane geometry provides an important constraint on the direction of
magma flow, and their fast cooling preserves evidence of laminar flow of viscous
magma in the alignment of rigid grains (e.g., Cañón-Tapia, 2004). The AMS
technique has been successfully applied to magma flow in dikes in studies since
Knight & Walker (1988) empirically showed that mean K1 reflects macroscopic
lineation directions and thus can be equated to magma transport direction in
Hawaiian dikes. In the intervening twenty years, the AMS technique has become
an accepted and robust geologic tool for defining igneous, metamorphic, and
even sedimentary rock fabrics (Cañón-Tapia, 2004).
26
A B Velocity Profile
K1
K3
K2
Dike Margin Magma Flow
Imbrication Angle C E D W N Flow Plane
Magma Flow Flattening
Magma Flow Plane
Figure 7. The AMS ellipsoid and models of magma flow in dikes. (A) The output of low field magnetic measurement of oriented cubic samples is the length and orientation of three mutually perpendicular axes that define the AMS ellipsoid: K1 (maximum), K2 (intermediate), K3 (minimum). K1 is the magnetic lineation direc- tion, used as a proxy for magmatic flow fabric. The K1-K2 plane (perpendicular to K3) defines the magnetic foliation (after Knight & Walker, 1988; Ferré et al., 2004). (B) Schematic three dimensional view of magma flow in a dike. The flow velocity profile is controlled by friction between the magma and the wall rock (after Callot et al., 2001). (C) The flow velocity gradient produces an imbrication angle between elongate and flattened mineral grains and dike margins. Lower hemisphere equal area projection showing resultant K1 axes for eastern (white squares) and western (black squares) dike margins (after Martín-Hernández et al., 2004). (D) S/C-type structure formed due to shear along dike margins caused by slower magma flow velocities (after Callot & Guichet, 2003).
27
Magma flow is assumed to align elongate or platy mineral grains (such as
plagioclase laths), which silicate grain structure may dictate the distribution of
late-crystallizing oxide grains (such as magnetite), as oxides represent late-stage
crystallization phases in most mafic melts (Ernst & Barager, 1992; Tauxe et al.,
1998). Oxide crystallization can also be coincident with dike emplacement
(Lanza & Meloni, 2006). Following the formation of initial flow emplacement
fabrics, further rigid grain rotation is unlikely because it would be strongly
opposed by the high viscosity of the cooling basaltic magma. During dike
emplacement, laminar flow of a viscous magma responds to friction along the
dike walls, producing a velocity profile with the fastest flow in the center of the
dike (Fig. 7B). K1 directions measured across the width of a dike often exhibit a bimodal distribution at low angles (about 5-30°) to a dike’s chilled margins, interpreted as imbrication of flow fabrics formed by such velocity gradients
(Knight & Walker, 1988; Fig. 7C). Magmatic flow structures appear as lineation and foliation, tectonic layering, folds, boudinage, and S/C structures (Fig. 7D).
The presence of tiling structures and grain imbrications that produce these structures are associated only with magma dynamics and not with plastic deformation (Callot & Guichet, 2003). Thus, determination of flow vectors from
AMS data in dikes commonly accounts for an imbrication angle of up to approximately 30° between flattened and elongate particles in the magma that define the magnetic ellipsoid and the dike margins (Knight & Walker, 1988; Callot
28
et al., 2001; Geoffroy et al., 2002). AMS data, when obtained from both margins
of a dike, can yield not only a flow lineation orientation but also a unique magma flow direction, e.g., upward or downward (Tauxe et al., 1998; Fig. 7C).
Numerous studies using the AMS technique in conjunction with traditional
structural data obtained from field and petrographic measurements have
accomplished flow fabric analysis of dikes. Workers have repeatedly shown that
combined AMS and structural data provide intrusive directions of dikes with great
fidelity (Staudigel et al., 1992). The use of AMS analysis to investigate magma
flow during emplacement of dike structures is an active focus of recent research.
Previous AMS studies
Several recent studies of Precambrian intrusions in the North American
midcontinent have employed the AMS technique to elucidate magmatic
movement (Cadman et al., 1992; Ernst & Baragar, 1992; Ernst, 1994; Middleton
et al., 2004; Craddock et al., 2007; Maes et al., 2007; Palmer et al., 2007;
Craddock et al., 2008; Fig. 2). Intrusion of the 2.21 Ga Senneterre dikes in the
southern Superior Province (Fig. 2A) recorded overall horizontal flow patterns,
suggesting that the dikes may have been feeders for the coeval diabase
Nipissing sill intrusions (Palmer et al., 2007). The 2.20 Ga Kikkertavak dike
swarm and the younger Mesoproterozoic (1.64 Ga) Kokkorvik sills intruded into
gneisses of the Archean Hopedale block in en echelon arrays that are well
29
exposed today in eastern Labrador, Canada (Fig. 2B). The Kikkertavak dikes record subhorizontal primary magmatic flow directions that disagree with vertical crack propagation directions recorded by the en echelon crack geometry. This difference is interpreted as early vertical dike emplacement followed by later lateral flow of cooler, less buoyant magma in the same conduits (Cadman et al.,
1992). The subhorizontal Kokkorvik sills preserve magmatic flow directions consistent with those recorded by the geometry of dike offsets.
The 10- to 110-km-long dikes of the 2.06 Ga Kenora-Kabetogama giant radiating dike swarm extend across 30,000 km2 in northern Minnesota and western Ontario (Fig. 2C) and are overlain by the metasediments of the Animikie
Basin. The Kenora-Kabetogama dike swarm exhibits vertical magnetic fabrics in the south and horizontal magnetic fabrics in the northwest, indicating horizontal dike propagation to the north and west from a single vertical magma source located to the southeast near the Penokean Orogenic margin, in agreement with the locus of dike radiation (Craddock et al., 2008). At 1.877 Ga, two perpendicular pre-MRS ultramafic lamprophyre dikes intruded the rock that makes up Little Presque Isle near Marquette, Michigan (Fig. 2D). These narrow dikes (0.4 and 1.0 m) may represent the first evidence of ultramafic igneous intrusion related to the collisional Penokean Orogen far (400 km) to the south, perhaps due to subduction-induced flexure leading to local decompression melting. The older east-west dike records subhorizontal flow. The younger
30
north-south dike crosscuts the east-west dike and preserves vertical flow
(Craddock et al., 2007).
The 1.267 Ga Mackenzie giant radiating dike swarm sweeps across an
enormous swath of the North American midcontinent (Fig. 2E) and records both
vertical dike injection fabrics within 500-600 km of the locus of dike radiation and
horizontal magmatic flow fabrics traveling laterally at least 2,000 km beyond the
locus of dike radiation. This flow direction transition at 500-600 km may mark the
outer boundary of a hot mantle plume head of about 1,000 km diameter (Ernst &
Baragar, 1992). The 1.235 Ga Sudbury dike swarm in Ontario (Fig. 2F) also likely formed in response to a hot mantle plume and records lateral magmatic flow from a magma source to the southeast beneath the Grenville Province
(Ernst, 1994).
Coeval with MRS rifting, 1.108 Ga diabase Logan sills intruded near the
MRS triple junction (Fig. 2G) and preserve uniform flow directions away from the
rift margins. These subhorizontal intrusions record east-west flow adjacent to the north-south Nipigon Rift arm (the third rift arm of the MRS) and northwest- southeast flow near the triple junction, suggesting a distal melt source located along the rift axis (Middleton et al., 2004). Both primary emplacement and secondary flow structures of the 1.1 Ga Sonju Lake intrusion in the Duluth
Complex in northeastern Minnesota (Fig. 2H) point to a magma source and mode
31
of emplacement for magma flow involving lateral flow beneath a density barrier
(Maes et al., 2007).
A recurrent AMS dataset characteristic is that a significant proportion of studied dikes in a dike swarm display lateral to oblique flow fabrics. Overall vertical emplacement fabrics are documented in only a minority of recent dike emplacement studies, especially in settings where dike emplacement occurs in thin crust and at shallow crustal levels, such as along the Mid Atlantic Ridge axis in Iceland, where fissure eruption is common and the crust is approximately 30 km thick with magma chamber depths of about 1-5 km (e.g., Craddock et al.,
2008). The combination of vertical and horizontal flow in a dike swarm is often interpreted in terms of the dikes’ distance from a magma chamber. Vertical flow is inferred to occur above the magma chamber, and flow becomes progressively more oblique with increasing distance from the magma source, especially on the scale of entire dike swarms (Ernst & Baragar, 1992; Bates & Mushayandebvu,
1995; Archanjo et al., 2000; Raposo & Agrella-Filho, 2000; Herrero-Bervera et al., 2001; Craddock et al., 2008).
Increasingly, these and other studies are delineating the details of magma flow patterns in the dike structures that are associated with large-scale tectonic processes. Clearly, dike emplacement can be influenced by tectonic stress variations, preexisting structures, and mechanical barriers. The prevalence of
32
lateral and oblique flow in subvertical dike structures suggests that buoyancy forces and density contrasts are also important controls on magma flow direction.
Methodology
Field observation and sampling
This study avoids the central region of the outcrop area where the two swarms overlap (the Duluth and West Duluth Quadrangles and the majority of the Esko Quadrangle) in order to ensure that the two sample sets are from discrete dike populations (Fig. 6A). The field work entailed locating dike outcrops by means of the approximate locations recorded by Reichhoff (1987). The map position and associated UTM of each sampled dike outcrop were recorded (Fig.
6 and Table 2).
Dike erosion patterns in the field are striking. Basaltic dikes of the Carlton
County swarm preferentially erode relative to the surrounding grey metasedimentary Thompson Formation country rock, so well-preserved outcrops are rare. Good outcrops are located in the exposed bedrock below Thompson
Dam (Fig. 8A) and around the southern shore of Thompson Reservoir; at the edges of larger grey Thompson Formation whaleback outcrops, held up by the more resistant older rock (Fig. 8B); and in some stream channels (Fig. 8C). The extents of the Carlton County dikes in Fig. 6B are defined largely by geophysical data, with only scattered outcrop control. In many locations, the eroded swampy 33
dike
Thompson Formation
A
16 cm B
Thompson dike Formation Thompson Formation
C
D
1.0 mm E 1.0 mm Figure 8. Carlton County dike swarm description. (A) 7.0 m wide dike below Thompson Dam. (B) Typical dike outcrop: red-orange weathered, blocky jointed basalt, conspicuously less resistant to erosion than surrounding Thompson Fm. country rock. (C) 9.8 m wide dike, clear margins marked by channel erosion and a colunmar joint pattern perpendicular to the dike margin. View SW from Swing- ing Bridge over St. Louis River. (D) Stereoplot of poles to 13 dike orientations. Bulls-eye indicates pole to swarm mean dike orientation, plane: N28°E, 89°NW, with 95% confidence angle: 6.3°. Lower hemisphere, equal area projection. (E) Photomicrographs showing grainsize, plagioclase laths, opaques.
34
valleys represent dike positions. Where present, Carlton County dikes are
readily distinguishable from the metasedimentary country rock by their distinct
margins, which are defined by changes in rock type, joint patterns, and
weathered appearance. In outcrop, the Thompson Formation consists of dark to
light grey weathered metagreywackes with well-developed rock cleavage, which
contrasts with the distinct, blocky joint patterns in the dikes formed by a dike
margin parallel joint set and a margin perpendicular columnar joint set. The
black basaltic Carlton County dikes are most readily recognized in the field by
their red-orange weathered appearance in outcrop.
