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Tectonophysics 335 (2001) 211±228
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Crustal-scale rheological transitions during late-orogenic collapse
Olivier Vanderhaeghe*, Christian Teyssier
Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA
Abstract
Orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust,
culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and
cause late-orogenic collapse. Collapse of the Canadian Cordillera is expressed in the Early Tertiary Shuswap metamorphic core
complex, British Columbia, which displays a three-layer crustal section separated by two fundamental rheological discontinuities: (1) the brittle±ductile transition, across which high-angle normal faults in the upper crust control basin formation
merge into a low-angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic
to low-temperature conditions; and (2) the metatexite±diatexite transition across which the rocks lose their solid framework and
behave like a viscous magma. This transition has the potential to mechanically decouple the upper crust from the rest of the
lithosphere during a late-orogenic gravitational collapse. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: orogeny; gravitational collapse; brittle±ductile transition; migmatites; partial melting; crustal rheology; Canadian Cordillera;
Shuswap metamorphic core complex
1. Thermal and mechanical evolution of orogens
The rheological layering of the crust (Ord and
Hobbs, 1989) is likely to evolve during orogenesis
which typically involves an early stage of crustal
thickening and a late stage of gravitational collapse
(Fig. 1; Coney and Harms, 1984; Dewey, 1988).
Several authors (England and Houseman, 1988;
Molnar et al., 1993) propose that the transition from
crustal thickening to collapse is triggered by: (1) an
increase in the potential energy of the overthickened
crustal melt caused by removal of the mantle root and
subsequent asthenospheric upwelling; or (2) a
decrease in the tectonic forces applied to the boundary
of the system related to large-scale plate motion
reorganization. In this paper, we investigate another
* Corresponding author.
E-mail
address:
[email protected]
(O. Vanderhaeghe).
process, the changes in crustal rheology during
orogenesis (Fig. 1). Thermal models show that following a period of crustal thickening under a low geothermal gradient, thermal relaxation and radiogenic heat
production in the overthickened crust may lead to
signi®cant increase in temperature (e.g. England and
Thompson, 1986; DeYoreo et al., 1991; Thompson
and Connolly, 1995). Partial melting of fertile crustal
rocks is predicted after a characteristic time of 20±
30 My when the temperature exceeds the experimentally derived solidus of crustal rocks (T , 6508C in
metapelites, Vielzeuf and Holloway, 1988; Patino
Douce and Johnston, 1991; Gardien et al., 1995 and
T . , 8008C in amphibolites, Rushmer, 1991). In
fact, high-grade terranes exhumed in the hinterland
of collapsed orogens typically show high-temperature/low-pressure metamorphism (Thompson and
Ridley, 1987; Brown, 1993). In addition, the formation of metamorphic core complexes during lateorogenic gravitational collapse is often associated
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PII: S 0040-195 1(01)00053-1
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O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
Fig. 1. Conceptual model of orogenic evolution. (a) Continental convergence is accommodated by crustal thickening and accretion of an
allochthonous terrane to the margin of a continent. The sedimentary sequences of the passive margin are overthrusted by the allochthonous
terrane and decoupled from the crystalline basement along a major decollement. (b) Thermal maturation of the overthickened crust is caused by
radiogenic decay of radioactive elements highly concentrated in the marginal sequences and/or by input of heat from external sources. After a
few tens of My, the increase in temperature in the crust results in partial melting of the fertile sediments buried at mid-crustal depth. (c) Lateorogenic gravitational collapse of the overthickened crust is initiated and ampli®ed in the zone where the sedimentary sequences were
accumulated and partially molten. The characteristics of gravitational collapse are controlled by a new crustal-scale rheological layering
caused by the formation of a layer of partially molten rocks. Gravitational collapse is accommodated by brittle extension of the upper crust and
by ductile thinning of the lower crust. The layer of less viscous and less dense partially molten rocks buoyantly rises during this event to form
domes.
with the exhumation of large migmatitic domes such
as the Variscan Velay dome in the French Massif
Central (Dupraz and Didier, 1988; Burg and Vanderhaeghe, 1993; Lagarde et al., 1994) or the Thor±Odin
dome in the Shuswap metamorphic core complex
(Reesor and Moore, 1971; Vanderhaeghe and Teyssier, 1997). In regions of active continental collision
such as the India±Asia collision, the overthickened
crust is marked by the formation of a partially molten
layer (Fig. 1), as interpreted by the recent INDEPTH
seismic pro®le of southern Tibet (Nelson et al., 1996),
giving us a snapshot of incipient collapse. Therefore,
partial melting is likely to play an essential role in the
rheological behavior of mature orogens.
