Transcript An Introduction to Terrane Analysis in the Western North American

The Great Terrane Wreck
of the Cordillera
Allen J. McGrew
April 2005
What is a
Tectonostratigraphic
Terrane?
Terranes can be:
• Native – showing shared traits
with North American crust,
indicating an origin adjacent to
North America.
• Exotic – Far-traveled, not born
adjacent to North America
• Superterranes – amalgamated
with other terranes before
accretion to the continental
margin.
Evidence of origins?
Stratigraphic
similarities
 Faunal affinities
 Sedimentary
provenance
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– e.g.,detrital
zircon dating
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Paleomagnetic
evidence
Paleomagnetic Evidence
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Paleomagnetism
– Inclination
– Declination
– Polarity
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Paleopoles and
Paleolatitude
Apparent Polar
Wander Paths
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Comparison with
APWP for North
America allows
assessment of
whether terrane could
have been close to
No. America at time
of deposition of the
measured rock.
Limitations of Paleomagnetic Analysis
Paleolatitude North or South?
 No constraint on paleolongitude.
 What if paleomagnetism was not acquired
when we thought it was? (fold tests)
 How well do we know paleohorizontal at
the time magnetism was acquired?
 What if paleomagnetic vectors changed in
orientation after deposition? (e.g., due to
flattening or deformation)

An Example: Paleomagnetic Reconstruction
of the Alexander Terrain
(after Butler, Gehrels & Bazard, GSAB, 1997)
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Ordovician-Silurian:
No dependable
paleolatitude.
Early Devonian:
Karheen Fm 14º ± 4º
possibly Australia or pref.
Baltica
Middle Devonian Pennsylvanian: no
dependable information.
Alexander Paleogeographic
Reconstructions (cont’d)
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Permian: Halleck volcanics &
three other formations indicate
paleoposition of amalgamated
Alexander-Wrangellia terrane
at 25º - 30º N off the west
coast of No. America.
Triassic - 10º - 20º N at
paleolatitude of northern
Oregon (Seven Devils &
Huntington Arcs).
Jurassic & Cretaceous – no
primary magnetizations;
Alexander-Wrangellia accreted
to North American continental
margin and dispersed
northward as far as Alaska by
translation on strike-slip faults.
Recognizing Amalgamation & Accretion
• “Stitching Plutons” –
plutons intruded into
adjacent terranes
demonstrating that they had
to be adjacent at the time
of pluton injection.
• “Overlap Assemblages”
Stratigraphic packages
deposited across terrane
boundaries indicating that
terranes had to be adjacent
at time of deposition.
A History of Terrane
Accretion
Mesozoic
Paleogeography and
Tectonic History of
Western North America
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Images and text modified from a poster
session presented to the Annual Meeting of
the Geological Society of America, Seattle,
Nov. 2003 by Ron Blakey and Paul
Umhoefer, Department of Geology, NAU
http://jan.ucc.nau.edu/~rcb7/mz_paleogeog_
wus.html
Permian Tectonics and Paleogeography
(290 Ma)
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Mc Cloud arc fragmented
during late Paleozoic truncation
of SW North America
Assembly of western Pangaea
completed
Transform margin coupled
McCloud arc with arcs built on
Chortis and South America
Havallah back arc basin (HB)
and Slide Mtn back arc
separated northern arc,
Quesnellia (Q), from Antler belt
(A) and western North America
Blakey and Umhoefer, 2003
Triassic Tectonics and
Paleogeography (240 Ma)
Early to Middle Triassic
(240 Ma)
 McCloud arc fragmented
and accreted to North
America resulting in
Sonoman orogeny
 Caborca terrane moved SE
along truncated continental
margin
 Subsequent Cordilleran arc
was continental to south
and marine to north
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Blakey and Umhoefer, 2003
Early Jurassic Tectonics and
Paleogeography (180 Ma)
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Early to Middle Jurassic
(180 Ma)
?? Fringing Cordilleran
arc accreted to western
North America including
Bridge River, North
Cascades, western parts
of Eastern Oregon
terranes, and western
terranes of Klamaths
and Sierra Nevada
?? Older Mesozoic back
arc basins closed
Blakey and Umhoefer, 2003
