Tectonophysics 384 (2004) 91–113 www.elsevier.com/locate/tecto

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

a Department of 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 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 of Laurentia after 480 Ma in western Ireland resulted in the deformation, magmatism and metamorphism of the Grampian , 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 rollback, into the new trench formed after subduction polarity reversal following collision. The Achill Beg , in particular, along the southern edge of the North Mayo Dalradian , 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 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: De´partment de Mine´ralogie, Universite´ de Gene´ve, Rue des Maraıˆchers 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 92 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 similar in size, duration and geology to the Grampian orogen (Dewey and Mange, 1999). Geochemical evidence from the bulk global com- In this paper, we focus on the later stages of this position of indicates that it was collision by documenting the nature of orogenic formed principally in subduction zone settings (e.g., collapse in this part of the Grampian . Dewey and Windley, 1981; Ellam and Hawkesworth, We further show how the tectonic and sedimentary 1988; Rudnick and Fountain, 1995). However, be- processes that occurred during orogenic collapse are cause of the loss of crust by tectonic along related directly to the changes in plate boundary zone many continental arc margins (e.g., Dewey, 1980; von strains linked to subduction polarity reversal. Huene and Scholl, 1991), continued continental crust- al growth must be largely achieved through the development of oceanic island arc complexes and 2. Regional geology their accretion to continental margins (Clift and Vannucchi, in press). Also, although arguments for The Lough Nafooey Arc forms the southern edge subduction initiation along passive continental mar- of the South Mayo Trough and comprises a series of gins have been made (e.g., Erickson, 1993), arc– earliest Ordovician basaltic lavas overlain by Trem- continent collision is also likely the principal origin adocian andesitic lavas and epiclastic breccias inter- for development of active continental margins bedded with pillowed flows, which are succeeded by (Dewey and Bird, 1970; Clift et al., 2003).The an Arenig sequence of andesitic to rhyolitic lavas, collision of Taiwan with the passive margin of south- breccias and volcaniclastic sediments. These rocks are ern China is the simplest known example of such a associated with an f 6-km-thick series of Early- process in the modern oceans (e.g., Teng, 1990; Middle Ordovician volcaniclastic and siliciclastic sed- Lundberg et al., 1997; Lallemand et al., 2001), yet, imentary rocks (Figs. 1 and 2). The sedimentary rocks because of limited sampling in the adjacent Okinawa in the South Mayo Trough accumulated prior to, Trough and Ryukyu Arc, the rates of orogenesis, during and after the arc–continent collision event. gravitational collapse and sedimentation in this area The oceanic Lough Nafooey Arc can be considered an are known only in outline. along-strike equivalent of the Lushs Bight Arc of A well-dated example of arc–continent collision is Newfoundland (e.g., Coish et al., 1982), and the known from the western Irish Caledonides. In the Shelburne Falls Arc of New England (Karabinos et South Mayo Trough and adjacent metamorphic ter- al., 1998). An up-section trend within the Lough ranes of Connemara and North Mayo (Fig. 1), a clear Nafooey volcanic rocks to more siliceous and light Early Ordovician collision event between the Lough rare earth element (LREE) enriched compositions is Nafooey Arc and the passive margin of Laurentia has consistent with the arc colliding with the passive been recognized (Dewey and Shackleton, 1984; margin of Laurentia during the Early Ordovician Dewey and Ryan, 1990; van Staal et al., 1998; Dewey (after 480 Ma; Draut and Clift, 2001). This is due to and Mange, 1999; Draut and Clift, 2001).New the progressively increasing recycling of LREE- Guinea illustrates the Miocene collision of an arc with enriched continental material through the subduction the north Australian continental margin (Davies et al., zone as the passive margin is subducted. Collision 1987). Here, arc/suprasubduction-zone ophiolite ob- precipitated the Grampian Orogeny in the British Isles duction developed the subjacent Bismarck Orogen, and the Taconic Orogeny in North America (e.g., P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 93

Fig. 1. Regional geological map of western Ireland showing the Connemara and North Mayo metamorphic 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. 94 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 and time

