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Encouragingtheextrusionof...
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Encouraging the extrusion ofdeep-crustal rocks in collisional zones
A. HYNES*
Department of Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, Canada H3A 2A7
ABSTRACT
Most unroofing mechanisms invoked for the exhumation of blueschist-plus-eclogite terranes, includingcorner-flow and extensional collapse of the orogenic wedge, predict steep unroofing paths for thedeeply-buried rocks and are applicable only to unroofing from depths within the crust. Many high-Pand ultrahigh-P rocks of continental affinity are derived from greater depths than this. Their lack ofwarming during unroofing, together with indications that they may rest directly on less deeply buriedequivalents, are suggestive of shallow unroofing paths similar to those for the subduction-channelmodel. They are interpreted to have been emplaced by the upward extrusion of coherent slices ofcontinental crust, bounded below by thrust faults and above by normal faults, with unroofing pathsessentially reversing the original burial paths.Where continental crust has been subducted into the mantle, upward extrusion is probably driven
largely by buoyancy forces, although examples of upward extrusion without subduction into the mantleindicate that buoyancy forces may not be essential. Two features in addition to buoyancy may promoteupward extrusion. Slab breakoff may reduce the pull from the descending slab, and subduction-zonegeometry may change as a continental margin is dragged into the subduction zone. Both features maypromote the extrusion of continental crust at precisely the time at which it has been partially subducted.A close spatial relationship between a lateral ramp and a lobate zone of extruded high-P rocks in the
Mesoproterozoic Grenvillian orogen indicates that lateral ramps may be important in localizingextrusion. Lateral ramps disturb the two-dimensional flow, with channelling of material into the regionof the lateral ramp as it is extruded. Many exhumed ultrahigh-P terrains are associated with jogs in thetrends of orogenic fronts that may reflect the presence of lateral ramps at depth. Ultrahigh-P rocks maybe expected to be concentrated at such jogs, and may record the channelling in their deformationhistory.
KEYWORDS: blueschist, collision zones, eclogite, Grenville orogeny, subduction-channel model, unroofing
mechanism, ultra-high pressure terranes.
Introduction
SINCE the first recognition of the conditions under
which they occur, mechanisms for the preserva-
tion of blueschists and eclogites at the surface
have been the subject of much debate. The
preservation of these high-P-low-T metamorphic
rocks requires their burial in regions with
unusually low geotherms, followed by unroofing
without warming to normal geotherms.
Mechanisms for the burial are not particularly
problematic. Any deep-seated thrust system, and
particularly one associated with a subduction
zone, produces the requisite conditions in its
footwall (e.g. Oxburgh and Turcotte, 1974;
England and Richardson, 1977; England and
Thompson, 1984). Unroofing without warming,
however, requires either rapid unroofing, or
unroofing under conditions that prevent
warming, and the search for such mechanisms
has been challenging. The now widespread
recognition of high-P and ultrahigh-P (in the
field of coesite stability; >~2800 MPa) metamor-
phosed rocks with continental affinities (e.g.
Compagnoni et al., 1977; Seidel et al., 1982;
# 2002 The Mineralogical Society
* E-mail: [email protected]: 10.1180/0026461026610013
Mineralogical Magazine, February 2002, Vol. 66(1), pp. 5–24
Chopin, 1984; Griffin et al., 1985; Okay et al.,
1989) has broadened this challenge, since it
indicates that continental crust partially subducted
into the mantle may fairly commonly be returned
to the surface. In this paper, I review evidence for
the possible paths followed by deeply buried
rocks on their way back towards the surface, and
the mechanisms that have been suggested for such
paths. I also show that formerly deeply-buried
rocks appear to occur at very specific localities
along orogenic belts, which may reflect features
of the geometry in the subsurface that were
instrumental in effecting their unroofing.
Paths to the surface
Blueschists, high-P and ultrahigh-P continental
rocks and associated eclogites almost certainly
develop their mineral assemblages in the foot-
walls of reverse faults, where the requisite high P
may be achieved at low temperatures. Such
reverse faults could develop within the orogenic
wedges of crustal-thickening zones, in which case
the hanging wall would be crustal, or could be the
reverse faults associated with subduction zones, in
which case the hanging wall could include mantle
material. In the case of many blueschist-plus-
eclogite terranes, the melange-like character of
the assemblage and the incorporation of frag-
ments of ophiolite complexes provide compelling
evidence for their formation in subduction-zone
settings (cf. Ernst, 1973, 1984). In the case of
high-P metamorphosed continental rocks, meta-
morphic pressures of 2000 MPa and higher are
indicative of burial to depths greater than that of
the thickest continental crust, and therefore also
require a subduction-zone setting. In this paper,
discussion is largely restricted to rocks that are
assumed to have been metamorphosed during
burial in subduction zones. There are essentially
two paths that might be followed by such buried
material in its passage back to shallower depths.
