Available online at www.sciencedirect.com
Tectonophysics 448 (2008) 115 – 124
www.elsevier.com/locate/tecto
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)
John P. Craddock a,⁎, Bryan C. Kennedy a , Avery L. Cook a , Melissa S. Pawlisch a ,
Stephen T. Johnston b , Mike Jackson c
b
a
Macalester College, 1600 Grand Avenue, St. Paul, MN 55105 USA
Department of Earth and Ocean Sciences, University of Victoria, Victoria, B.C., Canada
c
Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55445 USA
Received 25 May 2006; received in revised form 19 November 2007; accepted 22 November 2007
Available online 21 December 2007
Abstract
Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility
(AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic
boundary. The dykes are b 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic
crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical Kmax
orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada
over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate
geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes
preserve a vertical Kmax orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama
dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The
unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single
intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical Kmax orientation for the southerly locations (2 dykes, n = 53) and
horizontal Kmax for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite
dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the
magnetic foliation (the plane containing Kmax and Kint, normal to Kmin) is parallel to the dyke planes, which gives us confidence that the magnetic
lineations (i.e., Kmax orientations) are parallel to magmatic flow.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Mafic dykes; Intrusion mechanisms; AMS; Tectonics
1. Introduction
Intrusion of magmatic fluids into crustal rocks has always
been a mechanical paradox contrasting hot weak fluids being
forced into cold, stiff host rocks with minimal metamorphic
alteration or deformation in the host. Field and petrographic
observations in dyke swarms are often complex (forking
⁎ Corresponding author.
E-mail address: [email protected] (J.P. Craddock).
0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2007.11.035
directions, cryptic layering in composite dykes, xenolith
alignment, etc.; Philpotts and Asher, 1994) and indicate
multiple intrusive flow directions. Knight and Walker (1988)
and Ernst (1990) have pioneered the application of AMS
methods to Proterozoic mafic dyke swarms (Ernst and Baragar,
1992; Ernst and Duncan, 1995) as a proxy for primary
magmatic flow directions during dyke intrusion. AMS studies
on dyke swarms in other tectonic settings have been completed:
Hawaii (Knight and Walker, 1988); the Troodos ophiolite
(Staudigel et al., 1992); Makhtesh Ramon, Israel (Baer, 1995)
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J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
and the Independence dyke swarm, California (Dinter et al.,
1996) and others (see Cañón-Tapia, 2004 for a recent review). A
variety of complicating factors are now recognized, including
the “inverse” AMS of single-domain magnetite (Rochette et al.
1992; Ferre, 2002; Potter and Stephenson, 1988), the “distribution anisotropy” of clusters of magnetic grains (Stephenson,
1994; Hargraves et al., 1991), non-flow-parallel grain alignment
by viscous fluid flow (Cañón-Tapia and Chávez-Álvarez,
2004), and tectonic overprinting of primary flow fabric (Park
et al., 1988). Aware of these caveats, our goal in this study was
to use the AMS technique to document igneous flow fabrics in
the Proterozoic Kenora–Kabetogama swarm in cratonic North
America (Archean Superior province; ~ 50 km thick) and
compare this with Eocene dykes in an accreted terrane (Yukon–
Tanana terrane; ~ 30 km thick), and with ridge-parallel dykes in
thin, active Iceland (30 km thick, with magma chambers at 1–
5 km depth) along the Mid-Atlantic ridge.
