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Tectonophysics, 139 (1987) 107-122 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
107
Accretion of southern Alaska
JOHN W. HILLHOUSE
U.S. Geological Survey, 345 Middlefield Road Menlo Park, CA 94025 (U.S.A.)
(Received March 7,1986; revised version accepted June 20,1986)
Abstract
Hillhouse, J.W., 1987. Accretion of southern Alaska. In: D.V. Kent and M. Krs (Editors), Laurasian Paleomagnetism and Tectonics. Tectonophysics, 139: 107-122.
Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago, and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation.
Introduction
A current theme in Alaskan geology is the growth of the Alaska continental margin by successive accretion of terranes carried by oceanic plates from the Pacific Basin. Ever since Packer and Stone (1972) presented the first paleomagnetic evidence that Alaska has moved northward rela- tive to the North American craton, additional support for the accretionary model has come from paleomagnetic studies of southern Alaskan ter- ranes, such as the Wrangellia (Hillhouse and Gromme, 1984), Peninsular (Stone and Packer, 1979; Stone et al., 1982), and Prince William terranes (Plumley et al., 1983). The catalogue of Alaskan paleomagnetic poles has grown to a size such that paleolatitudes and the timing of accre- tion of the major terranes are roughly constrained. For example, the travel histories of some southern Alaska terranes have been interpreted from plots of paleolatitude against time (Stone et al., 1982; Panuska and Stone, 1985a). Although the plots
employ paleomagnetic data of variable quality and the details of the implied terrane movements are debatable, the general trend of northward drift is clear.
With the development of the plate tectonic model, a common goal of many geologists has been the explanation of Alaskan geologic events in terms of relative motions between North America, Asia, and the oceanic plates of the Pacific region (e.g., Scholl et al., 1975; Cooper et al., 1976). The most difficult aspect of this goal has been the reconstruction of the motions of the oceanic plates relative to Asia and North America. A quantita- tive method for reconstructing the relative mo- tions between the plates was presented by Engebretson (1982), who used the fixity of hot- spots in the Pacific and Atlantic basins to provide a reference frame for the plate interactions. Using this method, Engebretson et al. (1985) modeled oceanic plate velocities at the North American margin from 175 Ma ago to the present. The Farallon plate is believed to have undergone sub-
0040-1951/87/$03.50 0 1987 Elsevier Science Publishers B.V.
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duction beneath the Alaskan margin during
Mesozoic time, followed by subduction of the
Kula plate 85-43 Ma ago and the Pacific plate
from 43 Ma ago to the present. The rates and
directions of oceanic plate movements are likely to
have left their imprints on the geology of the
Alaskan margin, both in the emplacement of sub-
duction-related plutons and in the accretion of
far-travelled terranes (Wallace and Engebretson,
1984).
As determined from the plate reconstructions,
much of the oceanic crust that has arrived at the
Alaskan continental margin originated far to the
south, consistent with the trends in paleolatitude
observed from paleomagnetic study of southern
Alaskan terranes. Oceanic crust is found in some
terranes, but for the most part the terranes consist
of continental margin deposits and volcanic arc
assemblages. Thus, although the terranes were
probably transported to higher latitudes by the
oceanic plates, they were not created at oceanic
spreading centers. Instead, transcurrent motion of
oceanic plates along the continental margins must
be an important mechanism for the transport of
Alaskan terranes.
The Yakutat terrane (Fig. l), which is now
attached to the Pacific plate, is a modern example
of the transcurrent accretion process
PACIFIC OCEAN
(Plafker.
Fig. 1. Latitudinal displacements inferred from paleomagnetic studies (Table 1) of Lower Cretaceous and older rocks in Alaska.
Displacements are in degrees of latitude, assuming terranes originated in the northern hemisphere; displacements to higher latitudes
are indicated by upward arrows. Downward arrows indicate displacements to lower latitudes relative to the craton. Length of arrow is
proportional to the amount of displacement. Selected terranes of Howell et al. (1985): A-Alexander. Chugach; C-Chulitna;
NF-Nixon Fork; P-Peninsular, Stikinia; W-Wrangellia, Yakutat; YT-Yukon-Tanana. Overlap assemblages: Cr-undifferen-
tiated Cenozoic rocks; IU-Jurassic and Cretaceous flysch.
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1983). The eastern boundary of the Yakutat ter-
rane is the Fairweather fault, which offsets
moraines and drainages in the right-lateral sense.
During the last 1000 years, the rate of offset on
the Fairweather fault has been similar to the rate
of displacement between North America and the
Pacific plate as determined from marine magnetic
anomalies (Plafker et al., 1978). Transcurrent mo-
tion gives way to thrusting at the northwest
boundary of the terrane where it is colliding with
the Prince William terrane near the eastern end of
the Aleutian trench. Accretion of the Yakutat
terrane is only the latest event in the episodic
accretion of southern Alaska, a progression of
collisions involving terranes such as Prince Wil- liam, Chugach, and Wrangellia.
My main goal in this paper is to propose a scenario for the accretion of southern Alaska, integrating the paleomagnetic data from terranes with the plate-reconstruction model. The first step is to review Alaskan paleomagnetic data to iden- tify the strong and weak points in paleolatitude determinations. The data set undoubtedly con- tains errors in excess of the analytical uncertain- ties, as for example, some formations give inter- nally inconsistent results. The second step is to compare the stronger paleomagnetic constraints on terrane displacements with relative plate mo- tions as proposed by Engebretson et al. (1985) to seek a mutually consistent model for the accretion of southern Alaska.
Paleomagnetic data from Early Cretaceous and
older rocks in Alaska
The following discussion is keyed to Fig. 1,
showing the distribution of paleomagnetic studies
in pre-upper Cretaceous rocks of Alaska. The base
map shows the fault boundaries of selected
tectonostratigraphic terranes, as defined by How-
ell et al. (1985). Each terrane is defined by a
distinctive stratigraphy, provenance, and paleon-
tology which distinguish it from neighboring ter-
ranes. The localities of paleomagnetic studies were
compiled from the literature and are plotted on
the terrane map. The paleolatitude and 95%confi-
dence limit were determined for each site, then a
paleolatitude anomaly and 95%confidence limit
were calculated (Table 1). The paleolatitude
anomaly, represented by arrows in Fig. 1, is de-
fined as the predicted latitude, assuming the site
has remained fixed to the north American craton,
minus the measured paleolatitude from the
paleomagnetic studies. As a starting point for the
discussion, each paleolatitude is assumed to be
within the northern hemisphere, although the op-
tion of placing some terranes at certain times
within the southern hemisphere must be kept open.