Lambert and McKerrow, 1976; Ryan and Dewey, Laurentian margin (e.g., Lambert and McKerrow, 1991; van Staal et al., 1998; Dewey and Mange, 1976; Dewey and Shackleton, 1984; Harris et al., 1999; Soper et al., 1999). 1994). Connemara is now displaced from its original Despite earlier work indicating a protracted Gram- location and lies along strike and south of the units pian Orogeny in Connemara (e.g., Elias et al., 1988; that comprise the Lough Nafooey Island Arc. This Cliff et al., 1996; Tanner et al., 1997), more recent displacement cannot be the result of southward studies have now demonstrated a duration of less thrusting of high-grade Connemara over South Mayo than 15 m.y., with peak metamorphism at f 470 Ma because much of this has never exceeded anchi- in Connemara (Friedrich et al., 1999a,b; Dewey and metamorphic conditions. Rather, the position of Mange, 1999). Ar–Ar dating of the main Connemara today must have been achieved by fabric in the Ox Mountains Dalradian suggests that post-Grampian strike-slip tectonism that affected cooling had started by at least 460–450 Ma in this the Laurentian margin [e.g., the Southern Uplands area (Flowerdew et al., 2000). The Connemara and Fault, the Fair Head-Clew Bay Line (equivalent to North Mayo Dalradian metamorphic terranes are the Highland Boundary Fault in Scotland) and the considered to be metamorphosed fragments of the Great Glen Fault; Hutton, 1987]. P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 95

The Connemara Terrane have reached close to its ren, 1987; Burchfiel et al., 1992). In western Norway, modern position before the Late Llanvirn to Lower high-pressure and ultrahigh-pressure metamorphic Llandovery (464–443 Ma) based on current and rocks were exhumed beneath a gently dipping late provenance data from the Derryveeny Conglomerate orogenic detachment (Dewey et al., 1993). A similar (Graham et al., 1991). Additional later Silurian and situation is envisaged for the Irish Dalradian. Devonian strike-slip faulting through Clew Bay also In Connemara, exhumation is also achieved by displaced the South Mayo Trough relative to the synchronous thrusting and detachment faulting that North Mayo Dalradian (Hutton, 1987),butthe combine to extrude deeply buried meta-sedimentary amount of displacement was probably not large (Ryan rocks to the surface. The Mannin Thrust places the et al., 1995). Dalradian towards the south over anchizone meta- Connemara is one of the best-dated segments in rhyolites of the Delaney Formation (Fig. 1),a the Caledonian–Appalachian Orogen and differs deformed equivalent of the syn-collisional arc se- from the North Mayo Dalradian in that the meta- quence, the Tourmakeady Volcanic Group (Dewey sedimentary rocks are intruded by a number of Early- and Mange, 1999; Draut and Clift, 2002). This Dela- Mid Ordovician gabbros and quartz diorites (Leake, ney Dome arc fragment must have been transposed 1989). Trace element and Nd isotopic geochemical into its present location with the Connemara Dalra- evidence shows that these gabbros were emplaced in dian, after the Grampian Orogeny. South southeasterly a subduction setting, with substantial involvement of motion on the Mannin Thrust may be constrained new mantle melts, relative to crustal recycling, in relative to the formation of other regional structures, their petrogenesis (Draut et al., 2002, in press). post-dating the D3 fabric and pre-dating the D4 fabric Therefore, the Connemara Dalradian can be inter- defined by Leake et al. (1983) and Tanner et al. preted as both the deformed margin of Laurentia (1989). Movement on the thrust is indirectly con- and the mid-crustal roots to a syn-collisional volcanic strained by isotopic dating of the syn-orogenic arc arc (e.g., Yardley et al., 1982). intrusive rocks to 470–462 Ma (Friedrich et al., Following peak metamorphism in the Connemara 1999b) and is considered to date from the period of Dalradian, indicates that the orogen rapid cooling in Connemara, i.e., post-dating peak experienced rapid cooling (>35 jC/m.y.; Friedrich et metamorphism. Motion on the Mannin Thrust may al., 1999a,b), typically explained as a response to have been related to the early phases of subduction orogenic collapse. Power et al. (2001) used fluid polarity reversal (Dewey and Mange, 1999). Identifi- inclusion techniques to estimate rates of exhumation cation of related extensional detachment has been in Connemara of at least 7 km in 10 m.y., similar to more controversial. We present here newly recognized modern massifs that exhibit the fastest exhumation evidence for major detachment faults in North Mayo rates (e.g., Nanga Parbat; Zeitler et al., 1993). We now and Connemara. explore the exhumation and orogenic collapse of the Irish Dalradian and assess implications for the tectonic 3.1. Detachment faulting in North Mayo and Achill evolution of this area and for the process of arc– Island continent collision. A key feature of all major detachment systems is the presence of a significant metamorphic contrast, 3. Detachment faulting with low-grade rocks in the hanging wall and higher- grade exhumed rocks in the footwall. Across Clew Low-angle