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Tectonophysics 384 (2004) 91 – 113
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Rapid tectonic exhumation, detachment faulting and orogenic
collapse in the Caledonides of western Ireland
Peter D. Clift a,*, John F. Dewey b, Amy E. Draut c,1, David M. Chew d,2,
Maria Mange b, Paul D. Ryan e
a
Department of Geology and Geophysics/MS 8, Woods Hole Oceanographic Institution, 360 Woods Hole Road,
Woods Hole, MA 02543-1541, USA
b
Department of Geology, 174 Physics/Geology Building, University of California, One Shields Avenue, Davis, CA 95616-8605, USA
c
Woods Hole Oceanographic Institution-Massachusetts Institute of Technology Joint Program in Oceanography, Woods Hole, MA 02543, USA
d
Department of Geology, Trinity College, Dublin 2, Ireland
e
Department of Geology, National University of Ireland, Galway, Ireland
Received 11 February 2003; accepted 25 March 2004
Available online 07 June 2004
Abstract
Collision of the oceanic Lough Nafooey Island Arc with the passive margin of Laurentia after 480 Ma in western Ireland
resulted in the deformation, magmatism and metamorphism of the Grampian Orogeny, analogous to the modern Taiwan and
Miocene New Guinea Orogens. After f 470 Ma, the metamorphosed Laurentian margin sediments (Dalradian Supergroup)
now exposed in Connemara and North Mayo were cooled rapidly (>35 jC/m.y.) and exhumed to the surface. We propose that
this exhumation occurred mainly as a result of an oceanward collapse of the colliding arc southwards, probably aided by
subduction rollback, into the new trench formed after subduction polarity reversal following collision. The Achill Beg Fault, in
particular, along the southern edge of the North Mayo Dalradian Terrane, separates very low-grade sedimentary rocks of the
South Mayo Trough (Lough Nafooey forearc) and accreted sedimentary rocks of the Clew Bay Complex from high-grade
Dalradian meta-sedimentary rocks, suggesting that this was a major detachment structure. In northern Connemara, the
unconformity between the Dalradian and the Silurian cover probably represents an eroded major detachment surface, with the
Renvyle – Bofin Slide as a related but subordinate structure. Blocks of sheared mafic and ultramafic rocks in the Dalradian
immediately below this unconformity surface probably represent arc lower crustal and mantle rocks or fragments of a high level
ophiolite sheet entrained along the detachment during exhumation.
Orogenic collapse was accompanied in the South Mayo Trough by coarse clastic sedimentation derived mostly from the
exhuming Dalradian to the north and, to a lesser extent, from the Lough Nafooey Arc to the south. Sediment flow in the South
Mayo Trough was dominantly axial, deepening toward the west. Volcanism associated with orogenic collapse (Rosroe and
* Corresponding author. Fax: +1-508-457-2187.
E-mail address: [email protected] (P.D. Clift).
1
Present address: US Geological Survey/University of California, Santa Cruz, 2801 Mission St. Extension, Santa Cruz CA 95060, USA.
2
Present address: Départment de Minéralogie, Université de Genéve, Rue des Maraı̂chers 13, CH-1205 Geneva, Switzerland.
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.03.009
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Mweelrea Formations) is variably enriched in high field strength elements, suggesting a heterogeneous enriched mantle wedge
under the new post-collisional continental arc.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Caledonides; Exhumation; Detachment; Ireland
1. Introduction
Geochemical evidence from the bulk global composition of continental crust indicates that it was
formed principally in subduction zone settings (e.g.,
Dewey and Windley, 1981; Ellam and Hawkesworth,
1988; Rudnick and Fountain, 1995). However, because of the loss of crust by tectonic erosion along
many continental arc margins (e.g., Dewey, 1980; von
Huene and Scholl, 1991), continued continental crustal growth must be largely achieved through the
development of oceanic island arc complexes and
their accretion to continental margins (Clift and
Vannucchi, in press). Also, although arguments for
subduction initiation along passive continental margins have been made (e.g., Erickson, 1993), arc –
continent collision is also likely the principal origin
for development of active continental margins
(Dewey and Bird, 1970; Clift et al., 2003). The
collision of Taiwan with the passive margin of southern China is the simplest known example of such a
process in the modern oceans (e.g., Teng, 1990;
Lundberg et al., 1997; Lallemand et al., 2001), yet,
because of limited sampling in the adjacent Okinawa
Trough and Ryukyu Arc, the rates of orogenesis,
gravitational collapse and sedimentation in this area
are known only in outline.
A well-dated example of arc –continent collision is
known from the western Irish Caledonides. In the
South Mayo Trough and adjacent metamorphic terranes of Connemara and North Mayo (Fig. 1), a clear
Early Ordovician collision event between the Lough
Nafooey Arc and the passive margin of Laurentia has
been recognized (Dewey and Shackleton, 1984;
Dewey and Ryan, 1990; van Staal et al., 1998; Dewey
and Mange, 1999; Draut and Clift, 2001). New
Guinea illustrates the Miocene collision of an arc with
the north Australian continental margin (Davies et al.,
1987). Here, arc/suprasubduction-zone ophiolite obduction developed the subjacent Bismarck Orogen,
similar in size, duration and geology to the Grampian
orogen (Dewey and Mange, 1999).
In this paper, we focus on the later stages of this
collision by documenting the nature of orogenic
collapse in this part of the Grampian orogenic belt.
We further show how the tectonic and sedimentary
processes that occurred during orogenic collapse are
related directly to the changes in plate boundary zone
strains linked to subduction polarity reversal.
2. Regional geology
The Lough Nafooey Arc forms the southern edge
of the South Mayo Trough and comprises a series of
earliest Ordovician basaltic lavas overlain by Tremadocian andesitic lavas and epiclastic breccias interbedded with pillowed flows, which are succeeded by
an Arenig sequence of andesitic to rhyolitic lavas,
breccias and volcaniclastic sediments. These rocks are
associated with an f 6-km-thick series of EarlyMiddle Ordovician volcaniclastic and siliciclastic sedimentary rocks (Figs. 1 and 2). The sedimentary rocks
in the South Mayo Trough accumulated prior to,
during and after the arc – continent collision event.
