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Tectonophysics 384 (2004) 91–113

Rapid tectonic exhumation, detachment faulting and orogenic

collapse in the Caledonides of western Ireland

Peter D. Clifta,*, John F. Deweyb, Amy E. Drautc,1, David M. Chewd,2,Maria Mangeb, Paul D. Ryane

aDepartment of Geology and Geophysics/MS 8, Woods Hole Oceanographic Institution, 360 Woods Hole Road,

Woods Hole, MA 02543-1541, USAbDepartment of Geology, 174 Physics/Geology Building, University of California, One Shields Avenue, Davis, CA 95616-8605, USA

cWoods Hole Oceanographic Institution-Massachusetts Institute of Technology Joint Program in Oceanography, Woods Hole, MA 02543, USAdDepartment of Geology, Trinity College, Dublin 2, Ireland

eDepartment 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

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2004.03.009

* 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: Department de Mineralogie, Universite de Geneve, Rue des Maraıchers 13, CH-1205 Geneva, Switzerland.

P.D. Clift et al. / Tectonophysics 384 (2004) 91–11392

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 similar in size, duration and geology to the Grampian

Geochemical evidence from the bulk global com-

position 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, be-

cause of the loss of crust by tectonic erosion along

many continental arc margins (e.g., Dewey, 1980; von

Huene and Scholl, 1991), continued continental crust-

al 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 mar-

gins 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 south-

ern 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 ter-

ranes 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 ob-

duction developed the subjacent Bismarck Orogen,

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 Trem-

adocian andesitic lavas and epiclastic breccias inter-

bedded 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 Early-

Middle Ordovician volcaniclastic and siliciclastic sed-

imentary 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 LREE-

enriched 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.,

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.

P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 93

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 Gram-

pian 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 anchi-

metamorphic 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].


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 meta-

sedimentary rocks are intruded by a number of Early-

Mid 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 inter-

preted 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 metamor-

phic rocks were exhumed by orogen-perpendicular

extension beneath the Zanskar and South Tibetan

Detachments, synchronous with thrusting (e.g., Her-

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 meta-

rhyolites of the Delaney Dome Formation (Fig. 1), a

deformed equivalent of the syn-collisional arc se-

quence, the Tourmakeady Volcanic Group (Dewey

and Mange, 1999; Draut and Clift, 2002). This Dela-

ney Dome arc fragment must have been transposed

into its present location with the Connemara Dalra-

dian, 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 con-

strained 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). Identifi-

cation 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 higher-

grade exhumed rocks in the footwall. Across Clew

Bay, a clear difference in grade has long been recog-

nized 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,

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).



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 ClewBay 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 sedimenta-

ry rocks. Furthermore, to the north, Dalradian meta-

morphic rocks of North Mayo (now in the hanging

wall of this structure) preserve an earlier south-dip-

ping 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

Fig. 4. Geological cross section through Achill Island (after Chew, 2001)

Dalradian, contrasting with the NNW-vergent D1 deformation. See Fig. 3

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 struc-

tures 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 anticlock-

wise to the S2 foliation (Figs. 5A and C) and exten-

sional 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 withthe 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 contem-

poraneous with the F3 asymmetric buckle folds based

on the absence of apparent overprinting relationships.

On a vertical surface, the extensional shear bands give a

showing the south-vergent asymmetry of the D2 deformation in the

for location of section. Modified after Chew (2001).

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 crenulation-

related 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 accre-

tionary wedge, whose tectonic fabric would dominant-

ly 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-


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

Fig. 6. Field photographs of (A) The Achill Beg Fault on the west side of A

motions. Note the tectonized, yet more coherent South Achill Beg Formati

Example of the pervasively deformed Dalradian meta-sedimentary rocks of

reaches upper greenschist grade in this region. (C) Tight, recumbent folding

Formation, east side of Achill Beg. (D) Coarse-grained gabbro pod within

South Mayo Trough. Likely, the rotation is linked to

transpressional deformation, whose age is not con-

strained 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

chill Beg. The brittle fault trace is likely reactivated after Ordovician

on overlain by a zone of coarse tectonic breccia, 5–10 m thick. (B)

the Argyll Group exposed in southern Achill Island. The Dalradian

in shaley low grade, metasedimentary rocks of the South Achill Beg

Dalradian metasedimentary rocks on coast west of Gowlaun.


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 accre-

tion 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, late-

stage 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

Fig. 7. Diagram showing how rotation about a horizontal axis, driven by t

reversal of dip on the Achill Beg Fault, as well as placing the Clew Bay Com

region to their original orientations results in tectonic relationships more

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-sedimen-

tary 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. Conse-

quently, 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-

ranspressional tectonics in the Clew Bay area, may have caused the

plex enigmatically over the South Mayo Trough. Restoration of this

consistent with what is known from modern subduction zone.