extensional faulting synchronous with Bay, a clear difference in grade has long been recog- thrust faulting has been recognized as a common nized between the low-grade South Mayo Trough and tectonic process during and immediately following the higher-grade Dalradian in North Mayo. Much of orogeny. In the High Himalaya, high-grade metamor- the contact is covered by Silurian and Carboniferous phic rocks were exhumed by orogen-perpendicular sedimentary rocks (Fig. 1), except on the island of extension beneath the Zanskar and South Tibetan Achill Beg, located in NW Clew Bay (Figs. 1 and 3), Detachments, synchronous with thrusting (e.g., Her- where a major high-angle fault, the Achill Beg Fault, 96 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 97 places accretionary complex sedimentary rocks of the of the Grampian , i.e., the top to the Clew Bay Complex under the youngest Dalradian north tectonic transport during D1 and D2 at least in rocks seen in the North Mayo inlier—the upper Argyll South Achill Island (Fig. 5C). Group (Chew, 2003). Although the Fault has been The northern boundary of the Achill Beg reactivated since the Carboniferous (Chew et al., in Zone, which we tie to D3 and D4 deformation during press), both the Clew Bay Complex and the Dalradian cooling, is not sharply defined, but instead, D3 struc- on Achill Beg show a dominant northerly dipping tures become gradually less intense to the north. Two structural fabric (45–60jN; Fig. 4), as does the Clew discrete elements have been recognized in the D3 Bay Complex on the southern edge of Clew Bay, west deformation episode in North Mayo and Achill Island: of Westport (Fig. 1). asymmetrical buckle folds with axial planes anticlock- Gravity (Ryan et al., 1983) and deep-penetrating wise to the S2 (Figs. 5A and C) and exten- reflection seismic (Klemperer et al., 1991) studies sional cleavages (Platt and Vissers, 1980) show that the Clew Bay Complex is associated with that cut the S2 foliation in a clockwise sense (Chew et a north-dipping fabric that can be traced to the Moho, al., in press). The main S2 foliation is generally defined implying that post-orogenic extension effected the by muscovite, chlorite and equigranular quartz grains whole of the continental crust. At outcrop, the with interlobate grain boundaries. The later extensional north-dipping fabric is observed to be a penetrative crenulation cleavages make an angle of about 29j with shear fabric within metamorphosed clastic sedimenta- the S2 foliation in a clockwise direction (Fig. 5B) and ry rocks. Furthermore, to the north, Dalradian meta- consistently give a dextral sense of shear. Identical in morphic rocks of North Mayo (now in the hanging style to the normal-slip of Dennis and wall of this structure) preserve an earlier south-dip- Secor (1990), they are believed to be broadly contem- ping fabric that is not observed south of Clew Bay that poraneous with the F3 asymmetric buckle folds based is attributed to the Grampian event. These fabrics are on the absence of apparent overprinting relationships. consistent with structural arguments about the initial 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). 98 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 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 the geometry of modern accretionary complexes, the of this orientation. forearc basin might be expected to overlie the accre- Near Westport, Graham et al. (1989) mapped the tionary wedge, whose tectonic fabric would dominant- accretionary complex rocks of the Clew Bay Complex ly parallel the dip of the subduction zone, i.e., dip as dipping north and structurally overlying the rocks of towards the arc volcanic front, not away from it, as it the South Mayo Trough. This is unusual because given does in the modern setting. The current steep north- P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 99 dipping fabric of the Clew Bay Complex implies shear South Mayo Trough. Likely, the rotation is linked to and accretion in a north-dipping subduction zone, transpressional deformation, whose age is not con- inconsistent with the subduction polarity inferred from strained by this study. Sedimentary structures and the South Mayo Trough and with the relative location biostratigraphy along the northern arm of the South of the arc volcanic rocks of the Lough Nafooey and Mayo Trough demonstrate that this is not overturned, Tourmakeady Volcanic Groups. The modern dip of the although the strata may have been more north-dipping tectonic fabric within the Clew Bay Complex and its when originally deposited. The contact between the overlying relationship with the South Mayo Trough strongly rotated Clew Bay region and the South Mayo can, however, be understood if it is recognized that the Trough is now covered by Silurian sequences exposed entire region has been tilted and overturned following west of Westport (Fig. 1). Their location in this belt exhumation, as a result of the D3 dextral shearing may reflect the presence of a major fault in that area, described above. decoupling the South Mayo Trough and the Clew Bay Fig. 7 shows the proposed model in which the Complex. region around northern Clew Bay has been rotated, Hence, prior to late Grampian (D3) dextral shearing, while not apparently affecting the main part of the 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 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. 