One path is steep, transporting material up
through the accretionary prism or orogenic wedge
towards the surface. A path such as this was
advocated by Cowan and Silling (Cowan and
Silling, 1978) in an analysis of accretionary
prisms. In their model, the upward flow results
from the inhibition of continued flow of low-
viscosity material down the subduction zone, due
to the presence of a rigid buttress at the rear of the
accretionary wedge. This type of flow is generally
referred to as ‘corner flow’, because the low-
viscosity material flows around the corner defined
by the point at which the subduction zone and the
rigid buttress meet (Fig. 1a). The path followed
by material is a function of the geometry of this
corner and of the position of the material in the
wedge with respect to the buttress. Upward flow
occurs throughout the wedge, which is treated as a
viscous fluid with uniform rheology.
A similar path was invoked by Platt (1986) who
showed that it is a predictable consequence of
wedge dynamics if there is any significant amount
of underplating in the rear of the wedge. As the
wedge is underplated, it is thickened beyond its
critical-taper geometry and responds by
extending. For continuous or repeated under-
plating, such a mechanism could achieve a
substantial amount of unroofing of deeper parts
of the wedge. The steep path followed by the
rocks reflects simply the fact that material has
been removed from above them, probably largely
by normal faulting. One such geometry for the
normal faulting is shown on Fig. 1b, although
there are others (Platt, 1993).
A steep path could also be achieved through the
development of sloping upper surfaces to an
orogenic wedge where it is floored by locally
steep ramps, as suggested by Jamieson and
Beaumont (Jamieson and Beaumont, 1988;
Fig. 1d, this paper). In this model, unroofing of
the rocks is due to greater erosion rates at the
steep upper surface of the wedge where it overlies
the ramp at depth. Erosion alone would give rise
to a vertical path; continued inward flow of
material into the wedge concomitant with erosion
would lead to a path inclined in the same sense as
the thrusting. In this scenario, the most deeply
buried rocks would be exposed above and slightly
in front of the underlying ramp. Although erosion
has traditionally been regarded as a slow
unroofing mechanism, some recent estimates
(Burbank et al., 1996, 2002) indicate that rates
of erosion in the Himalayas may be adequate to
satisfy the time constraints required by the
preservation of metamorphic assemblages.
An alternative path that could be followed by
material is essentially to reverse the path of
subduction. Such a path was invoked by Cloos
(1982) and Cloos and Shreve (1988) in a variant
of the corner-flow model, with flow restricted to a
narrow ‘subduction channel’ above the subduc-
tion zone, due to the limited width of the region of
low-viscosity material (Fig. 1c).
For all the models of Fig. 1 except the
subduction-channel model, the maximum depths
from which metamorphic rocks could be unroofed
6
A.HYNES
are limited by the thickness of the crust overlying
the subduction zone. The pressures of meta-
morphism in the ultrahigh-P terranes of the
Western Alps (Chopin et al., 1991), the Western
Gneiss region of Norway (Andersen et al., 1991)
and the Dabie Shan (Okay et al., 1993) are,
however, as high as 3000 MPa, indicating
exhumation from significantly below the base of
the continental crust (cf. Andersen et al., 1991).
Although, in principle, there is no limit to the
depths from which material might be derived in
the subduction-channel model, very low viscos-
ities are required in the channel, and the resulting
exhumed high-P material should be relatively
incoherent, in contrast to the characteristics of
many ultrahigh-P terranes (cf. Platt, 1993).
Although the subduction-channel model per se,
with low viscosities in the channel, may not be
appropriate for the unroofing of coherent, high-
and ultrahigh-P terranes in collisional orogens,
the terranes may have followed similar paths. As
was pointed out by Ernst (1975), high-P terranes
are commonly well organized, typically with
increasing pressures towards the structural top of
the exhumed complex, and faults dipping
uniformly in one direction. Ernst (1975) attributed
FIG. 1. Proposed mechanisms and paths for the return of rocks deeply buried at convergent plate-boundaries. The
paths are shown by the heavy dashed lines, and the material unroofed has the dotted ornament. (a) Corner flow
(adapted from Cowan and Silling, 1978). (b) Wedge spreading due to underplating (adapted from Platt, 1993). (c)
Subduction-channel-flow (adapted from Cloos and Shreve, 1988). (d) Enhanced erosion above a basal ramp (adapted
from Jamieson and Beaumont, 1988). Scales estimated by author; all figures have approximately 26 vertical
exaggeration.
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
7
these relationships to the successive underplating
of tectonic slices in a subduction-zone setting,
followed by buoyant return of the earlier
underplated slices, on a pathway that essentially
reversed the path of the original burial process.
The deep-seated rocks travelled upwards with
respect to both the underlying rocks and the
immediately overlying rocks, in the direction of
vergence for the collisional zone. In a process of
this kind, the slices would be bounded by thrust
faults below and normal faults above, both of
which had deep-seated roots. This process is a
form of extrusion, but in the vertical plane rather
than the horizontal plane as in the extrusion
associated with continental escape (e.g. Molnar
and Tapponnier, 1975).
10
0k
m
Sino-Korean cratonYangtze craton Wudan
block
mantle
mantle
oceanic crust
Great Valley sequence
oceanic crust
central belt
eastern belt
50
km
10
0k
m5
0k
m
LAURENTIABALTICA
Mid-Silurian (pre 425 Ma)
Late Silurian - Early Devonian
a
b
c
FIG. 2. Three examples of upward extrusion reversing the path of subduction: (a) From the Franciscan of California
(after Harms et al., 1992); (b) from the Dabie Shan of SE China (after Ernst and Liou, 1995); and (c) from the
Norwegian Caledonides (after Hurich, 1996).