(Study No. 3, in Table 1, was assigned a southern
hemisphere paleolatitude because its declination is
too similar to the declinations from study No. 2
and study No. 4 to warrant switching its polarity.
In another exception, study No. 24, the Pybus
Formation was presumably magnetized during the
Permo-Carboniferous Reversed-Polarity Su-
perchron, which would require placement of the
site in the southern hemisphere.) A positive
anomaly implies that a given site has moved to a
higher latitude or northward relative to stable
North America. Paleomagnetic poles from each
locality were compared to an appropriate-aged
reference pole either from Irving and Irvings
(1982) compilation for North America or from
other sources, as noted in Table 1, to derive the
paleolatitude anomalies. Confidence limits on the
paleolatitudes and anomalies were adjusted
according to the method of Demarest (1983).
The purpose of Fig. 1 is to convey the general
temporal and spatial trends in the paleolatitude
anomalies, as indicated by the length of the arrows.
It is readily apparent that although a general sense
of northward drift is depicted for terranes south of
the Denali fault, the amounts of drift are not
mutually consistent. The differences in paleolati-
tude anomalies between sites in coeval rocks from
the same terrane sometimes exceed the confidence
limits. Clearly, the overall data set contains hid-
den errors in excess of the analytical errors, which
were primarily derived from the internal scatter of
magnetic directions determined at each site. Such
hidden errors might be due to unrecognized post-
depositional components of magnetization, insuf-
ficient averaging of secular variation to meet the
axial dipole assumption, or incorrect corrections
for structural tilt. The fold test, which is a com-
parison of directional dispersion before and after
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TABLE 1
Paleomagnetic data from Alaska: Lower Cretaceous and older rocks *
No. Age/ Identifier
Site S N n Thermal Fold Rever- Paleolat. Paleolat. Refer- Refer-
lat. long. demag. test sal f) anomaly ence ence
() () test (1 pole for
data
E. Cretaceous
1 HND-8B
2 LCL-1
3 LCL-5
4 LCL-6
5 NUT-l.3
6 NBS-3
Jurassic
7 CHL-2,4,5
8 WRG-3
9 WRG-4,5
10 TXD-4,5,7,8,9
11 TXD-2
12 SLD-1
13 SLD-2
14 SHP-5
15 SHP-X
Triassic
16 Hound Island
17 CWM-1.2
18 MNT-1.3
19 NBS-2
20 NBS-5.6
21 Mt. Hayes
22 McCarthy
Permian
23 Nuyukuk Lake
24 Pybus
25 Hasen Creek
Permian and
Carboniferous
26 Station Creek
Carboniferous
27 Ladrones-Klawak
28 Peratrovich
Devonian
29 Port Refugio
30 Wadleigh
Ordovician
31 Descon, Iavas
32 Telsitna Ridge
33 Novi Mountain
55.2 199.0 1 6 6
60.3 205.6 1 15 ?
60.3 205.6 1 7 ?
60.3 205.6 1 14 ?
62.4 217.4 1 11 11
62.6 217.0 1 11 11
56.3 201.1 3 ? 31 28.7 i 21.7 31.1 f 25.4 b 3
61.5 217.2 13 ? 25.8 i 19.7 28.7 f 24.6 c 2
61.5 217.2 1 11 ? 20.0 + 8.7 34.5 f 17.2 c 2
60.0 207.3 5 ? 70 39.6 rt 13.8 13.1 * 16.3 d 3
60.2 207.3 1 14 ? 29.9 + 10.2 23.0 f 13.3 d 2
59.4 208.3 1 11 ? 12.6 f 7.1 31.7 k 8.1 e 2
59.4 208.3 19 ? 5.6 k 9.1 38.7 F 9.9 e 2
61.7 212.3 1 12 ? 4.0* 9.1 41.2 k 9.9 e 2
61.9 212.5 1 11 ? 18.6 & 5.3 26.7 f 6.6 e 2
56.9 226.1 12 12 139 *
63.1 213.0 2 6 28
62.9 216.1 3 ? 30
62.6 216.9 2 ? 11
62.4 217.0 2 9 17
63.2 214.0 6 46 292 *
61.6 217.4 5 50 91 *
59.9 201.0 9 9 23 *
57.3 225.8 ? 12 24
61.7 217.7 1 11 11 *
61.7 217.4 1 10 10 *
55.4 226.9 4 33 33 *
55.4 226.8 5 28 28 *
55.3 226.7 5 30 30 *
55.5 226.9 5 31 31 *
55.1 226.6 2 8 85
63.9 206.3 7 14 90 *
63.9 206.4 9 19 89 *
_
48.4 & 19.1 21.3 + 22.8 a I
13.7 & 4.1 56.8 + 13.1 a 2
16.1 ? 12.8 86.4 f 17.9 a 2
1.5 rf- 3.4 71.8 i 12.9 a 2
18.6 i 7.6 48.2 + 14.6 a 2
22.8 * 3.3 44.3 + 12.9 a 3
* 46.7 + 8.6 -9.2 + 9.7 f 4 15.5 f 11.8 26.8 + 12.3 g 2, 5
1.5 It: 5.5 39.9 * 6.6 g 2. 5
22.8 f 2.0 18.2 i 4.1 g 2, 5
1.0 f 9.7 39.7 i 10.3 g 2, 5
13.9 + 3.8 28.2 &- 6.0 g 6
* 10.4 f 2.7 29.6 k 5.3 g 6, 7
38.4 i 7.8 2.9 i 9.5 h 8
-9.1 * 7.0 40.9 i 7.7 i 9
7.4 + 5.2 29.5 i 6.0 i 10
5.2 + 9.1
* 7.7 * 7.0 11.2 f 9.1 k 14.0 i 8.7 4.9 + 9.8 k
20.7 k 6.2
10.3 * 5.9
1.4 + 22.1
33.2 f 4.5
37.1 * 4.5
31.0+ 9.6 J
-6.6 + 8.4 I
3.6 + 8.4 I
28.6 f 24.9 m
4.9 * 10.2 m
-1.1 f 16.1 m
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TABLE 1 (Continued)
No. Age/ Site SN n Thermal Fold Rever- Paleolat. Paleolat. Refer- Refer-
Identifier lat. long.