2. Methods
2.1. Magnetic instrumentation
Oriented hand samples (Iceland, Aishihik, KK) or cores
(KK, some composite dykes) were collected from the various
field locations, and oriented cores (or cubes) were prepared for
AMS analysis (Table 1; Tauxe et al., 1998). The “Roly-Poly” is
a low-field AC magnetic susceptibility bridge with an
automated sample handler for determining anisotropy of
susceptibility at room temperature, and is housed at the Institute
for Rock Magnetism at the University of Minnesota. An
alternating current in the external “drive” coils produces an
alternating magnetic field in the sample space with a frequency
of 680 Hz and an amplitude of up to 1 mT. The induced
magnetization of a sample is detected by a pair of “pickup”
coils, with a sensitivity of 1.2E− 6 SI volume units. For
anisotropy determination, a sample is rotated about three
orthogonal axes, and susceptibility is measured at 1.8° intervals
in each of the three measurement planes. The susceptibility
tensor is computed by least squares from the resulting 600
directional measurements. Very high precision results from the
large number of measurements; in most cases principal axis
Table 1
AMS results
Sample
Dykes
sampled
n=
Anisotropy
(%)
Lineation Foliation
Iceland basaltic dykes
Aishihik dykes,
Yukon
Kenora–Kabetogama
dykes
Single intrusion
Composite intrusions
13
11
153 14.9
111 13.65
11.86
7.09
3.16
6.43
247 14.21
92 11.9
3.14
7.6
2.06
6.8
21
17
4
Explanation: anisotropy (Kmax − Kmin / Kmean); lineation (Kmax − Kint / Kmean);
foliation (Kint − Kmin / Kmean) for the Roly-Poly instrument.
orientations are reproducible to within two degrees, and axial
ratios to within about 1%.
2.2. X-ray fluorescence
Eleven of the Aishihik Lake dyke samples were analyzed for
major and trace element composition using X-ray fluorescence
as there was no geochemical data on these intrusions. Samples
were split with a vise wedge and only pieces lacking weathered
or saw-marked edges were selected to be used in XRF analyses.
Powders were prepared by further splitting the sample and
reducing these pieces to a fine powder in a Spex 8510
Shatterbox. The use of pre-contaminated bowls (iron for trace
element and tungsten for major element powders) reduced the
chance of cross contamination between the samples.
Pressed powder pellets were prepared for trace element
analyses by mixing exactly 10 g of rock powder with 15 drops
of 2% polyvinyl alcohol and pressing the mixture into pellets on
a stainless steel mold under a pressure of 6 tons. Major elements
were prepared by first heating approximately 10 g of each major
element powder to 1000 °C to drive off all water. The amount of
water lost in heating (loss on ignition) was recorded after
samples had cooled and these numbers were taken into account
in the reporting of element totals. Exactly 1 g of dried powder
was mixed with 5 g lithium metaborate/tetraborate flux and
0.1 g NH3NO4. Once mixed, the powder was placed in a
platinum crucible and 2 drops of HBr were added. The crucibles
were heated on a Spex Fluxy until the material was molten, then
poured into a platinum mold creating a homogenous glass disk.
The pellets and beads were analyzed by a Phillips PW-2400
XRF.
3. Field relations and results
3.1. Iceland ridge-parallel swarm
Iceland is one of the best-exposed and best-studied
geological settings in the world, combining a mid-ocean
ridge, a hotspot plume, active glaciation, and little vegetation
covering the rocks which range from 16–0 Ma (Gudmundson
and Kjartansson, 1996). The interplay of these geologic
processes results in some very complex local geodynamics, as
recorded by a variety of methods: seismicity and focal
mechanism solutions, fault slip data, GPS and borehole
strainmeter surveys, tiltmeter-elevation change and gravity
surveys, and hydrofracture and overcoring measurements. Most
recently, Linde et al. (1993; Mt. Hekla) and Stefansson et al.
(1993; South Iceland Lowland project), have made significant
advances in monitoring and predicting both volcanic eruptions
and destructive seismic events, respectively. Dyke intrusion has
been observed seismically (Einarsson and Brandisdottir, 1980)
and studied extensively by Sigurdsson (1980), Gudmundsson
and Brynjolfsson (1993) and Paquet et al. (2007).
