USGS Professional Paper 1697 - Alaska Resources Library

elt of late Tertiary plutons that approximately coincide with
the Pemberton belt of late Tertiary and Quaternary volcanic
rocks (Woodsworth and others, 1991; Souther, 1991). K-Ar
ages of the pluton rocks progressively decrease northward,
from between 35 and 16 Ma in the south, to about 7 Ma in the
north. These relatively young plutonic rocks and associated
volcanic rocks are part of the Cascade volcanic-plutonic belt
of the U.S.A. Pacific Northwest and southern British Columbia
(Nokleberg and others, 1994c, 1997c, 2000).
In the Canadian Cordillera, the Cascade volcanic-plutonic
belt consists of Pleistocene and Holocene basalt, andesite, and
dacite eruptive centers, and late Eocene(?), Oligocene, and
Miocene plutons (Chilliwack and Mount Barr batholiths). In
the U.S. Pacific Northwest, the belt consists of volcanic rocks
of stratovolcanoes, mostly andesite but ranging from basalt
to rhyolite. The belt includes interbedded fluvial and lacustrine
deposits and minor tonalite to granodiorite plutons. In
Washington, parts of belt are included in the Ohanapecosh, the
Fifes Peak, and the Northcraft Formations (Vance and others,
1987; Smith, 1993). The youngest active volcanoes in the belt
are Mount Jefferson, Mount Hood, Mount Adams, Mount St.
Helens, and Mount Rainier.
The Cascade volcanic plutonic belt and corresponding
Cascade continental-margin arc is interpreted as forming in
response to subduction of the Juan de Fuca Plate (Wells and
Heller, 1988; England and Wells, 1991; Monger and Nokleberg,
1996; Nokleberg and others, 2000). Remnants of the
subducting plate are preserved in the Siletzia, Olympic Core,
and Hoh terranes along branches of the Cascadia megathrust.
Middle Tertiary Metallogenic Belts (20
to 10 Ma) (figs. 125, 126)
Overview
The major middle Tertiary metallogenic belts in the Russian
Far East and the Canadian Cordillera are summarized in
table 3 and portrayed on figures 125 and 126. The major belts
are as follows: (1) In the Russian Northeast, the Central Kamchatka
(CK) belt, which contains granitic-magmatism-related
deposits, and the East Kamchatka (EK) belt, which contains
Au-Ag epithermal vein deposits, are hosted in the Central
Kamchatka Volcanic and Sedimentary Basin and are interpreted
as forming during subduction-related granitic plutonism
that formed the Kamchatka Peninsula part of Northeast Asia
continental-margin arc. (2) In southern Alaska, the Alaska
Peninsula and Aleutian Islands (AP) belt, which also contains
granitic-magmatism-related deposits, is hosted in the Aleutian
volcanic belt and is interpreted as forming during subductionrelated
granitic plutonism that formed the Aleutian continental-margin
arc. (3) In the southern Canadian Cordillera,
continuing on from the early Tertiary was the Owl Creek (OC)
belt, which contains granitic-magmatism-related deposits and
Middle Tertiary Metallogenic Belts (20 to 10 Ma) (figs. 125, 126) 271
is hosted in the Cascade volcanic-plutonic belt, is interpreted
during subduction-related granitic plutonism that formed the
Cascade continental-margin arc. In the below descriptions of
metallogenic belts, a few of the noteable or signficant lode
deposits (table 4) are described for each belt.
Metallogenic-Tectonic Model for Middle Tertiary
(20 to10 Ma; fig. 127)
During the middle Tertiary (Miocene - 20 to 10 Ma), the
major metallogenic-tectonic events were (fig. 127; table 3) (1)
continuation of a series of continental- margin arcs, associated
metallogenic belts, and companion subduction-zone assemblages
around the Circum-North Pacific, (2) back-arc spreading
behind the major arcs, (3) opening of major sedimentary
basins behind major arcs, (4) in the eastern part of the Circum-
North Pacific, a continuation of dextral transpression between
the Pacific oceanic plate and the Canadian Cordillera margin
and a continuation of orthogonal transpression between the
Pacific Plate and the southern Alaska continental margin, and
(5) continued sea-floor spreading in the Arctic and eastern
Pacific Oceans.
Specific Events for Middle Tertiary
(1) After accretion of various terranes in the early Eocene
and outward stepping of subduction, the Northeast Asia arc
commenced activity. Parts of this arc are preserved in the
East Japan volcanic-plutonic belt (ej), Kuril volcanic arc (ku),
and the various parts of the Kamchatka arc consisting of the
Central Kamchatka volcanic belt (kc), Central Kamchatka
Volcanic and Sedimentary Basin (ck), and West Kamchatka
Sedimentary Basin (wk). To the northeast, the Okhotsk-Chukotka
arc ceased activity. These two major Andean-type arcs
overlapped previously accreted adjacent terranes in both the
Russian Southeast and to the south and extended for a distance
of about 3,000 km. Associated with these arcs was subduction
of part of the Pacific oceanic plate (PAC) along the Kuril-
Kamchatka megathrust (KK) to form the Kuril-Kamchatka
(KUK) subduction-zone terrane. Intra-arc faulting resulted
in tectonic doubling of the Kamchatka-Koryak (kk) arc that
started to become extinct as the Central Kamchatka arc (kc)
enlarged. Forming in the Kamchatka part of the Northeast Asia
arc was the Central Kamchatka (CK) metallogenic belt, which
contains granitic-magmatism-related deposits, and is hosted in
the Central Kamchatka Volcanic and Sedimentary Basin.
(2) Regional extension associated with back-arc spreading
behind the northern Japan part of the Northeast Asia arc
(East Japan volcanic-plutonic belt, ej), resulted in marine eruption
of the Sea of Japan back-arc unit (sj), which consists of
mainly tholeiitic basalt and associated rocks.
(3) In the Sea of Okhotsk, back-arc spreading occurred
behind the Kuril Island and Kamchatka Peninsular part of the
Northeast Asia arc (B.A. Natal’in in Nokleberg and others,
1994a), resulting in marine and continental eruption of tholei-