Displaying equally striking erosion patterns, green-grey weathered basaltic
dikes of the Duluth swarm jut out of pink weathered NSVG lavas, often extending
into the water and forming small promontories along the north shore of Lake
Superior (Fig. 9A). Duluth swarm dike outcrops also exist in stream cuts just
inland of the lake shore, where less erosion has worked to carve out the basaltic
dikes from the basaltic lava country rock (Fig. 9B). NSVG lava flows are massive
with minor vesiculated layers. Blocky joint patterns present in the Duluth swarm dikes are similar to those observed in the Carlton County swarm dikes. Some
Duluth swarm dikes exhibit clear chilled margins (Fig. 9E). One sampled dike
(dike 17, Fig. 6C) intrudes a finely laminated interflow sandstone deposit (Fig.
9F).
35
1.0 mm
A D 1.0 mm
B C
Interflow Dike Sandstone NSVG Lava Flow 15 cm Dike
E 15 cm F
Figure 9. Duluth dike swarm description. (A) Typical dike outcrop on the North Shore of Lake Superior: 4m-wide dike, green-grey weathered, blocky-jointed basalt, displays resistance to erosion relative to pink basalt of the North Shore Volcanic Group lava flows and juts into the lake. (B) This swarm also outcrops in stream cuts adjacent to the lake shore. (C) Stereoplot of poles to 13 dike orien- tations. Bulls-eye indicates pole to swarm mean dike orientation, plane: N18°E, 72°NW, with 95% confidence angle: 14.2°. Lower hemisphere, equal area projection. (D) Photomicrographs showing grainsize, plagioclase laths, opaques. (E) Chilled margin. (F) Dike intruded interflow sandstone deposit.
36
A total of 26 dikes were sampled, 13 dikes in each swarm (Fig. 6B and C),
with two oriented blocks collected for AMS analysis from each dike at different
distances from the dike margins. One sample (designated m) was taken
relatively closer to a dike margin, and a second sample (designated c) was taken
relatively closer to the center of the dike (Table 3). The sample collection
method used to obtain AMS data in this study involves acquiring oriented block
samples that are at least about 15 cm3 (at least the size of two fists or one-and-a-
half grapefruits). Dike margin (m) samples were located at least 10 cm from the
dike margin to avoid wall effects and quenched textures (Cadman et al., 1992;
Tauxe et al., 1998), and dike center (c) samples were located away from the very center of the dike where possible to avoid turbulence effects.
Dike and country rock characteristics and the equivalent Reichhoff (1987)
sample number, which provides dike polarity measurements, were noted at each
site (Table 3). Of the sampled Carlton County dikes 85% (11 out of 13) are
reversely polarized. All of the sampled Duluth swarm dikes are normally
polarized. No macroscopic flow structures in the form of visible imbricated
elongate phenocrysts, stretched vesicles, or linear and ropy lobate flow
structures (Philpotts & Philpotts, 2007) were observed in the field. The dikes in
both swarms are composed of aphanitic to medium grain sizes (Reichhoff, 1987;
Figs. 5E and 6D). Undeformed, filled vesicles (about 0.5-1.5 cm) were observed
in a few Duluth swarm dikes. A minority of the dikes sampled in this study
37
display variable strike directions (noted in Table 3). The Duluth swarm dikes sometimes show evidence of compound, multiply intruded dike structures. In one locality, a second, later dike intruded between the sampled dike and the lava flow country rock, as evidenced by 0.5 cm veins of the second dike that filled the dike margin parallel joints of the sampled dike.
Sample Preparation
The magnetic analyses and sample preparation for this study were performed at the Rock Magnetic Laboratory at Southern Illinois University.
Sample preparation entailed cutting oriented 16-20 mm cubic replicates from 1 or
2 oriented block samples for each of the 26 dikes sampled in the field (Fig. 10, top row). The block samples were reoriented in a sand box with the dip amount and horizontal strike line recorded on the block in the field, using a Brunton compass and a level, and a horizontal strike line was traced around the entire circumference of the block sample with a Sharpie marker, using a wooden slat laid horizontally across the top of the sand box. In order to cut horizontal slabs, the strike line was visually aligned with the non-magnetic, stainless steel slab saw blade, and the block sample was secured with a vice. As many 16-20 mm thick horizontal slabs as possible were cut from the block by making parallel cuts without repositioning the block in the vice (Fig. 10, bottom left). The cut block was reassembled in order to orient the slabs. The top of each horizontal slab 38
Figure 10. Steps in the process of obtaining AMS data. (Top Row) Two oriented hand samples were collected from each dike in the field. (Bottom Row) Oriented slabs and cubes (16-20 cubic mm) were cut from each hand sample. Cube labels: hatch marks point north and the labeled face is the horizontal plane. (Center) KLY-4S Kappabridge Spinning Specimen Magnetic Susceptibility Anisot- ropy Meter (and KLY-3 Kappabridge Pick-Up Unit, not pictured) ready to perform a low field AMS measurement at the Rock Magnetic Laboratory at Southern Illinois University. Each cube was measured in 15 positions plus a bulk suscepti- bility measurement, which took about 4 minutes per cube (the spinning meter requires that each cube be placed in the sample holder in only 3 positions).
39
was labeled with “H0 Top,” sample number, and a north arrow (measured from
the oriented face of the block sample with a protractor). The thickness of each
slab was measured with calipers. A grid was drawn on the top of one or more of
the slabs, positioned to produce cubes with N-S and E-W vertical faces (as
illustrated in Fig. 7A) and to produce maximum cube yield (at least 8 cubes per
sample for statistically significant results in these basalts), accounting for trim
saw blade thickness (2 mm) between cubes. The top of each cube was labeled
with sample number, cube number, and north-pointing hatch marks on the cube’s eastern edge (Fig. 7A). Each labeled slab was cut into oriented 16-20 mm cubes on the trim saw (Fig. 10, bottom right), and any cubes that broke along fractures
during cutting were retained. Pieces of broken cubes, however small, were
superglued in the correct orientation, after drying completely on a hotplate
(superglue is diamagnetic and does not effect magnetic measurements). This study includes analysis of 32 block samples (18 from Carlton County swarm dikes and 14 from Duluth swarm dikes) and production of a total of 530 cubes
(with a mean of about 17 and a range of 9-32 cubes per sample) by this method.
Low-field AMS analysis
Following sample preparation, the cubes underwent low-field magnetic
measurement, using a KLY4-S Kappabridge Spinning Specimen Magnetic
Anisotropy Meter at field intensity 300 A/m and a KLY3-S Kappabridge Pick-Up
40
Unit (Fig. 10, center). The cubes were left in the low-field magnetic lab overnight to allow the rock to equilibrate to room temperature (around 20°C) and over
handling was avoided during analysis, as AMS measurement is temperature sensitive. Prior to analysis, the instrument was calibrated to a standard, and the
susceptibility of the sample holder (a 5-sided, diamagnetic, plastic cube that
holds a cube up to 20 mm in size) was measured. The actual volume of each
cube was set to 10 cm3, which was later corrected for during data processing by
multiplying the volume-dependent bulk magnetic susceptibility (km) measurement
for each cube by a correction factor (the actual volume of each cube in cm3 / 10 cm3). During analysis, measurement error was monitored and kept under 1%.
A cubic rock sample was positioned in the sample holder and held in place
during analysis with small slips of diamagnetic styrofoam padding and a plastic
screw (tightened with a plastic screwdriver). Each cube was analyzed in a set
series of 3 positions. The spinning magnetometer rotates the cube in each
position, allowing for measurement in a total of 15 positions. A bulk magnetic measurement was also recorded for each cube. AMS measurement took approximately 5 minutes per cube.
High-field hysteresis analysis
High-field magnetic room temperature hysteresis loop data were
produced, using a MicroMag VSM 3900-04 Vibrating Sample Magnetometer,
41
in order to determine magnetic mineralogy characteristics such as magnetic grain
size from a bulk estimate of single-domain (SD), pseudosingle-domain (PSD),
and multi-domain (MD) magnetite grains. This indicates whether a grain contains one magnetic field or many. The induced magnetization that defines the AMS
response of a ferromagnetic mineral is a result of the applied field’s ability to
modify the arrangement of magnetic domain walls within the ferromagnetic
mineral grains, which is a function of grain size (Lanza & Meloni, 2006). SD
grains are too small (< 1.0-0.03 μm) for a magnetic domain wall to form within
them and change magnetizations by rotation without high energy input, because
a change in the domain state is opposed by grain shape and crystal structure in a
small SD-size grain. Susceptibility measured parallel to the long dimension of an
SD grain is zero. In larger (> 1.0-10.0 μm) MD titanomagnetite grains,
magnetization can be changed more easily, as there are more magnetic domain
walls present. With the domain walls free to move in MD grains, the direction of easy magnetization coincides with the physical grain elongation direction. PSD behavior occurs when MD grains exhibit aspects of SD behavior, which is inherent in small MD grains (0.1-1.0 μm) due to magnetic domain wall displacement and pinning (Dunlop & Özdemir, 1997; Lanza & Meloni, 2006).
High-field magnetic hysteresis data were produced for 75% of the
samples, using 1 cube per sample (24 cubes total, 14 from the Carlton County swarm and 10 from the Duluth swarm). Additional opaque microscopy work
42
could independently evaluate opaque grain sizes and potentially help to
determine which minerals contribute to the magnetic signal.
Data Analysis
Scalar AMS data
To process the scalar AMS data, it is necessary to know the true value of
bulk susceptibility (Km) for each cube, which is a volume-dependent statistic
requiring determination of the actual volume of each cube. The three dimensions
of each cube were measured individually with digital calipers (and any missing
volume was calculated and subtracted from each total). Using the actual volume,
Km measured during analysis was converted to true Km.
The scalar values of the principle susceptibilities that define the
susceptibility tensor (K1 ≥ K2 ≥ K3), allow for determination of a series of scalar
parameters, as defined by Jelinek (1981). These parameters include magnetic lineation (L = K1 / K2), magnetic foliation (F = K2 / K3), anisotropy degree (P = K1 /
2 2 2 K3), corrected anisotropy degree (Pj = exp √ { 2 [(η1 - η) + (η2 - η) + (η3 - η) ] }, where η = (η1 + η2 + η3) / 3; η1 = ln K1; η2 = ln K2; and η3 = ln K3), shape factor
(Tj = [ln F - ln L] / [ln F + ln L]), and difference shape factor (U = [2 K2 – K1 –K3] /
[K1 – K3]). Pj is the quantity of the anisotropy. Tj and U describe the anisotropy quality. Tj is a representation of the shape only, independent of the amount of
43
anisotropy present. If anisotropy degree is low (i.e., if the ellipsoid is a spheroid),
then U is approximately equal to Tj (Jelinek, 1981).
Comparison of anisotropy parameters allows for assessment of the shape
of the magnetic ellipsoid. This is accomplished by plotting Tj against both Km and
Pj to determine whether the shape of the AMS ellipsoid is primarily oblate
(flattened; 0 < Tj ≤ 1;) or prolate (elongated; -1 ≤ Tj < 0). The amount of
anisotropy present (Pj) may not be much higher than 1 (Pj = 1 indicates a spherical AMS fabric); however, AMS measurement is suitably sensitive to small degrees of anisotropy. In most rocks, Pj is > 1.3-1.4, but Pj can be as low as
about 1.005, reflecting a very low susceptibility and magnetic mineral alignments
on the order of 1% (Lanza & Meloni, 2006). Also, paramagnetic versus
ferromagnetic input to the magnetic signal can be determined by comparing Pj and Km, as ferromagnetic minerals such as magnetite have relatively higher
degrees of anisotropy and relatively higher bulk magnetic susceptibilities.