() ()
demag. test sal test
() anomaly
()
ence pole
ence for data
Ordovician and
Silurian
34 Descon, seds. 55.1 226.8 3 14 14 * * * 6.8 f 9.4 25.2 f 18.1 m II
* Paleolatitudes and paleolatitude anomalies are given with 95% confidence limits, in degrees. Positive anomalies imply displacement to a higher latitude. S. number of sites, N number of time samples, n total number of specimens. Key lo reference poles: a-64 o N, 163 E, A,, = 16 ; Irving and Irving (1982); 140 Ma. b-61 o N, 13S E, A,, = 17 ; Irving and Irving (1982): 150 Ma. c-70 N, 102O E, A - 19O; Irving and Irving (1982); 160 Ma. d--74ON, 102OE, A,, = 11; Irving and
95- Irving(1982); 170Ma. e--66ON,93OE, A 9s = 5 ; Irving and Irving (1982); 190 Ma. f-65.3O N, 94.2O E, A,, = 5.9 ; Hillhouse and Gromme (1980); Late Triassic. g-61.4 N, 92.5 o E, A,, = 4.6 ; Hillhouse and Gromme (1984); Middle Triassic. h-46 o N, 117OE, A,, = 7O; Karl and Hoare (1979); Permian. i--48N, llSOE, A,, = 4O; Irving and Irving (1982); 260 Ma. j--43ON, 126E, A,, = 4; Irving and Irving (1982); 280 Ma. k-31.3ON, 124.6E, A,, = 7.5 O; Kent and Opdyke (1985); Carboniferous. I-28N, 120E, A,, = 8.1; Van der Voo and Scotese (1981); Devonian. m-32N, 147.8E, A9, = 19.9; Plumley (1984); Ordovician. Key to references: 1 -Stone and Packer (1979); 2-Stone et al. (1982); J-Packer and Stone (1974); I-Hillhouse and Gromme (1980); S-Stone (1982); 6-Hillhouse and Gromme (1984); 7-Hillhouse (1977); a-Karl and Hoare (1979); 9-Panuska and Stone (1985a); IO-Panuska and Stone (1981); If-Van der Voo et al. (1980); 12-Plumley (1984).
corrections are made for tilt of the bedding, tests if the magnetization was acquired before folding occurred. This test also reaffirms the accuracy of the tilt corrections. The consistency of reversals test is met when the means of normal and re- versed-polarity directions do not deviate signifi- cantly from antipolarity, increasing the likelihood that spurious or secondary components of magne- tization have been completely removed.
Statistical weights of the data points are not equal for the following reasons: (1) Reliability criteria, such as the fold test or consistency of reversals test, may or may not be satisfied. (2) Thoroughness of the demagnetization treatments, whether both alternating field and thermal meth- ods were applied, and rigor of magnetic vector analyses may vary. (3) The number of specimens and time span represented by each collection are highly variable. For example, results from study No. 22 (Table 1) are from a total of 91 indepen- dently oriented specimens collected from 50 lava flows. The five sites were distributed over a maxi- mum distance of 30 km. The results pass the fold test and consistency of reversals test. In contrast, site 1 from the Alaska Peninsula is based on six specimens from a single outcrop which provided
no opportunity for a fold test or consistency of reversals test.
To help in assessing the reliability of a given paleomagnetic result, the following factors are tabulated: number of localities, number of time samples, total number of independently oriented specimens, availability of thermal demagnetization results, fold test, and consistency of reversals test. In most cases, the number of localities refers to outcrops that are at least several kilometers apart and have different structural attitudes. A time sample is defined as an individual lava flow, cool- ing unit, or sedimentary bed. All studies in Table 1 employed alternating-field demagnetization to obtain the final results, so only the utilization of thermal demagnetization to assess thermal stabil- ity of the remanence is noted.
The more reliable results are from the Triassic and older rocks of the southern terranes both in terms of having the greater number of specimens and more positive reliability factors. In particular, lava flows of the Middle and (or) Upper Triassic Nikolai Greenstone (sites 21, 22) in the Wrangellia terrane have the largest distribution of samples and satisfy the most reliability factors. In the Alexander terrane of southeastern Alaska, good
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reliability can be assigned to the studies of the
Paleozoic rocks (sites 27-31, 34) and the Hound
Island Volcanics (site 16) of Late Triassic age, but
the area of the sampling is quite small.
While the Jurassic sites (7-15) are well distrib-
uted in southern Alaska, the results are weak
because they lack thermal demagnetization and
the conventional tests for magnetic overprints.
The Jurassic data for the most part, however, yield
consistent paleolatitudes, so overprinting might
not be a serious problem. Results from the Lower
Cretaceous rocks are very weak due to the small
number of samples and the very poor internal
consistency of the paleolatitudes, as for exampl.e,
sites 2, 3, and 4. Magnetizations determined from
the Early Cretaceous sites may very well be con-
taminated by overprints; however, this possible
problem cannot be evaluated with the data in
hand.
Paleopositions of the southern terranes prior to the Late Cretaceous
The pattern of paleolatitude anomalies from
the Paleozoic and Triassic rocks indicates that the
Nixon Fork (sites 32, 33) and Alexander terranes
have moved little in terms of latitude relative to
North America, while large poleward displace-
ments are inferred for the Wrangellia terrane. No
results from Triassic rocks of the Peninsular ter-
rane are available. Before the original paleolati-
tude of Wrangellia can be reconstructed, it is
necessary to determine whether the Nikolai basalts
(sites 17-22) cooled mainly during a period of
normal polarity or during a period of reversed
polarity. Knowing the polarity would set the sign
of the paleolatitude, either north or south of the
paleoequator. Although the magnetic directions
are dominantly of one polarity (upward and to the
northeast), the rapid succession of polarity transi-
tions during the Late Triassic makes it impossible
to choose the sign of the resultant pole. The
options are as follows: (1) If the larger group of
directions is normally polarized, then the terrane
originated in the southern hemisphere. (2) If the
polarity is reversed, the terrane originated in the
northern hemisphere. Panuska and Stone (1981)
presented preliminary results from Pennsylvanian
and Permian volcaniclastic rocks (Nos. 25, 26)
which presumably were magnetized during the
Permo-Carboniferous Reversed-Polarity Super-
chron, to solve the ambiguity concerning the origi-
nal position of Wrangellia. Magnetic directions
from these Paleozoic volcanic rocks are similar to
directions from the overlying Triassic lavas at
McCarthy, leading Panuska and Stone to choose
the tropical zone of the northern hemisphere as
the original position of Wrangellia. If true, the
northern option implies that the part of Wrangel-
lia now in south-central Alaska has moved to a
higher latitude relative to North America by about
3o.