8
A.HYNES
Three schematic representations of upward
extrusion reversing the path of subduction in
specific geographic localities are depicted in
Fig. 2, including one in which the descending
plate was oceanic and two in which extrusion
occurred during continental collision. All involve
initial subduction of the rocks to depths beneath
the mantle of the overlying plate. Two variants of
this model are what Wheeler (1991) called the
‘pip’ model and the ‘continental-sheet’ model,
depending on whether the extruding body was
continuous to the surface (Fig. 3).
Upward extrusion of deeply buried rocks by
reversal of the path of subduction has been
invoked increasingly in the recent literature,
particularly to explain the unroofing of high-P to
ultrahigh-P continental terranes. Examples include
the Dora Maira massif in the European Western
Alps (Hsu, 1991; Wheeler, 1991), the Dabie Shan
in China (Maruyama et al., 1994; Ernst and Liou,
1995), the Western Gneiss region in the
Caledonides of Norway (Hurich, 1996), the
Maksyutov Complex in the southern Urals
(Hetzel et al., 1998), the Galicia Massif in the
Variscan Belt of northwestern Iberia (Matte,
1998), and the Phyllite-Quartzite nappe of the
Cretan Hellenides (Wijbrans et al., 1993; Jolivet et
al., 1996). Extrusion models have also been
invoked for the classic Franciscan blueschists
(Ernst, 1975; Harms et al., 1992; Ernst and Liou,
1995).
There are two principal reasons for appeal to an
extrusion path that reverses the subduction path.
First, there is evidence in many cases that the
a
b
crust
crust
lithospheric mantle
lithospheric mantle
100
km
100
km
FIG. 3. The ‘pip’ (a) and ‘continental sheet’ (b) models for extrusion of high-P rocks, after Wheeler (1991).
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
9
high- to ultrahigh-P metamorphosed rocks have
experienced the bulk of their decompression
without significant heating, or even with cooling
during decompression (Hacker and Peacock,
1995). Decompression without associated
warming could be achieved only through very
rapid unroofing, or through unroofing while being
continually underthrust by cooler material (cf.
Davy and Gillet, 1986). Cooling during decom-
pression requires associated continual under-
thrusting of cool material regardless of the
unroofing rate, and is strongly suggestive of an
unroofing path that reverses the subduction path
(Hacker and Peacock, 1995). A second line of
evidence for reversal of the subduction path
comes from the observation that high-P conti-
nental rocks are typically in tectonic contact with
lower-P underlying rocks to which they are
possibly related. That is, the high-P rocks have
been transported over rock units that may have
been their equivalents, without the interposition of
exotic tectonic slices. Lithological units of the
Dora Maira massif in the European Western Alps,
for example, are lithologically similar to those of
the lower-grade Brianconnais zone (Borghi et al.,
1984; cited in Wheeler, 1991), which underlies
them structurally. The Western Gneiss Region of
the Norwegian Caledonides has similarities to the
Precambrian basement in the orogenic foreland
(Gorbatschev, 1985), and the Dabie Shan
Complex of southeastern China, which is
thought to have been derived from the Yangtze
block, is in direct tectonic contact with under-
thrust basement of the Yangtze block (Okay et al.,
1993). These kinds of relationship, although not
as compelling as the metamorphic arguments,
given the problems with identifying the prove-
nance of metamorphosed terranes, are difficult to
explain with pathways to the surface steeper than
those along which the rocks were originally
buried.
As discussed above, the preservation of high-P
metamorphic assemblages does not require rapid
unroofing if unroofing occurs along the subduc-
tion path. Indeed, a slow rate of unroofing would
provide further support for an unroofing path that
reversed the subduction path, since it would
increase the need for a mechanism for keeping
the rocks cool. In this context, it is noteworthy
that unroofing rates for the ultrahigh-P rocks of
the Dabie Shan appear to have been slow enough
to have permitted substantial thermal relaxation
(Hacker et al., 2000). The absence of evidence for
such thermal relaxation therefore provides strong
support for the suggested subduction-zone
unroofing path in this particular case. There is,
however, evidence in other cases that extrusion
occurred shortly after burial. In the Aegean, some
rocks were decompressed 400�600 MPa within
4 Ma of their burial (Jolivet et al., 1996), and the
Tso Morari eclogites of the northwest Himalaya,
which are interpreted as partially subducted parts
of India (de Sigoyer et al., 1997), were
decompressed from ~2000 MPa to ~900 MPa in
only ~8 Ma (de Sigoyer et al., 2000). The balance
of this paper is concerned with a review of
features that might enhance such rapid extrusion
up the subduction-zone path.
