153
~ec~~n~~~~~~c,s, 119 (1985) 153-179
Etsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
PALEOMAGNETISM AND THE OROCLINE HYPOTHESIS
S. ELDREDGE,
V. BACHTADSE
and R. VAN DER VOO
of Geological Sciemvs, The Uniwrsirryof Michigan, Ann Arbor, Ml 4RIUo /trXA.)
&parfment
(Received
October
18, 1984; accepted
December
18, t984)
ABSTRACT
Eldredge,
S., Bachtadse,
N.L. Carter
V. and Van der Voo, R., 1985. Paleomagnetism
and S. Uyeda
(Editors),
Collision
Tectonics:
and the orocline
Deformation
hypothesis.
of Continental
In:
Lithosphere.
Tectonophysics, 119: 153-179.
Oroclines
were originally
feast straighter
curved
mountain
belt, regardless
Since the occurrence
tectonic
setting.
analyzed
relative
to determine
Western
belts investigated
Jura
significant
and
bending.
and unrotated
nappes
age, in the Northern
Hercynian
strated)
data
in the change
from six potential
displayed
of declination
oroclines
by the change
as a function
have been compiled
of paleomagnetic
of
and
declination
Arc and the Umbrian
are: the Sicilian-Calabr~~n
the central
portion
overthrust
of the Appalachian
belt of western
Arc of Italy. the Swiss
Mountains
(from Pennsylvania
to
America
and the Hercynides
of
North
Arc
shows oroclinal
significant
in the Late Tertiary
Apennines,
Europe
the Pennsylvania-Virginia
‘The Wyoming-Idaho
portion
belt
of the Appalachians
shows
a combination
fail
of rotated
to show
(possibly
thrust sheets.
In the Sicilian-Calabrian
Calabria-Peloritani
or at
to include any
Europe.
Mountains
oroclinal
oroclinai)
has been broadened
shape.
is best recorded
of rotation
the Wyoming-Idaho
and Central
The
belts which initially were straight.
in strike of the fold belt.
of the Jura Mountains,
U.S.A.).
bending
and structural
the amount
to the change
Virginia,
by Carey as curved mountain
of its original
of oroclinal
paleomagnetic
The arcuate
portion
defined
than they are today. In the last few years, the definition
bending
caused
can be demonstrated.
also shows oroclinal
(the only belt included
bending
oroclinat
bending
by the impingement
while the Umbrian
of the
Arc of similar
on a smaller scale.
in which deformation
(at least SO”) as well as a marked
of basement
original
rocks can be demon-
curvature
(70”) in its western
part.
Common
continental
to all the orocIines
margin
during
studied
the orogeny,
in this paper
causing
is the probable
subsequent
deformation
impingement
and bending
of a rigid block or
of the fold belt.
INTRODUCTION
The term orocline was first defined by Carey (1955) as “an erogenic system which
has been flexed in plan to a horse-shoe shape”. In recent years the term has been
used to describe any orogen of arcuate shape, regardless of its deformationa~ history.
~~~-1951/~5/~3.3~
0 1985 Elseviet Science Pubfishers
B.V.
154
This extension
restoring
of the original
the primary
originally
linear,
paleomagnetism
and structural
term has arisen
shape of mountain
or at least less bent,
to be a powerful
history
than
of the difficulties
Recent
tool for reconstructing
years have shown
such a paleogeographical
the world, we have chosen six
data base for our purpose:
Arc of Italy. the Jura Mountains
in
if an orogen was
1980).
belts throughout
in this study that have a suitable
and the Umbrian
it is today.
(Van der Voo and Channell,
From the bulk of curved mountain
to analyze
largely because
belts, and in determining
of Switzerland,
the Calabrian
the Appalachian
Mountains
of eastern and the Wyoming overthrust bett of western North America.
and the European Hercynian m~~untain belt. The structural and pale~)m~gneti~ data
from an area must meet several criteria before an orogen can be used in this study:
(1) The curvature of the orogen must be defined by a systematic change in the
trend of fold-axes.
(2) Geographic
and
geological
position
of the rocks
sampled
must
be well
documented.
(3) If rocks of more than one age are used. appropriate
must be available.
(4) The remanent
magnetizations
age of magnetization
established
must pass modern
reference
reliability
declinations
criteria,
either by a fold test or by comparison
and the
of paleolati-
tude with the reference apparent polar wander path (Irving, 1977).
Based on theoretical considerations,
three possible magnetic signatures
can be
expected in the analysis of arcuate mountain chains:
(1) If the belt had an originally curved shape, then the magnetic declinations
should not correlate with the change in fold-axis trend, but remain parallel.
(2) If the belt had a linear configuration,
and was subsequently
bent, then the
change in magnetic declinations
should mimic the change in fold-axis trend on a one
to one basis.
(3) Bending of a preexisting arcuate belt will lead to a smaller change in magnetic
declination
than in the trend of the fold-axis or strike direction.
In order to quantify the variation in paleomagnetic
direction as a function of
regional change in structural
trend, we use the linear regression technique.
A11
paleomagnetic
data have been transferred to uniform polarities wherever necessary
to avoid ambiguities
caused by dual polarities. The observed declinations
(D,,) are
subtracted from the reference declination (D, - D,,). Thus, a clockwise rotation away
from the reference gives a negative value. and a counterclockwise
rotation gives a
positive value. Where available, the stable magnetization
of contemporaneous
and
undeformed
rocks is used as a reference. If reliable paleopoles are not obtainable.
then an average reference declination
is chosen. The exact value of the chosen
declination
( Dr?, is not critical, only that it is constant. For the purpose of this study
155
_ Original
Fig. 1. Graphical comparison
Dedinotion - Deformed
Declination
of change in strike vs. change in declination.
Ideal oroclinal bending would
display a line of unit positive slope. The best fit line is calculated by linear regression, and the correlation
coefficient
is for change in strike and change in declination.
S, is the estimated
original strike of the
orogen.
the paleomagnetic inclinations are of no importance, except to be used to calculate
error limits (AD = sin-‘{
a,,/cos[Incl.]})
on the declinations
(Beck, 1980). These
values might be too large (Demarest, 1983) but do not affect the results of our study.