The Wrangellia terrane is recognized in fault-
bounded blocks along the Pacific coast of British
Columbia on the Queen Charlotte Islands and
Vancouver Island. The southernmost part of the
terrane is in Hells Canyon in eastern Oregon
(Jones et al., 1977). Paleomagnetic studies on
Vancouver Island (Irving and Yole, 1972; Yole
and Irving, 1980) and in Oregon (Hillhouse et al.,
1982) gave similar paleolatitudes, all consistent
with the determinations from the Nikolai Green-
stone in Alaska (Hillhouse and Gromme, 1984).
Apparently, this once-coherent terrane has been
dismembered by faulting and fragments have dis-
persed to their present positions along the con-
tinental margin. Under the northern option, the
fragment in Oregon has changed little in latitude
relative to the craton. However, declinations from
the Triassic Wrangellian rocks of Oregon and
Vancouver Island imply rotations on the order of
180, if the northern option is correct (Yole and
Irving, 1980; Hillhouse et al., 1982). The small
data set from the Pennsylvanian and Permian
sedimentary rocks at McCarthy does not prove
beyond any doubt that Wrangellia originated in
the northern hemisphere, because it is possible
that the older rocks have been remagnetized by
numerous dikes and hypabyssal bodies associated
with the Nikolai flows. Until the result is con-
firmed by a broader distribution of samples, the
southern-hemisphere option should be kept open.
Total poleward drift of the part of Wrangellia that
is now in south-central Alaska would be about
60 if the terrane travelled from the southern
hemisphere.
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Van der Voo et al. (1980), in their study of the Alexander terrane (sites 27-31, 34), proposed that the Paleozoic rocks had moved nor~ward about lo-15 from an original position near Cali- fornia. However, a controversy has arisen concern- ing the North American reference poles used in reconstructing the paleoposition of the Alexander terrane. The controversy stems from the recent detection of overprints of Permian age in the Late Carboniferous and Devonian data sets from the eastern part of the craton (Irving and Strong, 1984; Kent and Opdyke, 1985). Given the polarity that Van der Voo et al. (1980) assigned to the Alexander magnetic directions, and using the re- vised reference poles for the Carboniferous, then a Late Paleozoic position of the Alexander terrane near its present latitude relative to the craton is favored.
In southeastern Alaska, the Hound Island Volcanics (site 16) of Late Triassic age cap the Alexander terrane. Paleomagnetic results from the Hound Island Volcanics (Hillhouse and Gromme, 1980) indicate no significant paleolatitude anomaly on the basis of Triassic and Early Jurassic refer- ence poles. This result is not consistent with paleomagnetic evidence recently obtained by Irving et al. (1985) from British Columbia in mid- Cretaceous plutons of the southern part of the Coast Plutonic Complex of Lappin and Hollister (1980). The plutons give significantly low inclina- tions. The Coast Pluto& Complex, which extends along the length of the British Columbian and southeastern Alaska coast, intrudes the major accretionary terranes. Although large-scale tilt of the complex was proposed as a possible explana- tion for the low inclinations, a vast region would have to be tilted uniformly, so northward transla- tion of the Coast Plutonic Complex was the favored explanation. The results imply that a com- posite block, including part of the Wrangellia, Alexander, and Stikinia terranes in British Col- umbia, was displaced about 2400 km northward after 90 Ma ago.
As with Wrangellia, the polarity of most Alexander paleomagnetic poles cannot be selected with certainty, so it is possible that the terrane moved to a position in the southern hemisphere during the Permian and Triassic (Panuska and
Stone, 1985a). For this option, reversed polarity would be assigned to the upward, eastward direc- tions from the Ordo~cian, Devonian, and Carboniferous rocks and reversed polarity would be assigned to the Pybus Formation (site 24) of Permian age. Reversed polarity would also be assigned to the steeply inclined, downward and southwestward directions from the Hound Island Volcanics. Under the southern-he~sphere option the terrane would move about 50 to the south following deposition of the Devonian Port Re- fugio Formation and then move northward to a position 85 higher in latitude relative to the craton in post-Triassic time. Moving the Alexander terrane to the southern hemisphere during the Permian and Triassic has the advantage of getting the terrane out of the way of the apparently translated Coast Plutonic Complex. Compared to the northern option, however, the southern option has the disadvantage of requiring much larger displacements and more changes in the direction of transport.
The main evidence for the southern hemisphere option is from the Pybus Formation, which was deposited during the time of the Permo-Carbonif- erous Reversed-Polarity Super&on. If the magne- tization is indeed of Permian age, then the down- ward inclinations measured in the Pybus Forma- tion would require location of the Alexander ter- rane in the southern hemisphere. However, the possibility of remagnetization in the Pybus cannot be ruled out, because a conclusive fold test could not be mide from the limited collection of weakly magnetized samples (Panuska and Stone, 1985a). More corroboration of the Permian paleomagnetic pole is needed from the Alexander terrane to prove the validity of the southern hemisphere op- tion.
For Jurassic rocks in the range of ages from 190 to 140 Ma (studies Nos. 7-15) the paleolati- tudes of the Peninsular terrane and the northern fragment of Wrangellia are similar to paleolati- tudes determined from the Triassic rocks. Paleo- latitude anomalies from the Jurassic sites average about 20 of poleward drift, assuming the north- em option used in Table 1. Hence, it is reasonable to assume close proximity of the northern part of Wrangellia to the Peninsular terrane since Early
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114
Jurassic time. By the Middle Jurassic, the two
terranes were overlapped by sedimentary rocks
that require similar displacements of the terranes
after that time.