Factors contributing to extrusion
The net effect of extrusion is that the zone in
which the crust is thicker than normal becomes
wider (and less thick) than it was before. The
geometric redistribution is therefore similar to
that which would occur with gravitational
spreading of thickened and elevated continental
crust, but the redistribution is probably not driven
by relative elevation. A major potential driving
force for extrusion is presumably the buoyancy of
the continental crust relative to the mantle, when
the continental crust has been partially subducted
into the mantle. This buoyancy supplies a
substantial, upward-directed body force. The
potential importance of buoyancy as a drive for
extrusion was discussed by Platt (1987), was
emphasized by Hsu (1991) and Ernst and Liou
(1995), and has received strong support in recent
years from the analogue modelling studies of
Chemenda and co-workers (Chemenda et al.,
1995, 1996). Although it has been suggested that
the buoyancy drive for the uplift of crustal rocks
from the mantle could be effective only until such
rocks reached the base of the crust, it was pointed
out by Wheeler (1991), and amply confirmed by
the analogue studies of Chemenda et al. (1995,
1996), that if there is still crustal material within
the mantle below that which has already reached
the Moho, the drive is still there.
The buoyancy of partially subducted crustal
material undoubtedly provides a drive tending to
restore these rocks to shallower depths. In and of
itself, this drive does not guarantee the path of this
restoration, but the path is most likely governed
by the disposition of weak regions in the
neighbourhood of the buoyant rocks and in the
zones overlying them (cf. Wijbrans et al., 1993).
In Chemenda’s analogue modelling, it is the
10
A.HYNES
lowermost crust that is the weakest zone,
modelled with a material of lower yield strength,
and the crust above this zone travels as a unit
above it. In reality, the rheology of the crust and
surrounding mantle is considerably more complex
than has so far been addressed in analogue
models. The variation of yield stress in the
region of a subduction zone may be assessed by
combining estimates of the temperature distribu-
tion in subduction zones with estimates of the
strength of the rocks (Fig. 4). Figure 4 was
derived from modelled temperature distributions
for the subduction of continental lithosphere (van
den Beukel, 1992), combined with the assumption
that rocks yield by the lower of the stress required
by Byerlee’s law (Byerlee, 1978) and the stress
necessary for creep. The temperature-dependent
flow laws for creep of wet olivine (Chopra and
Paterson, 1984) and diabase (Shelton and Tullis,
1981) were used for the mantle and the crust
respectively, assuming a strain rate of 10�15 s�1.
Strength variations are illustrated for the tempera-
ture distributions in initially relatively cool
(Fig. 4a) and relatively warm (Fig. 4b) conti-
nental lithosphere. Both temperature distributions
result in a relatively weak zone coincident with
the crust in the descending slab, due to the lower
viscosity of diabase compared with olivine,
overlain by a strong lid of upper mantle, due to
the low temperatures in the mantle there.
Buoyancy forces begin to take effect as soon as
continental crust passes beneath the Moho (A, on
Fig. 4) and increase in intensity as more crust is
subducted. From Fig. 4, it is clear that the return
route followed by crust subducted to point A, or
anywhere deeper, would probably follow the
route of original subduction, which is the
channel in which strength is lowest. Its return
towards the surface by a steeper route is prevented
by the strong lid of mantle material overlying the
descending slab. In Fig. 4, weakening of the
mantle lid due to the generation of melts above
the subduction zone is not considered. This melt
generation occurs, however, typically only above
FIG. 4. Isotherms (8C) and contours of yield strength (MPa) in the neighbourhood of a continent/ocean lithospheric
boundary subducting beneath a continental lithospheric plate. Temperature distributions after van den Beukel (van
den Beukel, 1992). (a) For continental lithosphere with an initial surface heat flow of 50 mWm�2. (b) For continental
lithosphere with an initial surface heat flow of 90 mWm�2.
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
11
regions in which the Benioff zone has reached
depths of 100 km (Dickinson, 1973) in accor-
dance with the depths estimated for volatile
release from the descending slab (Peacock,
1990); i.e. at depths beyond the right hand side
of Fig. 4, and is therefore unlikely to affect the
return route followed by the crust.
Although buoyancy is undoubtedly instru-
mental in the restoration of partially subducted
crustal material to shallow levels, there may be
other changes at subduction zones that produce
geometric adjustment in the convergent zone.
Two such possibilities are slab breakoff and
change in the configuration of the lower litho-
spheric plate. Both such effects could promote the
extrusion of subducted continental material.
Slab breakoff is a process envisioned by Davies
and von Blanckenburg (1995), in which the partial
subduction of a continental margin gives rise to
extensional failure of the descending plate. This
failure allows access of asthenospheric material to
the upper plate behind the subduction zone and,
more importantly in the context of this paper,
significantly reduces the slab pull on the lower
plate. In the scenario envisaged by Davies and
von Blanckenburg (1995), this results in the
development of a crustal-scale duplex, the lower
sheets of which were originally subducted into the
mantle but were subsequently extruded into the
lower parts of the accretionary wedge. The
driving force for the upward extrusion is again
the buoyancy of the partly subducted continental
crust, but before slab breakoff, this buoyancy
force was effectively counteracted by the slab-
pull effect. This slab-breakoff model has been
used by von Blanckenburg and Davies (1995) to
explain extrusion of the Dora Maira massif in the
Western Alps.
Based on Davies and von Blanckenburg’s
(1995) mechanical arguments, slab breakoff
might be a direct result of the attempted
subduction of a continental margin, so that the
associated extrusion might be expected wherever
continental margins are partially subducted (cf.
Wong et al., 1997). The slab-breakoff model is
testable, in that access of asthenospheric mantle to
the base of the upper plate should raise geotherms
and produce mantle-derived partial melts,
although such melts would also be produced by
foundering of the lithospheric mantle of the upper
plate as envisaged by Houseman et al. (1981).