The strike data are analyzed in a similar fashion to the declinations. The reference
direction of strike (S,)
has been assumed as the average strike as determined by
analyzing the shape of the arcuate orogen or to represent the primary trend of the
orogen as a first approximation. The observed strikes (S,) have been determined
either from the original publications or from geological maps of appropriate scale.
The numerical values for S, - S, are plotted on the abscissa, and D, - Do on the
ordinate of an orthogonal coordinate system. When plotting the data set in such a
fashion there are three endmember situations which can result (Fig. 1):
(1) If an erogenic belt displays perfect oroclinal behavior, then the data points
should lie on the line of unit positive slope.
(2) If the erogenic belt contains parallel, nearly linear, thrust fronts, while it
consists of thrust sheets that underwent various amounts of rotation, then the data
points should run in the ordinate direction.
(3) If the belt has an original arcuate shape that has seen no tightening, then the
data points should fall along the abscissa.
SICILIAN-CALABRIAN
ARC
The Sicilian-Calabrian
arcuate belt (Fig. 2) includes most of Sicily and the
Southern Apennines of Italy, which are separated by the Calabria-Peloritani
nappes
of northeasternmost Sicily and the “toe” of the boot of Italy. The arcuate trend of
TYRRHENIAN
SEA
IONIAN
E
i UC
----
Fig. 2. Map of the Sicilian-Calabrian
Arc. with arrows
from
numerical
Channel1
declinations;
et al. (1980)
while
E = Calabria-Peloritani
teeth on the upper
arc;
SEA
E = transcurrent
plate:
nappes:
D = main tectonic
lines: D, -Sangineto.
showing
sites are from
C = Overthrust
windows
mean site declinations.
Catalan0
front of the Apennines
(Apennines
Dz -Taormina.
outcrop)
Modified
Lettered
et al. (1976).
site
and Sicily nappes.
in the Calabrian
from Catalan0
sites are
A = mean
Peloritani
et al. (1976).
the orogen is defined by the regional variation in the strike of fold axes and by the
prevalent
thrust fronts. These structures
strike N40”W and have a northeastern
vergence in the Southern
across Sicily. The thrust
Cretaceous
margin
parts
age which
(Bernoulli
of Apulia
Apennines,
and strike east-west with a southern vergence
sheets consist of platform and basinal carbonates
of Late
originally
and Jenkyns,
on the “heel”
formed
parallel
to the ancient
southern
1974). Only the Iblei zone in southeast
of Italy
are considered
Tethys
Sicily, and
to be autochthonous
(cf.
Channel1 et al.. 1980). Paleomagnetic
data for Upper Cretaceous basalts from the
Iblei zone (Schult, 1973; Barberi et al., 1974; Gregor et al., 1975) as well as bauxites
of the same age from Apulia are available in the literature (Channel1 and Tarling.
1975). Paleomagnetic
data from contemporaneous
rocks in these two autochthonous
areas agree not only with each other (Schult, 1976). but with the apparent polar
wander path of Van der Voo and French (1974) for Africa, implying that autochthonous Italy and Sicily are part of an African promontory.
This relationship
has
been suggested by Channel1 et al. (1979) and VandenBerg et al. (1978). Channel1 et
al. (1980) further suggest that the Calabria-Peloritani
nappes appear to be a discrete
block thrust in the Miocene (Aquitanian)
onto the carbonate platform.
Paleomagnetic
studies throughout
the Sicilian-Calabrian
arc (Catalone
et al.,
157
1976; Channel1
et al., 1980) concentrated
ceous to Paleocene
fold test and hence reflect pre-folding
The results
from several Sicilian
al., 1980) are compared
the autochthonous
the internal
of the Iblean
thrust
of Late Creta-
of these rocks pass the
sheets (Catalone
et al., 1976; Channel1
declination
(0,)
foreland
(Schult,
sheets show a progressive
from 10” on the thrust
(northernmost)
limestone
magnetizations
magnetizations.
thrust
with a reference
basalts
from the allochthonous
to Iblei, ranging
on the Scaglia
age. The characteristic
of 345” determined
1973). The declinations
clockwise
rotation
sheet closest to the autochthon,
thrust sheet, near Palermo,
et
from
northern
relative
to 140” on
Sicily (Fig. 2). The
average strike of the Sicilian thrust fronts is east-west, and these are compared to an
overall average strike for the orogen (S,) of N23”E. The range in declination
differences ( Dr - Do) is much greater than strike differences (S, - S,), causing the
Sicilian portion of the plotted data to have an elongated distribution
parallel to the
ordinate direction (Fig. 3). This implies that Sicily itself has seen more rotation upon
SICILIAN
- CALABRIAN
ARC
.-00
-100
.-I20
S,.= 23O
.-I40
1
Fig. 3. Diagram
showing
arc. S, is the reference
observed
declination.
cc=0.92
the strike deviations
strike,
relative to declination
S, is the observed
C, is the correlation
coefficient.
also text. Solid line with unit slope represents
calculated
by linear regression.
strike,
deviations
Dr is the reference
Error bars calculated
ideal oroclinal
See Table 1 for numerical
data.
bending.
for the Sicilian-Calabrian
declination,
according
Dashed
and
&
is the
to Beck (1980), see
line is a best-fit
line
‘TABLE
1
Paleomagnetic
Site
data for the Sicilian-Calabrian
Locality
Are *
Strike
&cl.
1x1.
s,,
s,
Q -- 4,
%Y
References
23
355
+ 35
0
- IO
7
Channell et al.
M. Genuardo
108
4x
+ 4U
- 85
-- 63
x
(1980)
M. Barracu-
101
279
-- 44
- 7x
-114
M. Rose
117
2.80
-40
-94
-115
6
M. Bonifato
125
63
1-41
~- 103
-7x
16
.. 107
-131
36
.- 92
-140
M. San Calogero
12
M. Colomba
Sagana-Belmonte
130
116
-t36
Terrasini
115
125
t 3x
23
165
-26
1hlei
11
0
0
8
Barberi et ai..
