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Table 17.--Srrontlum-Isoto Ic anal ses and selected chmtcal data for volcanlc rocks of the
nataPnus&m ~a~beetna%fimni---
[See f4gure 53 for ggcatggn of samples. halysesgp slbld-source mass spectrometry: analyst,
D.G.0rooklns. SF/ Srr +bnormalfze to Sr/ Sr0.1194. EandA.SrC0
o.omi. y o n a R , 1 . 0 9 1 . ~oalytical uncertainty. t0.6~~E~~t.
( ), measured ratlo; ( )o, rat10 corrected for age of rock. StOZ analyses by X-ray
fluorescence; analysts: V. kDanlel, G. Kdwaklca. sna 0. Hopping]
Nap No. Fleld Rock S!O2 Rb Sr
(fig. 5-31 NO. type (wt ,DPII, (87~r/86~r),,, (8'~r/86~r)a
1 77AGr-AR3 Basalt flow------ 47.92 5 370 0.7042 0.7042
5 78ASi-MlsJ do ------------- 45.9 10 500 .7M1 .7041
16 78ASi-N21 do------------- 50.46 29 640 -7004 .I007
--
lnrruslve volcanic rocks of the tlatanuska Valley
8 78ASf-R2A Rhyolltestock---74.80 49 54 0.7056 0.7042
II 78ASI-H45 Basalt 51 \I------ 49.9 10 183 ,1042 .lo47
10 78AS1-n9 Oactte stock----- 69.31 25 115 .704 1 .7037
17 78ASI-MI2 Msal t I------ 46.40 10 102 .7037 .7036
with the higher (0.7060.707) values measured in sandstone
of the Orce and Valdez Groups and in the granodiorite
believed to be anatectically derived Prom
them. According to Hudson and others (1979), the anatexis
followed subduction and accretion of the Orca
and Valdee Croups (Chugach and Prince William terranes)
to coastal southern Almka during the Late Cretaceous
or early Tertiary (MacKevett and Plafker,
1974; Plofker and others, 1977). It appears unlikely
that the volcanic rocks of the Matanuska Valley area
originated from melting of older crustal material.
Instead, their strontium-isotopic ratios and geologic
setting suggest that these rocks were emplaced In a
continental margin with young crust. This crust was
composed of alIochthonous terrmes that had been
accreted and welded by plutonkm to nuclear Alaska
during Late Cretaceous and PaIeocene time, and it
included thick prisms of newly deposited Paleocene
and Eocene continental sedimentary rocks. At least
some of these volcanic rocks may be the remnants of a
late Paleocene and Eocene arc that was emplaced on
this young continental margin in response to subduction
in the present Gulf of Alaska-Chugach Mountains
we&.
The age and stratigraphic and structural relations
of the volcanic and sedimentary rocks of the
Metanwks Valley area also help conatrain be history
I of movement on the Castle Mountain fault system.
Juxtaposition of the coeval, but UthologicaLly distinct,
Arkose Ridge end Chickdoon Formations indicates
that large-scale strike-slip motion on the Castle Mountain
rinht-lateral fault was post-late Paleocene (fossilplant-&ernblage
ages) and (or) post-early or middle
Eocene (potassium-ar~on ages of volcanic rocks within
and ~nt~ding these lormaths). Contiguity and slmilar
structural orientation of the mafic dike field across
the eastern part of the Caribou strend of the Castle
Mountain fault system (~rantz, 1B60) suggest that this
strikedp motion was largely complete during the
Eocene, although this suggestion requires confirmation
from radiometric ages of some of these dikes. Pelsic
stocks, plugs, and dikes occur on both sides of the
Castle Mountain strand of the fault system in the
upper Matanuska Valley (Detterman and others,
1976). South of the Castle Mountain strand, these
rocks have been radiometrically dated at late Eocene
(table 16). If, as we suspect, the fekic rocks north and
south of the strand are coeval, then these relations
would support the proposed minimum age of large-
scde right-lateral motion on the fault system. None
of our data preclude postintrusive large-scale mainly
vertical motion on the fault system.
The potassium-argon sges reported here, within
the context of the geology of the Matansuka Valley
area, suggest that volcanism and major lateral slip on
the Castle Mountain fault system were both largely
Eocene events. Strong temporal association is sup-
ported by the previously noted west-northwestward
preferred orientation of the Eocene rnafic dikes in Me
southeastern Talkeetna Mountah. The trend of these
dikes Indicates that at the time of their emplacement,
the maximum and minimum principal stresses In the
region were generelly horizontal, with the maximum
stress oriented west-northwest~t-southe~~tward.
This orientation is appropriate for right-lateral move-
ment on the high-angle to vertical east-northeest-
trendlng Castle Mountain fault system.
REPERENCBS CITED
Bejkman, H. M., compiler, 1974, Preliminary geologic
map of the southeast quadrant of Alaske: U.S.
Geological Survey Miscellaneous Pield Studies
Map MP-612, scale 1:1,000,000, 2 sheets.
Detterrnan, R. L., Plafker, George, Tysdal, R. G., and
Hudson, Travis, 1976, Geology and surface
features along part of the Talkeetna segment of
the Castle Mountain-Caribou fault system: U.S.
Geological Survey Miscellaneous Field Studies
Map MP-738, scale 1:63,360.