The aim of this paper is to present a model for the
orogenic evolution of the Canadian Cordillera and
emphasis is laid on the role of partial melting of a
signi®cant part of the crust in modifying its rheology.
During the early stage of crustal thickening, shortening was accommodated by major mechanical
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
decoupling along a crustal suture and/or along a basement-cover decollement (Fig. 1a). After thermal
maturation and partial melting (Fig. 1b), a new crustal
rheological layering developed, enhancing major
mechanical decoupling at the brittle±ductile transition and also across the top of a newly-formed
partially-molten layer deeper in the crust (Fig. 1c).
2. Rheology of partially molten rocks
The geometric relationships between the melt and
solid fractions during partial melting are marked by
several thresholds controlling the migration of the
melt fraction (liquid produced by partial melting)
and the mobility of the magma (liquid combined
with part of the residual solid) (Fig. 2; Wickham,
1987; Burg and Vanderhaeghe, 1993; Sawyer, 1994;
Brown et al., 1995; Vigneresse et al., 1997). The ®rst
melt produced is localized in the pore space (Mehnert
et al., 1973) and it is able to migrate and segregate
from the solid as soon as a melt interconnected
213
network is formed. Under thermodynamic equilibrium the morphology of the melt fraction is
controlled by the balance of Gibbs' free energies of
solid±liquid and solid±solid interfaces de®ning the
wetting angle of the melt with respect to the solid
(Bulau et al., 1979). For granitic systems, experimentally determined wetting angles range from 30 to 608,
in which case an interconnected network is formed for
a few percent of melt (Jurewicz and Watson, 1985;
Laporte, 1994). The formation of an interconnected
network of melt, which corresponds to the ®rst percolation threshold, does not necessarily have a drastic
effect on the rheology of the partially molten rock,
although increasing melt fraction enhances diffusion
creep in experiments performed on ®ne grained aggregates (Dell'Angelo and Tullis, 1988). Nevertheless,
connection of the melt fraction allows percolation of
the melt through the solid framework and segregation
at the macroscopic scale by compaction (McKenzie,
1984; Brown et al., 1995) leading to the formation of
the syn-migmatitic layering (Fig. 2; Brown et al.,
1995; Vanderhaeghe and Teyssier, 1997). Although
Fig. 2. Rheology of partially molten crust. (a) The rheological critical melt percentage at the transition from solid-dominated rocks to liquiddominated rocks. Experimental and theoretical data from Arzi (1978) and from Van der Molen and Paterson (1979). The RCMP corresponds to
a drastic decrease in the strength of the partially molten rocks observed in experiments at about 20% of melt. For crystallising systems, the
effective viscosity is marked by a catastrophic increase for about 30% of melt according to Arzi (1978) or for about 75% according to Philpotts
and Caroll (1996). This latter value depends on the shape and distribution of the particles. For increasing melt fraction, the RCMP is related to
the loss of continuity of the solid framework. We compare the RCMP to the metatexite±diatexite transition in migmatitic terranes illustrated by
pictures from the Thor-Odin dome (Plate 2). (b) Schematic crustal strength pro®le after thermal maturation and before late-orogenic collapse.
The strength of the upper unit is controlled by Byerlee's law. The emplacement of leucogranite laccoliths is associated with a drastic decrease in
the strength which is likely causing localised deformation and activation of the detachments. The strength of the middle unit is controlled by
creep modelled by a power law equation. The metatexite±diatexite transition corresponds to the RCMP described in a) across which there is a
drastic decrease in crustal strength. The question marks indicate that appropriate values for strength of magmatic rocks are ill-de®ned.
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O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
compaction is only ef®cient in segregating melt from
a few centimeters to a few meters, the connection of
the segregated liquid veins into a network of dikes and
sills through fracturing might lead to larger-scale melt
segregation (Clemens and Mawer, 1992) and to the
formation of the homogeneous leucogranite bodies
which are emplaced in most orogens.