Thirteen single-intrusion basaltic dykes were sampled
around Iceland and all but one (sample 4) strikes parallel to
the ridge axis (Fig. 1; site data). The dykes are generally b1 m
wide and can be traced a few tens of meters along strike. In ten
J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
117
Fig. 1. Digital elevation map of Iceland showing dyke locations and typical field exposure of a dyke. AMS results are plotted on lower hemisphere projections (Kmax —
solid circles, Kmin — open circles) with representative vertical flow on the left (all sites except 4, 11 and 12) and dike-parallel horizontal flow (sites 11 and 12) and a
curious result for the ridge-normal dike (site 4) on the right.
of thirteen dykes Kmax is sub-vertical and Kmin plots
horizontally and normal to the plane of the dyke. The steep
magnetic lineations suggest sub-vertical flow. Two ridgeparallel dykes (samples 11 and 12; horizontal Kmax and vertical
Kmin) and one ridge-normal dike (#4; vertical Kmax and
horizontal Kmin) exhibit anomalous fabrics, with Kmin and
Kmax both within the plane of the dyke (see Rochette et al.,
1992; Cañón-Tapia and Chávez-Álvarez, 2004). Sample
anisotropy percentages are quite high, ranging from 5–34%
for the Icelandic suite (153 cores), with an average lineation and
foliation of 12.23% and 3.16%, respectively (Table 1).
Thermomagnetic analysis (using a Kappabridge KLY-2
susceptibility bridge with a CS-2 furnace) shows that these
dykes have a bimodal magnetic mineralogy: a relatively low-Ti
titanomagnetite Fe 3 − x Ti x O 4 with a Curie temperature
Tc ~ 510 °C (TM10, x ~ 0.10) and a more Ti-rich composition
with Tc ~ 180 °C (TM60, x ~ 0.60). Hysteresis measurements
show that for both compositions the major domain state is
pseudo-single domain (PSD): the ratio of saturation remanence
to saturation magnetization (Mr/Ms) ~ 0.1.
3.2. Aishihik dyke swarm, Yukon, Canada
The Yukon–Tanana terrane is sutured between the Coast
Plutonic and Stikinia terranes bounded on the north by the
Tintina dextral fault, on the south by the Denali dextral fault,
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J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
and is composed of metamorphosed marine sediments of
Devonian–Permian age crosscut by a variety of Mesozoic
plutons (Johnston and Timmerman, 1993; Johnston and
Erdmer, 1995). The dynamics of accretion of the various
terranes is complex and debated (Jackson et al., 1991; Johnston
et al., 1996) but it is assumed that the Aishihik dykes were
intruded after accretion of the Yukon–Tanana terrane to another
terrane (Stikinia?) or N. America in the Triassic. The porphyritic
dykes crosscut a variety of rock types (Fig. 2B) including the
Cretaceous–Paleocene Ruby Range batholith; the dykes
probably fed and do not intrude the Eocene Mount Creedon
volcanics, giving an age constraint. Within this terrane the
~ N–S Aishihik dyke swarm intrudes an area of 200 by 60 km
(Fig. 2A). Dykes are generally b 4 m in width (Fig. 2B) and
can be traced for a few hundreds of meters in places, and we
sampled the southern 100 km of the swarm. Petrographically, the
dykes are unaltered (some chlorite after olivine) porphyritic
andesites (Fig. 2C). Major and trace element XRF analysis of the
dykes, which have an aphanitic groundmass and sodic
phenocrysts, indicates a consistent REE chemistry and a calcalkaline single source magma with a within-plate tectonic setting
(Fig. 2E; Table 2; Cook, 2001). Ten dykes were sampled and 111
cores were analyzed by AMS; eight of the ten dykes preserve a
vertical Kmax orientation within the plane of the dyke, suggestive
of vertical intrusion (Fig. 2D; site data, Table 1). The remaining
two dikes preserve complex Kmax and Kmin plots within the dyke
plane. The average anisotropy is 13.65%, with average lineations
of 7.09% and foliations of 6.43%.