Middle Jurassic Tectonics and
Paleogeography (160 Ma)
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Middle to Late Jurassic (160 Ma)
Major arc magmatism
Initial collision between southern
Wrangellia and Cordilleran
margin at approximately the
latitude of Klamaths and Sierras
Fringing arcs south of collision
zone
Ophiolites obducted in collision
zone and in inter arc region
between Cordilleran arc and
fringing arcs
Foreland basin in Utah and
thrusting in Nevada
Rift basin in southwestern North
America related to opening Gulf
of Mexico
Blakey and Umhoefer, 2003
Late Jurassic Tectonics and
Paleogeography (145 Ma)
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Major plate reorganization -change to sinistral motion of
Farallon plate relative to North
America causes Wrangellia to
move southward
Complex series of events at SW
margin of North America: Chortis
and related terranes shift SE as
Atlantic and Proto-Caribbean
expand; transform fault and
oblique rift systems developed
along waning continental arc
(McCoy-Bisbee; Mojave-Sonoran
megashear); early Franciscan
mÈlange formed farther
outboard
Nutzotin Ocean between
northern Wrangellia and North
America remained open
Oblique convergence shut down
much of Cordilleran arc
Blakey and Umhoefer, 2003
Early Cretaceous Tectonics and
Paleogeography (125 Ma)
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Guerrero arc collided and
subduction zone rebuilt to west.
South-moving Wrangellia linked
to North Cascades, TyaughtonMethow, and adjacent terranes
to form Baja BC
Tectonic escape of Intermontane
terrane and parts of California
northward Great Valley forearc
basin and Franciscan subduction
complex built on reorganized
Cordilleran margin; as Baja BC
moved southward, northern
Great Valley a transpressive
basin
North Slope terrane rotated CCW
as Canadian basin opened
Early Sevier thrust belt and
foreland basin formed
Blakey and Umhoefer, 2003
Late Early Cretaceous Tectonics
and Paleogeography (105 Ma)
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Two large volume magmatic belts
formed (Peninsular, Sierra
Nevada, Idaho; Coast Plutonic
complex built on Baja BC);
plutonic complexes may have
been fed by volatiles from underthrusting of North American
miogeocline
South-moving Baja BC
approached maximum S position
at latitude of southern Arizona
Baja BC west of classic Great
Valley and Franciscan complexes;
Great Valley as interarc basin(?)
Rotated North Slope terrane
collided with Yukon-Tanana and
related terranes
Sevier orogeny continued and
foreland basin expanded
eastward
Blakey and Umhoefer, 2003
Late Cretaceous Tectonics and
Paleogeography (85 Ma)
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Plate reorganization as Kula
separated from Farallon Plate;
dextral transpression with North
America; Kula-Farallon ridge
moved northward along coast
Baja BC transported northward
with obliquely converging Kula
plate
Major magmatic activity in two
belts (110-85 Ma) followed by
strong decline and null in
magmatic activity (80-40 Ma)
Andean-style segmentation of
Farallon arc from Central
America to Southern California
Sevier thrusting continued and
foreland basin shifted locus of
subsidence southward in
response to shallow
Laramide ‘flat-slab’ subduction
Blakey and Umhoefer, 2003
Cretaceous-Tertiary Tectonics and
Paleogeography (65 Ma)
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Triple junction and Baja BC moved
rapidly northward
Intermontane terranes translated
to north
Forearc sedimentation (FranciscanGreat Valley) continued along
southern margin
Uplift of Coast plutonic complex
generated large deep-sea fan
deposits (Chugach flysch)
Strong Sevier thrusting and
foreland basin
Laramide orogeny: Farallon Plate
shallow subduction caused
expansion NE of foreland uplifts
and basins
Blakey and Umhoefer, 2003
Eocene Tectonics and
Paleogeography (50 Ma)
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Intermontane and Baja BC blocks
continue northward translation
along with north-migrating KulaFarallon plate boundary; many
terranes approach their present
latitude with respect to North
America
Olympic terrane accreted
Extensive fore arc sedimentation
along most of Cordilleran margin
Rocky Mountain foreland uplift,
deformation, and shallowsubduction-related magmatism;
interior sedimentation mostly
restricted to Rocky Mountain
foreland basins
Blakey and Umhoefer, 2003