On the other hand, if the melt is not well segregated, i.e. if the melt production rate is faster than
the melt segregation rate, the melt fraction in the
rock increases. Experimental and theoretical studies
(Fig. 2; Arzi, 1978; Van der Molen and Paterson,
1979) show that for a melt fraction of about 30±
50% the partially molten rock loses its strength and
becomes a magma which behaves as a dense suspension. This drastic rheologic transition (RCMP, Fig. 2)
is related to the loss of continuity of the rock solid
framework. In the ®eld, the transition from partially
molten rocks with a continuous solid framework
which accommodates deformation, to melt-supported
magma is recognized as the metatexite±diatexite
transition described in migmatitic terranes (Fig. 2;
Brown, 1973; Burg and Vanderhaeghe, 1993; Vanderhaeghe and Teyssier, 1997). Note that during crystallization the equivalent rheologic threshold to the
RCMP is probably not reached for exactly the same
liquid±solid proportions (Vigneresse et al., 1997).
Recent observations of ¯ood basalts indicate that a
continuous skeleton of crystals is formed for solid
fractions from 25 to 35% (Philpotts and Caroll,
1996). Although the rigid structure obtained is likely
to be very delicate, the corresponding solid fraction
(25%) is well below the 70±75% predicted assuming
random close packing of the solid particles (Einstein,
1906; Roscoe, 1952).
3. Case study: Shuswap metamorphic core
complex
In this paper, we investigate the exhumed hinterland of an orogen, the Early Tertiary Shuswap metamorphic core complex (hereafter Shuswap MCC),
British Columbia (Figs. 3±5) which formed during
late-orogenic gravitational collapse of the Canadian
Cordillera (Brown and Journeay, 1987; Parrish et
al., 1988; Carr, 1992; Vanderhaeghe and Teyssier,
1997). This metamorphic terrane provides a unique
window to interpret the evolution of crustal-scale
rheological transitions during late-orogenic collapse
because it displays a complete crustal section from a
migmatitic core to upper crustal units (Fig. 3).
The present structure of the Canadian Cordillera is
the result of (1) Mesozoic accretion of terranes to the
western margin of the North American craton and
associated crustal thickening, and shortening forming
the Rocky Mountain foreland thrust belt, followed by
(2) late-orogenic collapse of the overthickened crustal
welt and formation of the Shuswap MCC during
Paleocene time, and (3) E±W regional extension
and extrusion of basaltic lavas starting in Eocene time.
Accretion of outboard terranes comprising magmatic arcs and oceanic-type sediments (Monger et
al., 1982; Gabrielse et al., 1991; Roback et al.,
1994) started after Late Jurassic time as constrained
by the age of the youngest sediments deposited
offshore of ancient North America involved in the
accretion (Monger et al., 1982; Price, 1986). This
time also corresponds to the early subsidence of the
Alberta foreland basin formed as a consequence of
tectonic loading of the continental margin (Fig. 3;
Price and Mountjoy, 1970). Oblique subduction and
convergence resulted in partitioning of deformation
into a strike-slip component and horizontal shortening
associated with crustal thickening (Coney and Harms,
1984; Price and Carmichael, 1986; Struik, 1993).
Convergence was accommodated by large-scale
thrusting of the allochthonous terranes towards the
NE, over the sedimentary sequence deposited on the
North American passive margin, and major decollement and thrusting of the sedimentary cover over the
Proterozoic and early Paleozoic North American basement (Brown et al., 1986; Price, 1986; Brown and Journeay, 1987). This period of crustal accretion resulted in
the formation of a 55±60 km thick crustal welt by the
end of the Mesozoic (Coney and Harms, 1984).
Following this period of crustal thickening, lateorogenic gravitational collapse of the Canadian
Cordillera began during the Mesozoic±Tertiary transition and was characterized by the formation of the
Shuswap MCC (Crittenden et al., 1980). This MCC
straddles the NW±SE-trending suture between the
paleocontinental margin of Ancestral North America
and the most inboard Intermontane superterrane.
Across N±S-trending normal faults and detachments,
the MCC displays the juxtaposition of upper crustal
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
215
Fig. 3. The Shuswap metamorphic core complex (modi®ed after Wheeler and McFeely (1991). The Shuswap MCC straddles the suture between
accreted terranes and parautochthonous North American terranes. Numbers and associated black lines indicate the location of the sections
presented in Figs. 4 and 5.