The oceanic Lough Nafooey Arc can be considered an
along-strike equivalent of the Lushs Bight Arc of
Newfoundland (e.g., Coish et al., 1982), and the
Shelburne Falls Arc of New England (Karabinos et
al., 1998). An up-section trend within the Lough
Nafooey volcanic rocks to more siliceous and light
rare earth element (LREE) enriched compositions is
consistent with the arc colliding with the passive
margin of Laurentia during the Early Ordovician
(after 480 Ma; Draut and Clift, 2001). This is due to
the progressively increasing recycling of LREEenriched continental material through the subduction
zone as the passive margin is subducted. Collision
precipitated the Grampian Orogeny in the British Isles
and the Taconic Orogeny in North America (e.g.,
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
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Fig. 1. Regional geological map of western Ireland showing the Connemara and North Mayo metamorphic terranes separated by the South
Mayo volcanic island arc terrane. The strike-slip faulted boundary between the Connemara and South Mayo terranes is buried under Silurian
strata, while the Achill Beg Fault separates low grade arc and trench rocks of South Mayo from the high-grade rocks of North Mayo.
CLW = Cashel – Lough Wheelan; DCD = Dawros – Currywongaun – Doughruagh.
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Fig. 2. Schematic stratigraphy of the South Mayo Trough Ordovician, showing the variability from south to north limb of the syncline and time
Lambert and McKerrow, 1976; Ryan and Dewey,
1991; van Staal et al., 1998; Dewey and Mange,
1999; Soper et al., 1999).
Despite earlier work indicating a protracted Grampian Orogeny in Connemara (e.g., Elias et al., 1988;
Cliff et al., 1996; Tanner et al., 1997), more recent
studies have now demonstrated a duration of less
than 15 m.y., with peak metamorphism at f 470 Ma
in Connemara (Friedrich et al., 1999a,b; Dewey and
Mange, 1999). Ar – Ar dating of the main nappe
fabric in the Ox Mountains Dalradian suggests that
cooling had started by at least 460 –450 Ma in this
area (Flowerdew et al., 2000). The Connemara and
North Mayo Dalradian metamorphic terranes are
considered to be metamorphosed fragments of the
Laurentian margin (e.g., Lambert and McKerrow,
1976; Dewey and Shackleton, 1984; Harris et al.,
1994). Connemara is now displaced from its original
location and lies along strike and south of the units
that comprise the Lough Nafooey Island Arc. This
displacement cannot be the result of southward
thrusting of high-grade Connemara over South Mayo
because much of this has never exceeded anchimetamorphic conditions. Rather, the position of
Connemara today must have been achieved by
post-Grampian strike-slip tectonism that affected
the Laurentian margin [e.g., the Southern Uplands
Fault, the Fair Head-Clew Bay Line (equivalent to
the Highland Boundary Fault in Scotland) and the
Great Glen Fault; Hutton, 1987].
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
The Connemara Terrane have reached close to its
modern position before the Late Llanvirn to Lower
Llandovery (464 – 443 Ma) based on current and
provenance data from the Derryveeny Conglomerate
(Graham et al., 1991). Additional later Silurian and
Devonian strike-slip faulting through Clew Bay also
displaced the South Mayo Trough relative to the
North Mayo Dalradian (Hutton, 1987), but the
amount of displacement was probably not large (Ryan
et al., 1995).
Connemara is one of the best-dated segments in
the Caledonian – Appalachian Orogen and differs
from the North Mayo Dalradian in that the metasedimentary rocks are intruded by a number of EarlyMid Ordovician gabbros and quartz diorites (Leake,
1989). Trace element and Nd isotopic geochemical
evidence shows that these gabbros were emplaced in
a subduction setting, with substantial involvement of
new mantle melts, relative to crustal recycling, in
their petrogenesis (Draut et al., 2002, in press).
Therefore, the Connemara Dalradian can be interpreted as both the deformed margin of Laurentia
and the mid-crustal roots to a syn-collisional volcanic
arc (e.g., Yardley et al., 1982).
Following peak metamorphism in the Connemara
Dalradian, radiometric dating indicates that the orogen
experienced rapid cooling (>35 jC/m.y.; Friedrich et
al., 1999a,b), typically explained as a response to
orogenic collapse. Power et al. (2001) used fluid
inclusion techniques to estimate rates of exhumation
in Connemara of at least 7 km in 10 m.y., similar to
modern massifs that exhibit the fastest exhumation
rates (e.g., Nanga Parbat; Zeitler et al., 1993). We now
explore the exhumation and orogenic collapse of the
Irish Dalradian and assess implications for the tectonic
evolution of this area and for the process of arc –
continent collision.
3. Detachment faulting
Low-angle extensional faulting synchronous with
thrust faulting has been recognized as a common
tectonic process during and immediately following
orogeny. In the High Himalaya, high-grade metamorphic rocks were exhumed by orogen-perpendicular
extension beneath the Zanskar and South Tibetan
Detachments, synchronous with thrusting (e.g., Her-
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ren, 1987; Burchfiel et al., 1992). In western Norway,
high-pressure and ultrahigh-pressure metamorphic
rocks were exhumed beneath a gently dipping late
orogenic detachment (Dewey et al., 1993). A similar
situation is envisaged for the Irish Dalradian.