In the western part of the Peninsular terrane,
Jurassic paleomagnetic poles are generally dis-
placed 110 o in the clockwise sense from coeval
reference poles. Across the northern part of
Wrangellia, the Triassic poles (Nos. 21, 22) are
dispersed along a small-circle arc of about 80 o
due to localized block rotations within the terrane
(Hillhouse and Gromme, 1984). Under the north-
em-hemisphere option, the Wrangellian poles are
displaced from the reference poles in the counter-
clockwise sense, suggesting that the western part
of the Peninsular terrane has undergone a large
rotation relative to the northern fragment of
Wrangellia. Stone et al. (1982) and Panuska and
Stone (1985a) favored the option of placing the
two terranes in the southern hemisphere during
Jurassic time by reversing the polarity of the
Jurassic data and thereby minimizing this rota-
tion. The southern option, however, maximizes the
latitudinal displacement of Wrangellia and the
Peninsular terrane, requiring major southward
transport during the Early Jurassic followed by
substantial northward drift during the Late
Jurassic and Cretaceous. Given the current state
of the Jurassic data set, which lacks thermal de-
magnetization results and reliability tests, choos-
ing one option over the other is probably not
warranted at this time.
Paleolatitudes of the southern Alaska terranes
during Early Cretaceous time are highly conject-
ural, because the paleomagnetic sampling is very
sparse and the reliability of the results is difficult
to evaluate. For the Peninsular terrane, the paleo-
latitude determination from site 1 is considerably
greater than the two determinations from the
Wrangellia terrane (sites 5 and 6). In turn, the
Peninsular paleolatitude is considerably greater
than results from three sites in the more northern
Jurassic and Cretaceous flysch (Nos. 2, 3, 4). The
poor internal consistency of these results is prob-
ably caused by unremoved secondary components.
Hence, speculations on the Early Cretaceous
paleopositions of the southern terranes are not
warranted, at least until reliability tests are availa-
ble.
Pakopositions of the southern terranes during the Late Cretaceous and Tertiary
Paleomagnetic studies of Late Cretaceous and
younger rocks of southern Alaska were reviewed
by Coe et al. (1985). Their tabulation of results
included paleolatitudes, paleolatitude anomalies,
determinations of rotations, and reliability factors.
Their analysis showed a systematic discrepancy
when paleolatitude determinations from volcanic
rocks and sedimentary rocks were compared. The volcanic rocks gave significantly higher paleolati- tudes. They explained the discrepancy as being due to depositional inclination errors and unde- tected magnetic overprints in the sedimentary rocks. Also, studies of the volcanic rocks generally satisfied a greater number of reliability factors and, hence, should be weighted more heavily in the tectonic interpretation. A possible exception is a recent study of the MacColl Ridge Formation, an Upper Cretaceous (Campanian or Maastrich- tian) sequence of arkosic sandstones (Panuska, 1985). MacColl Ridge is located within Wrangellia near its southern boundary in the McCarthy quadrangle (near site 9 in Fig. 1). The paleo- magnetic results pass the fold test and antipolarity of reversals test; however, the specimens gave erratic demagnetization paths during thermal de- magnetization at temperatures from 250 o to 350 o C.
Paleomagnetic poles from four studies of Up- per Cretaceous and Lower Tertiary volcanic flows
are shown in Fig. 2 along with Cretaceous and
Tertiary reference poles from the North American
compilation of Irving and Irving (1982). The
paleomagnetism of Paleocene and Lower Eocene
volcanic rocks was determined along a transect
that crosses northern Wrangellia, deformed
Jurassic and Cretaceous flysch, several smaller
terranes within the flysch, and the Denali fault
(Hillhouse and Gromme, 1982; Hillhouse et al.,
1985). North of the Denali fault, Paleocene ande-
sites of the Cantwell Formation yielded a paleo-
latitude of 81 & 8, which is 9 o _t 8 higher than
predicted from coeval reference poles. Similar re-
sults were obtained from Lower Eocene andesites
and dacites in the northern Talkeetna Mountains
where the paleolatitude determination was 76 k
-
Fig. 2. Paleomagnetic poles (stars) and 95% confidence limits
from Upper Cretaceous and Lower Tertiary volcanic rocks that
overlap the Wrangellia, Peninsular, and Yukon-Tanana ter-
ranes of Howell et al. (1985). The studies are: 1 -Cantwell
Formation (Paleocene, Hillhouse and Gromme, 1982);
Z-volcanic rocks of the northern Talkeetna Mountains
(Paleocene and (or) Lower Eocene, Hillhouse et al., 1985);
3-volcanic rocks of Hagemeister Island (68 Ma; Globerman
and Coe, 1984a); I-volcanic rocks near Lake Clark (66 Ma;
Thrupp and Coe, 1986). Reference poles from Upper Creta-
ceous and Tertiary rocks of North America (Irving and Irving,
1982) are indicated by connected dots; ages given in Ma.
10 O. In the southern Talkeetna Mountains, Eocene volcanic rocks along the southern boundary of the Peninsular terrane yielded a paleolatitude of 80 + 9 (Panuska and Stone, 1985b). Another trans- ect from Lake Clark to Iliamna Lake covers Lower Tertiary andesites and basalts that overlap the Peninsular terrane (Coe et al., 1985; Thrupp and Coe, 1984, 1986). Near Lake Clark, lava flows dated at about 66 Ma gave a paleolatitude of 63 f 9, which is lower than the predicted lati- tude by 9 f 11. Further west, uppermost Creta- ceous (68 Ma) volcanic rocks near Hagemeister Island in Bristol Bay yielded a paleolatitude of 65 f 4 (Globerman and Coe, 1984a, b).