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O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
Fig. 4. Section across the Thor-Odin dome (see localisation on Fig. 3). Cross-section constructed from ®eld data and extrapolated over a few km
at depth. This section shows that the major fabric developed in the presence of melt from the migmatites of the lower unit to the detachments.
The complexity of the transposition of the pre-existing fabric is not represented and emphasis is put on the syn-melt deformation.
levels that have preserved Mid-Cretaceous and older
K±Ar cooling ages, overlying high-grade rocks characterised by Paleocene±Eocene K±Ar ages (Ewing,
1981; Mathews, 1981; Parrish et al., 1988). Although
the signi®cance of the high-grade fabric is still
debated, Vanderhaeghe and Teyssier (1997) argue
that it is dominantly transposed during the period of
late-orogenic gravitational collapse starting in the
Paleocene. They proposed that late-orogenic collapse
of the Canadian Cordillera was accommodated by
ductile ¯ow of the lower crust and associated brittle
dislocation of the upper crust coeval with the reactivation of major thrusts in the foreland thrust belt
(Price and Mountjoy, 1970; Covey et al., 1994).
Late-orogenic collapse was followed by a period of
regional E±W crustal extension during Eocene time
characterized by high-angle normal faults affecting all
the rocks at the present exposure levels and associated
with the emplacement of large volumes of basaltic
lavas (Ewing, 1981; Parrish et al., 1988; Struik,
1993; Wingate and Irving, 1994; Constenius, 1996).
4. Structural units
At present exposure level, the Shuswap MCC
consists of three juxtaposed units representing a
,15 km thick structural section through the collapsed
Canadian Cordillera crust (Figs. 3±5; Vanderhaeghe,
1997; Vanderhaeghe and Teyssier, 1997). A lowangle detachment zone separates remnants of
dismembered upper crustal rocks from the exhumed
ductile-deformed core of the complex. Below the
detachment zone, the exhumed ductile-deformed
high-grade rocks are subdivided into an amphibolite-facies middle unit, dominantly metasedimentary, and a lower migmatitic unit appearing in the core
of domes intruding the amphibolite-facies cover.
4.1. Upper unit
The upper unit (Figs. 4 and 5) consists of a metamorphic basement that experienced upper crustal
conditions since Late Jurassic time as indicated by
Rb±Sr and K±Ar cooling ages which range from
157 to 147 Ma (Mathews, 1981; Parrish and
Armstrong, 1987; Colpron et al.,1996). This unit is
affected by an array of high-angle normal faults and
steep strike-slip faults active during Late Mesozoic
and Early Tertiary time (Ewing, 1981; TempelmanKluit and Parkinson, 1986; Van Den Driessche and
Maluski, 1986; Armstrong et al., 1991; Struik, 1993;
Fig. 5. Section of the Canadian Cordillera from Vernon to Calgary (modi®ed after section of Price and Mountjoy (1970). The section across the Shuswap MCC is modi®ed
according to our ®eld data. Interpretation at depth is based on an interpretation of the Lithoprobe seismic pro®le (Cook et al., 1988). The lower crust underneath the Shuswap is
characterized by a large number of re¯ectors and by a ¯at Moho which are interpreted to represent transposed lithological layering during horizontal ¯ow of the lower crust.
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
217
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O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
Plate 1. Leucogranite in the Shuswap. (a) Leucogranite emplaced below the detachment at Trinity Hills showing a pervasive C/S fabric. (b)
Interconnected network of concordant and discordant granitic veins in an hornblende tonalite along the Trans-Canada Highway 1. (c) Garnets
with symmetrical pressure shadows ®lled by granitic material. Photo taken on the southern ¯ank of Mt Symonds, to the south of the Thor-Odin
dome.
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
Wingate and Irving, 1994). Block faulting of the
upper unit resulted in the development of basins.
These are ®lled by up to a thousand meters of thick
immature and coarse sediments. The base of the sedimentary sequence is dominated by volcaniclastic
fanglomerates and the top by slide blocks of the metamorphic basement including quartzite and granite±
gneisses. Vitrinite re¯ectance values (R0 ˆ 1.16,
Enderby Basin, Fig. 4) in coal-bearing sediments
found at the base of the basin indicate temperatures
of 150±1758C (Mathews, 1981). The basin ®ll is
capped by Early Eocene ma®c volcanics of the
Maroon Formation dated between 47 and 49 Ma
(Mathews, 1981; Tempelman-Kluit and Parkinson,
1986.