(1974)
7
Southern
90
Custonaci
7.5
+28
67
-93
17
Schult (1976)
Apenninrr
IO
Apulia Platform
23
342
-t 3b
0
0
4
C’atalano et al.
11
Gargano
13
335
+3X
0
7
7
(1976)
7
Basin
8
Matese
320
322
1-28
63
20
6
M. Ma&ore
255
322
+31
9x
20
I
Capri
31x
286
i 54
65
56
18
M. Cerviero
306
2X7
+46
54
55
17
12
Manzoni
(1975. in
Channel1 et al.,
1980)
+ S, = 23”.
Dr = 34S” (Sicily).
Dr = 342”
(Italy).
DecI.
and
incl.
are declination
degrees. oqs is. the radius of the cone of confidence at the 95% probability
the reference strike minus the observed strike. 0, ‘-- D,, is the reference declination
declination.
and inclination
Level (Fisher,
in
1953). S, - S,, is
minus the observed
S,, was measured from regional maps.
dCcollement
surfaces. probably
consisting
of Triassic evaporites
(cf.
Bernoulli, 1978), than actual bending of the entire thrust system, including
Hsii and
basement
rocks.
The data from the Southern Apennines
(Catalan0 et al., 1976) are less complete
than the data from Sicily. Results are available for four sites in the allochthonous
Southern
Apennines,
and two sites from the Apulian
autochthon.
Results of
paleomagnetic
surveys in the aut~hthonous
Gargano region (Fig. 2) by Channel1 et
al. (1980) can be used as a reference declination
(D, = 342”) for comparison
with
allochthonous
southern Italy. The resulting normalized declinations
(D, - 0,) show
a counter-clockwise
rotation ranging from 7” to 56” for the thrust sheets of the
Southern Apennines (Table I).
When the data are plotted according to the method of Schwartz and Van der Voo
((1983), a best fit line can be calculated by linear regression. This line (Fig. 3) fails
nearly parallel to the line of ideal (i.e., unit slope) oroclinal bending (Fig. I). The
correlation
coefficient
(C,) of 0.92 suggests
strong
correlation
between
the changes
159
in declination and the changes in trend of the orogen, and therefore indicates
oroclinal bending as the dynamic process responsible for the evolution of the
Sicilian-flabby
arc. Supporting evidence for our results has been enumerated by
Catalan0 et al. (1976):
(1) Palinspastic restoration of the thrust sheets in the Southern Apennines and
Sicily leads to stratigraphic overlap (uniess bending is assumed).
(2) There is increased shortening in the sedimentary cover in both Sicily and Italy
near the ~alabria-Peloritani
allochthon.
(3) The depth of burial (or metamorphic grade) af the sediments increases
towards Calabria.
(4) The age of tectogenesis becomes older towards the Calabrian nappes.
(5) There are more internal units found near Calabria.
All of the evidence listed here, including the sense of rotations for the thrust
sheets of Sicily and the Southern Apennines, is consistent with an Early Miocene
impingement of the Calabria-Peloritani
nappes and the synchronous oroclinal
bending of the continental margin (Catalan0 et al., 1976).
IJMBRIAN
ARC
The Umbrian Arc is a convex-eastward Late Miocene-Pliocene orogen in the
Northern Apennines of Italy (Fig. 4). The northern limb of the orogen trends NW,
while the southern limb trends to the NNE, with fold axes throughout the orogen
remaining roughly tangential to the arc. This area has been a subject of major
paleomagnetic investigations in Italy since the mid 70’s (Lowrie and Alvarez, 1975;
VandenBerg and Wonders, 1976; Channel1 et al., 1978; VandenBerg et al., 1978),
and there are several controversial interpretations of the results (cf. VandenBerg and
Zijderveld, 1982). Notable among these interpretations is one by VandenBerg and
Wonders (1976). These authors postulate that the whole Umbrian fold belt, together
with its basement, has been rotated. This inte~retation contrasts sharply with that
of Channel1 et al. (1978), who explain the declination variations by systematic
rotations of ~l~hthonous sheets. We concur here with the second inte~retation.
Paleomagnetic data are available for 51 sites in the basinal facies of the Scaglia
Rossa limestone (Channel1 et al., 1978) of Late Cretaceous to Eocene age (Table 2).
The magnetic results show a systematic variation in declinations from north to south
which roughly parallels the change in trend of the orogen. Every site is dated
paleontologically, and the changes in declinations can not be explained by variations
in age (Channel1 et al., 1978). These data are best explained by some type of tectonic
rotations about vertical axes.
It must be noted, however, that recent investigations on the pal~ma~etism
of
Early Cretaceous limestones from Umbria by Hirt and Lowrie (1984) did not reveal
this pattern of declination anomalies, but these results are as yet preliminary and
only in abstract form. Numerical data for the Umbrian Arc are not directly available
43.5ON
Scaglia
Marne
I
Fig. 4. Geological
or groups
sketch map of the Umbrian
of sites are indicated
have been measured
Arc. The direction
old
a Fucoidi
Palrodrclinatlon~
of
singte tttrs or groups
of rites.
of paleodeclinations
by arrows (after Channel1 et al., 1978). Structural
of individual
and paleomagnetic
sites
data
from this map.
from the original publications
and therefore had to be determined
geometrically
from sketchmaps
in the publication
of Channel1 et al. (1978, Fig. 4). As an
illustration
of the consequences
of the choice of reference strike and reference
declination,
we will discuss our first attempt at analyzing
the Umbrian
data set.
When the data were plotted with an average measured strike (S,) of due north, the
best fit line, calculated by least squares method, fell 20” from the origin. By choosing
a new S, of 339’, the best fit line retains, of course, the same slope but its trace is
much improved and is now through the origin (Fig. 5). This suggests that if oroclinal
bending did take place in the Umbrian Arc system, then the two limbs of the arc
161
TABLE 2
Paleomagnetic data from Upper Jurassic blue micrites in the Jura Mountains *
Site
Strike
Decl.