In Connemara, exhumation is also achieved by
synchronous thrusting and detachment faulting that
combine to extrude deeply buried meta-sedimentary
rocks to the surface. The Mannin Thrust places the
Dalradian towards the south over anchizone metarhyolites of the Delaney Dome Formation (Fig. 1), a
deformed equivalent of the syn-collisional arc sequence, the Tourmakeady Volcanic Group (Dewey
and Mange, 1999; Draut and Clift, 2002). This Delaney Dome arc fragment must have been transposed
into its present location with the Connemara Dalradian, after the Grampian Orogeny. South southeasterly
motion on the Mannin Thrust may be constrained
relative to the formation of other regional structures,
post-dating the D3 fabric and pre-dating the D4 fabric
defined by Leake et al. (1983) and Tanner et al.
(1989). Movement on the thrust is indirectly constrained by isotopic dating of the syn-orogenic arc
intrusive rocks to 470 – 462 Ma (Friedrich et al.,
1999b) and is considered to date from the period of
rapid cooling in Connemara, i.e., post-dating peak
metamorphism. Motion on the Mannin Thrust may
have been related to the early phases of subduction
polarity reversal (Dewey and Mange, 1999). Identification of related extensional detachment has been
more controversial. We present here newly recognized
evidence for major detachment faults in North Mayo
and Connemara.
3.1. Detachment faulting in North Mayo and Achill
Island
A key feature of all major detachment systems is
the presence of a significant metamorphic contrast,
with low-grade rocks in the hanging wall and highergrade exhumed rocks in the footwall. Across Clew
Bay, a clear difference in grade has long been recognized between the low-grade South Mayo Trough and
the higher-grade Dalradian in North Mayo. Much of
the contact is covered by Silurian and Carboniferous
sedimentary rocks (Fig. 1), except on the island of
Achill Beg, located in NW Clew Bay (Figs. 1 and 3),
where a major high-angle fault, the Achill Beg Fault,
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Fig. 3. Geological map of Achill Beg Island and the boundary between the relatively low-grade metamorphic rocks of the South Achill
Beg Formation, essentially part of the Clew Bay Accretionary Complex and the high-grade rocks of the North Mayo Dalradian. Section through
A – AV is shown in Fig. 4. Modified after Chew (2001).
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
places accretionary complex sedimentary rocks of the
Clew Bay Complex under the youngest Dalradian
rocks seen in the North Mayo inlier—the upper Argyll
Group (Chew, 2003). Although the Fault has been
reactivated since the Carboniferous (Chew et al., in
press), both the Clew Bay Complex and the Dalradian
on Achill Beg show a dominant northerly dipping
structural fabric (45 – 60jN; Fig. 4), as does the Clew
Bay Complex on the southern edge of Clew Bay, west
of Westport (Fig. 1).
Gravity (Ryan et al., 1983) and deep-penetrating
reflection seismic (Klemperer et al., 1991) studies
show that the Clew Bay Complex is associated with
a north-dipping fabric that can be traced to the Moho,
implying that post-orogenic extension effected the
whole of the continental crust. At outcrop, the
north-dipping fabric is observed to be a penetrative
shear fabric within metamorphosed clastic sedimentary rocks. Furthermore, to the north, Dalradian metamorphic rocks of North Mayo (now in the hanging
wall of this structure) preserve an earlier south-dipping fabric that is not observed south of Clew Bay that
is attributed to the Grampian event. These fabrics are
consistent with structural arguments about the initial
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vergence of the Grampian nappes, i.e., the top to the
north tectonic transport during D1 and D2 at least in
South Achill Island (Fig. 5C).
The northern boundary of the Achill Beg Shear
Zone, which we tie to D3 and D4 deformation during
cooling, is not sharply defined, but instead, D3 structures become gradually less intense to the north. Two
discrete elements have been recognized in the D3
deformation episode in North Mayo and Achill Island:
asymmetrical buckle folds with axial planes anticlockwise to the S2 foliation (Figs. 5A and C) and extensional crenulation cleavages (Platt and Vissers, 1980)
that cut the S2 foliation in a clockwise sense (Chew et
al., in press). The main S2 foliation is generally defined
by muscovite, chlorite and equigranular quartz grains
with interlobate grain boundaries. The later extensional
crenulation cleavages make an angle of about 29j with
the S2 foliation in a clockwise direction (Fig. 5B) and
consistently give a dextral sense of shear. Identical in
style to the normal-slip crenulations of Dennis and
Secor (1990), they are believed to be broadly contemporaneous with the F3 asymmetric buckle folds based
on the absence of apparent overprinting relationships.
On a vertical surface, the extensional shear bands give a
Fig. 4. Geological cross section through Achill Island (after Chew, 2001) showing the south-vergent asymmetry of the D2 deformation in the
Dalradian, contrasting with the NNW-vergent D1 deformation. See Fig. 3 for location of section. Modified after Chew (2001).
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Fig. 5. (A) Stereographic plot of the orientation of reverse-slip crenulation-related structures in South Achill (F3 fold hinges and poles to the S3
foliation) along with the mean orientation of the S2 foliation. (B) Rose diagram illustrating the mean orientation of normal-slip crenulationrelated structures in South Achill (dextral shears measured on horizontal surfaces). The mean orientation of the S2 foliation is also illustrated.
(C) Stereographic plot of the orientation of the preexisting foliation-slip surface in South Achill (poles to the S2 foliation).
down to the south shear sense, consistent with a
detachment fault of this orientation.
Near Westport, Graham et al. (1989) mapped the
accretionary complex rocks of the Clew Bay Complex
as dipping north and structurally overlying the rocks of
the South Mayo Trough. This is unusual because given
the geometry of modern accretionary complexes, the
forearc basin might be expected to overlie the accretionary wedge, whose tectonic fabric would dominantly parallel the dip of the subduction zone, i.e., dip
towards the arc volcanic front, not away from it, as it
does in the modern setting. The current steep north-
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
dipping fabric of the Clew Bay Complex implies shear
and accretion in a north-dipping subduction zone,
inconsistent with the subduction polarity inferred from
the South Mayo Trough and with the relative location
of the arc volcanic rocks of the Lough Nafooey and
Tourmakeady Volcanic Groups. The modern dip of the
tectonic fabric within the Clew Bay Complex and its
overlying relationship with the South Mayo Trough
can, however, be understood if it is recognized that the
entire region has been tilted and overturned following
exhumation, as a result of the D3 dextral shearing
described above.