Multiple magnetic phases and domain states were identified
in these dykes. Pyrrhotite was identified in some samples by a
Curie point Tc ~ 325 °C and a low-temperature magnetic
transition near 34 K (Dekkers et al., 1989; Rochette et al.,
1990), with dominantly PSD–SD domain states indicated by
the remanence ratio Mr/Ms ~ 0.4. In other samples, nearly pure
multi-domain (MD) magnetite (Tc ~ 565 °C, Mr/Ms ~ 0.01) was
the major susceptibility source.
3.3. Proterozoic Kenora–Kabetogama swarm
The Kenora–Kabetogama (KK = Fort Frances = Marathon;
swarm A25 and A26 in Ernst et al., 1996) mafic dyke swarm
intrudes Archean granitic, metavolcanic, and metasedimentary
rocks of the southern Archean Superior Province of the
Canadian Shield, represented in northern Minnesota and
southwestern Ontario by four lithotectonic terranes, the
Wabigoon, Quetico, Wawa, Minnesota river valley (MRV;
see Schmitz et al., 2006)) sub-provinces (Fig. 3). Recent
reviews of the geology and evolution of these sub-provinces
were given by Card (1990), Blackburn et al. (1991), Williams
(1991), and Williams et al. (1991). The Wabigoon (northern)
and Wawa (southern) sub-provinces are “granite–greenstone”
terranes considered to represent accreted slices of continental
platform, ocean floor, and volcanic arc rocks. They are
juxtaposed with the intervening metasedimentary Quetico
sub-province. These terranes are considered to have been
accreted by 2600 Ma into the currently ENE-trending belts
bounded by north-dipping thrust faults, and were of a thickness
of 40–50 km at the time of dyke intrusion. The Superior
province has been tectonically stable for ~ 2 Ga as the KK dikes
are not offset along any terrane boundaries.
The outcrop extent of the KK dyke swarm is approximately
30,000 km2 (Southwick and Day, 1983). The western terminus
of the swarm, like the Superior province boundary, is however
obscured by a thick overburden of Phanerozoic sedimentary
rocks and glacial drift. Samples of basalt and diabase were
obtained from drill cores (at depths of a few hundred meters)
that intersected four of the hundreds of strong NW-trending
normal and reversely-polarized linear magnetic anomalies in
northwestern Minnesota (Chandler, 1991). Considering these
magnetic anomalies to be KK dykes, and exposures of mafic
dykes in the Minnesota River valley, the total extent of the
swarm approaches 100,000 km2 , ranking it among the largest
dyke swarms in the world.
At the southern boundary of the Superior Province in
northeastern Minnesota (Fig. 3), the KK dykes are overlain by
metasedimentary rocks of the Animikie Basin, thus constraining
their intrusion to the Early Proterozoic. K–Ar ages for the dykes
range from 2240 to 1520 Ma (Hanson and Malhotra, 1971) and
probably record Ar loss and/or resetting during post-emplacement metamorphism. A Rb–Sr whole-rock isochron age of
2120 ± 67 Ma was determined from a composite dyke near Lake
Kabetogama by Beck and Murthy (1982). A regression line fit
to Sm–Nd whole-rock and mineral data yields an age of 2065 ±
120 Ma (Wirth et al., 1995; Wirth and Vervoort, 1995) and is
within analytical error of preliminary U–Pb analyses of zircons
from the southeastern (2075 ± 2 Ma; L. Heaman, pers.
commun.) and southern (2.067 ± 2 Ma, Schmitz et al., 2006;
TIMS zircon age, Franklin dykes) part of the swarm. Early
major element geochemical work on the KK dyke swarm
suggested the existence of two distinct groups of dykes: low-Ti/
high-Mg and high-Ti/low-Mg (Manzer, 1978). Southwick and
Day (1983) studied the field geology of the swarm, as well as
the detailed petrography and mineral chemistry of a composite
dyke from Lake Kabetogama. In a study of the major and trace
element characteristics of the swarm, Southwick and Halls
(1987) proposed a common magmatic origin for all of the
dykes, suggesting that they represent a cogenetic differentiation sequence from an evolving magma system that was
tapped at two stages. Schmitz et al. (1995) have identified two
sub-groups in the KK swarm based on incompatible trace
element abundances, low-ITE (Zr b 100 ppm) and high-ITE
(Zr N 120 ppm) groups, and both seem to have been intruded
from an aesthospheric mantle-enriched source without crustal
contamination.