Incl.
s, - s0
Q - 0,
+64
-28
-25
3
a95
6
91
35
7
74
216
-59
-11
-26
17
8
75
15
+59
-12
-5
6
9
73
15
+60
-10
-5
8
17
58
19
+65
-9
4
19
73
3
+47
-9
7
13
30
73
358
+47
-11
12
5
31
97
18
+46
-34
32
96
9
+54
-33
33
76
312
+65
-13
35
68
30
+49
-5
36
50
4
+61
13
6
8
39
53
0
+57
10
10
10
40
52
11
+52
11
41
55
336
+61
8
42
73
7
+53
-10
43
72
16
+44
-9
-6
44
73
15
+57
-10
-5
45
74
1
+54
-12
46
82
22
+ 35
-19
47
90
356
+64
-27
48
89
350
+55
-25
49
81
22
+61
-18
-12
15
52
103
23
+47
-40
-13
12
5
Reference
Johnson et al. (1984)
7
-8
1
58
-20
4
5
22
-1
9
34
12
3
13
17
14
9
6
14
10
-12
20
6
9
* S, = 63”, D, = 10”. See also Table 1.
may not have seen an equal amount of rotation. It may be that the original trend of
the orogen was closer to that of the northern limb, and that the southern limb has
swung
through
a larger angle of rotation.
This hypothesis
is comparable
with the
change in structural
style between northern and southern Umbria. Fold axes are
more variable to the south (Channel1 et al., 1978), and the structures are generally
more complex, possibly suggesting greater tectonic mobility in this region. These
rotations may have occurred on a decollement
surface in a Triassic evaporitic unit
(Channel1 et al., 1978; Hsti and Bernoulli, 1978). With this structural interpretation,
and the strong correlation
between change in declination
and change in strike, the
Umbrian
arc can properly be called an orocline. However, it must be noted that
deformation
does not include autochthonous
Italy, and as such does not appear to
involve lithospheric
bending; in this, Umbria resembles the Sicilian-Calabrian
Arc
where the autochthonous
basement (Iblei and Gargano) also shows no rotation.
UMBRIAN
ARC
Or - Dn
/
-10
-20
Sr= 3390
Cc- 0.72
-30
-40
Fig. 5. Diagram
JURA
showing
the strike deviations
relative
to declination
deviations
(as in Fig. 3) for the
arc. Dr = 329”.
Umbrian
MOUNTAINS
The
Jura
Switzerland
Mountains
form
and eastern
France.
while the southern
tary rocks thrust
a gently
arcuate
The northern
of the Alps,
in
limb of the foid belt trends nearly
belt,
E,
limb trends NE. The orogen consists
in the latest Miocene
(Laubscher,
northwest
of largely Jurassic
sedimen-
1961) from SE to NW over a
dCcollement in Triassic evaporites (cf. Rutten, 1969). Johnson et al. (1984) studied a
series of Late Jurassic (Oxfordian-Kimmeridgian
and Callovian)
blue and tan
limestones in the Jura (Table 3). Detailed paleomagnetic
analysis suggests that the
tan micrites carry a secondary magnetization
and are diagenetically
altered, but that
the blue micrites appear to carry a primary magnetization.
Statistical analysis of the
site means determined
for the blue micrites revealed a positive fold test (Johnson et
al., 1984), significant at the 99% confidence level (McElhinny,
1964).
The dataset from the blue n&rites were analyzed and plotted with reference strike
(S,) of 63’. It must be noted that the sampling sites are distributed over a little more
than half the entire belt: nevertheless there should be sufficient variation in strike to
163
TABLE
3
Paleomagnetic
Site
data from the Pennsylvania
Strike
DeCl.
Incl.
157
+28
salient and Virginia
s* - s*
D, - Do
12
-3
re-entrant
References
11
Van der Voo and French (1977)
1
38
JU
2
36
351
-28
-1
JU
3
48
339
-35
-13
JU
4
48
4
-42
-13
JU
5
63
342
-30
-18
JU7
22
351
-39
JUS
43
175
+30
-8
-7
5
JU9
49
1‘73
+27
-14
-4
30
JU 10
49
341
-47
-14
JU 12
34
349
-37
1
-1
29
JU 13
22
193
+22
13
-25
14
JU 14
35
154
+32
0
20
-2
9
II
10
-16
7
35
10
-2
8
37
15
JU 15
55
164
+33
-20
JU 16
48
173
+34
-13
-4
JU17
43
349
-18
-8
-1
5
24
12
BLA
72
24
-23
-37
-24
10
BLB
81
2
-30
-46
-2
10
BLC
82
0
-28
-47
BLD
82
4
-28
-47
BLE
49
343
-34
-14
-14
BLF
49
0
-29
BL L
23
336
-37
BLM
58
20
-35
-23
87
353
-24
-52
1
14
17
14
-4
12
BLP
1
5
24
4
20
8
6
CA
2
21
174
+7
14
-11
12
CA
3
21
168
-2
14
-4
8
CA
4
28
154
+10
7
10
11
CA
5
27
161
+12
8
2
11
CA
6
28
164
i-6
7
0
9
CA
7
35
160
+15
0
4
6
CA
8
35
163
+11
0
1
10
RH
1
29
328
-51
6
11
6
RH
2
26
317
-38
9
22
7
RH
2A
27
335
-45
8
4
15
RH
4
45
11
-49
-10
-32
14
RH
5
47
343
-46
-12
-4
14
RH11
49
329
-30
-14
RH 17
33
353
-27
* JU = Juniata,
BL = Bloomsburg,
10
CA = Catskill,
-14
Roy et al. (1967)
5
-20
2
Mountains
a95
JU
13
in the Appalachian
Van der Voo et al. (1979)
French (1976), French (1978)
French
14
14
RH = Rose Hill. Modified
Voo (1983). .S, = 35”, tt, varies with each formation,
and Van der Voo (1977)
French and Van der Voo (1979)
see text for details.
after Schwartz
See also Table
1.
and Van der
*
JURA
MOUNTAINS
T
Dr-
D,
/
,’
/
//’
/
/’
T i’
I
Fig. 6. Diagram
Mountains.
analyze
showing
L-60
the strike deviations
Note that reversed
for oroclinal
’
polarities
bending.
relative to declination
have been inverted
The regression
deviations
to normal.