Fig. 7 shows the proposed model in which the
region around northern Clew Bay has been rotated,
while not apparently affecting the main part of the
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South Mayo Trough. Likely, the rotation is linked to
transpressional deformation, whose age is not constrained by this study. Sedimentary structures and
biostratigraphy along the northern arm of the South
Mayo Trough demonstrate that this is not overturned,
although the strata may have been more north-dipping
when originally deposited. The contact between the
strongly rotated Clew Bay region and the South Mayo
Trough is now covered by Silurian sequences exposed
west of Westport (Fig. 1). Their location in this belt
may reflect the presence of a major fault in that area,
decoupling the South Mayo Trough and the Clew Bay
Complex.
Hence, prior to late Grampian (D3) dextral shearing,
the present-day subvertical, north-dipping Dalradian
Fig. 6. Field photographs of (A) The Achill Beg Fault on the west side of Achill Beg. The brittle fault trace is likely reactivated after Ordovician
motions. Note the tectonized, yet more coherent South Achill Beg Formation overlain by a zone of coarse tectonic breccia, 5 – 10 m thick. (B)
Example of the pervasively deformed Dalradian meta-sedimentary rocks of the Argyll Group exposed in southern Achill Island. The Dalradian
reaches upper greenschist grade in this region. (C) Tight, recumbent folding in shaley low grade, metasedimentary rocks of the South Achill Beg
Formation, east side of Achill Beg. (D) Coarse-grained gabbro pod within Dalradian metasedimentary rocks on coast west of Gowlaun.
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
nappes would have originally been flat-lying along the
northern margin of Clew Bay (e.g., Sanderson et al.,
1980; Chew, 2003; Fig. 4). Consequently, we infer that
the original dip of the Achill Beg Fault and the tectonic
fabric of the Clew Bay Complex were not steeply
towards the north but, instead, towards the south. In
this reconstructed orientation, the Clew Bay Complex
along the southern edge of Clay Bay dips southward
under the South Mayo Trough, consistent with accretion in a south-dipping subduction zone. Also, on
Achill Beg, the low-grade Clew Bay Complex (Fig.
6C) would have originally overlain the high-grade
Dalradian (Fig. 6B), thus implying late extension not
shortening across the Achill Beg Fault. Brittle, latestage shear-sense indicators along the fault (Fig. 6A)
that indicate a thrusting motion in the present sense
(i.e., top to the SSE; Fig. 5A) imply a top to the south –
southeast extension after restoration.
Once the effects of later deformation have been
removed, the original geometry of the collision
zone in the region is more apparent. The dominant,
high-strain deformation event in North Mayo (D1)
is associated with bedding-parallel shear zones with
a NNW-directed transport direction (Chew, 2001,
2003). In contrast, the D2 event that caused much
of the large-scale folding observed has a clear
southward vergence. NNW-directed transport in
the Dalradian and Clew Bay Complex is consistent
with a collision between a passive margin and a
south-dipping subduction zone (Dewey and Ryan,
1990). That the upper Argyll Group meta-sedimentary rocks are juxtaposed against the Clew Bay
Complex also make geodynamic sense in that these
are recognized as the youngest units within the
Dalradian stratigraphy in western Ireland. Consequently, it is the Clew Bay Complex and the upper
Argyll Group meta-sedimentary rocks of the North
Mayo Dalradian that would have been immediately
overthrust by the accretionary wedge of a colliding
arc. Having been overthrust, deformed and meta-
Fig. 7. Diagram showing how rotation about a horizontal axis, driven by transpressional tectonics in the Clew Bay area, may have caused the
reversal of dip on the Achill Beg Fault, as well as placing the Clew Bay Complex enigmatically over the South Mayo Trough. Restoration of this
region to their original orientations results in tectonic relationships more consistent with what is known from modern subduction zone.
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
morphosed, the upper Argyll Group was still at the
top of the Dalradian structural stack when a detachment fault, exploiting a location close to, or
along the original overthrust, brought the upper
Argyll Group back to the surface.
3.2. Detachment faulting in Connemara
The nature of detachment faulting in northern
Connemara has been a source of controversy. Wellings (1998) proposed that extension along an east –
west trending, north-dipping tectonic discontinuity
through northern Connemara, known as the
Renvyle –Bofin Slide, was responsible for exhumation of the main Dalradian of the Twelve Bens,
whereas Williams and Rice (1989) favored a role
for low-angle extensional faults south of Connemara.
Although the Renvyle – Bofin Slide is a major extensional structure, it is not considered to be the
master structure because no major metamorphic
break is observed across it. Because of the strikeslip emplacement of Connemara south of the South
Mayo Trough, it is possible that the original root
zone of the detachments was not preserved within
this block or has been buried by later sedimentation.
By comparison with North Mayo, a detachment
might be expected to separate high-grade rocks in
Connemara from a lower grade arc towards the
south, similar to the model proposed Williams and
Rice (1989). This southern repeat of the arc is not
well preserved, but is likely represented by the
rhyolites of the Delaney Dome Formation that correlate with the arc volcanic rocks of the Tourmakeady Formation (Dewey and Mange, 1999; Draut
and Clift, 2002). However, the hypothesis that detachment faults in Connemara should lie along its
southern edge assumes that the Connemara block has
not been rotated within the strike-slip zone since
exhumation. Paleomagnetic and GPS studies of active strike-slip margins clearly demonstrate that major block rotations are to be expected in such areas
(e.g., Transverse Ranges, California, Nicholson et al.,
1994; Aegean Sea, Clarke et al., 1998). Indeed, the
top-to-the-north geometry of the Renvyle – Bofin
Slide and the southward motion on the Mannin
Thrust are consistent with a major north-dipping
detachment along the northern edge of the Connemara Dalradian. This scenario also compares closely
101
with the High Himalaya exhumation along a southward-verging Main Central Thrust and a northward
extending South Tibetan Detachment, both with
north-dipping geometries.