4.2. Detachment zone
The transition between the upper and middle units
is marked by a shallowly dipping detachment zone
(Fig. 4) de®ned to the west as the Okanagan detachment (Tempelman-Kluit and Parkinson, 1986, and to
the east as the Columbia River detachment (Read and
Brown, 1981). The detachment zone is characterized
by progressive overprinting of amphibolite-facies
fabric, by a greenschist-facies mylonitic fabric
which grades into cataclasite, breccia, and pseudotachylite zones. Most high-angle normal faults in
the upper unit do not penetrate the middle unit and
apparently sole into the low-angle detachment zone.
The detachment zone is associated to the emplacement of leucogranite laccoliths probably generated
at lower crustal levels which migrate through the
amphibolite-facies middle unit. The leucogranites
show strong ductile fabrics representing deformation
during the magmatic stage overprinted by solid-state
deformation under greenschist-facies conditions
(Plate 1).
4.3. Middle unit
Below the low-angle detachment zone, the middle
unit (Fig. 4) is mainly composed of a metasedimentary
sequence affected by high-temperature/low-pressure
metamorphism and partial melting (T , 7008C;
P , 6±7 kbars; Reesor and Moore, 1971; Ghent et
al., 1977; Journeay, 1986; Lane et al., 1989; Sevigny
et al., 1989; Nyman et al., 1995). In addition to the
presence of in situ partial melts, the middle unit is
219
permeated by a network of granitic sills and dikes
connecting the leucogranite laccoliths emplaced
within the detachment zone (Plate 2). Map-scale relations suggest that the granites originated from the
lower migmatitic unit which is consistent with their
similarity in geochemical signatures (Sevigny et al.,
1989; Brandon and Lambert, 1993). Previously
published Uranium±Lead age determinations on
zircon and monazite from these granites range from
Cretaceous to Eocene. Small, synkinematic sill-like
sheets yield U±Pb ages of ,100 Ma (Sevigny et al.,
1989, and the voluminous Ladybird leucogranite
yields ages from 60 to 55 Ma (Carr, 1992; Parkinson,
1992). Widespread crystallization of pegmatites
occurred until 50 Ma (Parkinson, 1992) SHRIMP
analysis of zircons from the leucogranite emplaced
below the western detachment at Trinity Hills give a
mean U±Pb age of 59.8 ^ 1 Ma (Vanderhaeghe,
1997). In addition cathodoluminescent imaging
revealed the presence of inherited cores yielding
discordant U±Pb ages. Ar-thermochronology of hornblende, white micas, biotite and K-feldspar indicates
that the footwall of the detachments recorded a period
of rapid cooling (50±1008C/Ma) between 55 and
50 Ma (Vanderhaeghe, 1997).
The foliation in the amphibolite facies middle unit
is concordant with the attitude of the low-angle
detachment, except around migmatitic cores where
it steepens and underlines the shape of the domes
(Figs. 3-5). The major foliation carries a NE±E-trending lineation de®ned by the alignment of biotite and
the preferred orientation of sillimanite and quartz
ribbons (Vanderhaeghe and Teyssier, 1997). This
major fabric is dominantly symmetrical except toward
the detachment where asymmetric kinematic criteria
such as delta and sigma porphyroclasts (Passchier and
Simpson, 1986), C/S structures (Berthe et al., 1979),
shear bands, and asymmetric boudins (Hanmer, 1986)
indicate east-side down on the eastern side, and westside down on the western side of the complex. The
presence of granitic material as veins and pods
concordant with the major foliation, in boudin
necks, shear zones, pressure shadows around garnets
and in®lling fractures affecting the garnets, indicates
that the presence of melt was coeval with the major
deformation (Plates 1, 2).