(as in Fig. 3.) for the Jura
For numerical
line displays
data see Table 2.
a far flatter
slope than
would be expected for oroclinal bending (Fig. 6) indicating that the orogen had an
original arcuate shape. The correlation coefficient of 0.20 is indicative of the random
scatter
of the declination
data with respect
to the corresponding
study suggests that the Jura Mountains
cannot be described
there is little evidence of post-depositional
rotation in plan.
APPALACHIAN
site strikes.
as an orocline
This
because
MOUNTAINS
In order to make our review more complete we summarize
analysis of a small portion of the Appalachian
Mountains
the results of an earlier
that has already been
studied with respect to oroclinal bending by Schwartz and Van der Voo (1983). Only
a small s-curved segment of the Appalachians
between the Pennsylvanian
salient and
the Virginia re-entrant
has been studied, and this displays a change in trend from
NE-SW in the north, through N-S, to NE-SW again in the south. Paleomagnetic
data from four studies (Table 4) fullfilling the set of reliability criteria mentioned
above are available at present: the Middle Silurian Rose Hill Formation
(French,
165
TABLE
4
Paleomagnetic
Site
data from the Wyoming-Idaho
Strike
Decl.
Incl.
overthrust
belt *
S, - S‘,
f’, - 0,
Referencses
Prospect thrust sheet
T
129
111
-17
29
41
7
Grubbs
s
111
268
+30
47
70
16
(1976)
X
121
131
- 10
27
14
B
160
9
i23
-2
J
170
155
-25
-12
H
174
193
-8
-16
1
336
310
i56
-3
37
-31
and Van der Voo,
5
3
-35
8
13
20
9
Schwartz
and Van der
Voo (1984)
Darby and Absaroka thrust sheets
22
6
Grubbs
9
(1976)
V
129
316
+23
29
A
135
342
+23
23
F
164
336
+19
-6
2
6
K
164
157
-8
-6
1
11
D
158
146
-16
0
12
7
E
158
321
+13
0
17
6
-4
-5
C
183
163
-11
2
318
343
+60
12
3
324
329
+47
6
1
12
20
5
32
13
-25
4
318
310
+51
5
332
298
+61
-2
!
352
3187
i63
+73
-22
-22
from regional
geologic
and
Schwartz
-:::
14
;
sites, D, = 330” for numerical
sites. Observed
the Devonian
Catskill
A.N., 1978; French,
Formation
(Van
and
reference declination
has been selected based on the average
average strike (S,) is measured as N35”E.
A.N.
der Voo et al.,
1979) the Late Silurian Bloomsburg Formation
(Roy et al., 1967) and
cian Juniata Formation
(Van der Voo and French, 1977). The different
four formations
require a different reference declination
(0,) for each
Because there are few reliable paleopoles available from cratonic North
When the
shows a very
coefficient of
probably not
and concluded
Appalachians
strikes
(Schwartz
units.
R.B. and Van der Voo, 1977; French,
Van der Voo, 1979)
and Van der
Voo (1984)
maps, and sites 2 and 4 and sites 6 and 7 are averaged
Van der Voo, 1984). See text for stratigraphic
R.B., 1976; French,
14
-13
* S, = 158’ for all sites, D, = 158” for alphabetical
are measured
and Van der Voo
the Ordoviages of the
formation.
America, a
for each age unit. An
data are plotted by the standard method (Fig. 7) the line of best-fit
poor relationship
to the ideal oroclinal bending line, and a correlation
0.34 shows that the changes in strike and the changes in declination
are
related. Schwartz and Van der Voo (1983) have interpreted
these data
that the Pennsylvanian
Salient-Virginia
re-entrant
transition
in the
does not. display oroclinal bending. It is possible that the curvature in
APPALACHIAN
VIRGINIA-PENNSYLVANfA
Fig. 7. Diagram
showing
Pennsylvania
salient
been inverted
to reversed.
the strike deviations
and Virginia
re-entrant
For numerical
this area is a result of an original
embayment
deformation.
Rogers,
MOUNTAINS
VALLEY
of the Iapetus
relative
RIDGE
PROVINCE
to declination
of the Appalachian
deviations
Mountains.
(as in Fig. 3) for the
All normal
irregularity
Ocean,
in the cratonic
that was present
before collision
by several authors
1976; Thomas,
margin,
OVERTHRUST
have
1977). Further
(Fleming
e.g., an ancient
and subsequent
and Sumner,
work is being
portions of the Appalachians
in an effort to unravel the deformational
more complete section of the orogen (see also Alterman, 1984).
WYOMING
polarities
data see Table 3.
This theory is upheld
1975; Rankin,
AND
1975;
done in other
history
of a
BELT
The Wyoming-Idaho
overthrust belt displays a pronounced
change in trend that
is convex eastward in the region southwest of the Teton and the Gras Ventre Ranges
and west of the Game Hill reverse fault (Fig. 8). This portion of the Rocky
Mountain orogen consists of at least three major, eastwardly verging, thrust sheets:
167
u
Thrust
Fault
-
Normal
Fault
Fig. 8. Map of the Wyoming-Idaho
Van der Voo, 1984) and lettered
site-mean
declinations,
declinations
lies in the stable foreland,
Schwartz
overthrust
belt. Numbered
sites are Triassic
from Jurassic
and its declination
sites are Jurassic
sites (Grubbs
sites are reversed
is used as a Triassic
locations
(Schwartz
and Van der Voo, 1976). Arrows
relative
reference
to the Triassic
declination.
and
show
sites. Site G
Modified
from
and Van der Voo (1984).
the Prospect, the Dar-by, and the Absaroka. These rocks have been deformed in the
Late Mesozoic to Early Tertiary Sevier orogeny, during which thrusting progressed
from west to east with older thrust sheets overlying the younger. Thus movements of
the young frontal thrust sheets carried the older sheets “piggy-back”
with them. Two
paleomagnetic
analyses of rotations have been undertaken
in this region (Table 5).
The first study (Grubbs and Van der Voo, 1976) sampled 14 sites in Triassic redbeds
found mostly in the easternmost
thrust sheets, while a later study by Schwartz and
Van der Voo (1984) added data from seven Jurassic sites in the western sheets.