We propose that fragments of a detachment
surface are mapped close to and along the unconformity between the Dalradian and the overlying
Silurian sedimentary rocks, along the northern edge
of Connemara (Fig. 8). There is a preferential
development of serpentinite, ultramafic and mafic
blocks along, and just south of this contact but not
within the main outcrop of the Connemara Dalradian. These mafic and ultramafic rocks, which
include coarse-grained gabbros (Fig. 5D), occasionally cut by dykes of ophitic meta-dolerite, form
coherent, relatively undeformed pods 10 – 200 m
across, surrounded by strongly sheared Dalradian
metasedimentary rocks.
The coarse-grained character of most of the gabbro blocks (grains >1 cm across) is inconsistent with
an alternative origin in which the small mafic bodies
were intruded into the sedimentary rocks and then
deformed. The coarse grain size of some of the
gabbros indicates original slow cooling within a
large intrusion, followed by tectonic dismembering.
In this respect, they contrast with the clearly
intruded, kilometer-scale Cashel – Lough Wheelan
and Dawros – Currywongaun – Doughruagh gabbro
bodies (Leake, 1986; Fig. 1), although some smaller
intrusions, complete with chilled margins, related to
these larger bodies are also noted. Furthermore, the
fact that such mafic and ultramafic bodies are only
found either close to the unconformable northern
contact of the Connemara Dalradian or close to the
Achill Beg Fault is difficult to reconcile with an
original intrusive origin. The serpentinized peridotites, moreover, have a tectonized internal texture
consistent with a mantle origin rather than as an
ultramafic intrusive. Because the bulk of the Dalradian outcrop comprises metasedimentary rocks, the
occurrence of small mafic and ultramafic rocks only
at the structural top suggests that these have been
emplaced tectonically.
We suggest that the unconformity represents an
exhumed and partly eroded detachment surface. In
this scenario, the presence of lower crustal or mantle
rocks can be explained as blocks entrained along a
crustal scale structure that brings Dalradian rocks to
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Fig. 8. Geological map of the NW Connemara coast showing the Early Silurian Kilbride and Lettergesh Formations lying unconformably over
Dalradian metasedimentary rocks. Note the preferential occurrence of mafic and ultramafic pods along the unconformity contact.
the surface from beneath the colliding arc. The hanging wall was completely removed and the exposed
metamorphic rocks were covered by the fluviatile
Silurian Lough Mask Formation. This relationship
might be consistent with a ‘‘rolling hinge’’ type of
detachment (Buck, 1988). Metamorphic core com-
plexes in the US Basin and Range and those on
slow-spreading mid-ocean ridges expose deeply buried rocks at the surface where they may be eroded and
transgressed by sediments.
The marine Kilbride and Lettergesh Formations,
which transgress over the Dalradian, are dated as
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Upper Llandovery (435 –420 Ma) implying a significant hiatus between the time of most rapid cooling
(470 – 455 Ma; Friedrich et al., 1999b) and final
transgression. However, this time gap may be less
than it initially appears. Although the rocks cooled
rapidly through the biotite Ar closure temperature
(f 300 jC; Lovera et al., 1989), exhumation at
shallower levels after 455 Ma below the closure
temperature could have continued at slower rate. This
would reduce the amount of time that the detachment
would have been exposed at the surface and subject to
erosion prior to Silurian transgression.
4. Sedimentary response to collapse
Orogenic exhumation in western Ireland was accompanied by erosion, resulting in rapid, coarsegrained sedimentation preserved as the Rosroe,
Maumtrasna and Derrylea Formations within the
South Mayo Trough (Fig. 2). On the south side of
the South Mayo Trough, the Rosroe Formation is
dated as Early Llanvirn (lower artus graptolite zone,
467 –464 Ma according to the time scale of Tucker
and McKerrow, 1995). It comprises very coarse conglomerates and sandstones, including abundant granite and volcanic clasts (Archer, 1977; Fig. 9B –D).
The Rosroe Formation was interpreted originally to
represent the deposits of large submarine fan deltas
eroding a volcanic and plutonic arc source south of
the modern outcrop region (Fig. 1; Archer, 1977,
1984). The Rosroe Formation has been correlated
with the Derrylea Formation on the northern limb of
the South Mayo Trough (Dewey, 1963; Dewey and
Ryan, 1990; Fig. 2), although Williams (2002) considered the Derrylea Formation to have been a separate system, deposited in a different subbasin. Upsection, the depositional environment of the South
Mayo Trough shallows, culminating in the middle
Llanvirn Mweelrea Formation, which comprises a >3km-thick package of west-flowing fluvial sandstones
derived from a rapidly eroding Dalradian source
(Pudsey, 1984).
The sedimentary system in western Ireland related
to Dalradian exhumation bears many similarities in
terms of facies and relationships to related tectonic
units with the sedimentation associated with exhumation in the Taiwan arc – continent collision zone. In
103
Taiwan, alluvial fans derive material from the exhumed metamorphic rocks of the Central Ranges,
where sediment eroded from the collisional orogen
is washed along-strike from the alluvial Ilan Plain,
through the Huapingshu Canyon into the post-orogenic collapse basin of the Okinawa Trough (e.g., Yu
and Hong, 1993). A depositional model for the South
Mayo Trough featuring two canyons, one in the
center and one in the east of the southern margin of
the basin (Archer, 1977), is consistent with this
modern analogue. Pleistocene sedimentation rates in
the southern Okinawa Trough exceed 325 cm/k.y. at
ODP Site 1202 (Salisbury et al., 2001). In comparison, Graham et al. (1989) estimated 1350 m of
Rosroe Formation deposited at 464 – 467 Ma, a
long-term average rate of >45 cm/k.y., also a very
high rate of accumulation.