Directional AMS data
To process the directional AMS data for each of the 32 individual block samples analyzed, the orientations of K1, K2, and K3 for each of the block
samples’ replicates (cubes), were plotted on stereonets, using the program
Stereo32 v. 0.9.4 (Röller & Trepmann, 2007). The vector means of the three
principal axes, including the corresponding cones of 95% confidence, were
44
calculated for each sample. The Stereo32 software assumes a Fisher
distribution to calculate the 95% confidence cone for each vector mean. The
resultant site averaged K1, K2, and K3 directional data are positioned relative to the orientation of each respective sampled dike. On a stereonet, these directions were all rotated by hand from the reference frame of their specific dike’s orientation into the plane of the swarm mean dike orientation, so that the directional AMS results can be compared to one another on the scale of each dike swarm.
High-field data
The high-field magnetic remnance data are plotted on a Day et al. (1977)
plot and compared to the theoretical magnetization curves of Dunlop (2002) to
determine effective magnetic grain size by graphing remnant magnetization (Mr)
divided by saturation magnetization (Ms) versus remnant magnetic coercivity (Hcr) divided by magnetic coercivity (Hc). The measured remnant magnetism for the
samples shows which magnetic minerals contribute to the signal and whether
they have SD, PSD, or MD grain sizes.
During low-field magnetic measurement, MD and PSD magnetite grains
have a normal magnetization, and SD magnetite grains have an inverse
magnetization (Fig. 11B). This range leads to normal AMS fabrics, where AMS
axes coincide with magnetic grain shape anisotropy, and inverse AMS fabrics, 45
Crystal Shape Axes Z A
a ≥ b ≥ c b a
c Y X Grain response to an MD B SD applied magnetic field
AMS Ellipsoids K1 C K1 ≥ K2 ≥ K3 K3
K2 K2 K3 K1
AMS Axes Y Y D lower hemisphere stereoplots ▲ ▲
■ K1 ■ ■ ▲ K2 X ● X● Z Z ● K3
Normal AMS Fabric Inverse AMS Fabric
Figure 11. Normal vs. inverse AMS fabrics as functions of magnetic grain size. Normal and inverse magnetic fabrics are both controlled by the grain shape anisotropy of the phase that carries the magnetic susceptibility. Normal magnetic fabrics result from MD-size grains, in which a number of magnetized domains form in response to an applied field, producing K1 and K3 orientations that corre- spond to the mineral grains’ long and short axes respectively. Inverse magnetic fabrics result from SD-size grains, in which all magnetic moments are mutually parallel, producing swapped K1 and K3 orientations relative to the physical crystal shape (after Ferré, 2002; Lanza & Meloni, 2006).
46
where AMS axes are swapped relative to grain shape (Figs. 11C and D).
Intermediate AMS fabrics result from a mixture of normal and inverse magnetic
components, and occur when a proportion of SD magnetite is present (Rochette
et al., 1992; Ferré, 2002).
In a dike where flow is confined between the dike walls, AMS fabrics are
said to be normal when the magnetic lineation and foliation orientations fall within
about 30° of the dike plane, and thus the minimum K3 axis is perpendicular or at
a high angle to the dike plane. AMS fabrics are termed inverse when the
magnetic lineation and foliation orientations are perpendicular or at high angles
to the dike plane and thus the minimum K3 axis is approximately in the dike
plane, within about 30°. Primary laminar magmatic flow in the plane of a dike
would not produce lineation and foliation fabrics at high angles to the dike margin, however such anomalous fabrics can still indicate magmatic flow
directions. The contribution of tiny SD magnetite grains produces inverse
magnetic fabrics that, like normal magnetic fabrics, are a function of grain shape
and can likewise reveal the flow orientation of grains (Fig. 11). AMS fabrics are
labeled intermediate where the intermediate K2 axis is nearly perpendicular to the
dike orientation (e.g., Archanjo & Araújo, 2002; Lanza & Meloni, 2006).
47
Results
Field results
At each outcrop measurements of the dike’s orientation and width were
recorded (Table 3). The Carlton County swarm mean dike orientation for the 13
sampled dikes is N28°E, 89°NW, with a 95% confidence angle of 6° (Fig. 8D).
The Duluth swarm mean dike orientation for the 13 sampled dikes is N18°E,
76°NW, with a larger 95% confidence angle of 14.2°, reflecting more variability in
Duluth swarm dike plane orientations (Fig. 9C). Dikes with relatively narrow
widths, about 1-5 m, were selected; however, the range of dike widths actually
sampled is 0.8-15.3 m, as sampling was also controlled by outcrop availability in
the field.
Scalar AMS data results
The scalar low-field AMS data consist of site-averaged, volume-corrected
bulk magnetic susceptibility (Km) values, site-averaged principle susceptibility (K1
≥ K2 ≥ K3) values, and site-averaged scalar parameters (Tables 4 and 5). The
bulk magnetic susceptibility for the 530 cubes analyzed has a mean value of
3.0×10-2 SI volume, 1.4×10-2 and 5.0×10-2 SI volume for the Carlton County and
Duluth swarms respectively, meaning that the magnetic signal is likely dominated
-3 by the fabric of ferromagnetic phases (indicated when Km > 5×10 ; Cañón-Tapia,
2004). The AMS signal is robust as the bulk magnetic susceptibility is well within
48
the instrument sensitivity (2×10-8 SI at 300 A/m; Pokorný et al., 2004). The mean corrected degree of magnetic anisotropy (Pj) for both swarms is 1.036 ± 0.006
with a range of 1.002 to 1.142. For the Carlton County swarm the mean
corrected anisotropy degree is 1.045 ± 0.008 with a range of 1.002 to 1.142. The
mean corrected anisotropy degree for the Duluth swarm is slightly lower, 1.024 ±
0.004 with a smaller range of 1.010 to 1.057. A plot of Pj versus Km also
supports the result that ferromagnetic phases control the AMS signal, as all of
the samples fall within the ferromagnetic field (Fig. 12A). Low-field magnetic
susceptibility plots that incorporate the shape parameter (Tj) describe the shape
of the AMS ellipsoid (Tj versus Pj and Tj versus Km; Figs. 12B and C). The
magnetic fabrics are predominantly oblate in the Carlton County swarm and
mixed to prolate in the Duluth swarm.
High-field results
In order to determine effective magnetic grain size, the high-field magnetic
remnance data from 24 of the low-field samples (Table 8) are plotted on a Day et al. (1977) plot (Fig. 12D). About 30% of these data fall within the pseudosingle-
domain (PSD) field. The trend of the data follow the theoretical mixed SD and
MD grain population magnetization curves of Dunlop (2002). This finding
suggests that both the Carlton County and Duluth mafic dike swarms host
magnetic grains of PSD grain size (0.1 to 1.0 µm), which may reflect a mixture of 49
1.2 1 A C j
Ferromagnetism j P
, T
e , r e r e t g e e D m Oblate a y r p
a 0 o P r
Prolate t 1.1 e o s p i a n h A
S c i t e n g a M
Paramagnetism -1 100 1000 10000 100000 1000000 1.0 -6 Magnetic Susceptibility, K m (10 SI) 100 1000 10000 100000 1000000 -6 Carlton County dike swarm (n=18) Duluth dike swarm (n=14) Magnetic Susceptibility, K m (10 SI) Carlton County dike swarm (n=18) Duluth dike swarm (n=14)
1 SD n=24 1 B j T
, r e t e s m Oblate PSD a r 0 0.1 a / M r
P Prolate
M e p a h S
-1 MD 1.00 1.04 1.08 1.12 1.16 D Magnetic Anisotropy Degree, Pj 0.01 Carlton County dike swarm (n=18) Duluth dike swarm (n=14) 1 2 3 4 5 6 Hcr / Hc
Carlton County dike swarm (n=14) Duluth dike swarm (n=10)
Figure 12. Low-field (A-C) and high-field (D) magnetic susceptibility plots. (A) Pj vs. Km. (B) Tj vs. Pj. (C) Tj vs. Km. Data listed in Tables 4 and 5. (D) Day et al. (1977) plot of remanent magnitization (Mr) / saturation magnitization (Ms) vs. remnant magnetic coercivity (Hcr) / magnetic coercivity (Hc). Theoretical mixed single-domain (SD) and multi-domain (MD) grain population magnitization curves of Dunlop (2002) included, with pseudosingle-domain (PSD) grain size (0.1 to 1.0 µm) field in between. Data listed in Table 8.
50
SD and MD grains.
Directional AMS results
On the scale of individual dikes, the directional low-field AMS data reveal
distinct magnetic fabric patterns in each dike swarm (Table 6 and Figs. 13 and
14). The principal directions cluster well at most sample sites. The Carlton
County swarm dikes yield overall normal AMS fabrics (Fig. 13). Fifty-six percent
of the Carlton County dikes display vertical to subvertical mean magnetic
lineation (K1) orientations. Twenty-eight percent of the Carlton County dike
samples have inclined K1 orientations, and 16% have subhorizontal K1
orientations. The Carlton County dikes produce K1 orientations and magnetic
foliation (K1-K2 plane) attitudes located in the plane of the dike, within ~35°,
except for one dike that displays subhorizontal K1 orientations at about 90° to the dike margin. This dike has an inverse magnetic fabric. One other analyzed
Carlton County swarm dike has a high enough angle between K2 and the dike
plane (83°) to be considered intermediate. Approximately one-third of the
Carlton County dike samples exhibit a linear spread in the data clusters of both
the K1 and K2 axes. This clustering configuration indicates maximum and
intermediate magnetic susceptibility axes of nearly equivalent lengths and thus
oblate magnetic fabrics, an interpretation supported by the shape parameter
finding of predominantly oblate magnetic fabrics in the Carlton County swarm. 51
1 2 3
4 5 6
7 8 9
10 11 12
13 Figure 13. AMS results, 13 Carlton County swarm dikes (sampled dikes 1-13). Normal (pink), inverse (blue), and intermediate (green) AMS fabrics. Hatch marks point north. Lower hemisphere, equal area projections. Plotted: K1 (red squares), K2 (green triangles), and K3 (blue circles) axes directions, mean vector for each principal direction (open symbols) with associated cone of 95% confidence, dike planes (thick black lines), and magnetic foliation planes (thin blue lines). Darker symbols indicate c samples. 52
14 15 16
17 18 19
20 21 22
23 24 25
26 Figure 14. AMS results, 13 Duluth swarm dikes (sampled dikes 14-26). Normal (pink), inverse (blue), and intermediate (green) AMS fabrics. Hatch marks point north. Lower hemisphere, equal area projections. Plotted: K1 (red squares), K2 (green triangles), and K3 (blue circles) axes directions, mean vector for each principal direction (open symbols) with associated cone of 95% confidence, dike planes (thick black lines), and magnetic foliation planes (thin blue lines). Darker symbols indicate c samples.
53
Temporally, there is no difference observed between the AMS fabrics preserved
in the older, reversely polarized Carlton County swarm dikes, emplaced prior to
the magnetic polarity reversal at 1097±1 Ma, and the 15% of the sampled Carlton
County dikes that are normally polarized (Table 3; Reichhoff, 1987). The
normally polarized Carlton County swarm dikes in this study (dikes 3 and 9, Fig.
13) both have normal AMS fabrics with subvertical to southwest-inclined magnetic lineations.
The Duluth swarm dikes yield overall apparently inverse AMS fabrics (Fig.
14). Seventy-one percent of the Duluth swarm samples exhibit subhorizontal to
inclined K1 orientations and K1-K2 planes orientated at a high angle (60-90°) to
the dike margin, representing inverse magnetic fabrics. The remaining 29% of
the Duluth swarm dike samples have inclined K1 orientations approximately in the
plane of the dike, within ~35°. Of these, three dikes also have K1-K2 planes
approximately in the plane of the dike and thus represent normal magnetic
fabrics. The remaining dike displays an intermediate magnetic fabric where a
high angle (89°) between K2 and the dike plane creates a magnetic foliation perpendicular to the dike orientation.