The detailed geology and structure of the swarm have been
described by Southwick and Day (1983). In summary, the
trends of dykes fan from N15°E in the eastern part of the
swarm to N60°E in the south. Outcrops and aeromagnetic
signatures of individual dykes can be traced along strike for
over 200 km (Southwick and Halls, 1987). The dykes occupy a
NW-trending vertical fracture set perpendicular to the ENEtrending Archean fabric, with evidence for some local strikeslip movement along some dyke margins (Craddock and
Moshoian, 1995). Dips on the planar, sub-parallel dyke
J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
119
Fig. 2. Digital elevation map of southwest Yukon, Canada with terranes and bounding faults and the Aishihik dike swarm (A). B. Dike field photo. C. Photomicrograph
showing aphanitic groundmass with plagioclase phenocrysts. D. Lower hemispheres stereoplot of AMS data (Kmax = closed circles, Kmin = open circles) that shows vertical
flow (8 of 10 samples) E. Tectonic discrimination diagram (upper) and REE spider diagram for the Aishihik dykes.
120
J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
Table 2
Major and trace element geochemical data for the Aishihik dyke swarm
Sample 5
SiO2
TiO2
Al2O3
FeO
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
La
Ce
Rb
Ba
Th
U
K
Nb
Sr
Pb
P
Zr
Ti
Y
Ga
Cr
Ni
Co
Sc
V
Zn
61.6
0.476
17.67
0
5.04
0.04
1.91
4.82
4.42
2.44
0.233
0.4608
99.11
12.1
29.1
61.1
1444.6
4.8
2.9
2020
3.7
853
5.9
102
118.1
285
13.1
18.8
9.5
9.8
17
10.3
92.4
21.5
Sample 8
54.49
0.762
14.89
0
7.89
0.13
7.95
8.11
3.49
1.66
0.249
0.9777
100.6
10.2
29
47.2
832.1
2.9
1.1
1380
5.4
602.8
4.1
109
103.7
457
19.1
16.8
403.7
100.6
34.1
37
124.5
45.6
Sample 9
53.35
1.694
16.55
0
9.56
0.15
4.64
8.58
2.8
1.35
0.459
1.6184
100.75
18.1
44.7
39.5
948
3.5
1.7
1120
4.6
603.1
8.3
201
151
1016
27
20
67.8
13.4
30.8
26
150.2
125
Sample 10
Sample 11
Sample 12
Sample 13
Sample 16
66.81
0.37
16
0
3.32
0.04
2.05
3.75
4.32
2.8
0.141
0.8687
100.47
54.77
0.79
17.17
0
8.24
0.09
3.89
6.21
3.71
2.6
0.263
1.8038
99.54
66.39
0.365
16.36
0
2.93
0.04
1.33
3.92
4.17
3.26
0.191
0.5065
99.46
64.43
0.418
15.71
0
4.06
0.05
2.6
4.12
4.2
2.61
0.146
0.7819
99.13
61.4
0.489
17.24
0
5.11
0.1
2.42
6.09
4.15
1.62
0.167
0.6061
99.39
11
26.5
75.3
1464.8
7.9
2.7
2320
4.7
658
7.9
61
109.8
222
10.1
16.2
86
29.9
13.2
10
66
27.4
12
31.6
97.7
1664.5
5.8
2.7
2160
2.8
1102.7
4.5
115
116.9
474
18.2
19.5
18.7
13.3
33
25.5
153.1
42.9
contacts are within 10° of vertical, indicating that the dykes
have not rotated since intrusion. Dyke widths vary from less
than 10 cm to greater than 120 m. Most dykes have distinct
fine-grained margins at their contacts with the country rock.