The effect of changes in the geometry of the
descending slab has been suggested as a possible
mechanism for the unroofing of high-P rocks,
both where the descending slab is oceanic (Harms
et al., 1992) and for the attempted subduction of a
continental margin (Hynes et al., 1996). In the
Franciscan of California, Harms et al. (1992)
suggested that the extrusion of high-P rocks
60�50 Ma ago resulted from shallowing of the
underlying subduction zone. In essence, their
suggestion was that emplacement of the
Franciscan rocks resulted from adjustment of the
wedge geometry due to shallowing of the basal
plane. The Franciscan orogenic wedge appears to
have been built up against and partly beneath a
thick ophiolite sequence that underlies the Great
Valley, and was accreted to North America in the
Late Jurassic (e.g. Godfrey and Klemperer, 1998).
This ophiolite sequence may have provided a
relatively rigid cap to the wedge, thereby
explaining why adjustment of the wedge was
concentrated in its basal regions. Thus, a
relatively small shallowing of the deeper part of
the subduction zone may have driven the basal
unit of the overlying accretionary wedge a
considerable distance back up the subduction
zone. There is independent evidence for shal-
lowing of the subduction zone in the Franciscan
region at the appropriate time, from the distribu-
tion of subduction-related volcanic rocks (Coney
and Reynolds, 1977), providing support for the
hypothesis.
Hynes et al. (1996) conducted a theoretical
study of the effect of a continental margin arriving
at a subduction zone. They showed that thermally
mature continental-margin lithosphere would
have greater strength than typical oceanic litho-
sphere, and they investigated the effect on the
overlying orogenic wedge of the increase in
flexural strength of the subducting slab as the
continental margin enters the subduction zone. If
the load on the descending slab is held constant,
the deeper parts of the descending slab tend to
rise. If, on the other hand, the descending slab is
not permitted to rise, but is held in place, it
deepens at shallower depths. Neither scenario is
strictly correct, but the true situation probably lies
somewhere between them, and together they
permit an assessment of the effect on the
subduction zone of the arrival of a stronger plate
at the trench. The net effect of the change in
geometry is for the lower plate to exert an upward
pressure on the subduction zone at depth, and a
downward pressure at shallower depths (Fig. 5).
This produces a suction effect, tending to pull
material up from depth in the subduction zone.
Calculations (Hynes et al., 1996) suggest that the
12
A.HYNES
suction effects achieve their maxima in the depth
range between 30 and 70 km (Fig. 5), where
pressures due to the change in geometry exceed
600 MPa. These values are certainly adequate to
overcome the yield strength of the lower parts of
the continental crust (cf. Fig. 4). They provide, in
effect, a boost to the buoyancy forces operating
on partially subducted continental crust, which
occurs as a direct result of the conditions that
produce partial subduction of the continental crust
0 100 200 300 400
Dep
th(k
m)
Distance (km)
120
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0
20
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40
10
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0
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+
-
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-
-
-
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+
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+
continentocean
FIG. 5. Pressures exerted in a subduction zone as a result of the change in geometry of the descending slab as an
ocean-to-continent transition is subducted, after Hynes et al. (1996). The approximate position of the ocean-to-
continent transition is shown by the heavy stippled region. Increased pressures are shown by the shaded regions
marked with plus signs. Decreased pressures are shown by shaded regions marked with negative signs.
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
13
in the first place; i.e. the arrival of a continental
margin at the subduction zone.
Given all the possible aids to the extrusion of
continental crust outlined above, it is perhaps not
surprising that evidence for the unroofing of
partially subducted continental crust in continental
collision zones is quite common. Once continental
crust has been subducted any significant distance
into the mantle, the buoyant drive for its return,
combined with its progressive weakening as it
warms up with descent, would almost inevitably
lead to its return, although it may not in general
return all the way to Earth’s surface. This
tendency to return could probably be nullified
only if some adjustment to the subduction-zone
geometry worked in a sense opposite to that
addressed by Hynes et al. (1996). For example, if
progressively weaker lithosphere was subducted,
adjustments to the geometry of the subduction
zone would lead to pressure gradients that tended
to trap partly subducted continental material at
depth, despite its buoyancy.
Upward extrusion without a buoyancy drive
Probably the best support for upward extrusion of
metamorphic rocks as coherent slices bounded
above by normal faults and below by thrust faults
comes from the Himalayas, where Hodges and co-
workers (Hodges et al., 1992, 1996) have
documented the interplay of movements on
thrust faults (the Main Central Thrust system)
and overlying normal faults (the South Tibetan
Detachment Zone) in the Early Miocene exhuma-
tion of the Greater Himalayan crystalline terrane.
In this case, however, metamorphism in the
extruded sequences attained pressures of only
1000�1200 MPa (Hodges et al., 1996), consistent
with rooting of the faults at depths within the
orogenic crustal wedge rather than in the mantle.
Although there are metamorphic rocks in the
Himalayas that exhibit pressures consistent with
continental subduction (de Sigoyer et al., 1997)
they are at a higher structural level, and were
metamorphosed and unroofed much earlier (de
Sigoyer et al., 2000) than the rocks beneath the
South Tibetan Detachment.
