Studies in the Irish Caledonides and Taiwan
View of the Connemara Dalradian, a fragment of the Laurentian passive margin
metamorphosed by arc collision and forming the mid crust to the collisional arc sequence.
The collision of the Luzon Arc with southern China
represents the best example of arc-continent collision in the modern oceans,
and compares closely with the Early Ordovician (480-470 Ma) accretion of
the Lough Nafooey Arc of Connemara, Ireland, to the passive margin of Laurentia
(North America). Together with colleagues Amy Draut and Hans Schouten I have been developing
a general model for steady-state arc-continent collision in which arc crust
is progressively added to a passive margin during a process of compression,
metamorphism and magmatism lasting 3–10 million years at any one location
on the margin. This tectonic process is likely the principle origin
of continental convergent margins and the dominent process for building
the continental crust, at least during Phanerozoic times (<600 Ma).
Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion is more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events.
Plot from Draut et al. (EPSL, 2002) showing how the collisional arc volcanism is more La/Sm enriched than average continental crust and the local Dalradian country rock. Mixed with depleted pre-collisional oceanic arc volcanism this can create a crustal block with a bulk composition close to that of the continents.
During arc-continent collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2–3 million years) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.
Schematic model showing how the new Ryukyu subduction system is able to propagate
along the Chinese margin due to lithospheric tearing along the continent-ocean transition of the South China Sea (Clift et al., 2003).
Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion is more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events.
Plot from Draut et al. (EPSL, 2002) showing how the collisional arc volcanism is more La/Sm enriched than average continental crust and the local Dalradian country rock. Mixed with depleted pre-collisional oceanic arc volcanism this can create a crustal block with a bulk composition close to that of the continents.
During arc-continent collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2–3 million years) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.
Schematic model showing how the new Ryukyu subduction system is able to propagate
along the Chinese margin due to lithospheric tearing along the continent-ocean transition of the South China Sea (Clift et al., 2003).
Related References
Draut, A.E., and Clift, P.D. 2002. The origin and significance of the Delaney Dome Formation, Connemara, Ireland. Journal of the Geological Society, London, 159, 95–103.
Draut, A.E., Clift, P.D., Hannigan, R., Layne, G.D. and Shimizu, N., 2002. A model for continental crust genesis by arc accretion: rare earth element evidence from the Irish Caledonides. Earth and Planetary Science Letters, in press.
Clift, P.D., Schouten, H., and Draut, A.E., A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides. In Larter, R. and Leat, P., (Editors), Intra-Oceanic Subduction Systems; Tectonic and Magmatic Processes, Geological Society of London, special publication, in press.
Clift, P.D., Draut, A.E., Layne, G., and Blusztajn, J., Trace element and Pb isotopic constraints on the provenance of the Rosroe and Derrylea Formations, South Mayo, Ireland. Transactions of the Royal Society of Edinburgh, Earth Science, 93 (2), in press.
Draut, A.E., and Clift, P.D., 2001. Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland. Geology, 29, 543–546.
Clift, P.D., and Ryan, P.D., 1994. Geochemical evolution of an Ordovician Island Arc, South Mayo, Ireland. Journal of the Geological Society, London, 151, 329–342.
Draut, A.E., and Clift, P.D. 2002. The origin and significance of the Delaney Dome Formation, Connemara, Ireland. Journal of the Geological Society, London, 159, 95–103.
Draut, A.E., Clift, P.D., Hannigan, R., Layne, G.D. and Shimizu, N., 2002. A model for continental crust genesis by arc accretion: rare earth element evidence from the Irish Caledonides. Earth and Planetary Science Letters, in press.
Clift, P.D., Schouten, H., and Draut, A.E., A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides. In Larter, R. and Leat, P., (Editors), Intra-Oceanic Subduction Systems; Tectonic and Magmatic Processes, Geological Society of London, special publication, in press.
Clift, P.D., Draut, A.E., Layne, G., and Blusztajn, J., Trace element and Pb isotopic constraints on the provenance of the Rosroe and Derrylea Formations, South Mayo, Ireland. Transactions of the Royal Society of Edinburgh, Earth Science, 93 (2), in press.
Draut, A.E., and Clift, P.D., 2001. Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland. Geology, 29, 543–546.
Clift, P.D., and Ryan, P.D., 1994. Geochemical evolution of an Ordovician Island Arc, South Mayo, Ireland. Journal of the Geological Society, London, 151, 329–342.
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Page created by Peter Clift
Last Updated October 2002
Last Updated October 2002