On the scale of the dike swarms, the directional low-field AMS data
rotated into the plane of the swarm mean dike orientation also depicts distinct
magnetic fabric patterns in the two swarms (Table 7 and Fig. 15). The dike
samples of the Carlton County swarm cluster relatively consistently and include a
54
magnetic lineation that is subvertical to steeply inclined to the southwest with a
95% confidence angle of approximately 19° and a magnetic foliation in the plane
of the swarm mean dike orientation (Figs. 15A and 16).
In contrast, directional AMS results for the Duluth swarm are complex and
give poor statistics (Figs. 15B1 and 17); however, overall, the magnetic lineations
are subhorizontal to inclined. Plotted independently, the minority of the Duluth swarm dike samples that have normal and intermediate magnetic fabrics (K1
orientations approximately in the plane of the swarm mean dike orientation)
exhibit weak clustering (Fig. 15B2). This data group produces a mean K1 orientation that is oblique to the southwest with a 95% confidence angle of approximately 26° and is orientated in the plane of the swarm mean dike orientation as well as a magnetic foliation plane that approaches the swarm mean dike orientation. The reminder of the Duluth swarm samples that have apparently inverse magnetic fabrics (K1 orientations and K1-K2 planes orientated
at high angles, 60-90°, to the swarm mean dike orientation) also exhibit weak
clustering, producing a mean K1 with a 95% confidence angle of approximately
24°, and a magnetic foliation plane orientated nearly perpendicular to the plane
of the swarm mean dike orientation (Fig. 15B3). Figs. 15B2 and B3 would look
very similar if the maximum and minimum magnetic axes were reversed in Fig.
15B3, as could be the case if the inverse fabrics are the result of SD magnetic
behavior (and not produced by some other process; e.g., Ferré, 2002). 55
A B1 K2
K3
K1
B2 B3
K2
K2
K3 K1
K1 K3
Figure 15. Site-averaged AMS stereoplots. Low-field AMS results rotated into the plane of the swarm mean dike orientation (Carlton County: N28ºE, 89ºNW and Duluth: N18ºE, 76ºNW, plotted as black planes). Lower hemisphere, equal area projections. Mean vector for each principal direction located (open shapes) along with its’ associated cone of 95% confidence. Thin blue lines represent mean magnetic foliation planes. Thinner blue lines are mean magnetic foliation. (A) 18 Carlton County dike swarm samples. Subvertical swarm mean K1. 95% confidence angles: K1=18.8º, K2=19.9º, K3=11.6º. (B1) 14 Duluth dike swarm samples. Results are complex and give poor statistics. (B2) 4 Duluth swarm dikes where K1 falls approximately in the plane of the swarm mean dike orienta- tion. 95% confidence angles: K1=25.7º, K2=42.2º, K3=30.3º. (B3) Remaining 10 Duluth swarm dikes where K1 is oriented 50-90º to swarm mean dike plane. 95% confidence angles: K1=22.6º, K2=31.5º, K3=31.4º. 56
Major Fold Axes
34 2 Pleistocene Dikes glacial deposits
1 1 Thompson 89 66 14 1 1 Formation 0 85 83 0 1 1 30 32 42 70 16 8 68
79 84 40 76 87 17 4 14 15 11 10 2 5 64 N 12 52 79 81 5
1.5 km
Normal 70 Dike orientation, inclined Site-averaged K1 orientation 10 (magnetic lineation) Inverse Dike orientation, vertical Site-averaged K3 orientation 75 Intermediate (pole to magnetic foliation)
Figure 16. Carlton County dike swarm magnetic fabric results. Sample site location map with dike orientations and site-averaged maximum (K1) and mini- mum (K3) AMS ellipsoid orientations plotted for each dike. Sites with apparently normal, inverse, and intermediate magnetic fabrics are indicated (map after Wright et al., 1970; Kiburg & Morey, 1977). 57
6969 43 22
666666 70 14 76 7070 Duluth Complex 70 25 7272 55 16 72 intrusive rocks 37 47 23 47
42
22 6565 6161 North Shore 65 17 Undivided Volcanic Group 51 intrusive rocks lava flows
5 75 4 84 75
20 71
23 Interflow 50 8787 sand
1 1 72 73 72 7281 60 Lake Superior N 74 80 61 8585 74 74 85 6 2 1 23 1.5 km 34
Normal 70 Dike orientation, inclined Site-averaged K1 orientation 10 (magnetic lineation) Inverse Dike orientation, vertical Site-averaged K3 orientation 75 Intermediate (pole to magnetic foliation)
Figure 17. Duluth dike swarm magnetic fabric results. Sample site location map with dike orientations and site-averaged maximum (K1) and minimum (K3) AMS ellipsoid orientations plotted for each dike. Sites with apparently normal, inverse, and intermediate magnetic fabrics are indicated (map after Boerboom et al., 2002; Green & Miller, 2008).
58
Magma flow orientations can be determined from the directional AMS
data, but these data do not provide a measure of the flow sense (i.e., upward or
downward), as the comprehensive sampling method put forth by Tauxe et al.
(1998) was not employed. However, overall upward transfer of mass in the
subvertical dikes studied is assumed. This is a reasonable assumption as
lithostatic pressure increases with depth in the upper crust and is zero at the
earth’s surface, so flow of relatively hot, buoyant magma at depth is likely to be
controlled by this pressure gradient on the whole.
Interpretations
Overall, the AMS analysis suggests differing magmatic emplacement
directions in the two dike swarms. The Carlton County swarm is interpreted to
have been emplaced predominantly in a vertical direction, as its mean magnetic
lineations yield a flow pattern that is predominantly subvertical to steeply inclined to the southwest. The Duluth dike swarm is interpreted to have been emplaced
more obliquely, as it displays, in the mean magnetic lineations of sample sites
with normal to intermediate AMS fabrics, a flow pattern that is inclined towards
the southwest. The Carlton County dike swarm has normal AMS fabrics in all but
one sampled dike, so the mean magnetic lineation can be interpreted to
represent magma flow directions in the swarm. As just under one-third of the
Duluth swarm samples have interpretable normal AMS fabrics, this interpretation
59
is more tenuous. However, it may be possible to utilize the K1 axes from the
inverse magnetic fabrics as a pole to magnetic foliation and the K3 axes as the
magnetic lineation direction, as it is clear that the inverse K1 axes are in
symmetry with the normal K3 axes in the Duluth swarm (e.g., Aubourg et al.,
2008; Figs. 15B1 and B2). This interpretation is valid if the apparently inverse
AMS results are truly magnetically inverse. This is because inverse magnetic fabrics, just like normal magnetic fabrics, are a direct consequence of the physical preferred orientation of the magnetically susceptible grains in a rock
(Cañón-Tapia, 2004; Fig. 11). If such an interpretation of the inverse fabrics is valid, the AMS data in these inverse-fabric dikes yield the same oblique to the southwest flow pattern preserved in the normal-fabic Duluth swarm dikes (Figs.
15B1 and B2).
In general, magmatic flow fabric studies tend to interpret the magma flow
patterns described by the majority of normal sample sites (e.g., Palmer et al.,
2007). However, when a significant proportion of apparently anomalous AMS
results exist, the source of these fabrics can often be discerned with additional
datasets. Violation of the basic assumptions of laminar flow and rapid cooling
could produce non-normal magnetic fabrics via non-Newtonian, non-laminar,
turbulent flow, convection cells in the cooling magma, eventual magma source
depletion, and gravitational effects on the cooling crystal mush, especially in
wider conduits (Palmer et al., 2007). This study attempted to avoid these
60
complications by focusing sampling on relatively narrow dikes. In general, the
very fine grain size of these basaltic dike rocks also argues for rapid cooling.
Rock magnetic properties, specifically the presence of an SD component (Fig.
11) or interference between two incongruent subfabrics (e.g., mafic silicate and
magnetite phases), can also account for non-normal AMS fabrics. Different
proportions of SD magnetite contribution to the signal can produce intermediate
to inverse AMS fabrics (Ferré 2002; Lanza & Meloni, 2006; Palmer et al., 2007;
Ferré et al., 2008).
Dike rock that cooled rapidly enough to produce tiny SD magnetite grains
would still likely preserve normal fabric lineations in the silicates. In this case, the
silicate grains would preserve lineations at low angles to the dike margin (within
imbrication angles of about 5-30°), even in dikes with inverse AMS fabrics. In
essence, if the inverse fabrics are a result of the contribution of inverse magnetic
signals from SD magnetite grains, then the SPO fabrics of the silicate grains
would not be expected to display the same patterns. Determination of the actual
magnetite grain sizes in thin section to compare with the high-field hysteresis
grain size estimates would further help to test this hypothesis, however, SD
magnetite grains may also exist in a rock as tiny, < 1μm, inclusions in plagioclase
and pyroxene crystals (Lanza & Meloni, 2006).
There is some correlation between apparently inverse AMS fabrics and narrower dikes and between inverse AMS fabrics and samples collected from
61
nearer the dike margin (Fig. 18). Inverse magnetic fabric results in this study are observed only within dikes that are less than about 6.0 m wide and within less
than 1.5 m of the dike margin. This observation may be supported by the
existence of magnetic grain size control in the form of SD magnetite contribution in these samples. Theoretical modeling by Ferré et al. (2008) predicts normal
AMS fabrics in a dike’s center, symmetrically surrounded by intermediate AMS fabrics and then inverse AMS fabrics along the margins. SD behavior occurs in smaller magnetite grains, producing inverse magnetic fabrics, where K1 and K3 are reversed, increasing towards dike margins cooled conductively against cold country rock. MD behavior occurs in coarser magnetite grains, producing normal magnetic fabrics, increasing towards dike centers. The two dikes in this study with demonstrably intermediate fabrics (dike 5, Fig. 13 and dike 16, Fig. 14), could be explained via such mixing of MD and SD grains (as is indicated in the
PSD magnetic grain size of these samples, Fig. 12D). The consistency of the inverse fabrics in the Duluth swarm dikes and the agreement between the inverted inverse fabrics and the normal fabrics in that swarm seem to suggest that mixing of SD and MD magnetite contributions is the likely cause.
A measure of the imbrication angle between K1 and the dike plane correlates well with apparently normal and inverse AMS fabric results (Fig. 18B, blue line at 37°, and Table 7). All of the dikes with normal AMS fabrics in both swarms have imbrication angles less than 37°, except for one Carlton County 62
25
20
15 ) m (
h t d i W
e k i D 10
5
CC D A 0 Normal 0 2 4 6 8 10 Sample Distance from Dike Margin (m) Inverse
Intermediate 90
80
Duluth Duluth 70
s CC i x a
1 60 K
f o
e
n 50 a l p
e
k Inverse i 40 d
o t
Normal e l Carlton g 30
n County A
20 Duluth 10 B 0 0.E+00 5.E+04 1.E+05 2.E+05 Bulk Susceptibility Km (10^-6 SI) Figure 18. AMS fabrics and dike properties plots. Plots of apparently normal, inverse, and intermediate AMS fabric results as a function of (A) sample position and dike width and (B) imbrication angle and bulk magnetic sus- ceptibility (after Cadman et al., 1992) for both the Carlton County (CC) and Duluth (D) dike swarms. Inverse fabrics are found only within the area outlined by the blue box in plot A and only above the blue line in plot B.
63
swarm dike (dike 12, Fig. 13). Even though this dike displays an imbrication
angle of 60°, K2 lies only 2° away from the dike plane orientation, and therefore
the fabric is not intermediate. This anomalous sample may be the result of
turbulent flow in a wide dike; this dike is at least 17 m wide (in the field both
margins were buried); or it may be an artifact of less robust AMS data from a
relatively more altered hand sample. The AMS data from this dike produces the most scattered AMS principal axes clusters in the swarm that are not attributable to oblate fabric effects. Bulk magnetic susceptibility does not correlate with imbrication angle in these dikes, except to say that three Duluth swarm dikes,
two with inverse and one with intermediate fabric results, have bulk magnetic
susceptibilities an order of magnitude higher than the rest of the analyzed dikes
in both swarms (Fig. 18B).