Lundberg and Dorsey (1990) demonstrated that
rapid Quaternary uplift of the Coastal Range, interpreted as a fragment of the accreted Luzon Arc
(e.g., Dorsey, 1992), has resulted in proximal sedimentation in the Longitudinal Valley overlying the
accreted arc and forearc. The Longitudinal Valley is
thus comparable with the syn-collisional South
Mayo Trough. Although erosion rates in the Coastal
Range are high, those in the higher and more
extensive Central Range of the Taiwan Orogen are
greater and contribute the bulk of the sediment
reaching the Okinawa Trough (Lundberg and Dorsey, 1990). This parallels the situation in South
Mayo where a rapid influx of material eroded from
the exhuming Dalradian metamorphic rocks is found
throughout the basin, recognized on the basis of the
high-grade metamorphic minerals (Dewey and
Mange, 1999), as well as the Pb isotopic composition of the detrital K-feldspar grains (Clift et al.,
2003).
4.1. Source of sediment
The provenance of sediment during orogenic collapse in the Irish Caledonides has been a source of
controversy. Apart from the apparent arc source to the
Rosroe Formation, Dewey and Shackleton (1984)
recognized the influx of ophiolitic material in the
upper parts of the South Mayo Trough, which they
related to the progressive unroofing of the forearc
basement, now preserved as the Deer Park Complex,
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
a mafic/ultramafic part of the Clew Bay Complex
(Fig. 1). Chemical data from the sediments of the
South Mayo Trough (Wrafter and Graham, 1989)
show a progressive increase in Cr, Ni and Mg upsection into the Derrylea Formation, although these
values drop in the middle of this unit. Dewey and
Mange (1999) correlated this drop with the first
appearance of high-grade metamorphic minerals (garnet, hornblende, staurolite and sillimanite) in the
middle of the Rosroe and Derrylea Formations, which
they related to an initial influx of detritus from a
newly exposed Dalradian source located to the north,
i.e., North Mayo. The sudden arrival of staurolite in
the upper Derrylea Formation, following erosion of an
ophiolitic source, suggests rapid exhumation of the
deformed Laurentian passive margin within a metamorphic complex.
Clift et al. (2002) examined the trace element
composition of granite boulders and the Pb isotopic
character of K-feldspar grains in the Rosroe Formation and inferred a Laurentian (i.e., Dalradian, rather
than an arc) source, in accordance with the model of
Dewey and Mange (1999). These boulders are mainly
of high-level quartz-rich, minimum melt type granites
that were, presumably, part of the cover to the
collapsing metamorphic pile. Paleo-current analysis
suggested that the Dalradian source was located
towards the NE, i.e., North Mayo (Fig. 9).
Fig. 9. Field photographs of (A) Rosroe Formation exposed on south side of Killary Harbour, 300 m NE of Rosroe Village. Note thick channel
sandstones interbedded with thin shaley overbank facies. (B) Debris flow deposit in the Rosroe Formation at Lough Nafooey comprising
mixture of granite and altered lava clasts. (C) Submarine fan deposit from the Rosroe Formation exposed on the south side of Killary Harbour, 1
km west of Leenane. (D) Debris flow deposit within the Derryveeny Formation on the west shore of Lough Mask (Fig. 1) dominantly
comprising granite, schist and foliated granite clasts from the Connemara Dalradian.
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
At outcrop, the provenance of the Rosroe and
associated formations is apparent. Along the southern
edge of Killary Harbour, the boulder conglomerates
are dominated by granite clasts with only minor lava
fragments (Fig. 9C). In contrast, at Lough Nafooey,
the proportion of lava, both fresh and altered, is much
higher (Fig. 9B). Both types of deposit contrast with
the younger (Llanvirn to Lower Llandovery; 464 – 443
Ma) Derryveeny Conglomerate (Figs. 2 and 9D),
which contains abundant schist and foliated granite
clasts and granites similar to those seen in the Rosroe
Formation. The provenance of sediments deposited
105
during exhumation is not uniform, but shows a
dominant influx from the North Mayo Dalradian and
equivalent sources to the NE (e.g., Ox Mountains,
southern Donegal), with variable, subsidiary input
from the Lough Nafooey Arc.
4.2. Sediment transport
The patterns of sediment dispersal during exhumation were summarized by Dewey and Mange (1999)
and Clift et al. (2002) (Fig. 10), combining paleocurrent data from across the South Mayo Trough.
Fig. 10. Rose diagrams showing the paleoflow directions in the South Mayo Trough during deposition of the Rosroe and Maumtrasna
Formations on the basis of conglomerate cobble orientations and cross-bedding. Although cobble orientation only specifies the axis of flow, we
infer the direction of flow from cross-bedding and pebble imbrication. The dominant southerly and westerly flow contrasts with the NNW-flow
in the later Derryveeny Conglomerate. Figure reproduced from Clift et al. (2002).
106
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Although some S – N flow is apparent along the
southern edge of the basin, a more axial flow towards
the west is seen in the central parts of the trough.
Williams (2002) argued that the presence of finergrained rocks in the Derrylea Formation, north of the
boulder conglomerates of the Rosroe Formation, precluded a sediment source to the north of the trough.
However, if the sediment flux is dominantly axial,
lateral changes in grain size could simply reflect
differences in tectonic subsidence rates focusing
coarser sediments into the depocenters. The sedimentary facies support the idea of basin deepening and
fining towards the SW. Fig. 9A shows the sands and
shales of the Rosroe Formation close to Rosroe
village (Fig. 1). These channel sands imply a more
distal, deeper water environment than the dominant
fanglomerate facies observed further east and north
(Fig. 9B and C).