Small northward displacements approaching the magnitudes of the confidence limits are implied for the Lake Clark and Bristol Bay studies, in contrast to the small southward displacements de- termined in the studies from the Denali-Talkeetna Mountains transect. The apparent differences in Late Cretaceous-Early Tertiary paleolatitudes
115
cannot be entirely explained on the basis of age, with the older rocks showing more northward translation. This is because K-Ar determinations from the Hagemeister Island flows are at most 3 m.y. older than determinations from the Cantwell flows, while the flows at Lake Clark and the Cantwell basin are nearly identical in age. Al- though an unrecognized suture might separate the southwestern sites from the Talkeetna Mountains to explain the paleolatitude differences, the re- gional geology suggests otherwise. The volcanic rocks of Lake Clark, The Cantwell Formation, and the northern Talkeetna Mountains appear to be part of a Late Cretaceous and Early Tertiary magmatic complex that overlaps the boundaries between Wrangellia, the Jurassic and Cretaceous flysch, and the Peninsular terrane. Therefore, it is reasonable to combine results from the studies of volcanic rocks. Taken as a whole, the data indicate that no significant change of latitude of southern Alaska relative to North America has taken place since 55-68 Ma ago. Therefore, large-scale north- ward drift of Wrangellia and the Peninsular ter- rane, as indicated by paleomagnetic studies of the Triassic and Jurassic rocks, was probably com- pleted during the Paleocene, certainly no later than 55 Ma ago.
Paleomagnetic results from the MacColl Ridge Formation (Campanian or Maastrichtian), in con- trast to results from the slightly younger volcanic rocks, imply that Wrangellia was about 40 south of its present position relative to cratonic North America. To reach the high latitudes determined from the volcanic rocks, the Wrangellia terrane would have to be transported at a high rate (at least 24 cm/yr.) between 70 and 55 Ma ago, or a tectonic suture exists between MacColl Ridge and the northern Talkeetna Mountains (Panuska, 1985). As there is no geologic evidence known for a fault zone within this part of Wrangellia which could accommodate thousands of kilometers of displacement, results from MacColl Ridge, if valid, constrain the major displacement of Wrangellia to have occurred in Maastrichtian or Paleocene time. Acceptance of this constraint on the timing of accretion awaits confirmation from a broader dis- tribution of Cretaceous rocks than was sampled in the MacColl Ridge pilot study.
-
116
The Prince William terrane, which makes up
the southern margin of Alaska, shows evidence of
large-scale northward drift in rocks possibly as
young as Early Paleocene. The most convincing
evidence comes from the southern shore of Kodiak
Island, where the Ghost Rocks Formation of
Moore et al. (1983) has been sampled for
paleomagnetism (Plumley et al., 1982; 1983). The
results have high reliability as indicated by posi-
tive fold tests at two sites and a positive reversal
test. The Ghost Rocks, a heterogeneous assemb-
lage of volcanic and sedimentary rocks, is divided
by faults into a nearshore facies with generally
coherent structure, and a deep-ocean facies com-
posed of pillow basalt, limestone, and argillite in a
melange (Moore et al., 1983). Planktonic for-
aminifers in limestone blocks within the melange
indicate poorly defined ages ranging from Late
Cretaceous to Paleocene.
A bimodal distribution of inclinations was ob-
tained from two areas within the Ghost Rocks. At
Alitak Bay, andesitic lavas interbedded with sand-
stone give a paleolatitude anomaly indicating X6
f 9O of northward drift (Coe et al.. 1985). At
Kiliuda Bay, pillow basalts from the melange and
andesites from the coherent unit give a northward
paleolatitude anomaly of 31 5 9O. When results
from the two areas are combined, as favored by
Plumley et al. (1983), the overall paleolatitude
anomaly is 25 o + 7 o northward.
In a geologic synthesis of the region, Moore et
al. (1983) interpreted the Ghost Rocks Formation
as an accretionary complex that formed before 62
Ma ago. Formation of the complex is constrained
by a belt of plutons that intruded the Ghost
Rocks 62 Ma ago after major deformation of the
unit occurred. Because the ages of the plutons and
the Ghost Rocks are so similar, the accretionary
complex must have formed in a collision zone that
was far south of the present position of Kodiak
Island. To reach its current position in Alaska, the
Ghost Rocks Formation was either: (1) abducted
onto Kodiak Island after being carried from the
central Pacific region, or (2) carried northward by
transcurrent faulting along the margin of North
America. A zone of convergence must lie between
the Alaskan Peninsula and the outer margin of
Kodiak Island to account for the different paleo-
latitudes of the Ghost Rocks and the Peninsular
terrane in Early Paleocene time.
Rotation of southwestern Alaska
A significant counterclockwise rotation of
southern Alaska can be inferred from the distribu-
tion of Late Cretaceous and Early Tertiary
paleomagnetic poles (Fig. 2). The declination
anomalies vary from 29 to 54O in the counter-
clockwise sense when the Alaskan poles are com-
pared with the appropriate reference poles (Coe et
al., 1985; Hillhouse et al., 1985). The extent of the
rotated domain is not known because the sam-
pling is currently restricted to the Peninsular ter-
rane, the Jurassic and Cretaceous flysch, and the
area of the Cantwell Formation. A constraint on
the timing of rotation has been obtained from the
volcanic rocks near Lake Clark (Thrupp and Coe,
1986) where Middle Eocene and Oligocene lavas
give no significant anomalies in declination.
Oceanic plate motions and the accretion of south- ern Alaska
Using the better-substantiated paleomagnetic
observations mainly from studies of volcanic rocks,
three main observations can be distilled from the
paleomagnetic literature of Alaska: (1) Retative to
the North American craton, Wrangellia and the
Peninsular terrane were lower in latitude by at
least 25 during the Triassic and Jurassic. Prob-
ably by 65 Ma ago and certainly no later than 55
Ma ago, poleward drift of the terranes relative to
the craton was completed. (2) During Paleocene
time, a volcanic complex at the southern rim of
the of the Prince William terrane was 25 o south of
the newly accreted Alaskan margin. (3) Between
68 Ma and 44 Ma ago, central and western Alaska
rotated as much as 50 countercl~kwise, pre- sumably about a hinge line near 146 W. The
areas of Alaska affected by these tectonic events
are depicted as tectonic domains (Fig. 3). The
northern extent of the Peninsular terrane in the
Bering Sea is not known, and a large part of the
Bering shelf could have moved with the Peninsular
terrane, The rotated domain might possibly ex-
tend westward into the Aleutian basin and north-
-
TECTONIC DOMAINS
0 Rotated Counterclockwise Canada Basin
68 - 44 Ma ago ARCTIC OCEAN
q Accreted after 60 Ma ago THEAST U.S.S.R.