Magmatic Flow Pattern Interpretation
The AMS measurements obtained in this study of dike swarm flow can be
interpreted as intrusion patterns that were influenced by an evolving tectonic
environment. The interpreted emplacement directions suggest that a single
locus of melt generation in the crust may have fed both swarms (Fig. 19). Earlier
in MRS formation (during time interval 2, Fig. 5), an approximately N60°W-S60°E oriented extensional stress regime exerted a tectonic force on the upper crust causing regional rifting of the old, cold, pre-rift upper continental crust. In this
64
context, the Carlton County dikes intruded into Paleoproterozoic metasediments, which fractured to accommodate a uniformly striking (about N30°E) set of subvertical dikes that record a relatively uniform subvertical to steeply inclined southwest to northeast flow emplacement pattern (the magnetic lineation axes for the whole swarm cluster consistently). Thus, the indicated magma source was
located at depth below and slightly to the south-southwest of the Carlton County
swarm.
As the MRS evolved (during time interval 3, Fig. 5), an approximately
N72°W-S72°E oriented extensional stress regime exerted a continued and
similarly oriented (within about 12°) tectonic force on the upper crust, which by
that time consisted of both pre-rift and young rift-related rocks in the region of the
study area. In this context of continued extension, the Duluth dikes exploited the
weaker rift rocks and intruded into the young, warm, weaker, and structurally
complex NSVG lava flows, interflow sands, and Duluth Complex intrusive rocks.
This structural and lithologic complexity led to more variability in dike orientation
(mean strike of N18°E with dips ranging from about 70°E to 70°W). The Duluth
dikes record an oblique southwest to northeast flow emplacement pattern
potentially emanating from the same magma chamber that earlier fed the Carlton
County swarm.
The Carlton County and Duluth dike swarms overlap in both time and
space. The fact that the Carlton County swarm continued to intrude after the 65
A B
NSVG lava * flow dike * dike inter- * flow dike NSVG sand- lava stone flow
Carlton County dike swarm Duluth Dike swarm On-axis Rift Rocks Off-axis Paleoproterozoic Metasedimentary Rocks (* )
Melt Source Melt Source
Figure 19. Tectonic interpretation model of an evolving rift environment. AMS directions (arrows and dashed lines) indicate that both swarms may have been fed from the same magma source at depth. Large arrows mark the extension directions recorded by each swarm. (A) Carlton County dikes intruded earlier, during a period of reverse magnetic polarity (Time 2, Fig. 2), into old, cold, Paleo- proterozoic metasedimentary rocks. (B) Duluth dikes intruded later, during a period of normal magnetic polarity (Time 3, Fig. 2), into warm, young, structurally complex, rift-related NSVG lava flows, interflow sands, and Duluth Complex intrusive rocks. Some Carlton County dikes were also emplaced during Time 3.
66
magnetic polarity reversal at 1097±1 Ma provides evidence that the melt source
for the Carlton County swarm was likely still actively supplying magma to the upper crust during time interval 3. The overall regional proximity of the two dike swarms, which both outcrop within a < 690 km2 area and actually overlap in the
center of this outcrop area, supports the viability of a single melt source for both
swarms.
Structurally, the MRS dike swarms mirror the general trend of the rift axis,
which in the region is about N30°E. The Carlton County dike swarm parallels the
trend of the adjacent rift axis and is at a high angle to older basement structures.
The more irregular dike orientations in the Duluth swarm may represent influence
from the complex local crustal structure of the multiply intruded Duluth Complex
and thick NSVG lava flows (Green et al., 1987; Reichhoff, 1987).
In the proposed tectonic emplacement model, the two dike swarms
preserve distinct flow patterns because, during successive time intervals in the
course of MRS development, a consistent extensional tectonic regime interacted
with different upper crustal structural environments located both directly above
and in the lateral vicinity of an enduring magma chamber. The existence of a
persistent regional magma source supports a combination of the rift models
presented in Fig. 1B. A long-lived plutonic complex, from a localized zone of melting in the lithosphere below, vertically fed dikes located above the melt zone, represented by the Carlton County dike swarm. And the same melt zone
67
obliquely fed dikes some distance away from the melt zone, represented by the
Duluth dike swarm.
Dike swarm emplacement from a long-lived paleo-melt zone has also
been interpreted in a recent AMS study of an Early Cretaceous dike swarm that
recorded initial rifting of the Gondwanan continent and opening of the equatorial
Atlantic Ocean with successive melt pulses over a period of about 20 million years. This more recent analog finds that a 120-km-long swarm of subvertical dikes in northeast Brazil was fed in a fan-like, anastomosing pattern from a single magma source located at depth below the center of the swarm. Like the Carlton
County and Duluth dike swarms, this dike swarm is rift-parallel (i.e., parallel to the modern continental margin) and not parallel to the regional structure of the
Precambrian basement. Interestingly, Bouguer gravity data locate an 80-km-long gravimetric low in the study area (within around 20 km of the dike swarm), which may represent the remains of a still-warm paleo-melt zone rooted in the mantle.
AMS fabrics in an adjacent swarm of similar provenance point to the gravity low and thus support the hypothesis that this anomaly could represent the ancient magmatic source zone (Archanjo et al., 2000).
Conclusions
Vertical to oblique dike flow patterns predominate in the Carlton County
and Duluth dike swarms. Horizontal dike emplacement was not an important
68
method of crustal construction in this region of the MRS. This study provides
evidence of vertical to oblique migration of rift melts from a long-lived regional
magma source at depth and finds no evidence of significant lateral, along-axis
magma propagation such as may occur in sheeted dike complexes in Mid Ocean
Ridge settings. In the process of continental rifting, tectonic, hot-spot-driven
extensional stresses necessarily interact with the extant country rock, which thus influences the mode of regional-scale igneous intrusion. As a rift evolves and the zone of separation widens, the structure and strength of the rift rocks represent a
zone of weakness in the upper continental crust and thus are likely to be rifted
themselves with continued extension.
As the MRS evolved, the upper crustal structural environment in the study
area developed from an “off-axis” setting, wherein the dikes intruded into ancient
country rock, to an “on-axis” setting, wherein the dikes intruded into the MRS
volcanic and intrusive sequence. This study demonstrates subvertical regional
magma flow in off-axis dikes (the Carlton County dike swarm) in response to
MRS extension breaking apart the old, cold continental crust. The data for the
on-axis Duluth dike swarm are more complex and yield a more oblique regional
flow pattern. This complexity in the Duluth swarm data may be due to
intermediate and inverse magnetic fabrics produced by the influence of SD
grains or may also be attributable to the influence of more complex geology
during emplacement within the rift environment (Lanza & Meloni, 2006). The
69
total stress state during dike emplacement may have been different on-axis,
where, in addition to the regional tectonic stress exerted by MRS extension,
stresses may have also resulted due to the heat of eruption, contraction with
cooling of the pile of young igneous rocks, and compaction under the lithostatic
load of the overlying magma.
The vertical to oblique emplacement directions and the location of these
two subvertical dike swarms above the MRS hot spot plume head is potentially
significant. Both dike swarms are located at a distance of approximately 300 km
from the triple junction, within a predicted 500-km-radius MRS starting plume head area (Jim Miller, personal communication; Campbell & Griffiths, 1990;
Griffiths & Campbell, 1990; dashed circle, Fig. 2). The 500-km-radius plume
area is consistent with dike emplacement patterns recorded by the Mackenzie
giant radiating dike swarm, where vertical dike emplacement may have occurred
above an approximately 500-km-radius mantle plume head, past which distance
dike emplacement involved lateral magma flow (Ernst & Baragar, 1992; Fig. 2E).
The Mackenzie giant radiating dike swarm is an enormous feature but the
continental lithosphere in this case did not rupture to form a continental rift. A hot
mantle plume may begin as a mushroom-shaped feature with a dome-shaped
head that applies pressure to the lithosphere above (Campbell & Griffiths, 1990;
Griffiths & Campbell, 1990). If the continental lithosphere gives way to the
pressure from the hot mantle below and continental rifting begins, then there is a
70
shift in the pressure environment of the plume. The mantle plume head rises highest where the pressure is lowest and therefore becomes focused below the rift axis and spreads laterally below the lithosphere, as depicted in Fig. 1A. This continental rifting model predicts vertical to subvertical flow in dike swarms above the plume head area, consistent with the AMS results for the Carlton County dike swarm, and horizontal to subhorizontal flow in sills, consistent with the AMS results in the MRS Logan sills (Middleton et al., 2004; Fig. 2G). The subhorizontal Logan sills intruded near the MRS triple junction and preserve uniform flow directions away from the rift axis, suggesting a melt source located along the rift axis. Additionally, the current study of MRS dikes finds local variations in stress state may cause a dike swarm emplaced on-axis to deviate from the predicted vertical emplacement model above the mantle plume head.
Future studies of magmatic flow patterns recorded by the other MRS dike swarms in the Lake Superior region could help to test this hypothesis that magma flow in dikes is largely subvertical within about 500 km of the MRS triple junction. Were the Logan, Baraga-Marquette, Grand Portage, Pukaskwa, and
Pigeon River dike swarms, which all also lie above the projected plume head area, also emplaced predominantly vertically? Additional dike swarm emplacement studies could also give further insight into temporal changes during
MRS development, as the Logan and Baraga-Marquette dike swarms were emplaced early in the rifting; the Grand Portage and Pukaskwa dike swarms
71
were emplaced, along with the Carlton County swarm, prior to the 1097±1 magnetic polarity reversal, and the Pigeon River dike swarm is coeval with
Duluth swarm emplacement. Also, these dike swarms are located around the triple junction, both on and off of the rift axis, and further research could provide more spatial information as to the nature of on-axis versus off-axis dike swarm emplacement.
References Cited
Archanjo, C.J., R.I. Trindale, J.W.P. Macedo, & M.G. Araújo, 2000, Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil, Journal of South American Earth Sciences, v. 13, p. 179-189.
Archanjo, C.J. & M.G.S. Araújo, 2002, Fabric of the Rio Ceará—Mirim mafic dike swarm (northeastern Brazil) determined by anisotropy of magnetic susceptibility and image analyis, Journal of Geophysical Research, v. 107, n. B3, 14 p.
Behrendt, J.C., A.G. Green, W.F. Cannon, D.R. Hutchinson, M.W. Lee, B. Milkereit, W.F. Agena, & C. Spencer, 1988, Crustal structure of the Midcontinent rift system: Results from GLIMPSE deep seismic reflection profiles, Geology, v. 16, p. 81-85.
Boerboom, T.J., J.C. Green, & M.A. Jirsa, 2002, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota, M-128, Minnesota Geological Survey, 1:24,000 scale.
Bates, M.P. & M.F. Mushayandebvu, 1995, Magnetic fabric in the Umvimeela dyke, satellite of the Great Dyke, Zimbabwe, Tectonophysics, v.242, p. 241-254.
Borradaile, G.J. & B. Henry, 1997, Tectonic applications of magnetic susceptibility and its anisotropy, Earth-Science Reviews, v. 42, p.49-93.
Buck, R.W., P. Einarsson, & B. Brandsdóttir, 2006, Tectonic stress and magma chamber size as controls on dike propagation: Constraints from the 1975-1984 Krafla rifting episode, Journal of Geophysical Research, v. 111, B12404.