100 P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 nappes would have originally been flat-lying along the zone in the region is more apparent. The dominant, northern margin of Clew Bay (e.g., Sanderson et al., high-strain deformation event in North Mayo (D1) 1980; Chew, 2003; Fig. 4). Consequently, we infer that is associated with bedding-parallel shear zones with the original dip of the Achill Beg Fault and the tectonic a NNW-directed transport direction (Chew, 2001, fabric of the Clew Bay Complex were not steeply 2003). In contrast, the D2 event that caused much towards the north but, instead, towards the south. In of the large-scale folding observed has a clear this reconstructed orientation, the Clew Bay Complex southward vergence. NNW-directed transport in along the southern edge of Clay Bay dips southward the Dalradian and Clew Bay Complex is consistent under the South Mayo Trough, consistent with accre- with a collision between a passive margin and a tion in a south-dipping subduction zone. Also, on south-dipping subduction zone (Dewey and Ryan, Achill Beg, the low-grade Clew Bay Complex (Fig. 1990). That the upper Argyll Group meta-sedimen- 6C) would have originally overlain the high-grade tary rocks are juxtaposed against the Clew Bay Dalradian (Fig. 6B), thus implying late extension not Complex also make geodynamic sense in that these shortening across the Achill Beg Fault. Brittle, late- are recognized as the youngest units within the stage shear-sense indicators along the fault (Fig. 6A) Dalradian stratigraphy in western Ireland. Conse- that indicate a thrusting motion in the present sense quently, it is the Clew Bay Complex and the upper (i.e., top to the SSE; Fig. 5A) imply a top to the south– Argyll Group meta-sedimentary rocks of the North southeast extension after restoration. Mayo Dalradian that would have been immediately Once the effects of later deformation have been overthrust by the of a colliding removed, the original geometry of the collision arc. Having been overthrust, deformed and meta-

Fig. 7. Diagram showing how rotation about a horizontal axis, driven by transpressional 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) 91–113 101 morphosed, the upper Argyll Group was still at the with the High Himalaya exhumation along a south- top of the Dalradian structural stack when a de- ward-verging Main Central Thrust and a northward tachment fault, exploiting a location close to, or extending South Tibetan Detachment, both with along the original overthrust, brought the upper north-dipping geometries. Argyll Group back to the surface. We propose that fragments of a detachment surface are mapped close to and along the uncon- 3.2. Detachment faulting in Connemara formity between the Dalradian and the overlying Silurian sedimentary rocks, along the northern edge The nature of detachment faulting in northern of Connemara (Fig. 8). There is a preferential Connemara has been a source of controversy. Well- development of serpentinite, ultramafic and mafic ings (1998) proposed that extension along an east– blocks along, and just south of this contact but not west trending, north-dipping tectonic discontinuity within the main outcrop of the Connemara Dalra- through northern Connemara, known as the dian. These mafic and ultramafic rocks, which Renvyle–Bofin Slide, was responsible for exhuma- include coarse-grained gabbros (Fig. 5D), occasion- tion of the main Dalradian of the Twelve Bens, ally cut by dykes of ophitic meta-dolerite, form whereas Williams and Rice (1989) favored a role coherent, relatively undeformed pods 10–200 m for low-angle extensional faults south of Connemara. across, surrounded by strongly sheared Dalradian Although the Renvyle–Bofin Slide is a major ex- metasedimentary rocks. tensional structure, it is not considered to be the The coarse-grained character of most of the gab- master structure because no major metamorphic bro blocks (grains >1 cm across) is inconsistent with break is observed across it. Because of the strike- an alternative origin in which the small mafic bodies slip emplacement of Connemara south of the South were intruded into the sedimentary rocks and then Mayo Trough, it is possible that the original root deformed. The coarse grain size of some of the zone of the detachments was not preserved within gabbros indicates original slow cooling within a this block or has been buried by later sedimentation. large intrusion, followed by tectonic dismembering. By comparison with North Mayo, a detachment In this respect, they contrast with the clearly might be expected