5. Magmatic response to collapse
Orogenic collapse and the establishment of an
active margin following the Grampian Orogeny was
accompanied by continued volcanism preserved as
tuffs within the Rosroe and overlying Mweelrea
Formation (Fig. 2). Like the preceding Tourmakeady
Volcanic Group, which was erupted prior to and
during peak metamorphism, these are high-silica tuffs,
up to 75% SiO2 (Clift and Ryan, 1994). Draut and
Fig. 11. Trace element discrimination diagram showing the wide range of REE and HFSE enrichment in tuffs from the Rosroe and Mweelrea
Formations compared to similar post-orogenic lavas from Taiwan (data from Chen et al., 1995; Shinjo, 1999). Note that lavas from both these
settings are very enriched in HFSEs compared to either the continental crust average and to oceanic arc melts. Tonga field is for a range of major
element glass compositions erupted since 7 Ma (Clift and Dixon, 1994). Tourmakeady and Lough Nafooey Group data are from Draut and Clift
(2001). Compositions of N-MORB, mantle and average continental crust are from Rudnick and Fountain (1995).
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Clift (2001) demonstrated that, whereas the Rosroe
and Mweelrea tuffs show similar high light rare earth
element (LREE) enrichment compared with the Tourmakeady Volcanic Group, they have a wider range of
relative enrichment in the high field strength elements
(HFSE; Fig. 11). Williams (2002) suggested that the
tuffs of the Rosroe Formation were erupted in an
oceanic island arc, but a comparison between siliceous
volcanic glass from a typical Neogene oceanic arc,
such as Tonga, and the Rosroe Formation shows that
the Rosroe Formation is much more enriched in REEs
and HFSEs. In contrast, there is a significant overlap
between the Lough Nafooey Group, erupted before
arc – continent collision, and the Tonga Arc. Draut and
Clift (2001) linked this change from the basaltic,
LREE-depleted Lough Nafooey to high-silica,
LREE-enriched Rosroe chemistry with the arc – continent collision.
The degree of continental crust recycling through
the collisional arc is best demonstrated through the
application of isotope chemical techniques, which are
not affected by fractional crystallization, as are the
REEs and HFSEs. Neodymium isotope data show
that magmatism during orogenic collapse in South
Mayo involved significant remelting of the continental crust. Initial eNd values of
8.4 to
10.6
reported by Draut and Clift (2001) from the Tourmakeady Volcanic Formation, Rosroe Formation and
Mweelrea Formation (472 –466 Ma) require significant mixing of radiogenic continental crust (Dalradian eNd = 12 to 22; O’Nions et al., 1983; Frost
and O’Nions, 1985; Jagger, 1985) with a typical
oceanic end member with eNd values of + 8 to
+ 10. The wide range of HFSE and LREE enrichments observed in the Rosroe and Mweelrea Formation tuffs points to a very heterogeneous source
compared with even the syn-collisional Tourmakeady
Volcanic Group. The scatter may reflect involvement
of more enriched mantle sources contrasted to that
feeding the Tourmakeady Volcanic Group. Extensional collapse of the orogen coupled with subduction
polarity reversal would change the flow of mantle
under the orogen and could draw in less depleted
mantle material under the volcanic arc. Alternatively,
gravitational loss of a dense arc lower crust during
collision could also provide space for upwelling and
influx of fresh mantle material under the volcanic
roots of the arc.
107
6. Discussion
The processes and effects of extensional collapse in
the Irish Caledonides after the Grampian Orogeny
bear similarities to other documented collisional belts
(Dewey, 1988), especially the Taiwan– China collisional orogen (Teng, 1996; Lee et al., 1997). Taiwan,
like the Grampian Orogen, is an oceanic arc – continental collisional environment involving subduction
polarity reversal and exhumation of a metamorphosed
continental margin sequence through the action of
major detachment faulting (Teng, 1996; Clift et al.,
2003). In Taiwan, the new continental Ryukyu island
arc is built on the remnants of the oceanic Luzon Arc.
In Connemara, the arc volcanic front of the succeeding continental arc is not exposed, but is known along
strike in Newfoundland as the Notre Dame Arc. The
lack of the arc in Ireland is presumed to reflect
tectonic excision during margin-parallel strike-slip
faulting. The Southern Upland accretionary complex
exposed in Scotland and Northern Ireland provided
evidence that a north-dipping subduction zone was
developed along this part of the Laurentian margin
(van Staal et al., 1998; Leggett et al., 1983).
Several tectonic processes may drive extensional
collapse following the metamorphic peak. Dewey et
al. (1993) invoked lower crustal loss to explain rapid
surface uplift and post-orogenic extension along major detachment faults following eclogitization in the
Norwegian Caledonides. Subsequently, Krabbendam
and Dewey (1999) invoked exhumation of the Norwegian eclogites in a sinistral transtensional pull-apart
setting. We consider both of these models to be highly
unlikely for exhumation in western Ireland. In any
case, because no arc lower crustal rocks are exposed
in western Ireland, any such hypothesis would be
speculative. Geophysical modeling of different lower
crustal lithologies and thermal regimes raises the
possibility of lower crustal loss and surface uplift
even without the formation of eclogite in arc settings
(Jull and Kelemen, 2001). However, Clift et al. (2003)
calculated that, given typical ocean arc crustal thicknesses (20 – 25 km), loss of an ultramafic lower crust
that had not been eclogitized could be expected to
drive < 10% of the topography in a collisional range.
Thus, other tectonic mechanisms may be required to
explain the degree of exhumation observed in western
Ireland (>25 km; Friedrich et al., 1999b).