Cadman, A.C., R.G. Park, J. Tarney, & H.C. Halls, 1992, Significance of anisotropy of magnetic susceptibility fabrics in Proterozoic mafic dykes, Hopedale Block, Labrador, Tectonophysics, v. 207, p. 303-314.
Callot, J.-P., L. Geffroy, C. Aubourg, J.P. Pozzi, and D. Mege, 2001, Magma flow directions of shallow dykes from the East Greenland volcanic margin inferred from magnetic fabric studies, Tectonophysics, v. 335, p. 313-329.
Callot, J.-P. & X. Guichet, 2003, Rock texture and magnetic lineation in dykes: a simple analytical model, Tectonophysics, v. 366, p. 207-222.
Campbell, I.H. & R.W. Griffiths, 1990, Implications of mantle plume structure for the evolution of flood basalts, Earth and Planetary Science Letters, v. 99, p. 79-93.
Cannon, W.F., M.W. Lee, W.J. Heinz, K.J. Shultz, & A.G. Green, 1991, Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America, Geology, v. 19, p. 207-210.
72
73
Cannon, W.F., 1994, Closing of the Midcontinent Rift—a far-field effect of Grenvillian compression, Geology, vol. 22, n. 2, p.155-158.
Cannon, W.F. & S.W. Nicholson, 1996, Middle Proterozoic Midcontinent Rift System, Archean and Proterozoic Geology, Lake Superior Region, 1993, p. 60-67.
Cañón-Tapia, E., 2004, Anisotropy of magnetic susceptibility of lava flows and dykes: A historical account, from: Martín-Hernández, F., Lüneberg, C.M., Aubourg, C., & Jackson, M. (eds.) Magnetic Fabric: Methods and Applications, Geological Society, London, Special Publications, 238, 205-225.
Clark, R.C., 1985, The structural geology of the Thompson Formation: Cloquet and Esko quadrangles, East-Central Minnesota, Unpublished M.S. Thesis, University of Minnesota Duluth.
Craddock, J.P., J. Anziano, K. Wirth, J.D. Vervoort, B. Singer, & X. Zhang, 2007, Structure, geochemistry and geochronology of a Penokean lamprophyre dike swarm, Archean Wawa Terrane, Little Presque Isle, Michigan, USA, Precambrian Research, v. 157, p. 50- 70.
Craddock, J.P., B.C. Kennedy, A.L. Cook, M.S. Pawlisch, S.T. Johnston, & M. Jackson, 2008, Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora-Kabetogama composite dykes (Minnesota and Ontario), Tectonophysics, v. 448, p. 115-124.
Day, R., M. Fuller, & V.A. Schmidt, 1977, Hysteresis properties of titanomagnetites: Grain size and compositional dependence, Physics of the Earth and Planetary Interiors, v. 13, p. 260-267.
Delaney, P.T. & D.D. Pollard, 1982, Solidification of basaltic magma during flow in a dike, American Journal of Science, v. 282, p. 856-885.
Dott, R.H., Jr., & D.R. Prothero, 1994, Evolution of the Earth, fifth edition, McGraw-Hill.
Dunlop, D.J. & Ö. Özdemir, 1997, Rock Magnetism: Fundamentals and frontiers, Cambridge University Press.
Dunlop, D.J., 2002, Theory and application of the Day plot (Mrs/Mr versus Hcr/Hc); 1. Theoretical curves and tests using titanomagnetite data. Journal of Geophysical Research, vol. 107, n. B3, 22p.
Ernst, R.E. & W.R.A. Baragar, 1992, Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm, Nature, v. 356, p. 511-513.
Ernst, R.E., 1994, Mapping the magma flow pattern in the Sudbury dyke swarm in Ontario using magnetic fabric analysis, Current Research 1994-E, Geological Society of Canada, p. 183-192.
Ferré, E.C., 2002, Theoretical models of intermediate and inverse AMS fabrics, Geophysical Research Letters, v. 29, n. 7, 4 p.
74
Ferré, E.C., F. Martín-Hernández, C. Teyssier, & M. Jackson, 2004, Paramagnetic and ferromagnetic anisotropy of magnetic susceptibility in migmatites: Measurements in high and low fields and kinematic implications, Geophysics Journal International, v. 157, p. 1119-1129.
Ferré, E.C., C.K. Ranaweera, M. Marsh, S.M. Maes, & J. Geissman, 2008, Magma flow sense in mafic dikes: is grain-size dependence an alternative to the “imbrication fabric” model?, abstract, American Geophysical Union, December 2008.
Geoffroy, L., J.P. Callot, C. Aubourg, & M. Moreira, 2002, Magnetic and plagioclase linear fabric discrepancy in dykes: a new way to define the flow vector using magnetic foliation, Terra Nova, v. 14, p. 183-190.
Green, J.C., T.J. Bornhorst, V.W. Chandler, M.G. Mudrey Jr., P.E. Myers, L.J. Pesonen, & J.T. Wilband, 1987, Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Mesoproterozoic midcontinent rift of North America, Mafic Dyke Swarms, Halls, H.C. and W.F. Fahrig (eds.), Geological Association of Canada Special Paper 34, p. 289- 302.
Green, J.C. & J.D. Miller Jr., 2008, Bedrock geologic map of the Duluth Quadrangle (preliminary version 2/15/08), www.d.umn.edu/~mille066/Publications/Duluth%20Q.pdf.
Griffiths, R.W. & I.H. Campbell, 1990, Stirring and structure in mantle starting plumes, Earth and Planetary Science Letters, v. 99, p. 66-78.
Herrero-Bervera, E., G.P.L. Walker, E. Cañón-Tapia, & M.O. Garcia, 2001, Magnetic fabric and inferred flow direction of dikes, conesheets and sill swarms, Isle of Skye, Scotland, Journal of Volcanology and Geothermal Research, v. 106, p. 195-210.
Holm, D.K., R. Anderson, T.J. Boerboom, W.F. Cannon, V. Chandler, M. Jirsa, J. Miller, D.A. Schneider, K.J. Schulz, & W.R. Van Schmus, 2007, Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation, Precambrian Research, v. 157, p. 71–79.
Hutchinson, D.R., R.S. White, W.F. Cannon, & K.J. Schultz, 1990, Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System, Journal of Geophysical Research, v. 95, n. B7, p. 10,869-10,884.
Jelinek, V., 1981, Characterization of the magnetic fabric of rocks, Tectonophysics, v. 79, p. T63- T67.
Karlstrom, K.E., K.-I. Åhäll, S.S. Harlan, M.L. Williams, J. McLelland, & J.W. Geissman, 2001, Long-lived (1.8-1.0 Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia, Precambrian Research, v. 111, p. 5-30.
Kiburg, J.A. & G.B. Morey, 1977, Reconnaissance geologic map of Esko quadrangle, St.Louis and Carlton Counties, Minnesota, M-25, Minnesota Geological Survey, 1:24,000.
75
Knight, M.D. & G.P.L. Walker, 1988, Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies, Journal of Geophysical Research, v. 93, p. 4301-4319.
Lanza, R. & A. Meloni, 2006, The Earth’s Magnetism: An Introduction for Geologists, Springer.
Maes, S.M., B. Tikoff, E.C. Ferré, P.E. Brown, & J.D. Miller Jr., 2007, The Sonju Lake Layered Intrusion, northeast Minnesota: internal structure and emplacement history inferred from magnetic fabrics, Precambrian Research, v. 157, p. 269-288.
Marshall, L.P & E.G. Lidiak, 1996, Geochemistry and paleomagnetism of Keweenawan basalt in the subsurface of Nebraska, Precambrian Research, v. 76, n. 1-2, p. 44-65.
Martín-Hernández, F., C.M. Lüneburg, C. Aubourg, & M. Jackson (eds.), 2004, Magnetic Fabric: Methods and Applications, Geological Society, London, Special Publications, 238.
Middleton, R.S., G.J. Borradaile, D. Baker, & K. Lucas, 2004, Proterozoic diabase sills of northern Ontario: Magnetic properties and history, Journal of Geophysical Research, v. 109, 12 p.
Miller, J.D., Jr. & V.W. Chandler, 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Middle Proterozoic to Cambrian Rifting, R.W. Ojakangas, A.B. Dikas, and J.C. Green, eds., Central North America: Boulder, Colorado, Geological Society of America Special Paper 312, p. 73-96.
Nicholson, S.W., & S.B. Shirey, 1990, Midcontinent Rift volcanism in the Lake Superior region: Sr, Nd, and Pb Isotopic evidence for a mantle plume origin, Journal of Geophysical Research, v. 95, n. B7, p. 10,851-10,868.
Nicholson, S.W., W.F. Cannon, & K.J. Schultz, 1992, Metallogeny of the Midcontinent rift system of North America, Precambrian Research, v. 58, p. 355-386.
Nicholson, S.W., S.B. Shirey, K.J. Schultz, & J.C. Green, 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development, Canadian Journal of Earth Science, v. 34, p. 504-520.
Ojakangas, R.W., G.B. Morey, & J.C. Green, 2001, The Mesoproterozoic Midcontinent Rift System, Lake Superior region, USA, Sedimentary Geology, v. 141-142, p. 421-442.
Palmer, H.C., R.E. Ernst, & K.L. Buchan, 2007, Magnetic fabric studies of the Nipissing sill province and Senneterre dykes, Canadian Shield, and implications for emplacement, Canadian Journal of Earth Science, v. 44, p. 507-528.
Philpotts, A.R. & D.E. Philpotts, 2007, Upward and downward flow in a camptonite dike as recorded by deformed vesicles and the anisotropy of magnetic susceptibility (AMS), Journal of Volcanology and Geothermal Research, v. 161, p. 81-94.
Pokorný, J., P. Suza, & F. Hrouda, 2004, Anisotropy of magnetic susceptibility of rocks measured in weak magnetic fields using the KLY-4S Kappabridge, in Magnetic Fabric: Methods and Applications, Martín-Hernández, Lüneberg, Aubourg, and M. Jackson, eds., Geological Society of London Special Publication 238, 69-76.
76
Poland, M.P., J.H. Fink, & L. Tauxe, 2004, Patterns of magma flow in segregated silicic dikes at Summer Coon volcano, Colorado: AMS and thin section analysis, Earth and Planetary Science Letters, v. 219, p. 155-169.
Raposo, M.I.B. & M.S. D’Agrella-Filho, 2000, Magnetic fabrics of dike swarms from SE Bahia State, Brazil: their significance and implications for Mesoproterozoic basic magmatism in the São Francisco Craton, Precambrian Research, v. 99, p. 309-325.
Reichhoff, J.A., 1987, Two Keweenawan basaltic dike swarms in the Duluth area, Minnesota, Unpublished M.S. Thesis, University of Minnesota Duluth.
Rochette, P., M. Jackson, & C. Aubourg, 1992, Rock magnetism & the interpretation of anisotropy of magnetic susceptibility, Reviews of Geophysics, v. 30, no. 3, p. 209-226.
Röller, K. & C. Trepmann, 2007, Stereo32 v. 0.9.4, unregistered educational software available for download from: www.ruhr-uni-bochum.de/hardrock/downloads.htm.
Ryan, M.P., 1994, Neutral-buoyancy controlled magma transport and storage in mid-ocean ridge magma resevoirs and their sheeted-dike complex: A summary of basic relationships, in Magmatic Systems, Michael P. Ryan ed., p. 97-138.
Seifert, K.E. & J.F. Olmsted, 2004, Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example – the 47th Avenue sill, Canadian Journal of Earth Science, 41, 829-842.
Soofi, M.A. & S.D. King, 2002, Post-rift deformation of the Midcontinent rift under Grenville tectonism, Tectonophysics, v. 359, p. 209-223.