to separate high-grade rocks in intruded, kilometer-scale Cashel–Lough Wheelan Connemara from a lower grade arc towards the and Dawros–Currywongaun–Doughruagh gabbro south, similar to the model proposed Williams and bodies (Leake, 1986; Fig. 1), although some smaller Rice (1989). This southern repeat of the arc is not intrusions, complete with chilled margins, related to well preserved, but is likely represented by the these larger bodies are also noted. Furthermore, the rhyolites of the Delaney Dome Formation that cor- fact that such mafic and ultramafic bodies are only relate with the arc volcanic rocks of the Tourma- found either close to the unconformable northern keady Formation (Dewey and Mange, 1999; Draut contact of the Connemara Dalradian or close to the and Clift, 2002). However, the hypothesis that de- Achill Beg Fault is difficult to reconcile with an tachment faults in Connemara should lie along its original intrusive origin. The serpentinized perido- southern edge assumes that the Connemara block has tites, moreover, have a tectonized internal texture not been rotated within the strike-slip zone since consistent with a mantle origin rather than as an exhumation. Paleomagnetic and GPS studies of ac- ultramafic intrusive. Because the bulk of the Dalra- tive strike-slip margins clearly demonstrate that ma- dian outcrop comprises metasedimentary rocks, the jor block rotations are to be expected in such areas occurrence of small mafic and ultramafic rocks only (e.g., Transverse Ranges, California, Nicholson et al., at the structural top suggests that these have been 1994; Aegean Sea, Clarke et al., 1998). Indeed, the emplaced tectonically. top-to-the-north geometry of the Renvyle–Bofin We suggest that the unconformity represents an Slide and the southward motion on the Mannin exhumed and partly eroded detachment surface. In Thrust are consistent with a major north-dipping this scenario, the presence of lower crustal or mantle detachment along the northern edge of the Conne- rocks can be explained as blocks entrained along a mara Dalradian. This scenario also compares closely crustal scale structure that brings Dalradian rocks to 102 P.D. Clift et al. / Tectonophysics 384 (2004) 91–113

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- plexes in the US Basin and Range and those on ing wall was completely removed and the exposed slow-spreading mid-ocean ridges expose deeply bur- metamorphic rocks were covered by the fluviatile ied rocks at the surface where they may be eroded and Silurian Lough Mask Formation. This relationship transgressed by sediments. might be consistent with a ‘‘rolling hinge’’ type of The marine Kilbride and Lettergesh Formations, detachment (Buck, 1988). Metamorphic core com- which transgress over the Dalradian, are dated as P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 103

Upper Llandovery (435–420 Ma) implying a signif- Taiwan, alluvial fans derive material from the ex- icant hiatus between the time of most rapid cooling humed metamorphic rocks of the Central Ranges, (470–455 Ma; Friedrich et al., 1999b)andfinal where sediment eroded from the collisional orogen transgression. However, this time gap may be less is washed along-strike from the alluvial Ilan Plain, than it initially appears. Although the rocks cooled through the Huapingshu Canyon into the post-oro- rapidly through the biotite Ar closure temperature genic collapse basin of the Okinawa Trough (e.g., Yu (f 300 jC; Lovera et al., 1989), exhumation at and Hong, 1993). A depositional model for the South shallower levels after 455 Ma below the closure Mayo Trough featuring two canyons, one in the temperature could have continued at slower rate. This center and one in the east of the southern margin of would reduce the amount of time that the detachment the basin (Archer, 1977), is consistent with this would have been exposed at the surface and subject to modern analogue. Pleistocene sedimentation rates in erosion prior to Silurian transgression. 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 4. Sedimentary response to collapse Rosroe Formation deposited at 464–467 Ma, a long-term average rate of >45 cm/k.y., also a very Orogenic exhumation in western Ireland was ac- high rate of accumulation. companiedbyerosion,resultinginrapid,coarse- Lundberg and Dorsey (1990) demonstrated that grained sedimentation preserved as the Rosroe, rapid Quaternary uplift of the Coastal Range, inter- Maumtrasna and Derrylea Formations within the preted as a fragment of the accreted Luzon Arc South Mayo Trough (Fig. 2). On the south side of (e.g., Dorsey, 1992), has resulted in proximal sed- the South Mayo Trough, the Rosroe Formation is imentation in the Longitudinal Valley overlying the dated as Early Llanvirn (lower artus graptolite zone, accreted arc and forearc. The Longitudinal Valley is 467–464 Ma according to the time scale of Tucker thus comparable with the syn-collisional South and McKerrow, 1995). It comprises very coarse con- Mayo