USGS Professional Paper 1697 - Alaska Resources Library

264 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
extended into the Russian Northeast (Fujita and others, 1997).
The rifting is interpreted as causing eruption of basalts in
the Chersky Range at about 37 Ma (Fujita and others, 1997).
The Lomonosov Ridge terrane (LO) is interpreted as forming
during the rifting of passive continental-margin units now
preserved in the Barents Sea region (out of field of fig. 123)
(Zonenshain and others, 1990). Sedimentation continued in the
large Amerasia (ab), Canada (cb) and Eurasia (eb) Basins.
(6) A short-lived period of marine arc volcanism formed the
Bowers (bw) and Shirshov (sh) Ridges in the Bering Sea. The arc
formed on the rear edge of the previously accreted Aleutia terrane
(al), a fragment of the Kula oceanic plate (Scholl and others,
1992, 1994). The Bowers Ridge volcanic belt consists chiefly of
intermediate-composition volcanic rocks, mainly altered andesite,
breccia, volcaniclastic sedimentary rocks, and lesser diatomaceous
siltstone (Cooper and others, 1992; Scholl and others, 1992).
Analysis of sparse dredge samples and Deep Sea Drilling Program
(DSDP) drill cores suggests a Miocene age for the volcanic rocks.
The presence of a trench filled with as much as 12 km of sedimentary
rocks located at the base of the northern and eastern slopes of
Bowers Ridge suggests that the unit formed in an early Tertiary
arc-trench system that faced toward the northeast. The Shirshov
Ridge volcanic belt consists chiefly of two assemblages—(1) A
relatively older oceanic assemblage is composed of amphibolite,
gabbro, diabase, basalt, and chert; chert contains Late Cretaceous
(Campanian to Maastrichtian) to early Paleogene microfauna; and
(2) a relatively younger volcanic-arc assemblage is composed of
altered andesite, volcaniclastic sedimentary rocks, and shale of
Miocene and younger age (Baranov and others, 1991; Scholl and
others, 1992). Alternatively, the Bowers and Shirshov Ridges may
be the northward extension of the Olyutorka-Kamchatka island arc
(OKA; Brandon and others, 1997, 1998). Also in the Bering Sea,
a thick sedimentary prism started to form in the Aleutian-Bowers
Sedimentary Basin (atb), which overlies the Aleutia terrane
(Plafker and Berg, 1994; Scholl and others, 1992, 1994).
(7) Tectonic escape (crustal extrusion) of terranes continued
to occur along major dextral-slip faults, including the Denali
(DE), Nixon Fork (NF), Kaltag (KA), and companion faults in
the area of the western Alaska the Bering Sea (Scholl and others,
1992, 1994). Dextral-wrench basins continued to form in association
with the major dextral-slip faults and were rapidly filled
with continental sediments. Coincident with crustal extrusion
was counterclockwise oroclinal bending of Alaska that perhaps
resulted from compression between Eurasia and North America
(Scholl and others, 1992, 1994; Plafker and Berg, 1994). To the
east, in the area of southern Alaska, displacement continued
along major dextral-slip faults, including the Denali, Nixon Fork,
and Kaltag Faults. These and similar dextral-slip faults probably
extended into the area of the Bering Sea.
(8) The Pacific oceanic plate (PAC) moved towards North
America as a result of sea-floor spreading along the Juan de
Fuca oceanic ridge. Along the Aleutian megathrust (AL), plate
convergence continued to vary from oblique-orthogonal in the
east to oblique in the west. Oblique-transpressive displacement
occurred between the Pacific oceanic plate (PAC) and
the southern Canadian Cordillera.
(9) As a result of trapping of part of the Kula oceanic
plate (underlying the Aleutian-Bowers Basin) and step-out of
subduction, the western part of Aleutian-Wrangell arc (al) was
initiated at about 40 Ma. This major Andean-type arc overlapped
the previously accreted Kula oceanic plate and initially
extended for a distance of about 3,000 km along the Bering
Straits and southern Alaska. Associated with the arc was subduction
of part of the Pacific oceanic plate (PAC) to form the
Prince William (PW) and Yakutat (YA) terranes along Aleutian
megathrust (AL).
(10) At about 25 to 30 Ma, a major tectonic change
occurred in the Southern Canadian Cordillera with tectonic
overriding of the northern segment of the Juan de Fuca oceanic
ridge (JFR) and resultant establishment of dextral-slip
along the Queen Charlotte Fault (QC). This tectonic change
ended subduction of the Farallon oceanic plate (FAR) and
started northward migration and subduction of the Yakutat
terrane (YAK), resulting in the beginning of volcanism in the
Wrangellia volcanic field (wr) in the eastern part of the Aleutian-Wrangell
arc (fig. 123). Total movement of the Yakutat
terrane is estimated at about 600 km (Plafker and Berg, 1994;
Plafker and others, 1994). Movement ceased along major
dextral-slip faults in interior Canadian Cordillera, including
the Tintina (TI) and Fraser Creek-Straight Creek (FS) Faults.
Between latitudes 51° and 60°, the Queen Charlotte transform
fault separated the Cascade arc and the Aleutian-Wrangell arc.
This fault forms the North American plate margin between
Vancouver Island, Canada. and northern southeastern Alaska.
(11) Offshore of the southern Canadian Cordillera, seafloor
spreading occurred along the Juan de Fuca oceanic ridge
(JFR). To the east, subduction of the Juan de Fuca oceanic plate
(JF) resulted in initiation of the Cascade continental-margin arc
(ca). Part of the subducting plate is preserved in the Siletzia
(SZ), Olympic Core (OC), and Hoh (HO) terranes along
branches of the Cascadia megathrust (CC). Forming along
transcurrent faults along the extension of the Cascade arc was
the Pinchi Creek (PC) metallogenic belt of Hg epithermal vein,
Sb-Au vein, silica-carbonate Hg deposits. Also forming in the
Cascade arc was the Owl Creek (OC) metallogenic belt, which
also contains granitic-magmatism-related deposits and is interpreted
as forming during subduction-related granitic plutonism.
Metallogenic Belts Formed in Tertiary Collision
of Outboard Terranes, Russian Southeast
Central Sakhalin Metallogenic Belt of Au Quartz
Vein and Talc Deposits (Belt CS) Sakhalin Island,
Southeastern Part of Russian Far East
The Central Sakhalin metallogenic belt of Au quartz vein
and talc deposits occurs in the Aniva subduction-zone terrane in
the central part of Sakhalin Island (fig. 102; tables 3, 4) (Nokleberg
and others, 1997b, 1998). The major deposit at Langeriiskoe
consists of Au-bearing quartz-sulfide veins in lenticular bodies