The Triassic data are available
for the Ankareh,
Chugwater,
and Woodside
setting
Structural
194
160 A
207
307
259
+19”“
+28 h
+oo b
203
216
203
217
265 a
272
255
240
Cabo de Peiias, Spain
Carteret
B, France
France
B, France
Crozon dolerites,
Rozel Group
Group
+29h
+OZh
France
+08 ’
Frehel redbeds.
195
Erquy-Cap
203
+06 h
290
France
h
272
redbeds,
t25
+19 *
+19 d
Zone Bocaine, France
Cambro-Ordovician
217
224
307
Lava1 syncline sediments.
France
Lava1 syncline tuffs, France
Northern limb
tienza, Spain
137
San Emiliano,
+13”
+ 34 .‘.h
144
135 d
125 a
San Pedro, Spain
Spain
188
d
to5
179
+ 36 ’ h
Incl
Europe
180 a
Decl
and central
180 a
Strike
data for western
Buqaco “ D”, Portugal
palaeomagnettc
Bucaco “ c”, Portugal
Southern limb
Ibero - Armorican arc
and Carboniferous
5
Devonian
TABLE
4
-30
-45
-62
- 55
- 80
L
-28
- 14
(1984)
(1984)
Perroud
Perroud
Perroud
7
10
Perroud
et al. (1983)
et al. (1982)
et al. (1982)
and Bonhommet
Jones et al.. (1979)
Jones et al. (1979)
14
6
12
Duff (1979)
Edel and Coulon
14
Edel and Coulon
13
-27
and Bonhommet
Van der Voo (1967)
Perroud
and Bonhommet
6
-6
~- 18
and Bonhommet
( 1981)
(1981)
(1981)
(198 I )
Perroud and Cobbold (1984)
Perroud
Perroud
Reference
18
12
-- 14
- 18
-49
-62
- 35
-28
-8
48
41
-5
-97
-97
50
85
75
30
30
*
redbeds,
dykes, France
sediments
Freshwater
’
data, indicating
limb of the Central
’ Remagnetized
European
directions
arc is represented
overprint.
b
and central
1-24
-04
+30
-02
+a2
i-20
-11
+10 b
+ 10
+ 16
+09
+17 h
+14
+20b
Europe.
-30
-30
-30
-30
-55
-70
-70
-70
-65
-65
-65
-65
-60
-60
by the northern
8
15
11
-9
-65
-55
-33
-62
-23
-10
-11
-24
- 14
-30
See also Table
1.
limb of the Ibero-Armorican
of the Bay of Biscay (see text).
for western
189
183
186
203
258
278
222
251
212
199
200
213
203
219
after the closing
240
240
240
240
265
280
280
280
275
275
275
275
270
270
late Hercynian
declination
paleomagnetic
’ The western
and Carboniferous
strike and magnetic
* Devonian
a Regional
“D”, W. Germany
Harz mountains
W. Germany
“c”,
Ham mountains
forest “D”, W. Germany
Franconian
France
forest “C”, W. Germany
“D”, Wales
“c’, Wales
Wales
France
Franconian
Massif Central,
Eastern limb
Central European arc
sediments
Freshwater
Mill Haven sediments“P-C”,
Brittany
North
Islands
Channel
Jersey dolerites,
granite,
France
France
Tregastel-Ploumana~‘h
Plourivo
granite,
France
Flamanville
Thouars,
arc.
19
18
8
8
12
15
7
8
9
7
12
15
18
Perroud
Brown (1983)
Brown (1983)
McClelland
Bachtadse
Bachtadse
Bachtadse
Bachtadse
(1984)
(1984)
(1984)
(1984)
Edel et al. (1981)
Brown (1983)
McClelland
et al. (1979)
McClelland
DeBouvier
Duff (1980)
Duff (1979)
Jones et al. (1979)
(1972)
and Van der VOO (1985)
Van der Voo and Klootwijk
170
Formations
(Grubbs
magnetizations.
average
The
of three Triassic
1976). A reference
and
reference
Jurassic
der Voo,
1976), and
(D,)
declination
sites from the stable
this value
is identical
display
of 158”
foreland
stable,
pre-folding.
was calculated
(Grubbs
for the
and Van der Voo,
to be the most representative
to the Triassic
reference
value
declination
and Van der Voo, 1984).
The Jurassic
zations,
Van
strike (S,) or 158” was found
by coincidence
(Schwartz
and
reference
and
rocks from the Oxfordian
Stump
Formation
they also pass the fold test (Schwartz
declination
Morrison
and
display
of 330” was taken from the contemporaneous
Formation
1975) and the same reference
Voo, 1984).
All data from the Prospect
sampled
in the Colorado
stable magneti-
Van der Voo. 1984). A
Plateau
and undeformed
(Steiner
strike (S,) of 158” was used (Schwartz
thrust
sheet are plotted
and Helsley,
and Van der
on Fig. 9a. Linear
regression
results in a significant correlation coefficient of 0.85 as well as a regression line that
is approaching
ideal oroclinal bending. On the other hand, for the data from the
Darby and Absaroka sheets (Fig. 9b), the correlation coefficient is 0.34, and the best
fit line does not correspond well to ideal oroclinal bending. As the Prospect thrust is
WYOMING OVERTHRUST
PROSPECT
THRUST
SHEET
BELT
I
171
WYOMING
OVERTHRUST
BELT
OARBY AND ABSAROKA THRUST SHEETS
Fig. 9. Diagrams
Prospect
thrust
showing
the strike deviations
sheet of the Wyoming
Wyoming
overthrust
belt. For numerical
(Schwartz
and Van der Voo, 1984).
thrust
relative
to declination
belt. b. For the Darby
deviations
(as in Fig. 3). a. For the
and Absaroka
data see Table 4. Strike data are measured
thrust
sheets in the
from regional
maps
the youngest thrust in the belt, any rotatjonal bending that the Prospect sheet has
undergone should have been transmitted to the overlying, older thrust sheets. As can
be seen from the graphs, this is not the case. The interior of the thrust belt shows no
systematic variation in declination, and this diminishes the likelihood that the
Wyoming-Idaho thrust belt can be termed an orocline. Detailed structural work in
the area suggests that the rotations seen in the Prospect thrust sheet are actually
local rotations caused by the crumpling of the frontal edge of the thrust belt as the
sheets were pushed against the ancestral Teton and Gros Ventre uplifts (Grubbs and
Van der Voo, 1976; Schwartz and Van der Voo, 1984).