Inward from the dyke margins, textures vary from basalt
through diabase; gabbroic textures are characteristic of dykes
greater than 20 m in width. Dykes of greater width often have
heterogeneous internal structures and are composed of
symmetrical and rhythmic compositional layers that are
truncated toward the cores of dykes. Most commonly these
layers are sub-parallel to the outer dyke margin, although
structures reminiscent of sedimentary cross bedding have been
noted. These structures suggest that they were formed by
multiple intrusions of magma. The discontinuity between
layers is wholly compositional, without chilled margins or
significant grain size variation (Schmitz et al., 1995). The
absence of chilled margins indicates that there was little time
for significant cooling between the emplacement of magma
pulses.
The KK dikes mostly contain pure magnetite (Tc ~ 580 °C)
with a range of grain sizes/domain states (Mr/Ms from 0.1 to
0.35). Some pyrrhotite is also present in a few samples.
10.8
25.6
84.8
1979.6
8.6
4.4
2710
5.4
763.1
7.4
83
126.5
219
13.1
16.6
16
9.8
11
8.1
62.6
24
10.7
26.9
67.7
1498.2
7.3
3.5
2170
4.3
620.4
7
64
114.9
250
10.8
16.4
102.5
36.3
16.4
11.9
85.3
34
12
29.5
55.5
1148.6
4.5
1.9
1340
3.1
705.6
10.6
73
114.7
293
13.5
16.8
21.7
11.1
18.4
14.6
94
62.9
Sample 17
66
0.295
17.75
0
3.11
0.07
0.68
3.73
4.5
2.67
0.173
0.514
99.49
23.8
44.8
63.5
1920.7
6.9
4.1
2210
4.9
909.3
12.4
76
176.3
177
15.9
17.5
3.7
5.8
8.7
6
29.4
28.9
Sample 18
Sample 19
61.39
0.575
16.75
0
5.83
0.1
2.56
5.12
3.97
2.21
0.273
0.6945
99.47
71.37
0.225
15.24
0
1.99
0.01
0.57
2.02
4.42
3.06
0.099
0.6512
99.66
17.1
37.8
53.7
1580.7
5.7
3.2
1840
4.6
806.3
7
119
145.9
345
18.5
17.7
21.4
10.6
18.8
14.5
108.6
58.6
14.1
30.6
65.9
1555
7.8
3.5
2540
5.7
467.7
9.9
43
116.6
135
9.5
15.9
7.1
6.3
8.1
5
19.2
14.5
3.4. Single-intrusion dyke AMS
Seventeen single-intrusion dykes were sampled (Fig. 3; site
data) and 247 cubes were analyzed by AMS (Table 1). Kmax axes
are all sub-horizontal within the plane of the dyke and Kmin axes
are all dike-normal and sub-horizontal for samples collected north
and west of the Penokean suture (Pawlisch et al., 1997). Different
AMS ellipsoid orientations were obtained from samples collected
along the southeast margin of the KK swarm near Virginia, MN
(strike N10°W) and in the Minnesota River valley near Franklin,
MN (strike N60°E), and the portion of the KK swarm nearest to
the Penokean subduction margin. Closest to the margin, Kmax is
vertical, while Kmin is horizontal and dike-normal (Fig. 3, white
stars). Two dikes in the northeast (Fig. 3, black squares) preserve
an intermediate AMS ellipsoid fabric: dike-parallel, horizontal
Kmax and a sub-vertical Kmin. However, the dominant trend
suggests a source area for the KK magmas in the south and east,
feeding the propagating swarm to the north and west (subhorizontal flow northwestward of the margin). The impingement
of the Penokean belt and Animikie basin, and younger Keweenaw
rift limit the exposure of KK dyke presumably nearer the source to
the southeast.
