Timing Of Obduction Research Articles

On the age of the Ballantrae Complex, SW Scotland

The Ballantrae Complex, SW Scotland, is an ophiolitic assemblage of mostly Early and early Middle Ordovician age (Tremadoc–Arenig in terms of the British Ordovician Series). Its varied components were generated and assembled in the Iapetus Ocean, then obducted on to the Laurentian continental margin by the earliest Llanvirn. The timing of obduction is constrained by biostratigraphic and radiometric data. It was most probably a polyphase process initiated at about the beginning of the Arenig, at around 478 Ma. However, parts of the Complex are significantly younger, with some recent evidence taken to suggest an earliest Llanvirn age of about 464 Ma for the emplacement of some of the volcanic and pelagic sedimentary rocks. The oldest strata in the succession that now unconformably overlies the Ballantrae Complex were deposited at about 463 Ma. Hence there may have been as little as one million years available for the final stages of the Complex's tectonic assembly, obduction, uplift, erosion and downfaulting. Obduction of the Complex has been invoked as a factor in the initiation of the Grampian Orogeny and, whilst there is a broad correlation in timing, the detail from Ballantrae militates against a causal relationship.

U–Pb zircon constraints on obduction initiation of the Unst Ophiolite: an oceanic core complex in the Scottish Caledonides?

The Unst Ophiolite is the best exposed of a chain of early Ordovician ophiolites in the Scottish Caledonides and is widely regarded as having formed in a supra-subduction zone setting within the Iapetus Ocean. Reinterpretation of sheeted dykes suggests that it formed as an oceanic core complex, presumably during subduction roll-back immediately prior to obduction onto the Laurentian margin. A new U–Pb zircon age of 484 ± 4 Ma for development of the metamorphic sole places a lower limit on the timing of obduction, which was subsequently followed by regional-scale crustal thickening and Barrovian metamorphism during the Grampian orogenic event. Supplementary material: Analytical methods, CL images of representative zircon grains and an LA-ICP-MS U–Th–Pb zircon data table are available at www.geolsoc.org.uk/SUP18814 .

Tracking the Oman Ophiolite to the surface — New fission track and (U–Th)/He data from the Aswad and Khor Fakkan Blocks, United Arab Emirates

The Oman Ophiolite in the United Arab Emirates (UAE) was formed in a supra-subduction zone environment at about 95Ma and was almost immediately obducted onto the eastern margin of Arabia. The timing of obduction is well constrained, but the post-obduction tectonic, uplift and exhumation history of the ophiolite and associated rocks are less well understood. We present twenty-one new fission track and (U–Th)/He analyses of apatite and zircon from the Hajar Mountains. The data show that the Oman Ophiolite had a complex exhumation history to present exposure levels in the Khor Fakkan and Aswad Blocks, resulting from at least three distinct exhumation events: 1) initial ophiolite obduction between ca. 93 and 83Ma is characterised by tectonic exhumation and rapid cooling, as revealed by zircon (U–Th)/He and apatite fission-track data, but it is not associated with major erosional exhumation; 2) data from the lower part of the ophiolite and the metamorphic sole document a second exhumation event at ca. 45–35Ma, interpreted to represent an early phase of the Zagros orogeny that led to reactivation of pre-existing structures and the differential exhumation of the Khor Fakkan Block along the Wadi Ham Shear Zone. This event led to significant erosional exhumation and deposition of a thick sedimentary succession in the Ras Al Khaimah foreland basin; and 3) Neogene exhumation is recorded by ca. 20–15Ma apatite (U–Th)/He data and a single apatite fission track date from the lowermost part of the metamorphic sole. This event can be linked to the main phase of the Zagros orogeny, which is manifested in large fans with ophiolite-derived debris (Barzaman Formation conglomerates). During this period, the metamorphic sole of the Masafi window stayed at temperatures in excess of ca. 120°C, corresponding to ca. 4km of overburden, only later to be eroded to present day levels.