THE EUROPEAN
HER~YNIDES
The European Hercynides between Poland in the northeast and Portugal in the
southwest are represented by an irregularly shaped mountain belt of approximately
6000 km length. The sedimentary and structural history has been studied in great
detail (cf. Rutten, 1969) and is well known.
The mountain belt itself eonsists of a central crystalline ridge containing Hercynian
syn- and post-erogenic granites and Precambrian and lower Paleozoic rocks, reactivated and metamorphosed by Hercynian high-temperature low-pressure meta-
173
morphism
on
both
sediments
between
(Zwart
sides
and Dornsiepen,
by
fold
belts
and volcanics.
The main
late Visean to Westphalian
The Hercynian
197X). The internal
consisting
Hercynian
of the mountain
ridge is bordered
metam~~rph~~sed
def~~rrnati~~nal events
(cf. Windley,
fold belt is characterized
E<xcluding the curvature
crystalline
of low-grade
Paleozoic
can be dated
1984).
by a complex
belt southwest
arcuate
shape (Fig.
of the Tornquist
IO).
line at the
boundary between the stable Kussian platform and mobile Europe. its arcuate shape
can be defined by the change in structural
trend from the so-called Variscan
direction (NE-SW) of central Europe to the western European Armorican direction
(NW -SE) in the Massif Central of France, followed hy the strong curvature of a
western branch around the Bav of B&cay \vhich gives rise to the Ib~r(~-,4r~~oric~Il
structural
Ardennes.
arc. There is approximately
After
restoring
X0 of curvature
the pre-Mesozoic
in the Massif Central
pale~~ge~)graphy
around
the
and the
Bay of
Biscay by rotating the Iberian peninsula about 35” clockwise (Van der Voo, 1969)
around an Eulerian pole located in the western Pyrenees (Carey. lY5S; Ries, 1Y7X)
the curvature
and the Iberian
Several
resulting
from the structural
peninsula
models
coherence
of the Paleozoic
totals 165” (Ries and Shackleton.
have been proposed
during
recent
of Britanny
1976).
years in order
to explain
the
arcuate shape of the Hercynian orogen. Argand (1924) and Carey (1955) originally
recognized the curvature of structural elements around the Bay of Biscay and Carey
postulated secondary processes leading to the deformation
of an originally straight
fold belt. In more recent times. Ries and Shackleton (1976). Perroud and Bonhommet (1981 f. Lorenz ( l9’76). Lorenz and Nicholls ( 1984) and Ziegler (19X4) interpreted the irregular shape 3s the result of the accretion of one or several microplates
onto the irregularly
shaped continental
margin of the North American--North
European continent
and the subsequent
tightening of pre-existing structures during
the final stages of the Hercynian collision (Perroud and Bonhommet.
1981). lmplying a different tectonic setting, Arthaud and Matte (1977) postulated the rotation of
discrete continental
segments along huge strike-slip systems in a Late Carboniferoust Early Permian dextral shear system between
Lefort and Van der Voo (1981) used the similarities
the Urals and Appalachians.
in structural evolution of the
Hercynides
and the H~rnaIay~ls (Moinar
and Tapponnier,
ii~dentati~~l~ by the West African Shield into eastern North
Europe causing
subsequent
displacenlent
and deformation
1977) to propose an
America and western
of the eastern
part of the
Hercynides analogous to the peri-Himalayan
structural setting.
Devonian
and Carboniferous
paleomagnetic
data from the autochthonous
and
parautochthonous
rocks of Hercynian Europe are now available for the central and
western part of the fold belt (Table 5: Fig. 10). These data enable us to test the type
of deformation
of the orogen. The reference declination
has been calculated for the
different regions analyzed from the Late C’arboniferous
pole position for stable
Europe (41”N, 166OE) (French, 1976). A reference declination
(D,.) for the Early
Devonian of Wales (McClelland
Brown, 1983) has been calculated from the Siluro-
Fig. 10 Geological
sketch map of the European
Voo, 1969) showing
Devonian
stipled.
the direction
to Carboniferous
A --Han
-Cantabria;
(3)
mountains;
of structural
magnetic
B -Franconian
Hercynides
(after closure of the Bay of Biscay-Van
trends (I ), the northern
declinations
Forest;
overthrust
(see also Table
C -Massif
5). Major
Central;
der
(2) and the directions
Paleozoic
D - Britanny;
massifs
E-Wales;
of
are
F
G -Calicia-Castilla.
Dr *Do
HERCYNIAN
-80
Fig. 11. Graph
Dashed
EUROPE
101
/
I’
T
of strike deviations
relative to declination
line is the best fit line calculated
deviations
using linear regression.
(as in Fig. 3) for Hercynian
For numerical
Europe.
data see Table 5.
174
Devonian
reference
pole position (Duff.
strike (SY 1. Linear
N = 28) correlation
coefficient
strike and declination.
than
expected
reference
1980). A strike direction of 210” has been chosen as
regression
reveals a reasonably
high (0.72784 for
and suggests
a strong
relation
The slope of the line of regression.
for perfect
oroclinal
bending
and
however,
differs
line with unit slope (Fig. 11). The flattening
between
change
I\ lower (21*)
significantly
from
of the regression
of Perroud
and Bonhommet
(1981) who showed
the
line can be
explained as the consequence
of less intense tightening of the Ibero-Armorican
than would be implied by its present curvature. This observation
is consistent
the results
in
arc
with
that 70” of structural
curvature are of primary, pre-Hercynian,
origin, whereas at least X0” of rotation are
of post-Early Carboniferous
age and can be ascribed to the secondary tightening of a
pre-existing
and already
curved
Europe. These results support
Hercynides
fold belt during
the oroclinal
and suggest the bending
the Hercynian
concept
collisions
for the bending
in western
of the European
is the result of either indentation
or tangential
stressfields.