!I83 Accreted 100 - 55 Ma ago
odiak Island
PACIFIC OCEAN
Fig. 3. Tectonic domains of southern Alaska, showing accreted areas and locations where counterclockwise-rotated declinations were
ward beyond Bering Strait, but more paleomag- netic studies are needed.
The goal is to fit the bettor-substantiated ter-
rane movements into a plate-tectonic model that
incorporates relative motions between old con-
tinental Alaska and the oceanic plates after 100
Ma ago. Engebretson et al, (1985) presented re-
constructions of oceanic plates in the Pacific basin
from 175 Ma ago to the present, which are the
basis of the following discussion. The greater un-
certainties in the reconstructions arise from possi-
ble motions of the hot-spots relative to each other,
errors in the deter~nation of hot-spot tracks, and
poor constraints on the locations of some plate
boundaries. Their best estimate for the combined
uncertainty associated with possible errors in the
hot-spot framework is 900 km for reconstructions
at 100 Ma. This does not include uncertainties in
the location of the ridge between the Kula and
Farallon plates for which isochrons and fractures
are not preserved. Placement of the Kula-Faral-
lon ridge is critical for reconstructions from 85 to
43 Ma ago, particularly for assessing directions of
slip along the northwestern margin of North
America (Fig. 4).
The Yakutat terrane is currently riding north-
ward with the Pacific plate. Offsets along the
Fairweather fault during the last 1000 years give a
rate of displacement of 4.8 cm/year (Plafker et al.,
1978) comparable to the relative velocity (6.3
cm/yr.) between North America and the Pacific
plate, as determined for the period O-5 Ma from
the hot-spot reference frame. Total poleward
transport that the Yakutat terrane has undergone
since Eocene time is controversial, with estimates
ranging from 30 o from inte~retation of microfos-
sil assemblages (Keller et al., 1984) to as little as
5 on the basis of sedimentary provenance
(Plafker, 1983). A recent abstract by Van Alstine
et al. (1985) proposed 13 of post-Eocene north-
ward transport, according to paleomagnetic data from the Yakutat Well in the Gulf of Alaska.
Integration of the plate models with the older
accretionary events is more easily approached by
working backward in time and starting with the
next-to-last terrane to be emplaced in Alaska, the
-
11x
a) 80 Ma
60 N
20 N -. H
56 hAa
1 PACIFIC FARALLON 3Y 20 N 0
I \ t
Fig. 4. Schematic piate reconstructions (adapted from En-
gebretson et al., 1985) and key Alaskan terranes at 80 Ma and
65 Ma ago. Heavy arrows indicate directions and amounts of
movement the plates underwent relative to the fixed hot-spots
(H-Hawaii; Y-Yellowstone) over a lo-Ma period. a. At 80
Ma ago, rapid northward movement of the Kula plate possibly
moved part of the Wrangellia terrane ( W) along the margin of
North America. b. By 56 Ma ago, part of Wrangellia was
lodged in Alaska, while the Ghost Rocks Formation (CR) of
Moore et al. (1983) was far south of Alaska. The counterclock-
ise rotation of southwestern Alaska was probably underway at
this time.
Prince William terrane. On Kodiak Island. struc-
tures within the Ghost Rocks Formation indicate
that at least part of the Prince William terrane
collided with an island arc or continental margin
by 62 Ma ago. If the Ghost Rocks are indeed of
Early Paleocene age as defined by Moore et al.
(1983), then the collision took place near a paleo-
latitude of 40 N, some 25 south of the forma-
tions present position relative to the craton. The most likely place for the collision would be along the coast between California and Vancouver Is- land (Fig. 4b). From 60 to 56 Ma ago, oblique subduction of the Kula plate beneath the British Columbian margin could produce poleward slip of the Ghost Rocks at an average rate of about 0.9/Ma, provided that the Ghost Rocks were partially decoupled from the Kula plate by trans- current faults. The Kula plate accelerated during the interval from 56 to 43 Ma ago, providing a poleward component of about 1.7/Ma. By 43 Ma ago these rates are sufficient to close the 25 gap that once separated the Ghost Rocks and the Paleocene volcanic rocks of the Peninsular ter- rane. The motion could be a~o~odated by transcurrent shear across a wide zone composed of the Prince William and Chugach terranes, and possibly the southern rim of the Peninsular ter- rane (Moore et al., 1983; Coe et al., 1985).
The final emplacement of the Ghost Rocks
near Kodiak Island was probably preceded by the
counterclockwise rotation of southwestern Alaska.
The rotation occurred 68-44 Ma ago either during
or just after the amalgamation of the Wrangellia
and Peninsular terranes with the central Alaskan
terranes. Tectonic forces during sea-floor spread-
ing in the Arctic Ocean and the North Atlantic
could account for the rotation. Counterclockwise
rotation of western Alaska to explain the arcuate
trends of the Denali and Tintina-Kaltag faults
was first proposed by Carey (1955), who suggested
that a wedge-shaped sphenochasm opened
within the Canada Basin to drive the rotation.
Opening of the Canada Basin, however. probably
occurred prior to Tertiary time according to inter-
pretations of geophysical data from the basin
{Sweeney, 1983) and could not have caused the
rotation.
Instead, sea-floor spreading in the North
Atlantic and the Labrador Sea, and consequent
compression between North America and Asia,
might explain the counterclockwise rotation of
western Alaska. This explanation does not neces-
sarily imply that the deformation of western
Alaska was driven by ridge push, as slab pull in
the opposing trenches could be the overall driving
force. Tectonic response related to sea-floor
-
119
spreading in the northern oceans has been pro- posed as an explanation for deformational trends across the Bering Strait (Patton and Tailleur, 1977) and in central Alaska (Grantz, 1966). Spreading in the Labrador Sea between Greenland and North America was active from Late Cretaceous to Early Oligocene time (Srivastava, 1978), while spreading in the North Atlantic and Eurasian Arctic basin has occurred since 90 Ma ago (Pitman and Talwani, 1972). The opening of these basins pre- dicts overlap or compression of continental crust along the Eurasia-North America plate boundary from 70 to 50 Ma ago. Although the plate boundary is not clearly defined by structures in the northeast U.S.S.R., it is generally assumed to be east of the Verkhoyansk Mountains. The com- pression between the continental plates might have been accommodated by deformation in the Bering Sea region, with western Alaska moving to the southeast. If so, the motion could produce rota- tion about a hinge line near 146 W and dextral slip along the Beringian continental shelf, eventu- ally creating the arcuate structural trends of southwestern Alaska. Rotation of western Alaska probably ceased by 50 Ma ago when a change in the pole of opening of the Arctic basin eased compression between North America and Eurasia (Harbert et al., 1985). Also, at about this time a fragment of the Kula plate was trapped behind the growing Aleutian volcanic chain (Cooper et al., 1976), as subduction began beneath the Aleu- tian trench.