With increasing metamorphic grade, the metasedimentary middle unit is progressively affected by
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O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
partial melting. This ®rst occurs in the most fertile
protoliths such as the metapelitic layers. Leucosomes
are segregated, but the compositional layering derived
from the sedimentary bedding and transposed into the
foliation is still identi®able (Plates 1, 2). Structurally
below these migmatitic paragneisses, the migmatites
are characterised by a continuous layering de®ned by
regular alteration of leucosomes, melanosomes and
mesosomes (Mehnert, 1968; Brown, 1973) which
are interpreted to represent small-scale melt segregation from partially molten material (Brown et al.,
1995). At this stage, we refer to the compositional
layering of the rock as synmigmatitic layering
(Vanderhaeghe and Teyssier, 1997). It represents an
original bedding transposed during deformation in
which the fertile layers are preferentially affected by
partial melting and segregation (Plate 2).
4.4. Metatexite/diatexite transition
Structural analysis in the Thor-Odin area indicates
that the migmatitic dome is cored by diatexites and
surrounded by metatexites (Figs. 3, 4; Vanderhaeghe
and Teyssier, 1997). Diatexites are dominated by the
granitic fraction in which the remaining solid is represented by discontinuous schlieren, restite, and xenoliths outlining the magmatic fabric. In contrast,
metatexites have preserved a continuous solid framework marked by regular alternations of melt layers
and residual solids de®ning a synmigmatitic layering
(Plate 2; Mehnert, 1968; Brown, 1973; Burg and
Vanderhaeghe, 1993). Although mesoscopic contacts
indicate that the diatexites are intrusive in the metatexites, the magmatic foliation and the synmigmatitic
layering are both steeply dipping and concordant with
the attitude of the metatexite±diatexite transition
which occurs over a few hundred meters and delineates the shape of the dome (Fig. 4; Plate 2). On the
southern limb of the Thor-Odin dome the magmatic
foliation carries a down-dip biotite and sillimanite
mineral lineation perpendicular to the horizontal E±
W trending mineral and stretching lineation of the
metatexites. The foliation is affected by shear bands
consistent with diatexites moving up with respect to
metatexites (Plate 2). Synmigmatitic way-up criteria
within the metatexites such as cauli¯owers, interpreted as incipient ®gures of diapirism, or asymmetric
vein clusters interpreted as melt accumulation under-
neath competent layers (Burg, 1991; Burg and
Vanderhaeghe, 1993) indicate outward tilting of the
synmigmatitic foliation with respect to the diatexites.
These observations suggest that the diatexites moved
en masse while the melt fraction was present and
deformed the metatexites to form the observed
domes.
4.5. Lower unit
The lower structural unit (Fig. 4) is formed by
diatexite migmatites (Plate 2) exposed in the core of
dome-shaped culminations aligned along the strike of
the belt. Previously published U±Pb geochronology
indicates slightly discordant ages on zircons with
upper intercepts of 2.2 Ga in paragneisses (Parkinson,
1991), to 1.87±2.10 Ga in orthogneisses (Wanless and
Reesor, 1975; Armstrong et al.,1991; Parkinson,
1991). On this basis, the protolith of the migmatites
has been considered to be mainly composed of Paleoproterozoic North American basement. However, in
the Thor-Odin dome, the diatexites show map-scale
cross-cutting contact with a quartzite marker
horizon (Reesor and Moore, 1971; Vanderhaeghe
and Teyssier, 1997) of either Proterozoic or Cambrian
age described by other authors to be unconformably
overlying the basement migmatitic gneisses (Scammel and Brown, 1990). In addition, SHRIMP analysis
of zircons from leucosomes of the migmatites of the
Thor-Odin dome yield a ,56 Ma U±Pb age for
magmatic rims surrounding inherited cores with
ages ranging from 1.8 to over 2.4 Ga. Metamorphic
assemblages suggest that partial melting occurred
under a high geotherm and was followed by isothermal decompression from 6 to 4 kbars (Duncan,
1984; Journeay, 1986). The magmatic fabric of the
diatexite, marked by the alignment of xenoliths and
restitic ma®c minerals, delineates the shape of the
dome (Plate 2). Microscopic analysis of the diatexite
(Plate 3) reveals preserved magmatic textures (inter®ngered minerals and mymekite) slightly overprinted
by solid-state deformation (checkerboard texture in
large quartz grains and locally chlorite- or muscovite-bearing shear zones). These results indicate that
crystallisation of the migmatites occurred in Early
Tertiary and proceeded rapid cooling recorded by
Ar-thermochronology by about 5 My.