Staudigel, H., J. Gee, L. Tauxe, & R.J. Varga, 1992, Shallow intrusive directions of sheeted dikes in the Troodos opholite: Anisotropy of magnetic susceptibility and structural data, Geology, v. 20, p. 841-844.
Tauxe, L., J.S. Gee, & H. Staudigel, 1998, Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way, Journal of Geophysical Research, v. 103, n. B8, p. 17,775-17,790.
Thomas, M.D. & D.J. Tesky, 1994, An interpretation of gravity anomalies over the Midcontinent Rift, Lake Superior, constrained by GLIMPSE seismic and aeromagnetic data, Canadian Journal of Earth Science, v. 31, p. 682-697. van der Pluijm, B.A. & S. Marshak, 2004, Earth Structure: An Introduction to Structural Geology and Tectonics, 2nd edition, W.W. Norton & Company, Inc.
Varga, R.J., J.S. Gee, H. Staudigel, & L. Tauxe, 1998, Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Opholite, Cyprus, Journal of Geophysical Research, v. 103, no. B3, p. 5241-5256.
Vervoort, J.D., K. Wirth, B. Kennedy, T. Sandland, & K.S. Harpp, 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism, Precambrian Research, v. 157, p. 235-268.
77
Wright, Herbert E., Jr., L.A. Mattson, & J.A. Thomas, 1970, Geology of the Cloquet quadrangle, Carlton County, Minnesota, Plate 1, GM-3, Minnesota Geological Survey, 1:24,000.
Zeyen, H., F. Volker, V. Wehrle, K. Fuchs, S.V. Sobolev, & R. Altherr, 1997, Styles of continental rifting: Crust-mantle detachment and mantle plumes, Tectonophysiscs, v. 278, p. 329- 352. Appendix
78 79
Dike UTM (NAD27) 1 15 545588E 5167928N 2 15 544717E 5166855N 3 15 544851E 5166614N 4 15 543238E 5167808N Swarm 5 15 544437E 5166662N
Dike 6 15 544991E 5168032N
7 15 545069E 5168037N 8 15 545652E 5167697N County 9 15 548641E 5166725N 10 15 548112E 5166543N arlton 15 543757E 5169668N C 11 12 15 545750E 5167973N 13 15 545599E 5166634N 14 15 570285E 5182853N 15 15 570229E 5182844N 16 15 569924E 5182662N 17 15 569980E 5183162N 18 15 577213E 5188510N Swarm 19 15 577513E 5188834N 20 15 577025E 5188287N Dike 21 15 575350E 5188567N 22 15 574733E 5186699N
Duluth 23 15 574782E 5186709N 24 15 573453E 5185543N 25 15 571804E 5185247N 26 15 572175E 5184520N
Table 2. Sampled dike locations 80
Previous Work+ Dike Measurements 2 oriented hand samples collected/dike Width Sam- Distance from Sam- Distance from Dike Polarity Equiv. Orientation (m) ple^ Margin (m) ple^ Margin (m) 1 reverse 70 7.0 N20°E, 85°NW1 m 2.1 from E c 2.5 from W 2 reverse 18# 15.3 N40°E, 87°NW m 2.4 from W c 5.1 from W 3 normal 16 3.1 N12°E, vertical m 0.3 from E c 0.4 from W 4 reverse 1$ 1.1 N33°E, 83°NW m 0.14 from W c 0.2 from E Swarm 5 reverse 14 7.0 N12°E, vertical m 1.6 from W c 3.0 from E
quadrangles) $
Dike 6 reverse 8 1.6 N10°E, vertical m 0.45 from E c 0.75 from E
7 reverse 9# 6.1 N36°E, vertical m 0.4 from E c 2.0 from E Esko 8 reverse 5$ 0.8 N20°E, vertical m 0.2 from W c 0.2 from E County
and
9 normal 67 24.0 N40°E, vertical m 2.4 from W c 9.3 from W 10 reverse 66 9.8 N22°E, vertical m 1.2 from E c 1.7 from E
Carlton 11* reverse 12 6.7-13? N30°E, vertical? m 4.7 from SE c 11.9 from E (Cloquet 12* reverse 3 17.0 N52°E, vertical ------c 1.5 from E # 13 reverse 68 4.0 N40°E, vertical m 0.5 from W c 1.5 from W 14* normal 33 5.0 N10°E, 74°NW m 1.0 from E c 1.5 from E 15 normal 36? 3.0 N35°E, 80°NW m 1.0 from E c 1.3 from E 16 normal 37$ 2.6 N40°W, vertical2 m 0.7 from W c 2.75 from W 17 normal 64 3.0 N30°E, 72°SE m 0.78 from W c 1.1 from W $
quadrangles) 18 normal 30 2.4 N40°E, 66°NW m 0.58 from NW c 0.7 from E
# Swarm 19 normal 31 > 4.0 N30°E, 69°NW m 0.6 from NW c 0.87 from W 20 normal 29# > 5.0 N21°E, 72°NW m 0.5 from NW c 1.5 from W Dike 21 normal 52# 7.0 N10°W, 70°SW m 1.6 from W c 2.8 from W Lakewood 22 normal 48 4.0 N5°E, 65°NW m 0.75 from SE c 0.9 from W 3 Duluth and
23 normal 49 1.5 N10°E, 61°NW m 0.4 from NW c 0.65 from W
h $ t 24 normal 46 3.9 N13°E, 75°NW m 0.75 from SE c 1.0 from E 25 normal 57# 3.0 N75°E, vertical m 1.0 from S c 1.0 from N
(Dulu 4 26 normal 47 5.7 N9°E, 87°NW m 0.6 from W c 1.1 from W * Margin buried, dike width and sample distances from margin approximate + from Reichhoff (1987) $ equivalent sample used in Reichhoff (1987) flow analysis study # equivalent sample discarded by Reichhoff (1987) due to lack of visible orientation ^ relative designations: m = sample nearer dike margin; c = sample nearer dike center 1 Variable Strike Observed: N20°E to N30°E 2 Variable Strike Observed: N40°W to NS 3 Variable Strike Observed: N10°E to NS 4 Variable Strike Possible
Table 3. Field data 81
Bulk Susceptibility Principle Susceptibilities + ^ Sample* n Km ±S.D. K1 ±S.D. K2 ±S.D. K3 ±S.D. ef001m 11 8.49E-03 6.79E-03 5.70E-03 4.68E-03 5.66E-03 4.64E-03 5.40E-03 4.38E-03 ef002c 12 1.36E-02 2.20E-03 8.46E-03 1.39E-03 8.23E-03 1.33E-03 8.03E-03 1.27E-03 ef002m 12 1.55E-02 3.36E-03 1.07E-02 2.28E-03 1.05E-02 2.24E-03 1.03E-02 2.18E-03 ef003c 9 1.25E-02 7.76E-04 8.07E-03 7.22E-04 7.94E-03 6.89E-04 7.81E-03 6.73E-04 ef003m 12 1.29E-02 1.84E-03 7.51E-03 1.48E-03 7.41E-03 1.44E-03 7.32E-03 1.44E-03 ef004m 15 1.96E-02 3.56E-03 1.19E-02 2.97E-03 1.17E-02 2.93E-03 1.14E-02 2.84E-03 Swarm ef005m 19 1.40E-02 1.93E-03 8.34E-03 1.59E-03 8.26E-03 1.57E-03 7.91E-03 1.50E-03 ef006c 24 1.16E-02 2.11E-03 8.32E-03 1.47E-03 8.23E-03 1.46E-03 7.98E-03 1.40E-03 Dike ef006m 25 1.67E-02 9.93E-04 1.20E-02 7.68E-04 1.18E-02 7.44E-04 1.13E-02 7.18E-04 ef007c 15 1.49E-02 1.40E-03 9.41E-03 1.22E-03 9.33E-03 1.20E-03 8.86E-03 1.14E-03
County ef007m 19 1.34E-02 2.29E-03 7.74E-03 1.85E-03 7.70E-03 1.85E-03 7.40E-03 1.75E-03
ef008m 24 1.21E-03 6.38E-05 8.63E-04 7.04E-05 8.62E-04 7.04E-05 8.61E-04 7.03E-05 ef009c 18 1.91E-02 1.91E-03 1.23E-02 1.65E-03 1.21E-02 1.57E-03 1.19E-02 1.52E-03
Carlton ef009m 13 1.67E-02 3.30E-03 9.60E-03 2.29E-03 9.52E-03 2.26E-03 9.39E-03 2.22E-03 ef010m 14 5.13E-03 1.62E-03 2.96E-03 9.02E-04 2.93E-03 8.91E-04 2.88E-03 8.64E-04 ef011c 21 2.21E-02 2.15E-03 1.58E-02 1.56E-03 1.56E-02 1.55E-03 1.50E-02 1.55E-03 ef012c 21 1.40E-02 1.36E-03 9.60E-03 1.27E-03 9.57E-03 1.27E-03 9.49E-03 1.25E-03
ef013m 17 1.69E-02 5.50E-03 8.88E-03 3.04E-03 8.79E-03 2.99E-03 7.84E-03 2.60E-03 ef014m 14 1.67E-01 1.07E-02 1.05E-01 1.30E-02 1.05E-01 1.29E-02 1.03E-01 1.27E-02 ef015m 12 1.83E-02 5.81E-03 9.82E-03 2.81E-03 9.75E-03 2.80E-03 9.67E-03 2.78E-03 ef016m 20 1.01E-01 1.39E-02 7.05E-02 1.07E-02 6.96E-02 1.05E-02 6.89E-02 1.03E-02 ef017m 15 1.46E-01 7.28E-03 9.87E-02 9.47E-03 9.79E-02 9.57E-03 9.37E-02 8.86E-03 m ef018m 12 3.30E-02 3.32E-03 1.87E-02 3.10E-03 1.85E-02 3.07E-03 1.83E-02 3.01E-03 ef019m 14 2.40E-02 4.27E-03 1.71E-02 3.32E-03 1.70E-02 3.31E-03 1.69E-02 3.31E-03 Swar
e ef020c 20 1.79E-02 5.62E-04 1.10E-02 5.74E-04 1.10E-02 5.53E-04 1.09E-02 5.54E-04 k ef020m 11 1.98E-02 5.90E-04 1.29E-02 6.57E-04 1.27E-02 6.38E-04 1.25E-02 6.22E-04 Di
h ef021m 11 5.20E-02 5.66E-03 3.70E-02 5.60E-03 3.64E-02 5.48E-03 3.57E-02 5.43E-03 t ef022m 11 2.46E-02 7.57E-04 1.68E-02 8.27E-04 1.65E-02 7.61E-04 1.65E-02 7.95E-04
Dulu ef023m 32 8.05E-04 2.56E-05 4.41E-04 6.64E-05 4.37E-04 6.59E-05 4.35E-04 6.55E-05 ef024c 25 3.28E-02 4.87E-03 2.01E-02 3.14E-03 1.98E-02 3.10E-03 1.95E-02 3.01E-03 ef025c 13 4.69E-02 3.10E-03 2.81E-02 3.09E-03 2.79E-02 3.12E-03 2.77E-02 3.07E-03