Undoing the rotation of western Alaska straightens out the Alaskan continental margin, bringing it into alinement with the British Col- umbia coast and providing a different continental geometry during the accretion of Wrangellia and the Peninsular terrane prior to 65 Ma ago. Paleomagnetic evidence from the Nixon Fork ter- rane indicates much smaller changes in latitude relative to the craton as compared to the changes measured in the Wrangellia terrane, so the Nixon Fork terrane and the Yukon-Tanana terrane are considered the backstop for the accretion. The Brooks Range, Koyukuk basin, and Seward Peninsula (Fig. 3) are bounded by major zones of deformation, and all have probably been displaced from their Early Mesozoic positions relative to
cratonic North America. By 100 Ma ago, however, these elements of Alaska had been assembled to- gether, as indicated by the general continuity of Upper Cretaceous marine deposits across the re- gion. In the northeastern U.S.S.R., paleomagnetic data support a similar conclusion that possibly displaced terranes such as the Kolyma block had amalgamated to the continent to be overlapped by Upper Cretaceous sedimentary deposits (Mc- Elhinny, 1973).
The northward movement of the Wrangellia and Peninsular terranes can best be explained by movement of the Kula and Farallon plates, with much of the poleward component of drift occur- ring 85-65 Ma ago (Fig. 4). The poleward velocity of the Kula plate relative to the Alaska mainland was about 150 km/Ma during this interval, according to the reconstruction by Engebretson et al. (1985). If the terranes were attached to the Kula plate, then the potential displacement is 3000 km or 27 of latitude, just sufficient to explain the displacement of Wrangellia from an original position in the northern hemisphere. A possible scenario to explain the current distribu- tion of the Wrangellian fragments starts at 85 Ma ago when the Kula-Farallon ridge became activated. Assuming that previous motion of the Farallon plate had carried Wrangellia to the main- land near Oregon, activation of the Kula-Farallon ridge would initiate right-lateral strike-slip motion along the continental margin, detaching parts of Wrangellia from the mainland and moving them northward (Fig. 4a). In this way, slivers of Wran- gellia could be left along the margin as the zone of shear moved westward. Eventually the last frag- ment of Wrangellia would be carried northward into Alaska, to what is now the central Alaska Range.
This model for the final accretion of Wrangellia and the Peninsular terrane is consistent with the timing of deformation and plutonism in the Jurassic and Cretaceous flysch (Fig. l), which was the zone of convergence between the moving ter- ranes and the mainland. Intense deformation of the flysch, which contains beds as young as Cenomanian, occurred after 100 Ma ago (Csejtey et al., 1982). Tremendous shortening of the flysch basin is interpreted from the isoclinal folding of
-
120
the strata, and a number of small Mesozoic ter- ranes, such as Chulitna, are structurally emplaced within the flysch. Relative motions between Wrangellia, the flysch, and the Nixon Fork ter- rane are constrained by the Alaska Range belt of plutons which cuts across the terrane boundaries (Reed and Lanphere, 1973). Plutons of the Mc- Kinley sequence, which perhaps were generated by the final closure of the flysch basin, were em- placed 57 Ma ago (Lanphere and Reed, 1985).
The whereabouts of Wrangellia prior to 100 Ma ago are virtually unconstrained except that the paleomagnetic data indicate near-equatorial lati- tudes during Triassic and Jurassic times. If the terrane were lodged along the margin of the Americas, there would be no driving force to move it northward until after 120 Ma, when the relative motion of the Farallon plate against North America shifted from eastward to northeastward. If the Wrangellia and Peninsular terranes were south of the equator at any time during the Mesozoic, then the most likely position would be within the Farallon plate in the west-central part of the Pacific basin. From a starting point in the southern hemisphere the terrane trajectory would proceed northeastward throughout the Jurassic and Early Cretaceous toward the margin of North America near California. In these models it is essential that by 85 Ma ago the terranes had reached a point north of the initial rift of the Kula-Farallon ridge. Otherwise, east-directed subduction of the Farallon plate beneath North America would prevent the terranes reaching Alaska.
Conclusions
The accretion of southern Alaska was ap- parently a two-step process according to a con- sensus drawn from the better substantiated paleomagnetic data. First, the Wrangellia and Peninsular terranes collided with the backstop consisting of the Nixon Fork and Yukon-Tanana terranes during the interval 100-55 Ma ago. The Kula plate probably provided the impetus to fi- nally close the latitude gap between Wrangellia and the mainland. Secondly, the Prince William terrane and southern margin of the Chugach ter-
rane arrived in Alaska after 55 Ma ago, also carried by the Kula plate. The counterclockwise rotation of southwestern Alaska occurred 68-44 Ma ago as the latitude gap was closing between the Prince William terrane and the mainland. The rotation of western Alaska may be a manifestation of sea-floor spreading in the Eurasian Arctic basin, Labrador Sea, and North Atlantic, which accom- panied deformation at the boundary between North America and Asia.
Acknowledgements
I would like to thank the local organizing com- mittee, especially Tomas Zelinka, Vladimir Kropacek, Vaclav Bucha, and Hana Prochazkova, for their hospitality and assistance during the 5th IAGA Assembly in Prague. Special thanks are owed to Miroslav Krs and Vladimir Kropacek for their contributions to the paleomagnetism field trip in Czechoslavakia. Improvements to the origi- nal manuscript were suggested by Dennis Kent, Kenneth Kodama, Jonathan Hagstrum, and two anonymous reviewers.
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