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The secona k~~-~ri?tation mmes that the mean Stone, D. B., Pmuska, B. C., and Packer, D. R., 2982,
magnetic vector obtained from the Mash Island Palmlatitude versus time for southern aka:
samples represents thermal remanent megnetizatIon Journal of Geophysical Research, v. 87, no. 5, p.
acquired during an unknown postdeposltlond thermal
3897-3707.
event. Unpublished paleornegnetic data (C. S. ~romrnd
and J. W. Hillhouse, oral commun., 1982) from
northern Prince of Wales Island, Etolin khd, and the lBe mrihem Gxiat m-etsrnme mmpw
I
Screen Islands, indicate mwetic vectors, uncorrected
for tectonic tilt, that point steeply downward to the
southeast, similar to the Marsh Island directions. The
important points are that: (1) The localities which
Grommb and Hillhouse sampled surround the Marsh
bland locality (fig. 781, and (2) rocks of different ages
and varying bedding attitudes aU show the same
anomalous magnetic direction. When corrected for
tectonic tilt, the Marsh Lsland, Prince of Wales Ishnd,
Etolin Island, and Screen Islands vectors diverge from
their common down-to-thesoutheast orlentatlon and
thus, taken together, fail the fold test. The obvious
conclusion k that a regional post-Albian
w- Abka and mxth~e~tem MtS8h
Columbia
SJI David & mDd Artb~W Ford
INTRODUCTION
The northern Caast plutonicmetamorphic
complex is but one Segment of an 8,000-km-long
batholithic complex that extee the length of the
North American Cordillera from Baja Californit3 to the
Aleutian Wands. As described here, the northern
Coast plutonic-metamorphi complex (area 1, fig. .72)
remagnetization affected all the above sample sites. lies between lat 55O and 60 N. The adjacent segment
I However, the steep down-to-thesoutheast direction to the north has been described only in reparts
1 does not correspond to any known or suspected post- concerned with regional gmlw (Kindle, 1953; Muller,
I Abian geomagnetic field, assuming that the sample 1954, 1967; Christie, 1957; Wheeler, 1861, 1963;
sites have been in their present orientation and at Campbell and Dcdds, 1975, 1878, 1979); the adjacent
their present latitude since Alblan time. Therefore, if segment to the south was described by Hutchison
post-Albian regional remagnetization in southeastern (1970) and Wick and Hutchison (1974) as well as .-
Alaska is to be assumed, additional depicted on various regional geologic maps.
postremagnetization regional tilting, rotation, and (or) Different segments of the Cordilleran batholithic
translation is required to establish the necessary complex have significantly different festlwrr. 7bb
correspondence between known geomagnetic-field report documents the main cheracteristlcs and
directions and the measured sample directions. inferred evolutionary history of the northern Coast
In summary, our data indicate that the dikes that plutonic-metamorphic complex that should facilitate
cut the Narsh Island section of the Gravlna-Nutzotin comparisons between segments.
belt did not remagnetize these beda, and 60 the beds The Coast plut onicmetamorphic complex is
pess the baked-contact test. Nonetheleas, we cannot defined here more or less according to the usage of
1 rule out the possibility of regional remagnetization. Douglas end others (1970) and Brew (1981) but not that
Further mmplfng is required to distinguish between the of Brew and Ford (1978). Thus, the northern Coast
two interpretations for the origin of the magnetization plutonic-metamorphic complex includes not only the
I described above. dominantly granitic and gneissic rocks of the Coast
Mountains, bui also the mebrnorphic rooks adjacent to
REFERENCES CTTBD both sides This definition poses no problems so far as
the northeast limlt of the complex is concerned
Berg, H. C., Jones, D. L, and Richter, D. H., 1972, because there the outer contact of the low-pressure
Gravina-Nutzotin belt-tectonic significanoe of facles-serles metamorphic rocks adjoining the
an Upper Mesozoic sedimentary and volcanic northeasternmost plutbns is relatively clear.
sequence in southern and southeastern Alaska, The southwest Iimit of the northern Coast
Geological Survey resesrch 1972: U.S. plutonic-metamorphic complex, however, is difficult
Geological Survey Professional Paper 800-D, p. to place because the mediumgressure f'aciesseries
Dl-D24. metamorphic rocks that contact the granitic and
Hillhouse, J. W., and ~rornrni, C. S., 1980, gnelssic rmks of the northern Coast plutonic-
Paleornagnetism of the Wassic Hound lshd metamorphic complex decrease in grade to the
Volcanics, Alexander terrane, southeastern southwest and appear to form a single wide
Alaska: Journal of Geophysical Research, v. 85, metamorphic and deformation belt. As noted below,
no. B5, p. 2594-2602. there may, instead, be two bells of different ages side
Irving, Bdward, and R.tliaiah, Ountur, 1976, Reversals by slde. Figure 79 shows this wide area of medium-
of the geomagnetic field, rnagnetostratigraphy, pressure low- to medium-temperature rnetarnorphlsm
and relative magnitude of paleosecular varfatian a9 the informally named western metamorphic zone; it
in the Phanemzoic: Earth-Science Reviews, v. haa as its southwest Emit the southwesternmost
12, no. 1, p. 35-64. contacts of flysch units within the U per Jurassic(?) to
McBUinny, M. W., 1913, P&eornmetism and plate
middle Cretctcecus Oravina belt &erg and others,
tectonics: London, Cambridge Unjversity Presf, 1978). The reason for selecting thb boundary is
358 p. twofold: (1) The generally low grade metamorphism in
Packer, D. R, and Stone, D. B., 1974, Paleornegnetism these units appears to grade northeastward into the
of Jurassic rocks from southern Alaska and their higher grade metamorphism in the granitic and
tectonic implications: Canadian Journal of earth gneissic rocks of the northern Coast plutonic-
Science, v. 11, no. 7, p. 976-997. metamorpMc complex; and (2) the deformational
120
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