Trough. Although erosion rates in the Coastal glomerates and sandstones, including abundant gran- Range are high, those in the higher and more ite and volcanic clasts (Archer, 1977; Fig. 9B–D). extensive Central Range of the Taiwan Orogen are The Rosroe Formation was interpreted originally to greater and contribute the bulk of the sediment represent the deposits of large submarine fan deltas reaching the Okinawa Trough (Lundberg and Dor- eroding a volcanic and plutonic arc source south of sey, 1990). This parallels the situation in South the modern outcrop region (Fig. 1; Archer, 1977, Mayo where a rapid influx of material eroded from 1984). The Rosroe Formation has been correlated the exhuming Dalradian metamorphic rocks is found with the Derrylea Formation on the northern limb of throughout the basin, recognized on the basis of the the South Mayo Trough (Dewey, 1963; Dewey and high-grade metamorphic minerals (Dewey and Ryan, 1990; Fig. 2), although Williams (2002) con- Mange, 1999), as well as the Pb isotopic composi- sidered the Derrylea Formation to have been a sepa- tion of the detrital K-feldspar grains (Clift et al., rate system, deposited in a different subbasin. Up- 2003). section, the depositional environment of the South Mayo Trough shallows, culminating in the middle 4.1. Source of sediment Llanvirn Mweelrea Formation, which comprises a >3- km-thick package of west-flowing fluvial sandstones The provenance of sediment during orogenic col- derived from a rapidly eroding Dalradian source lapse in the Irish Caledonides has been a source of (Pudsey, 1984). controversy. Apart from the apparent arc source to the The sedimentary system in western Ireland related Rosroe Formation, Dewey and Shackleton (1984) to Dalradian exhumation bears many similarities in recognized the influx of ophiolitic material in the terms of facies and relationships to related tectonic upper parts of the South Mayo Trough, which they units with the sedimentation associated with exhuma- related to the progressive unroofing of the forearc tion in the Taiwan arc–continent collision zone. In basement, now preserved as the Deer Park Complex, 104 P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 a mafic/ultramafic part of the Clew Bay Complex ophiolitic source, suggests rapid exhumation of the (Fig. 1). Chemical data from the sediments of the deformed Laurentian passive margin within a meta- South Mayo Trough (Wrafter and Graham, 1989) morphic complex. show a progressive increase in Cr, Ni and Mg up- Clift et al. (2002) examined the trace element section into the Derrylea Formation, although these composition of granite boulders and the Pb isotopic values drop in the middle of this unit. Dewey and character of K-feldspar grains in the Rosroe Forma- Mange (1999) correlated this drop with the first tion and inferred a Laurentian (i.e., Dalradian, rather appearance of high-grade metamorphic minerals (gar- than an arc) source, in accordance with the model of net, hornblende, staurolite and sillimanite) in the Dewey and Mange (1999). These boulders are mainly middle of the Rosroe and Derrylea Formations, which of high-level quartz-rich, minimum melt type granites they related to an initial influx of detritus from a that were, presumably, part of the cover to the newly exposed Dalradian source located to the north, collapsing metamorphic pile. Paleo-current analysis i.e., North Mayo. The sudden arrival of staurolite in suggested that the Dalradian source was located the upper Derrylea Formation, following erosion of an 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) 91–113 105

At outcrop, the provenance of the Rosroe and during exhumation is not uniform, but shows a associated formations is apparent. Along the southern dominant influx from the North Mayo Dalradian and edge of Killary Harbour, the boulder conglomerates equivalent sources to the NE (e.g., Ox Mountains, are dominated by granite clasts with only minor lava southern Donegal), with variable, subsidiary input fragments (Fig. 9C). In contrast, at Lough Nafooey, from the Lough Nafooey Arc. the proportion of lava, both fresh and altered, is much higher (Fig. 9B). Both types of deposit contrast with 4.2. Sediment transport the younger (Llanvirn to Lower Llandovery; 464–443 Ma) Derryveeny Conglomerate (Figs. 2 and 9D), The patterns of sediment dispersal during exhuma- which contains abundant schist and foliated granite tion were summarized by Dewey and Mange (1999) clasts and granites similar to those seen in the Rosroe and Clift et al. (2002) (Fig. 10), combining paleo- Formation. The provenance of sediments deposited current 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 distal, deeper water environment than the dominant southern edge of the basin, a more axial flow towards fanglomerate facies observed further east and north the west is seen in the central parts of the trough. (Fig. 9B and C). Williams (2002) argued that the presence of finer- grained rocks in the Derrylea Formation, north of