ef026m 19 2.46E-02 2.78E-03 1.71E-02 2.41E-03 1.69E-02 2.38E-03 1.67E-02 2.37E-03 * Total = 32 samples (CC = 18 ; Duluth = 14) + n = number of cubes analyzed per sample Total = 530 cubes Mean = 17 cubes/sample Range = 9 to 32 cubes/sample ^ measurements corrected for actual volume of each cube
Table 4. Site-averaged AMS tensor parameters 82
Sample n L F P Pj Tj U ef001m 11 1.01 1.04 1.04 1.05 6.89E-01 6.85E-01 ef002c 12 1.03 1.03 1.05 1.05 -3.25E-02 -4.54E-02 ef002m 12 1.01 1.03 1.04 1.04 3.18E-01 3.10E-01 ef003c 9 1.02 1.02 1.03 1.03 3.49E-02 2.68E-02 ef003m 12 1.01 1.01 1.03 1.03 -2.20E-02 -2.74E-02 ef004m 15 1.01 1.03 1.04 1.04 3.18E-01 3.09E-01 Swarm ef005m 19 1.01 1.04 1.05 1.06 6.15E-01 6.07E-01 ef006c 24 1.01 1.03 1.04 1.04 4.61E-01 4.54E-01 Dike ef006m 25 1.02 1.04 1.06 1.06 3.76E-01 3.64E-01 ef007c 15 1.01 1.05 1.06 1.07 7.41E-01 7.34E-01
County ef007m 19 1.00 1.04 1.04 1.05 7.84E-01 7.80E-01
ef008m 24 1.00 1.00 1.00 1.00 -2.58E-01 -2.59E-01 ef009c 18 1.02 1.01 1.03 1.03 -1.51E-01 -1.57E-01
Carlton ef009m 13 1.01 1.01 1.02 1.02 3.62E-01 3.57E-01 ef010m 14 1.01 1.02 1.03 1.03 3.24E-01 3.18E-01 ef011c 21 1.01 1.04 1.06 1.06 4.84E-01 4.74E-01 ef012c 21 1.00 1.01 1.01 1.01 3.27E-01 3.25E-01
ef013m 17 1.01 1.12 1.13 1.14 8.39E-01 8.30E-01 ef014m 14 1.01 1.01 1.02 1.02 3.34E-01 3.30E-01 ef015m 12 1.01 1.01 1.02 1.02 1.00E-02 6.42E-03 ef016m 20 1.01 1.01 1.02 1.02 -1.10E-01 -1.15E-01 ef017m 15 1.01 1.04 1.05 1.06 6.78E-01 6.71E-01
m ef018m 12 1.01 1.01 1.02 1.02 -1.10E-01 -1.15E-01 ef019m 14 1.01 1.01 1.02 1.02 3.11E-01 3.08E-01 Swar ef020c 20 1.01 1.00 1.01 1.01 -6.78E-02 -6.98E-02 e k ef020m 11 1.02 1.01 1.03 1.03 -1.66E-01 -1.71E-01 Di
h ef021m 11 1.02 1.02 1.04 1.04 1.32E-02 4.09E-03 t ef022m 11 1.02 1.00 1.02 1.02 -5.46E-01 -5.48E-01
Dulu ef023m 32 1.01 1.00 1.01 1.01 -2.56E-01 -2.59E-01 ef024c 25 1.02 1.02 1.03 1.03 6.68E-03 -1.28E-03 ef025c 13 1.01 1.00 1.01 1.01 -2.71E-01 -2.74E-01
ef026m 19 1.01 1.01 1.03 1.03 1.93E-02 1.31E-02 * calculated from measured principle susceptibilities
Table 5. Site-averaged AMS anisotropy factors* 83
K1 K2 K3 95% 95% 95% Sample n dec inc dec inc dec inc conf. conf. conf. ef001m 11 26.55 66.37 10.7 207.70 23.63 10.8 117.80 1.11 1.8 ef002c 12 43.26 39.86 1.6 251.20 46.57 2.3 145.60 14.30 2.0 ef002m 12 284.70 75.71 3.0 59.04 10.09 2.9 150.90 9.96 2.2 ef003c 9 193.40 80.62 6.1 36.54 8.73 6.4 306.20 3.98 3.2 ef003m 12 137.00 79.07 5.5 11.34 6.62 8.4 280.30 10.70 6.5 ef004m 15 241.90 31.70 2.2 44.43 57.12 2.2 147.00 8.05 1.2 Swarm ef005m 19 336.10 84.04 2.3 220.90 2.57 2.3 130.60 5.41 0.8 ef006c 24 204.70 70.23 1.8 4.89 18.69 1.6 97.02 6.23 1.2 Dike ef006m 25 192.00 67.50 1.4 11.00 22.39 1.2 101.10 0.35 1.4 ef007c 15 308.70 88.77 3.7 50.33 0.26 3.7 140.30 1.22 0.8
County ef007m 19 225.30 42.38 10.1 44.19 47.64 10.1 134.90 0.47 1.1
ef008m 24 293.90 15.75 3.1 179.10 54.91 8.4 35.97 29.73 8.0 ef009c 18 212.40 51.66 6.9 69.94 33.01 8.5 326.10 17.49 6.5
Carlton ef009m 13 15.16 79.23 14.3 213.20 10.11 14.4 123.70 2.11 2.4 ef010m 14 198.40 4.59 5.5 91.07 74.32 5.7 289.70 14.56 2.9 ef011c 21 233.40 33.80 5.6 35.60 53.29 8.2 138.00 2.01 7.7 ef012c 21 26.30 13.84 8.7 200.90 77.06 10.0 296.80 1.00 6.0
ef013m 17 258.10 64.32 11.3 46.73 22.37 11.2 141.00 12.19 1.5 ef014m 14 132.30 0.70 10.4 221.70 32.25 9.0 41.15 60.05 8.4 ef015m 12 90.65 5.59 7.2 181.30 7.72 7.7 325.00 80.58 3.2 ef016m 20 146.80 33.82 10.4 55.26 5.25 14.5 315.40 60.85 12.6 ef017m 15 172.00 1.81 12.4 81.22 17.37 12.6 269.60 73.39 3.3 m ef018m 12 246.60 15.89 4.4 347.60 30.34 15.1 136.10 55.29 14.6 ef019m 14 150.80 24.73 7.7 260.50 36.86 9.4 34.45 42.93 5.8 Swar
e ef020c 20 159.40 46.92 13.2 263.00 15.63 16.0 5.80 37.49 10.3 k ef020m 11 93.83 46.95 5.7 317.80 33.55 8.2 211.00 23.43 6.0 Di
h ef021m 11 270.90 75.59 3.2 171.00 2.56 4.2 80.35 13.91 2.9 t ef022m 11 29.95 41.82 7.1 171.50 37.65 11.2 282.10 22.22 11.9
Dulu ef023m 32 91.94 17.24 2.2 350.30 33.10 1.9 204.60 51.24 1.9 ef024c 25 71.79 4.93 2.3 162.10 3.22 3.0 286.30 84.35 2.2 ef025c 13 258.60 71.12 7.1 90.59 19.46 14.7 357.90 3.91 13.9
ef026m 19 45.57 22.69 5.6 151.00 28.73 8.9 279.10 50.34 7.9 * vector means and confidence angles calculated with Stereo32 stereonet software
Table 6. Site-averaged AMS principal directions* 84
rotated K1 rotated K2 rotated K3 angle with dike plane
Sample n dec inc dec inc dec inc K1 K2 ef001m 11 21.0 13.0 186.0 77.0 121.0 11.5 7 10 ef002c 12 1.0 77.0 190.0 12.0 100.0 0.5 7 25 ef002m 12 196.0 43.0 0.0 46.0 100.0 8.0 11 61 ef003c 9 354.0 88.0 4.5 6.0 95.0 2.0 1 67 ef003m 12 271.0 78.0 29.0 6.0 101.0 1.0 9 1 ef004m 15 304.0 69.0 41.0 1.5 130.0 21.0 32 14 Swarm ef005m 19 45.0 83.5 181.0 3.5 271.0 6.0 3 83 ef006c 24 196.0 72.0 33.0 19.0 302.0 6.0 6 15 Dike ef006m 25 191.0 69.0 27.5 21.0 297.0 0.0 1 1 ef007c 15 196.0 88.0 28.0 2.0 104.0 0.0 1 10
County ef007m 19 191.5 44.0 20.5 48.0 110.0 0.5 20 7
ef008m 24 114.0 16.0 231.0 56.0 12.0 28.0 74 13 ef009c 18 215.0 55.0 352.0 42.0 103.5 17.5 4 37
Carlton ef009m 13 53.0 80.5 215.0 9.0 304.5 3.5 5 31 ef010m 14 212.0 3.0 321.0 73.0 239.5 15.0 35 14 ef011c 21 233.4 33.8 35.6 53.2 138.0 2.0 32 3 ef012c 21 54.0 27.0 236.0 73.0 142.5 0.0 60 2
ef013m 17 180.5 56.5 19.5 31.0 286.0 9.5 17 17 ef014m 14 310.0 13.0 217.0 7.0 109.0 75.0 76 23 ef015m 12 278.0 18.5 142.0 65.0 14.0 16.0 87 86 ef016m 20 211.0 60.0 106.0 7.0 9.5 33.5 11 89 ef017m 15 50.0 18.0 158.5 43.0 299.5 42.0 68 40 m ef018m 12 281.5 58.0 174.0 10.5 79.0 30.0 37 30 ef019m 14 118.0 52.5 310.5 43.5 212.0 9.5 80 25 Swar
e ef020c 20 146.0 54.0 266.0 23.5 10.0 25.0 49 55 k ef020m 11 84.0 39.0 344.0 26.0 210.0 36.0 59 28 Di
h ef021m 11 195.0 14.0 330.0 71.5 101.5 13.5 5 5 t ef022m 11 66.0 63.0 171.5 4.0 266.0 28.0 49 42
Dulu ef023m 32 97.0 7.0 355.0 56.0 191.0 32.0 79 2 ef024c 25 73.5 13.0 340.0 10.0 208.5 72.0 89 45 ef025c 13 210.0 43.0 345.0 38.0 96.0 22.5 2 35
ef026m 19 76.5 66.0 339.0 6.0 294.0 21.0 57 50 * rotated by hand into plane of swarm mean dike orientation Carlton County swarm mean dike orientation = N28°E, 89°NW Duluth swarm mean dike orientation = N18°E, 76°NW
Table 7. Rotated site-averaged AMS principal directions* 85
Sample* Hcr/Hc Mr/Ms Ms Mr Hc Hcr ef001m 1.520 0.183 16.800 3.071 20.28 30.82 ef002m 1.586 0.232 23.990 5.554 23.66 37.53 ef003c 1.571 0.209 16.610 3.467 19.69 30.94 ef003m 1.495 0.193 20.050 3.870 19.00 28.41 Swarm ef005m 1.585 0.178 22.770 4.042 18.24 28.91 ef006c 1.404 0.237 23.440 5.566 25.41 35.68 Dike ef007c 1.545 0.195 20.190 3.927 19.43 30.02 ef007m 1.797 0.128 22.600 2.888 14.33 25.75 ef008m 3.081 0.157 0.049 0.008 11.06 34.08 County ef009c 1.588 0.215 30.480 6.555 22.73 36.09 ef009m 1.590 0.207 24.550 5.083 21.52 34.21
Carlton ef011c 1.673 0.206 30.770 6.342 19.64 32.85 ef012c 1.601 0.204 21.530 4.395 22.09 35.37 ef013m 1.394 0.250 23.290 5.818 22.93 31.96 ef014m 2.159 0.099 152.400 15.040 8.677 18.73 ef016m 2.167 0.145 100.200 14.510 10.24 22.19 ef017m 2.287 0.100 139.800 14.000 8.111 18.55 ef018m 2.409 0.088 27.840 2.457 8.525 20.54 Swarm ef019m 2.257 0.135 25.050 3.384 8.914 20.12
Dike ef020m 1.730 0.193 28.490 5.509 22.74 39.33
ef021m 2.824 0.079 56.470 4.451 7.483 21.13 ef022m 1.607 0.222 35.840 7.965 23.42 37.63 Duluth ef023m 4.005 0.017 2.667 0.045 6.709 26.87 ef026m 1.683 0.161 40.460 6.532 16.17 27.21 * each measurement performed on a single cube
Table 8. High-field magnetic hysteresis data