J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
121
Fig. 3. Aeromagnetic basemap of Minnesota (Chandler, 1996) with tectonic features identified. Kenora–Kabetogama dykes are found north and west of the red line
and appear as linear white lines with their extensions in black off the map. Inset explains dyke AMS stereoplots. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
3.5. Composite dyke AMS
Four composite dykes were sampled (n = 92) in great detail
(Fig. 3), two from the center of the swarm in the Quetico terrane,
and two from the northern terminus near Kenora, Ontario in the
northern Wabigoon terrane (Table 1). Each of the dykes are
~ 20 m wide and preserve multiple intrusions that are
increasingly coarse and felsic toward their interiors. Discordant
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J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
boundaries between different intrusive phases are common, as
are cryptic layering zones (“cross beds”). AMS results for both
dykes are clearly anomalous and cannot be interpreted in terms
of flow orientations: Kmax and Kmin axes have a variety of
trends and plunges.
4. Discussion
Visual observation of dyke intrusion is unlikely to occur, but
seismic tracking of dyke intrusion was monitored during the
1977–8 eruption of Krafla in Iceland (Brandsdottir and
Einarsson, 1979; Sigurdsson, 1980). During this eruption,
with measured caldera inflation–deflation cycles, seismic
events were recorded and located along the leading edge of
the advancing tabular sheet of magma which eventually
reached the surface through a drill hole or fissure within the
MAR axis. The eruptive source was at a depth of ~ 5 km and
the dyke intrusion front was both upward from the source and
northeastward within the ridge axis in the plane of the dyke
(see also Paquet et al., 2007). AMS studies of flow fabrics in
oceanic crust are limited to an ophiolitic suite and hotspotrelated dykes in Hawaii. The Troodos ophiolite preserves subhorizontal flow (Rochette et al., 1991; Staudigel et al., 1992).
The Koolau dyke swarm on Oahu, Hawaii preserves an
interpretable AMS fabric with flow directions in numerous
orientations within a well-organized swarm (Knight and
Walker, 1988). Our regional study of dykes in Iceland shows
a clear vertical orientation of magnetic lineations, which may
be interpreted as a proxy for vertical magmatic flow, perhaps
expected along a ridge axis where the crust is ~ 30 km (see
Allen et al., 2002) and basaltic magma reservoirs are 1–5 km in
depth. Geoffrey et al. (2002) and Callot and Geoffrey (2004)
report a mixed AMS flow result for Tertiary dikes in east
Greenland, where the majority (23 of 75) dikes preserve a
horizontal, southerly intrusion flow. “Inverse fabrics”, where
Kmax is dike-normal are best explained either because of the
inverse “single-domain effect”, or because the grain long axes
are not preferentially aligned with flow in the dyke, or some
combination of these (see Borradaile and Gauthier 2001;
Archanjo et al., 2002; Ferre, 2002).
Dyke swarms intruded in continental crust are numerous
(Ernst et al., 1995) and tend to be more prevalent in host rocks
that are supracrustal and of low metamorphic grade (as opposed
to granulites; Buchan and Halls, 1990) and are increasingly
studied by AMS techniques. Both radiating (Mackenzie,
1267 Ma, Ernst, 1990, Ernst and Baragar, 1992) and linear
(Labrador, early Proterozoic, Cadman et al., 1992; Abitibi,
1140 Ma, Ernst, 1990; Botswana, 1800 Ma, Ernst and Duncan,
1995) dyke swarms of Precambrian age preserve a vertical AMS
fabric near the source and a sub-horizontal fabric away from the
source. Baer (1995) found good correlation between AMS
ellipsoidal, petrofabric and dyke margin flow indicators in the
radiating Cretaceous Makhtesh Ramon dykes, Israel, but overall
complex local flow. The linear Jurassic Independence swarm
preserves a sub-vertical AMS ellipsoid at an acute angle to the
dyke walls suggesting a tectonic strain overprint during or after
intrusion (Dinter et al., 1996). Our results for the Eocene
Aishihik dyke swarm provide an intermediate crustal example:
vertical dyke intrusion in an accreted terrane with a thickness of
~ 30 km of an intermediate melt (andesitic), although we have
no constraints on the depth of the magma chamber. The tectonic
setting and age of these andesitic dikes are suggestive of a
shallow magmatic system.