CONCLUSlONS
Of the six arcuate deformed belts that we have analyzed in this study, at least
three show significant evidence of orochnal bending. The Jura Mountains
fail the
“orocline test”, and the variations in declinations
that can be seen are probably due
to local deformation
rather than a systematic rotation in plan. The arcuate nature of
the belt can be explained by an originai curvature that was present when the Jurassic
carbonates
were deposited, which was subsequently
compressed
uniformly
along
strike during the Alpine orogeny. The Pennsylvania
salient and the Virginia re-entrant of the Appalachian
Mountains also fail the paleomagnetic
criteria for oroclinal
bending.
Here the arcuate
the cratonic
Wyoming
margin
overthrust
tion of processes.
undergone
prior
of the belt can best be explained
to the collision
belt has a curvature
Certainty
systematic
nature
the frontal
rotation,
of North
that can be explained
portion
by irregularities
America
of the thrusted
and
Africa.
in
The
only by a combinarocks appears
albeit on a local scale. The ancestral
Teton
to have
and Gros
Ventre Ranges as well as the Game Hill reverse fault probably acted as buttresses
that impeded the forward progress of the thrust sheets. These allochthons
did not
react by rotating as an entity. but rather crumpled at the frontal edges. giving an
appearance
of oroclinal bending for the Prospect thrust sheet. The question still
remains as to the cause of the arcuate thrust traces west of the Prospect. One
explanation,
which seems unlikely, is that the sheets were originally more extensive
than today but have undergone
differential
erosion, effectively stripping off the
cover to the north, giving the thrust traces an arcuate exposure. A more likely
explanation
is that the rocks broke along an irregular line and were thrust without
significant rotation into their present position.
The three other belts show significant
evidence
of oroclinal
bending.
The
175
Sicilian-Calabrian
ward the middle
belt definitely
displays
of the arc, in addition
of a secondary curvature. The arcuate
pronounced
as in the Sicilian-Calabrian
although
asymmetric,
oroclinal
bending.
Sicilian
and Italian
rotation
systematic
arcs, being
declinations
belt is in a different
much larger in extent
geologic complexity,
the paleomagnetic
data
deformational
history of the belt, suggesting
to all the questions
are several different
pertaining
processes
to-
in favor
that
is evidence
for
than
as well as more pervasive
the
in
basement
seems clearly
Europe displays a great
utilized here shed some light on the
oroclinal bending as a contributing
cause of present-day
geometries.
Simply stating that an orogen has undergone
answers
sheets
earlier)
class of orogens
effect. In the Hercynian
belt, moreover, the lithospheric
involved in the large-scale bending. Although Hercynian
There
of thrust
(outlined
trend of the Umbrian
arc is not nearly as
arc, but it also displays a systematic,
of paleomagnetic
The Hercynian
rotation
to other evidence
oroclinal
bending
does not provide
to the causes of its deformational
that can be called upon
to produce
history.
oroclines:
(1) compressional
stresses and “buckling”;
(2) ball bearing effects in megashear zones (Van der Voo and Zijderveld,
1969;
Beck, 1976);
(3) impingement
of a hitherto straight zone against a rigid and irregular margin,
causing indentation;
(4) radial emplacement
off a rising highland.
The Sicilian-Calabrian
arc is a good example of the third model, with the
Calabria-Peloritani
block splitting away from Sardinia and Corsica (Alvarez et al.,
1974) and acting as a rigid block which impinged on the southern Tethyan margin of
Sicily and Italy during the Late Tertiary. The effect of this impingement
was the
thrusting of the platform and basinal carbonates onto the stable foreland of Apulia
and Iblei, and the formation
of an orocline.
some effect of shear as the Peloritani
clockwise
northeast
On a smaller
scale, it is possible
to see
block slides past the Sicilian region, causing
the
rotations seen in the thrust sheets of Sicily, while the reverse is seen to the
in Calabria.
The Umbrian
and Wyoming
arcs appear
to have had a very similar history to that
of the Calabrian arc, with impingement
being
Umbria and Wyoming, there are no “foreign”
to indent from behind,
against the autochthonous
but the entire
foreland.
the cause of bending.
In the case of
blocks such as the Peloritani nappes
sections have been compressed
and bent
The two pronounced
arcs within the Hercynian
mountain
belt both display a
strong relationship
in change of strike and declination
which justify them to be
called oroclines. The deformation,
however, can be due to one or more of the
following causes:
(I) Adjustment
of microplates
the cratonic Laurasian “Foreland”
(2) Major dextral
during their collision
(Lorenz, 1976).
shear (Arthaud
and Matte,
1977).
with the irregular
margin
of
176
(3) Paleo-east-- west directed
(4) Indentation
a Gondwana
derived
The distribution
microplate
be the location
(Lefort
of major
rocks.
subduction
high pressureelow
arcs
and collision
therefore
must
temperature
belts strongly
times. The post-Devonian
Ibero-Armorican
margin of Gondwana
or by
and Van der Voo, 19X1).
and paired metamorphic
Siluro-Devonian
and
Europe by the irregular
of ophiolitic
phism, nappe structures
during
stresses.
of Hercynian
of a microplate
bending
metamor-
suggests Britanny
of the Central
be interpreted
to
with Armorica
European
as a consequence
of
continued continental
convergence during the final suturing between Gondwana
and
Laurasia during the Carbonifercus
(Lefort and Van der Voo. 1981) in a tectonic
setting analogous to the one proposed for the Himalayas and the peri-Himalayan
region (Molnar
and Tapponnier.
1977).
As a result of this study on a small number
of orochnes.
it must be concluded
that
the most probable mechanism for oroclinal deformation
is the indentation
of rigid
crustal blocks into plastically-reacting,
more or less unconsolidated
fold belts. The
role of indenter
can be played either by promontory-like
irregular continental
margins (Channel1 et al.. lY79) or continental
blocks such as the Ebro-Aquitaine
microplate
(cf.
Ziegler.
1Y84). which
indented
Hercynian
Europe
during
the
Carboniferous.
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