the boulder conglomerates of the Rosroe Formation, pre- 5. Magmatic response to collapse cluded a sediment source to the north of the trough. However, if the sediment flux is dominantly axial, Orogenic collapse and the establishment of an lateral changes in grain size could simply reflect active margin following the Grampian Orogeny was differences in tectonic subsidence rates focusing accompanied by continued volcanism preserved as coarser sediments into the depocenters. The sedimen- tuffs within the Rosroe and overlying Mweelrea tary facies support the idea of basin deepening and Formation (Fig. 2). Like the preceding Tourmakeady fining towards the SW. Fig. 9A shows the sands and Volcanic Group, which was erupted prior to and shales of the Rosroe Formation close to Rosroe during peak metamorphism, these are high-silica tuffs, village (Fig. 1). These channel sands imply a more 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) 91–113 107

Clift (2001) demonstrated that, whereas the Rosroe 6. Discussion and Mweelrea tuffs show similar high light rare earth element (LREE) enrichment compared with the Tour- The processes and effects of extensional collapse in makeady Volcanic Group, they have a wider range of the Irish Caledonides after the Grampian Orogeny relative enrichment in the high field strength elements bear similarities to other documented collisional belts (HFSE; Fig. 11). Williams (2002) suggested that the (Dewey, 1988), especially the Taiwan–China colli- tuffs of the Rosroe Formation were erupted in an sional orogen (Teng, 1996; Lee et al., 1997). Taiwan, oceanic island arc, but a comparison between siliceous like the Grampian Orogen, is an oceanic arc–conti- volcanic glass from a typical Neogene oceanic arc, nental collisional environment involving subduction such as Tonga, and the Rosroe Formation shows that polarity reversal and exhumation of a metamorphosed the Rosroe Formation is much more enriched in REEs continental margin sequence through the action of and HFSEs. In contrast, there is a significant overlap major detachment faulting (Teng, 1996; Clift et al., between the Lough Nafooey Group, erupted before 2003). In Taiwan, the new continental Ryukyu island arc–continent collision, and the Tonga Arc. Draut and arc is built on the remnants of the oceanic Luzon Arc. Clift (2001) linked this change from the basaltic, In Connemara, the arc volcanic front of the succeed- LREE-depleted Lough Nafooey to high-silica, ing continental arc is not exposed, but is known along LREE-enriched Rosroe chemistry with the arc–con- strike in Newfoundland as the Notre Dame Arc. The tinent collision. lack of the arc in Ireland is presumed to reflect The degree of continental crust recycling through tectonic excision during margin-parallel strike-slip the collisional arc is best demonstrated through the faulting. The Southern Upland accretionary complex application of isotope chemical techniques, which are exposed in Scotland and Northern Ireland provided not affected by fractional crystallization, as are the evidence that a north-dipping subduction zone was REEs and HFSEs. Neodymium isotope data show developed along this part of the Laurentian margin that magmatism during orogenic collapse in South (van Staal et al., 1998; Leggett et al., 1983). Mayo involved significant remelting of the continen- Several tectonic processes may drive extensional tal crust. Initial eNd values of À 8.4 to À 10.6 collapse following the metamorphic peak. Dewey et reported by Draut and Clift (2001) from the Tourma- al. (1993) invoked lower crustal loss to explain rapid keady Volcanic Formation, Rosroe Formation and surface uplift and post-orogenic extension along ma- Mweelrea Formation (472–466 Ma) require signifi- jor detachment faults following eclogitization in the cant mixing of radiogenic continental crust (Dalra- Norwegian Caledonides. Subsequently, Krabbendam dian eNd = À 12 to À 22; O’Nions et al., 1983; Frost and Dewey (1999) invoked exhumation of the Nor- and O’Nions, 1985; Jagger, 1985) with a typical wegian eclogites in a sinistral transtensional pull-apart oceanic end member with eNd values of + 8 to setting. We consider both of these models to be highly + 10. The wide range of HFSE and LREE enrich- unlikely for exhumation in western Ireland. In any ments observed in the Rosroe and Mweelrea Forma- case, because no arc lower crustal rocks are exposed tion tuffs points to a very heterogeneous source in western Ireland, any such hypothesis would be compared with even the syn-collisional Tourmakeady speculative. Geophysical modeling of different lower Volcanic Group. The scatter may reflect involvement crustal lithologies and thermal regimes raises the of more enriched mantle sources contrasted to that possibility of lower crustal loss and surface uplift feeding the Tourmakeady Volcanic Group. Extension- even without the formation of eclogite in arc settings al collapse of the orogen coupled with subduction (Jull and Kelemen, 2001). However, Clift et al. (2003) polarity reversal would change the flow of mantle calculated that, given typical ocean arc crustal thick- under the orogen and could draw in less depleted nesses (20–25 km), loss of an ultramafic lower crust mantle material under the volcanic arc. Alternatively, that had not been eclogitized could be expected to gravitational loss of a dense arc lower crust during drive < 10% of the topography in a collisional range. collision could also provide space for upwelling and Thus, other tectonic mechanisms may be required to influx of fresh mantle material under the volcanic explain the degree of exhumation observed in western roots of the arc. Ireland (>25 km; Friedrich et al., 1999b). 