The Kenora–Kabetogama dyke swarm is one of the largest
and best-exposed radiating swarms with a presumed source
area in southeastern Minnesota postulated by convergence of
the swarm from the north and west. These are not equivalent to
the metamorphosed dykes along strike in western Wisconsin
and northern Michigan (Green et al., 1987; King, 1990; Beutel
et al., 1995). The KK dykes intrude the MRV, Wawa, Quetico,
and Wabigoon terranes of the Superior province (Chandler,
1991; Southwick and Chandler, 1996; Craddock and
Magloughlin, 2005) which are north of, and proximal to, the
Penokean suture and their extensions are buried by younger
accretion to the south (Fig. 3). Our single-intrusion AMS
fabrics suggest a magmatic source in the southeast near the
Penokean margin (vertical flow) and that the dykes propagated
horizontally outward to the west and north as part of a great
swarm. The distance between the two southerly sites where
vertical flow is preserved and the nearest dyke along strike
where horizontal flow is measured by AMS, is approximately
150 km, similar to other continental dyke swarms. This
relationship could be better resolved if the southern extent if
the KK swarm was not buried. AMS as a proxy for magmatic
flow in composite dykes of this swarm was inconclusive.
In all three of the cases studied here, we find a minority of
samples (3, 2, and 6 dikes in Iceland, the Yukon and KK
swarms, respectively) where the magnetic foliation (the plane
normal to Kmin) is oriented perpendicular to the dyke plane,
rather than parallel. Models of alignment of elongate particles in
the viscous fluid flow during emplacement all predict magnetic
foliation coinciding with the dyke plane (e.g., Cañón-Tapia and
Chávez-Álvarez, 2004). Viscous drag from the walls is expected
to produce a cross-dyke velocity profile, with a dyke-parallel
shear plane and dyke-normal velocity gradient. In the numerical
simulations of Cañón-Tapia and Chávez-Álvarez (2004),
involving a range of shear strains and grain shapes, a small
proportion (b5%) of resulting AMS fabrics had Kmin oriented in
the shear plane (type ‘i’ fabric), and in these cases Kmax
remained parallel to flow. This may be the origin of the minority
AMS fabrics that we observe, although the ubiquity of type ‘i’
orientations in our study compliments their rarity in theoretical
models.
5. Conclusions
Thin crust and shallow magmatic sources seem to favor the
propagation of vertical dyke intrusions that may also be fissure
eruptions, as is the case in Iceland. Magmatic systems that feed
great dyke swarms seem to flow upward in a central locale, then
horizontally for many hundreds of kilometers. The Kenora–
Kabetogama great dyke swarm appears to be the result of
horizontal flow from a single vertical source, although caution
must be exercised, at least with interpreting AMS data in such
J.P. Craddock et al. / Tectonophysics 448 (2008) 115–124
swarms, as to the erosional level that is sampled across a swarm
and the significance of composite dykes.
Acknowledgements
These projects were stimulated by discussions with Richard
Ernst and Henry Halls at the IDC-3 meeting in Israel, 1995. All
our magnetic measurements were made at the IRM, University
of Minnesota. The CUR-SURE summer fellowship program
supported Craddock and Pawlisch during the summer of 1996.
Our general understanding of the KK dyke swarm was enhanced
by discussions with Charlie Blackburn (Ontario Geological
Survey), and Dave Southwick and Mark Jirsa (Minnesota
Geological Survey). We are appreciative of the thorough reviews
by Jean-Paul Callot and an anonymous reviewer.
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