The existence of upward extrusion within the
Himalayan orogenic wedge, as distinct from
upward extrusion from beneath the Moho,
suggests that buoyancy resulting from the
subduction of crustal rocks to depths beneath the
Moho is not a necessary requirement for the
upward-extrusion movement pattern. In the
Himalayas, Hodges et al. (1996) appealed to
incremental adjustments to the orogenic wedge as
a mechanism for the extrusion, with former thrust
discontinuities supplying the planes of weakness
used as normal faults and thrust faults in the
adjustment. These adjustments could be in
response to episodic underplating (cf. Platt,
1986), but there is little support from deep
seismic soundings (e.g. Hauck et al., 1998) for
the requisite amount of underplating. A more
compelling case can be made, based on the
seismic data, for adjustment of the mantle wedge
due to shallowing of the dip of the base of the
wedge. In the palinspastic reconstructions of
Hauck et al. (1998, Fig. 8), rocks of the Greater
Himalayan belt and the Tethyan Himalayan belt
must mount a substantial crustal ramp during the
early stages of motion on the Main Central
Thrust. Once this ramp had been mounted, the
basal slope of the orogenic wedge should have
declined markedly. Response to this change alone
could have occasioned much of the adjustment of
the wedge evident in motion on the South Tibetan
Detachment. Later more minor adjustments would
also be expected with increasing transport of the
orogenic wedge southwards onto the Indian
foreland, due to a gradual decline in basal slope
dictated by the geometry of the relatively rigid
Indian lithospheric substrate. On the basis of the
southern Himalayan example, therefore, it is
perhaps unwise to consider buoyancy a necessary
condition for upward extrusion, although it is
probably a very important component of the drive
in circumstances in which continental crust has
been thrust beneath the Moho.
Lateral ramps and the localization of high-Procks in collision zones
Upward extrusion, driven by buoyancy or by
adjustments to the geometry of bounding rigid
bodies or some combination, may therefore be an
important feature of the tectonics of convergent
plate boundaries. There is evidence, in addition,
that lateral ramps may localize the extrusion. The
impetus for this suggestion comes from studies in
the Mesoproterozoic Grenvillian continental-
collision zone of eastern North America. There,
it can be shown that high-P rocks, metamor-
phosed at pressures up to 1800 MPa in a
continental-collisional setting, were extruded
towards the tectonic foreland shortly after their
burial (Indares et al., 1998, 2000; Hynes et al.,
2000). The high-P rocks are particularly well
14
A.HYNES
developed at the surface in a broad lobate zone
which is immediately adjacent to and in front of a
steep lateral ramp that has been delineated in the
subsurface based on seismic-reflection profiling
(Hynes and Eaton, 1999; Fig. 6, this paper).
Regional tectonic trends in the Grenvillian
orogen are ENE, but the subsurface ramp adjacent
to the high-P domain trends NNE (Fig. 6).
Lineation directions, parallel to transport direc-
tions in the high-P rocks, show clearly that the
high-P rocks at the surface have flowed out into
their present positions from the region of the
20 km
4
8
12
16
24
28
32 km
20
Gre
nvFr
illeon
t
lineation
Man
icouag
an Res.
52°N
68°W
69°W
69°W70°W
52°N
51°30’N
68°W
Atla
nticO
cean
Appal
achia
nsG
renvill
e
Superior
Yavapai-
Mazatzal
Tra
ns
-Hudson
Triassic impact-related volcanic rocks
High- rocks(MIZ)
P
PalaeoproterozoicLaurentian metasediments
Undifferentiated gneisses
Palaeoproterozoic meta-sediments & Archaean bsmnt.
Reworked LaurentianArchaean
Cratonal LaurentianArchean
FIG. 6. Structure contours, at 2 km intervals, on the surface forming the base of the high-pressure Manicouagan
Imbricate Zone (MIZ) in the Mesoproterozoic Grenvillian orogen of eastern Quebec, together with the directions of
lineations.
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
15
lateral ramp (Hynes et al., 2000; Fig. 6, this
paper).
Geometrical relationships in the Grenvillian
orogen make it clear that the presence of high-P
rocks at the surface is directly related to the
existence of the lateral ramp. Hynes and Eaton
(1999) interpreted this relationship as evidence
that the lateral ramp, because it gave rise to a
concavity in the lower surface of the orogenic
wedge, had produced a channel into which
material flowing from the deeper to the shallower
parts of the orogenic wedge was preferentially
concentrated (Fig. 7).
The ability of a concavity on the lower
confining surface (or a convex-upward irregu-
larity on the upper surface) of a flowing medium
to channel flow is not easy to demonstrate
rigorously, because it requires solution of the
FIG. 7. Schematic illustration of the concentration of flow in an orogenic wedge into the concavity produced by a
lateral ramp. The small arrows indicate the general direction of flow; the large arrows illustrate the increased flow
within and towards the region of the lateral ramp.
h
h3
h
h
h×1.25
due to and/or P�� �
due to shear
FIG. 8. General forms of the flow profiles between two planar, parallel plates, when the upper plate is moved parallel
to the lower. The flow velocity at each point consists of a component due to the shear of the overlying plate (to the
right) and a component due to changes in pressure or gravitational potential (to the left). The net flow due to the
combination is stippled. For a 25% increase in the distance, h, between the plates, illustrative of the flow within a
channel, there is a marked increase in the flow due to changes in pressure/gravitational potential.