morphosed, the upper Argyll Group was still at the

top of the Dalradian structural stack when a de-

tachment 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. Well-

ings (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 exhuma-

tion 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 ex-

tensional structure, it is not considered to be the

master structure because no major metamorphic

break is observed across it. Because of the strike-

slip 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 cor-

relate with the arc volcanic rocks of the Tourma-

keady Formation (Dewey and Mange, 1999; Draut

and Clift, 2002). However, the hypothesis that de-

tachment 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 ac-

tive strike-slip margins clearly demonstrate that ma-

jor 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 Conne-

mara Dalradian. This scenario also compares closely

with the High Himalaya exhumation along a south-

ward-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 uncon-

formity 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 Dalra-

dian. These mafic and ultramafic rocks, which

include coarse-grained gabbros (Fig. 5D), occasion-

ally 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 gab-

bro 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 perido-

tites, moreover, have a tectonized internal texture

consistent with a mantle origin rather than as an

ultramafic intrusive. Because the bulk of the Dalra-

dian 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

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 hang-

ing 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 bur-

ied 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


Upper Llandovery (435–420 Ma) implying a signif-

icant 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 ac-

companied by erosion, resulting in rapid, coarse-

grained 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 con-

glomerates and sandstones, including abundant gran-

ite 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) con-

sidered the Derrylea Formation to have been a sepa-

rate system, deposited in a different subbasin. Up-

section, the depositional environment of the South

Mayo Trough shallows, culminating in the middle

Llanvirn Mweelrea Formation, which comprises a >3-

km-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 exhuma-

tion in the Taiwan arc–continent collision zone. In

Taiwan, alluvial fans derive material from the ex-

humed 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-oro-

genic 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 compari-

son, 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, inter-

preted as a fragment of the accreted Luzon Arc

(e.g., Dorsey, 1992), has resulted in proximal sed-

imentation 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 Dor-

sey, 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 composi-

tion of the detrital K-feldspar grains (Clift et al.,

2003).

4.1. Source of sediment

The provenance of sediment during orogenic col-

lapse 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,


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 up-

section 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 (gar-

net, 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

Fig. 9. Field photographs of (A) Rosroe Formation exposed on south side

sandstones interbedded with thin shaley overbank facies. (B) Debris flo

mixture of granite and altered lava clasts. (C) Submarine fan deposit from t

km west of Leenane. (D) Debris flow deposit within the Derryveeny F

comprising granite, schist and foliated granite clasts from the Connemara

ophiolitic source, suggests rapid exhumation of the

deformed Laurentian passive margin within a meta-

morphic 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 Forma-

tion 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).

of Killary Harbour, 300 m NE of Rosroe Village. Note thick channel

w deposit in the Rosroe Formation at Lough Nafooey comprising

he Rosroe Formation exposed on the south side of Killary Harbour, 1

ormation on the west shore of Lough Mask (Fig. 1) dominantly

Dalradian.


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

Fig. 10. Rose diagrams showing the paleoflow directions in the South

Formations on the basis of conglomerate cobble orientations and cross-bed

infer the direction of flow from cross-bedding and pebble imbrication. The

in the later Derryveeny Conglomerate. Figure reproduced from Clift et al

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 exhuma-

tion were summarized by Dewey and Mange (1999)

and Clift et al. (2002) (Fig. 10), combining paleo-

current data from across the South Mayo Trough.

Mayo Trough during deposition of the Rosroe and Maumtrasna

ding. Although cobble orientation only specifies the axis of flow, we

dominant southerly and westerly flow contrasts with the NNW-flow

. (2002).


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 finer-

grained rocks in the Derrylea Formation, north of the

boulder conglomerates of the Rosroe Formation, pre-

cluded 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 sedimen-

tary 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

Fig. 11. Trace element discrimination diagram showing the wide range of

Formations compared to similar post-orogenic lavas from Taiwan (data fro

settings are very enriched in HFSEs compared to either the continental crus

element glass compositions erupted since 7 Ma (Clift and Dixon, 1994). To

(2001). Compositions of N-MORB, mantle and average continental crust

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

REE and HFSE enrichment in tuffs from the Rosroe and Mweelrea

m Chen et al., 1995; Shinjo, 1999). Note that lavas from both these

t average and to oceanic arc melts. Tonga field is for a range of major

urmakeady and Lough Nafooey Group data are from Draut and Clift

are from Rudnick and Fountain (1995).


Clift (2001) demonstrated that, whereas the Rosroe

and Mweelrea tuffs show similar high light rare earth

element (LREE) enrichment compared with the Tour-

makeady 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–con-

tinent 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 continen-

tal crust. Initial eNd values of � 8.4 to � 10.6

reported by Draut and Clift (2001) from the Tourma-

keady Volcanic Formation, Rosroe Formation and

Mweelrea Formation (472–466 Ma) require signifi-

cant mixing of radiogenic continental crust (Dalra-

dian 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 enrich-

ments observed in the Rosroe and Mweelrea Forma-

tion 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. Extension-

al 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.

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 colli-

sional orogen (Teng, 1996; Lee et al., 1997). Taiwan,

like the Grampian Orogen, is an oceanic arc–conti-

nental 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 succeed-

ing 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 ma-

jor detachment faults following eclogitization in the

Norwegian Caledonides. Subsequently, Krabbendam

and Dewey (1999) invoked exhumation of the Nor-

wegian 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 thick-

nesses (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).


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 rever-

sal, 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 Moun-

tains 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 Conne-

mara 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 discrep-

ancy 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

Fig. 12. Schematic tectonic model for the western Irish Caledonides in w

metamorphosed after collision with the Lough Nafooey Arc. Creation of a

compressive forces are removed. Extension is accommodated along low-a

Shortly after subduction polarity reversal, the margin is affected by strike

along the margin as the Connemara Terrane. The exposed detachment surf

excision of Connemara.

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 Forma-

tion. 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 up-

section. 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 syn-

orogenic 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 exhuma-

tion 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

hich a Neoproterozoic–Ordovician passive margin is deformed and

new trench south of the orogen allows the orogen to collapse as the

ngle detachment faults that entrain pods of lower crustal arc rocks.

-slip faulting that displaces a fragment of the Dalradian Supergroup

aces are transgressed by the Early Silurian Kilbride Formation after


entrained along the detachment fault are now pre-

served along the unconformity. The Renvyle–Bofin

Slide is considered to be a subordinate structure to

this master detachment. Alsop (1991) and Flower-

dew 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 recog-

nized in Vermont (Karabinos, personal communica-

tion, 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 transten-

sional 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 petro-

genesis 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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