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
Orogenic extension is caused by the topographic
load, facilitated by the low viscosity of the orogenic
lower crust. For the collision of an oceanic arc into a
rifted continental margin, once the compressive forces
that formed the mountains are released from the thrust
belt by renewed subduction following polarity reversal, there is a tendency for the structural stack to
collapse and extend under its own weight. The trench
thus forms a free space into which the orogen can
collapse, especially if aided by subduction rollback
(Dewey, 1980), triggering extension of the deformed
arc and passive margin sequences (Fig. 12; Teng,
1996). Detachment faulting is a common feature in
the Dalradian of the British Isles, where exposure has
permitted its recognition. Farther northeast of our
focus area, the Dalradian rocks in both the Ox
Mountains Inlier and southern Donegal are exhumed
along reactivated shear zones (the North Ox Mountains Slide, Flowerdew, 1998; the Lough Derg Slide,
Alsop, 1991; Alsop and Hutton, 1993). In northwest
Scotland, Powell and Glendinning (1990) documented
the reactivation of thrusts as extensional faults, similar
to our proposed motion on the Achill Beg Fault.
It is possible that some juxtaposition of high and
low-grade rocks in the Clew Bay region is caused by
Silurian sinistral strike-slip faulting. Such faulting is
pervasive in parts of Clew Bay, but does not, by itself,
explain the key juxtaposition of high- and low-grade
units that requires greater relative exhumation in
North Mayo and Achill Island, compared with the
South Mayo Trough. Most of the cooling in Connemara and North Mayo is, moreover, too old to have
been driven by the younger strike-slip faulting.
Sedimentation during exhumation was dominated
by erosion of the metamorphosed continental margin
series (Dalradian Supergroup). Although the cooling
ages (460 – 450 Ma; Flowerdew et al., 2000) of the Ox
Mountain Dalradian are slightly younger than the age
of Rosroe sedimentation (467 –464 Ma), this discrepancy may reflect moderate along-strike variability in
the timing and rate of exhumation. This is, however,
not required because the rocks now exposed in North
Mayo are clearly not the source of the sediment but
represent deeper levels of the source that was being
eroded at the time of deposition of the Rosroe Formation. Geodynamic models of mass motion through
orogenic belts (e.g., Willett et al., 1993; Royden,
1996) suggest that erosion can result in large fluxes
of material from the subducting/colliding plate (in this
case Laurentia), without erosional unroofing of any
deeper structural level at any one place through time.
Thus, the cooling ages of detrital minerals within the
eroded sediment series would become younger upsection. The cooling age preserved at the surface in the
source terrain after the end of rapid post-orogenic
sedimentation will record the time when that package
of igneous or metamorphic rock passed through the
closure temperature of the minerals analyzed. This age
must postdate the start of sedimentation. Depending on
whether the source cooling is driven by erosion or
tectonic exhumation, the cooling age of that source
may also postdate the end of sedimentation in a synorogenic basin, especially if this becomes filled, so that
additional eroded sediment will not accumulate but,
instead, is transported oceanward and/or along strike.
7. Conclusions
Orogenic collapse of the Grampian Orogeny in
western Ireland is inferred to have been rapid, with
radiometric data suggesting 10– 15 km of exhumation achieved between 470 and 460 Ma (Friedrich et
al., 1999b; Power et al., 2001). We propose that
rapid exhumation resulted from detachment faulting.
A south-dipping, now overturned, north-dipping
Achill Beg Fault is considered to represent a key
structure unroofing the North Mayo Dalradian from
under the low-grade rocks of the Clew Bay Complex
and South Mayo Trough. In Connemara, the hanging
wall of the detachment is not preserved, but the
detachment surface corresponds, at least locally, to
the Silurian unconformity along the north side of the
Connemara Dalradian. Mafic and ultramafic blocks
Fig. 12. Schematic tectonic model for the western Irish Caledonides in which a Neoproterozoic – Ordovician passive margin is deformed and
metamorphosed after collision with the Lough Nafooey Arc. Creation of a new trench south of the orogen allows the orogen to collapse as the
compressive forces are removed. Extension is accommodated along low-angle detachment faults that entrain pods of lower crustal arc rocks.
Shortly after subduction polarity reversal, the margin is affected by strike-slip faulting that displaces a fragment of the Dalradian Supergroup
along the margin as the Connemara Terrane. The exposed detachment surfaces are transgressed by the Early Silurian Kilbride Formation after
excision of Connemara.
P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
109
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P.D. Clift et al. / Tectonophysics 384 (2004) 91–113
entrained along the detachment fault are now preserved along the unconformity. The Renvyle –Bofin
Slide is considered to be a subordinate structure to
this master detachment. Alsop (1991) and Flowerdew et al. (2000) have described similar exhumation
along strike in the Grampian Orogen. It is likely that
Ordovician exhumation occurred in the Taconic Belt
of the Northern Appalachians from New York to
Newfoundland. To date, such exhumation structures
have not been described in detail, although Silurian –
Devonian external detachments have been recognized in Vermont (Karabinos, personal communication, 2003).
Our basic model is that extensional collapse and
exhumation in western Ireland was facilitated and
caused by the change in stress driven by subduction
polarity reversal, quite dissimilar from the transtensional exhumation of eclogites documented in the
Silurian of Norway (Krabbendam and Dewey,
1999). Influx of metamorphic detritus into the
South Mayo Trough at 467– 464 Ma marked the
start of exhumation, with most sediment derived
from the deformed Laurentian margin rather than
from the colliding volcanic arc. Erosion was rapid
and shed debris into a marginal-parallel trough as
fan deltas, feeding submarine fans that flowed
towards the west. Synchronous magmatism showed
extreme HFSE and LREE enrichment contrasted to
earlier volcanism. Volcanic chemistry implies petrogenesis in a continental subduction setting, with a
heterogeneous, locally enriched upper mantle
source.
Acknowledgements
Clift and Draut were partly supported by Woods
Hole Oceanographic Institution (WHOI). Clift and
Draut would like to thank Siobhan Power for
stimulating conversions and guidance in the field.
Dewey and Mange are grateful to British Petroleum
and the Leverhulm Foundation for support. We thank
Yildrim Dilek, Ian Alsop and Jean-Pierre Burg for
careful reviews of this paper that resulted in
significant improvements. We dedicate this paper to
the memory of Adrian Phillips who did so much to
promote the mapping and understanding of Irish
geology.
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