Plate 2. Migmatites of the Thor-Odin dome. (a)±(a 0 ) to (c)±(c 0 ) illustrate the metatexite/diatexite transition going towards the core of the Thor-Odin dome. (a) Metatexite of the
southwestern ¯ank of Mt Odin showing the continuity of the synmigmatitic foliation disrupted locally by small-scale shear zones ®lled by granitic material. (a 0 ) Detail of the
synmigmatitic foliation (width of the picture Ð 3 m). The meter-scale compositional layering characterized by alternations of dominantly felsic with dominantly ma®c layers is
interpreted as a remnant of a transposed bedding. Alternations of mesosomes with quartz-feldspar leucosomes surrounded by biotite melanosomes are interpreted to represent melt
segregation during transposition of the pre-existing fabric. (b) Metatexite±diatexite transition on the southwestern ¯ank of the Mt Odin. Increasing amount of granitic material is
associated with loss of continuity of the synmigmatitic foliation. (b 0 ) Shear zone at the contact between melt-dominated diatexite and solid-dominated metatexite (North of Mt
Thor). (c) Diatexite of Mt Odin with large amphibolite xenoliths ¯oating in a granitic matrix. (c 0 ) Detail of a diatexite on the southern ¯ank of Mt Odin showing the magmatic fabric
delineated by the alignment of xenoliths and schlieren.
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
pp. 221±222
Plate 3. Microstructures of rocks in the footwall of the detachments. (a) Photomicrograph of a leucosome/melanosome contact in a sample collected east of Mt Thor. Notice the
orientation of the biotite crystals parallel to the contact with the quartz-feldspar leucosome (crossed polars, width of picture: 5 mm). (b) Photomicrograph under crossed polars of
leucosome from a migmatite on the southern ¯ank of Mt Odin (width of picture: 6 mm). Surrounding a square feldspar crystal, quartz crystals with checkerboard deformation bands
sub-parallel and sub-perpendicular to C-axes. (c) Photomicrograph under crossed polars of melanosome in same sample as a) (width of picture: 3 mm) showing intergrown ®brous
sillimanite and biotite, and myrmekite. (d) Photomicrograph of Ladybird leucogranite (same sample as Plate l a) under crossed polars (width of picture: 6 mm). An early fabric,
marked by K-feldspar porphyroclasts surrounded by quartz ribbons and white micas, is overprinted by narrow shear bands marked by grain size reduction. (e) Photomicrograph of a
mylonitic granite on the eastern side of the Shuswap at Mt Hall under crossed polars (width of picture: 6 mm). Similar texture as d) with undulatory extinction of K-feldspar
porphyroclasts and increasing ®ne grained matrix (width of picture: 6 mm). (f) Same mylonitic granite as (e) with myrmekite developing at the boundaries of large K-feldspar
porphyroclasts (width of picture: 1.5 mm).
pp. 223±224
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
5. Discussion
Crustal thickening of the Canadian Cordillera is
deducted from the metamorphic conditions recorded
in the high-grade rocks of the Shuswap MCC, which
indicate a minimum burial of ,20±30 km (Ghent et
al., 1977; Duncan, 1984; Journeay, 1986; Nyman et
al., 1995. Crustal shortening was achieved by the
formation of nappes and large-scale thrusting
(Brown et al., 1986; Price, 1986). The high-grade
rocks of the Shuswap MCC are affected by hightemperature/low-pressure metamorphism, and widespread partial melting of fertile paleoproterozoic (?)
and paleozoic metapelites (Sevigny et al., 1989;
Brandon and Lambert, 1993), which are likely sources
for the leucogranites and pegmatites that intruded the
sequence from 60 to 50 Ma (Carr, 1992; Parkinson,
1992). Metamorphic assemblages of the high-grade
rocks indicate that the thermal peak was followed
by decompression (Duncan, 1984; Nyman et al.,
1995). Thermochronologic data suggest subsequent
high cooling rates at the Paleocene±Eocene boundary
(on the order of 50±1008C/My; Mathews, 1981;
Sevigny et al.,1990; Vanderhaeghe, 1997).
Based on our structural analysis and geochronological results, we propose that partial melting of the
middle and lower unit, transposition of the major
foliation in these units, upwelling of the lower unit
(Thor-Odin migmatite dome), and horizontal extension in the upper unit, are genetically related (Fig.