108 P.D. Clift et al. / Tectonophysics 384 (2004) 91–113

Orogenic extension is caused by the topographic Mayo are clearly not the source of the sediment but load, facilitated by the low viscosity of the orogenic represent deeper levels of the source that was being lower crust. For the collision of an oceanic arc into a eroded at the time of deposition of the Rosroe Forma- rifted continental margin, once the compressive forces tion. Geodynamic models of mass motion through that formed the mountains are released from the thrust orogenic belts (e.g., Willettetal.,1993;Royden, belt by renewed subduction following polarity rever- 1996) suggest that erosion can result in large fluxes sal, there is a tendency for the structural stack to of material from the subducting/colliding plate (in this collapse and extend under its own weight. The trench case Laurentia), without erosional unroofing of any thus forms a free space into which the orogen can deeper structural level at any one place through time. collapse, especially if aided by subduction rollback Thus, the cooling ages of detrital minerals within the (Dewey, 1980), triggering extension of the deformed eroded sediment series would become younger up- arc and passive margin sequences (Fig. 12; Teng, section. The cooling age preserved at the surface in the 1996). Detachment faulting is a common feature in source terrain after the end of rapid post-orogenic the Dalradian of the British Isles, where exposure has sedimentation will record the time when that package permitted its recognition. Farther northeast of our of igneous or metamorphic passed through the focus area, the Dalradian rocks in both the Ox closure temperature of the minerals analyzed. This age Mountains Inlier and southern Donegal are exhumed must postdate the start of sedimentation. Depending on along reactivated shear zones (the North Ox Moun- whether the source cooling is driven by erosion or tains Slide, Flowerdew, 1998; the Lough Derg Slide, tectonic exhumation, the cooling age of that source Alsop, 1991; Alsop and Hutton, 1993). In northwest may also postdate the end of sedimentation in a syn- Scotland, Powell and Glendinning (1990) documented orogenic basin, especially if this becomes filled, so that the reactivation of thrusts as extensional faults, similar additional eroded sediment will not accumulate but, to our proposed motion on the Achill Beg Fault. instead, is transported oceanward and/or along strike. 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 7. Conclusions pervasive in parts of Clew Bay, but does not, by itself, explain the key juxtaposition of high- and low-grade Orogenic collapse of the Grampian Orogeny in units that requires greater relative exhumation in western Ireland is inferred to have been rapid, with North Mayo and Achill Island, compared with the radiometric data suggesting 10–15 km of exhuma- South Mayo Trough. Most of the cooling in Conne- tion achieved between 470 and 460 Ma (Friedrich et mara and North Mayo is, moreover, too old to have al., 1999b; Power et al., 2001). We propose that been driven by the younger strike-slip faulting. rapid exhumation resulted from detachment faulting. Sedimentation during exhumation was dominated A south-dipping, now overturned, north-dipping by erosion of the metamorphosed continental margin Achill Beg Fault is considered to represent a key series (Dalradian Supergroup). Although the cooling structure unroofing the North Mayo Dalradian from ages (460–450 Ma; Flowerdew et al., 2000) of the Ox under the low-grade rocks of the Clew Bay Complex Mountain Dalradian are slightly younger than the age and South Mayo Trough. In Connemara, the hanging of Rosroe sedimentation (467–464 Ma), this discrep- wall of the detachment is not preserved, but the ancy may reflect moderate along-strike variability in detachment surface corresponds, at least locally, to the timing and rate of exhumation. This is, however, the Silurian unconformity along the north side of the not required because the rocks now exposed in North 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) 91–113 109 110 P.D. Clift et al. / Tectonophysics 384 (2004) 91–113 entrained along the detachment fault are now pre- References served along the unconformity. 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