16
A.HYNES
Navier-Stokes equation in three dimensions, but
indications of its probable effectiveness can be
derived from two-dimensional considerations.
Consider a Newtonian viscous fluid bounded
below by a stationary and rigid surface in which
there is concavity representing a channel, and
bounded above by a rigid surface moving at a
fixed rate relative to the lower surface (Fig. 8).
Flow of the viscous fluid is driven by the shear
stress exerted by the upper surface, pressure
differences along the path of the fluid, and the
buoyancy of the fluid. Flow outside the channel
may be approximated by flow between parallel
surfaces, which is readily determined using the
stream function (e.g. Turcotte and Schubert,
1982). Flow within the channel may be approxi-
mated likewise, but with a greater distance
between the upper and lower surfaces. Flow
between two surfaces due to the shear stress
from motion of the upper surface is proportional
to the distance between the surfaces, but flow due
to pressure differences along the path or buoyancy
is proportional to the cube of the distance between
the surfaces. There is, therefore, a dispropor-
tionate increase in flow within the channel
compared with that outside it (Fig. 8). A
channel should consequently concentrate flow
markedly if the flow is driven by pressure
gradients or buoyancy forces. It is therefore
clear in principle why a lateral ramp in the
subsurface might lead to a concentration of
exhumed high-P rocks in the region above the
ramp, as is observed in the Grenvillian orogen.
Recorded pressures in the high-P rocks of the
Grenvillian orogen do not exceed 1800 MPa,
which places them potentially still within
thickened continental crust. There is, furthermore,
no evidence to suggest that burial of these rocks
resulted from subduction of the leading edge of a
continent. It appears that the structural setting of
the preserved Grenvillian orogen was on the
northwestern edge of a southeast-facing Andean
margin (Rivers, 1997). The thrusting by which
rocks were deeply buried therefore forms part of a
retro-shear system, rather than a pro-shear system,
in the sense of Beaumont and Quinlan (1994), and
100 km
APENNINES
ADRIATIC
Po Plain
Jura
MolasseForedeep
Perialpine basins
Oligocene plutons
Ophiolitic units
Flysch
Austroalpinelow-grade
Austroalpinemedium-grade
Cret. metamorphism,W Alps; blueschist
Cret. metamorphism,W. Alps; eclogitic
DM
SL
MR
GP
A
FIG. 9. Tectonic map of the Alps, after Polino et al. (1990). A: Adula nappe; DM: Dora Maira nappe; GP: Gran
Paradiso nappe; MR: Monte Rosa nappe; SL: Sesia Lanzo nappe.
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
17
their probable correlatives, metamorphosed to
much lower pressures, structurally overlie them.
They have therefore been extruded within a
region of continental crustal thickening, rather
than in association with continental subduction.
Their setting is in this sense similar to the Early
Miocene extrusion, from mid-crustal depths, of
the Greater Himalayan sequence (Hodges et al.,
1996). The general theoretical considerations
discussed above, however, indicate that lateral
ramps might also be important in controlling the
locations of high-P rocks that had been extruded
from mantle depths. Some of the better-known
localities of high-P metamorphosed rocks world-
wide provide support for this suggestion.
The classic high-P locality of the Western Alps,
the Dora Maira massif, occurs at the western end
of the Alpine chain, in a region in which orogenic
trends undergo a pronounced leftward jog
(Fig. 9). Indeed, the eclogite-rich terranes of the
Monte-Rosa, Sesia Lanzo, Gran Paradiso and
Dora Maira nappes occupy a broad lobate region
around this leftward jog that is markedly similar
in form to that of the high-P rocks of the
Grenvillian orogen.
In the Aegean, the trends of the two zones of
high-P rocks, formed 45 and 25 Ma ago,
respectively (Jolivet et al., 1996), are at a high
angle to equivalent trends for the Hellenides, again
associated with a leftward jog of the trends, and
the stretching lineations that presumably illustrate
their transport directions are oblique to regional
orogenic trends (Fig. 10). Although the present
configuration of the Hellenic arcs is markedly
affected by subsequent extension in the Aegean
region, the Neogene extension resulted in little net
rotation of Crete (Angelier et al., 1982), and
restoration of the clockwise rotation of the main
Hellenic peninsula still leaves a substantial
leftward jog of the arc in the Early Miocene
(Kissel and Laj, 1988). Thus, emplacement of at
least the later of the two sets of high-P rocks
appears to have occurred at an offset along the
tectonic trend and in a direction subparallel to it.
In southern China, the high-P terranes in the
contact zone between the Sino-Korean and
Yangtze cratons, in the Hong’an, Dabie Shan
and Su-Lu regions (Hacker and Wang, 1995) lie
along and near a prominent leftward offset of the
zone associated with the Tan-Lu fault (Fig. 11),
Aegean Sea
Ionian Sea
Helle
nid
es
lineations
Hellenide tectonictransport direction
100 km
41°N
38°N
35°N23°E 28°E
N
HP
45M
a
HP
25M
a Crete
FIG. 10. Distribution of high-P metamorphic belts (heavily dotted regions) in the Aegean region, and stretching
lineations associated with them, after Jolivet et al. (1996).