5). Thermal maturation of the overthickened crust
(comprising thermal relaxation and heat generation)
caused signi®cant increase in temperature and, at 20±
30 km depth, partial melting of the thick and fertile
tectonic wedge with a thermal zonation leading to
what is observed today as the metatexite±diatexite
boundary. The middle crust, whilst undergoing partial
melting, developed a ¯at-lying foliation dominated by
bulk coaxial deformation transposing previous
fabrics. Rocks containing a large amount of melt,
which became diatexites, moved en masse like a
magma, intruding and steepening the ¯at-lying fabric
developed in the presence of melt in the mid-crustal
gneisses. Magma bodies of the Ladybird leucogranite
suite were most likely derived from melt segregation
from the middle and lower units, migrated to the base
of the brittle upper crust and ponded to make laccoliths traceable over tens of kilometers. Note that
225
the structures associated to crustal thickening and
potential nappes stacking are pervasively overprinted
and transposed during the late stages of ductile ¯ow in
the presence of melt.
This situation is comparable to present-day southern
Tibet, according to the interpretation of the INDEPTH
pro®le (Nelson et al., 1996) which indicates the
presence of a partially molten layer at mid-crustal
depth overlain by magmatic bodies. Accordingly,
we propose that two major rheological transitions
were established as a direct result of partial melting
of the thickened crust and formation of leucogranite
laccoliths: (1) the brittle±ductile boundary de®ned by
a sharp thermal gradient between cool upper crust and
crystallizing magma; (2) the metatexite±diatexite
transition, deeper in the crust. We propose that at
this point in the evolution of the orogen, the thickened
crust underwent a drastic decrease in strength and
could not support the mountain belt: The thickened
crust collapsed under its own weight and deformation
in the crust was potentially decoupled from subcrustal
lithospheric motion. Collapse of the Canadian Cordillera developed rapidly at about 55 Ma. The two
rheological transitions controlled the style of collapse:
(1) positive feedback between the emplacement of
leucogranite laccoliths and deformation localisation
triggered the activation of the brittle±ductile transition allowing mechanical decoupling during collapse
between a brittle upper crust undergoing normal faulting and a ductile lower crust ¯owing to accommodate
vertical thinning; (2) loss of the rock coherence
through the metatexite±diatexite transition allowed
gravitational instabilities to develop and the formation
of domes cored by migmatites.
Extension and thinning of the upper crust in the
core of the orogen, presumably driven by the orogen
topography is at least partly accommodated by
renewed thrusting in the Rocky Mountains foreland
belt, evidenced by out-of-sequence reactivation of
major thrusts.
The tectonic evolution of the Canadian Cordillera
illustrates the transition from (1) early stages of
orogeny dominated by plate tectonic convergence
and crustal thickening to (2) late-orogenic collapse
corresponding to ¯ow of the overthickened crust
under its own weight. Crustal thickening is accommodated in a relatively cold and rigid crust by mechanical decoupling along a major plate boundary
226
O. Vanderhaeghe, C. Teyssier / Tectonophysics 335 (2001) 211±228
marked by a suture and by decollement of the sedimentary cover over the old basement. The subsequent
thermal maturation of the overthickened crust causes
the formation of a layer of magma by partial melting
of the fertile middle crust inducing a drastic decrease
in the crustal viscosity. The resulting new crustalscale rheological layering controls the behavior of
the crust during late-orogenic collapse. The upper
brittle crust is affected by normal faulting whereas
the lower part of the crust is affected by ductile
thinning. Flat-lying detachments, associated to the
emplacement of leucogranite laccoliths, accommodate the difference in behavior at the brittle±ductile
transition. The ductile lower crust is affected by largescale boudinage and the buoyant low-viscosity
magmatic layer rises to form domes.
Acknowledgements
This work was supported by National Science
Foundation grant EAR 9509750. OV gratefully
acknowledges summer support and a Gruner±Emmons
fellowship from the Department of Geology and
Geophysics, University of Minnesota, Sigma Xi
Grant-in-Aid of Research and the Geological Society
of America Research Grants. CT gratefully acknowledges support from the Bush sabbatical grant, University of Minnesota as well as support from ETHZ
Zurich. In addition, OV would like to express his
thanks to ®eld assistants for their tenacity, friends in
Revelstoke for their hospitality, and collaborators
from the geochronology team at ANU for their
patience. This paper bene®ted from reviews by Stefan
Schmid and Alison Ord.
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