USGS Professional Paper 1697 - Alaska Resources Library

8 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Richard D. Lancaster and Kim Nguyen of the Geological
Survey of Canada for computer drafting of the lode mineral
deposit figures for the Canadian Cordillera. We thank Dani
Alldrick, Chris Ash, Derek Brown, Larry Diakow, Fil Ferri,
Trygve Höy, Dan Hora, David Lefebure, Jim Logan, Donald
MacIntyre, Bill McMillan, Mitch Mihalynuk, JoAnne Nelson,
Andre Panteleyev, Robert Pinsent, Gerry Ray, Paul Schiarizza,
and George Simandl for revisions of mineral deposit descriptions
for British Columbia, Canada. We also thank Marti L.
Miller and Suzanne Paradis for their constructive and very
helpful reviews.
Introduction to Phanerozoic Metallogenic
and Tectonic Model for the Russian Far
East, Alaska, and the Canadian Cordillera
In the Phanerozoic (Devonian through Recent) time-span
sections on metallogenesis of the region that follow, an interpretative
model is presented for the Phanerozoic metallogenictectonic
evolution of the Russian Far East, Alaska, and the
Canadian Cordillera. The model is derived from the descriptions
below of metallogenic belts and host rocks, and from a detailed
analysis of the Phanerozoic tectonic evolution of the region
(Nokleberg and others, 2000). The metallogenic-tectonic model
attempts to (1) integrate stratigraphic, age, structural, and paleomagnetic
data and field relations for the region, (2) integrate
data on metallogenic belts and contained lode deposits with
host-rock geology and structures, and (3) portray the regional
metallogenic-tectonic interactions between the North Asian
and North American continents. The model concentrates on
the Devonian through the Present. For more descriptions of the
regional geology and tectonics of the region, a detailed analysis
was published by Nokleberg and others (2000).
The metallogenic-tectonic model illustrates (1) major
metallogenic belts superposed, at approximate scale, over major
units, including cratons, craton margins, terranes, and overlap
assemblages, (2) geologic units that are proportional to those
on detailed terrane and overlaps assemblage maps (Nokleberg
and others, 1994a, 1997b,c; Monger and Nokleberg, 1996), and
(3) known or interpreted displacements along major strike-slip
and thrust faults. In most cases, however, the tectonic model
does not incorporate internal deformation of terranes or tectonic
erosion of terrane margins. In the following descriptions of the
metallogenic-tectonic model, the tectonic features of the model
are condensed. For complete description of tectonic features,
refer to the separate publication on Phanerozoic tectonic evolution
of the Circum-North Pacific (Nokleberg and others, 2000).
A dynamic (computer) version of the metallogenic-tectonic
model is published by Scotese and others (2001).
The metallogenic-tectonic model provides a guide for
future research by (1) integrating geologic, mineral deposit,
metallogenic belt, paleontologic, isotopic, and paleomagnetic
data from the Russian Far East, Alaska, the Canadian Cordil-
lera, the Pacific Ocean, and the Arctic Ocean, (2) proposing a
new, unified interpretation that spans the area from northeastern
part of the North Asian Craton to the northwestern part
of the North American Craton, and (3) identifying problems
with data and interpretations. Because of a lack of abundant
Proterozoic and older rock units exterior to the craton margins,
the model starts with the Devonian. For various published
tectonic reconstructions for the Proterozoic, which illustrate
highly different global interpretations, the studies of Hoffman
(1989, 1991), Moores (1991), Ross and others (1992), Scotese
(1997), Unrug (1997), and Karlstrom and others (1999) are
recommended.
An important complication of terrane recognition and
analysis is that the margins of terranes and parts of their preaccretionary
metallogenic belts, have been tectonically removed,
either by dislocation of terranes from distant locations, or
by tectonic erosion of the margins of terranes. In the case of
dislocation, place overlap assemblages, passive continentalmargin
assemblages, and cratonal assemblages should provide
the original site of origin. In the case of tectonic erosion, as in
the case of subduction-zone terranes, large parts of the original
unit (such as an oceanic plate) may have been thrust to great
depths and thereby may essentially have disappeared.
An important interpretation in the metallogenic-tectonic
model is that a succession of coeval single arcs and companion
subduction zones, and their contained, preaccretionary
metallogenic belts formed on or near the margins of the North
Asian and North American Cratons. One consequence of this
interpretation is that many of the complexities of the collage
of accreted terranes and contained metallogenic belts in
the region are the results of oblique subduction and resultant
strike-slip displacements within active continental margins,
rather than the migration of island-arc systems across ocean
basins to accrete eventually to the margins of plates. Substantiation
of this interpretation will require (1) determination of
the facing directions of the arcs with respect to cratons, (2)
correlation of coeval arc and tectonically linked (companion)
subduction zones to establish them as different parts of a
former, single, curvilinear arc/subduction-zone system, and (3)
determination of the linkage of arcs to cratons. This interpretation
is reasonably well established for most of the Mesozoic
and Cenozoic, but less so for the Paleozoic. For each time
interval (stage) in the metallogenic-tectonic model, specific
(numbered) tectonic events are described in a clockwise order,
according to similar tectonic environments, starting with the
area of the Russian Southeast and ending with the area of the
southern Canadian Cordillera. The time scale used for the
tectonic model is from Palmer (1983).
Paleomagnetic Dilemma: Loci of Accretion of
Wrangellia Superterrane
A major paleomagnetic dilemma exists for the loci
of accretion of superterranes to the margin of the North
American Craton in the mid-Cretaceous to early Tertiary. The