18
A.HYNES
which is believed to have been active as a left-
lateral fault in the Triassic, when the high-P rocks
were emplaced. Yin and Nie (1993) interpret this
fault as due to irregularities in the nature of the
pre-Triassic form of the northern margin of the
Yangtze block, whereas Okay et al. (1993)
consider the fault to have developed and offset
the margin of the block only in the Triassic. In
either case, it could have been the site of a side
ramp, and direct support for eastward thrusting
associated with it is evident in the eastward-
verging folds in the sedimentary rocks to the east
of the Dabie Shan Complex (Okay et al., 1993). A
recent interpretation of the dynamics of the region
(Hacker et al., 2000) incorporates eastward
extrusion of the ultrahigh-P rocks of the Dabie
region along the plate margin, towards an eastern
reentrant in the margin, with the re-entrant
channelling the extruding lithosphere much in
the manner argued above.
In the Norwegian Caledonides, the high-P
rocks of the Western Gneiss Region are also
associated with a marked change in direction of
the frontal thrusts of the orogen from their
regional NNE trend to a more easterly trend
(Fig. 12). In this case, the orogenic trends appear
to have jogged towards the right.
Transverse structures are not in and of
themselves evidence of the presence of subsurface
lateral ramps. The transverse structures associated
with the western Himalayan syntaxis, for
example, have been interpreted to indicate
simply unusual amounts of forelandward transport
due to the presence of weak layers in the orogenic
wedge (Burbank, 1983). In general, however,
broad changes in the directions of orogenic fronts
are interpreted to indicate irregularities in the pre-
collisional geometries of the colliding margins
(e.g. Thomas, 1977) and such irregularities will
tend to give rise to lateral ramps. The association
of all these high-P localities with large-scale
transverse structures is therefore highly sugges-
tive of a role for lateral ramps in their effective
transport to the surface.
Summary and conclusions
In summary, there is evidence from the Himalayas
that, even in the absence of a large buoyancy
drive, upward extrusion of major rock units from
deep in the orogenic wedge is an important
process. In circumstances in which continental
crust has been partially subducted into the mantle,
the buoyancy drive of the light continental crust
Pac
ific
140°E125°110°
095°
080°
20°N
20°N
50°N
600 km
Seaof
JapanSino-Koreancraton
Yangtzecraton
S
H
D
FIG. 11. Tectonic scheme for southern China in Triassic time, adapted from Yin and Nie (1993). Heavily stippled
regions are localities of high-P rocks extruded in the collisional zone between the Sino-Korean and Yangtze cratons,
after Ernst and Liou (1995), labelled H (Hong’an), D (Dabie Shan) and S (Su Tun).
ENCOURAGINGTHE EXTRUSIONOF HIGH-P ROCKS
19
provides a major boost to the extrusion process,
and this boost may be further enhanced by
adjustments in the geometry of the descending
slab due to arrival of the cool continental margin,
or by reduction of slab pull with detachment of
the oceanic part of the descending slab. Direct
evidence for the sensitivity of the wedge
geometry to the dip of the descending slab is
rare, but is supplied in one instance by the
synchronicity of slab shallowing with wedge
FIG. 12. Terrane map of the Scandinavian Caledonides, adapted from Stephens and Gee (1989), with eclogite
localities in the Western Gneiss region after Coleman and Wang (1995).
20
A.HYNES
inversion in the Franciscan. Extrusion may, then,
be the rule rather than the exception in
circumstances in which continental crust arrives
at a subduction zone on the lower plate and is
dragged partially into the mantle.
The close spatial relationship between a lateral
ramp and a broad lobe of exhumed high-P rocks
in the Grenvillian orogen indicates that a lateral
ramp may have played a role in the exhumation
process there. Many other high-P and ultrahigh-P
rocks are associated with jogs in orogenic trends
that may be indicative of lateral ramps. If material
transport is restricted to two dimensions, partially
subducted continental crust may be expected to be
extruded along paths similar to those along which
it was buried. Lateral ramps may, however, cause
substantial deviations from two-dimensional flow,
with concentration of the extrusion in the regions
of the lateral ramps themselves. In these
circumstances, high-P and ultrahigh-P rocks
may be expected to occur preferentially at right-
handed and left-handed jogs in orogenic trends
(Fig. 13). Confirmation of the importance of flow
patterns of the kind envisaged here could be
derived from regional studies of the spatial and
temporal variations in attitude of the structures
associated with extruded high-P complexes.
Acknowledgements
My work on the emplacement of high-P rocks
was sparked originally by stimulating discussions
with Reinhard Greiling and Zvi Garfunkel. It has
been supported throughout by grants from the
Natural Sciences and Engineering Research
Council of Canada, through its Operating Grants
and Lithoprobe Supporting Geoscience
Programmes. I am particularly grateful to the
organizers of and participants in the Metamorphic
Studies Group Meeting in Rennes for the
opportunity to air my thoughts further and for
many exciting discussions. Constructive
comments on the manuscript by Alexandre
Chemenda were much appreciated.
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[Manuscript received 20 December 2000:
revised 28 February 2001]
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A.HYNES