Geology Reference
In-Depth Information
Figure 11.8
N. American Cordilleran
belt: Canadian sector.
A.
Simplified
map showing the main structural
units of the orogenic belt. To the east
of the belt is the foreland, consisting
of Precambrian continental crust of
the American plate with its platform
sedimentary cover. The easternmost
unit of the orogenic belt is the foreland
fold-thrust belt; to the west of this unit
is a series of thrust- or fault-bounded
units, including basement slices, and
various displaced terranes consisting
mostly of volcanic arc material and
arc-trench deposits. The westernmost
unit is a late-orogenic to Recent
accretionary prism. The displaced
terranes have been accreted to the
American plate and subsequently slid
northwards along strike-slip faults.
The Columbia batholith has been
intruded into the western part of the
orogen after terrane amalgamation.
Terranes, from W to E: Cg, Chugach;
W, Wrangellia; Ax, Alexander; T, Taku;
TA, Tracy Arm BR, Bridge River; St,
Stikine; Ch, Cache Creek;YT, Yukon-
Tanana; E, Eastern assemblage.
Based on the terrane map of Coney
et al. (1980).
B, C.
Simplified sketch
sections to show how the orogenic
belt could have been formed by the
amalgamation of displaced terranes;
these are carried along on the oceanic
plate, then scraped off the descending
slab and pushed onto the leading edge
of the American continental plate. This
convergent movement causes the
formation of (mostly) easterly-directed
thrust sheets and a general thickening
and shortening of the orogen. A thrust
sheet with a core of metamorphosed
lower-crustal material escapes to
the surface, possibly as a result of
gravitational spreading. Note that the
terrane boundaries now are generally
steep faults.
Eastern (N American) side
metamorphic
complexes
foreland
fold-thrust belt
Ta
YT
accretionary
prism
fold-thrust
belt
metamorphosed
accretionary prism
Cg
YT
Pacific
Ocean
metamorphic
core complex
E
arc-trench deposits
Ax
arc-trench
deposits
foreland -
N American
plate
metamorphic complexes
accreted
terranes
W
St
ophiolite complexes
volcanic arc
complex
volcanic arc complex
E
W
BR
Western (Pacific) side
Ch
accretionary prism
ophiolites
A
major thrust
major strike-slip fault
600km
foreland
thrust
belt
accreted
terrane
platform
cover
island
arc
oceanic
crust
continental crust
mantle
lithosphere
mantle
lithosphere
B
batholith
basement
thrust
sheet
foreland
thrust
belt
displaced
terranes
ophiolite
platform
cover
oceanic
crust
accretionary
prism
continental crust
600km
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