Ophiolite obduction in the Quebec Appalachians, Canada — 40Ar/39Ar age constraints and evidence for syn-tectonic erosion and sedimentation1Geological Survey of Canada Contribution 20100430.2GEOTOP (Centre de recherche en géochimie et géodynamique) Contribution 2011-0002.3This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

Detailed field work conducted in the Dunnage zone of the Quebec Appalachians, is herein combined with 40Ar/39Ar dating on a series of ophiolitic massifs, crosscutting granites, and associated metamorphic rocks occurring along the Baie Verte–Brompton line, the Taconian suture between Laurentia and Lower Paleozoic peri-Laurentian oceanic terranes. Studied massifs are the Lac-Brompton ophiolite and the Rivière-des-Plante Ultramafic Complex in southern Quebec, and the Nadeau Ophiolitic Mélange in the Gaspé Peninsula. Our work suggests that these massifs form remnants of eroded ophiolitic nappes, which are unconformably overlain by the Saint-Daniel and Rivière-Port-Daniel mélanges, and correlate with the Thetford-Mines and Mont-Albert ophiolitic complexes. Our 40Ar/39Ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen. The timing of obduction and mélange formation varies according to the irregular geometry of the Early Paleozoic Laurentian margin, with earlier collision occurring along, or at the margins of promontories. Obduction was initiated with the formation of infraophiolitic metamorphic soles between ca. 479 and 472 Ma in southern Quebec and the Nadeau Ophiolitic Mélange, and possibly as late as ca. 470–466 Ma for the Mont-Albert Complex. These sole rocks were later exhumed and translated onto the Laurentian margin with the overlying ophiolites between 475 and 460–457 Ma. The uplifting and erosion of the orogenic wedge during the waning stages of obduction, has resulted in the sedimentation of olistostromal mélanges and onlapping flysch units above the ophiolitic nappes, as well as foredeep flysch successions during the latest Arenig(?) to earliest Caradoc.

U–Pb zircon ages from the Spontang Ophiolite, Ladakh Himalaya

The Spontang Ophiolite in the Ladakh Himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of India during the late Cretaceous. Precise isotopic ages of Himalayan ophiolites are presently unavailable. U–Pb dating of zircons from a dioritic segregation in the high-level gabbros of the Spontang ophiolite shows that the complex formed at 177±1 Ma (mid-Jurassic) and we suggest that it represents a fragment of Neo-Tethyan oceanic crust. This Jurassic age contrasts with late Cretaceous ages (91–95 Ma) previously reported for the formation of the Troodos and Oman ophiolite complexes further west along the Neo-Tethyan suture zone and also predates the early to mid-Cretaceous Masirah ophiolite (c. 150 Ma) and its alkaline volcanic seamounts (115–125 Ma). An andesitic rock from an arc sequence overlying the Spontang ophiolite was also dated using as 88±5 Ma. This Late Cretaceous age constrains the minimum age of initiation of subduction beneath the Spontang Ophiolite. The complex was emplaced during the Turonian–Maastrictian, some 80 Ma after its formation when the Indian continental margin collided with a Cretaceous arc about 25 Ma before the India–Asia collision. The lack of a sub-ophiolite metamorphic sheet beneath the Spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction.

The age of the Roberts Arm Group, north-central Newfoundland

Felsites from the Roberts Arm Group have yielded a Rb–Sr isochron age of 447 ± 7 Ma, with an initial 87Sr/86Sr of 0.7063, and a genetically related granite pluton has yielded an isochron age of 464 ± 13 Ma, with an initial ratio of 0.7064. The Mansfield Cove oceanic plagiogranite complex, which is in fault contact with the Roberts Arm Group, has yielded a minimum zircon 207Pb/206Pb age of 594 ± 10 Ma. This plagiogranite, and basaltic rocks of the Hall Hill complex which partly surround it, thus provide a basement of suitable age upon which the Roberts Arm Group may have been deposited. The age of this basement corresponds to that of the opening of the lapetus Ocean.The stratigraphic age of the Roberts Arm Group, indicated by these ages, is Middle or possibly Late Ordovician. The volcanics were deposited either before or slightly after the Caradocian black shales of the Notre Dame Bay region. In the former case, Ordovician volcanism around western Notre Dame Bay ceased at about the time of obduction of the allochthons of western Newfoundland. In the latter, volcanism of the Roberts Arm Group marks a north to westward shift of volcanism within the central mobile belt of Newfoundland that occurred in late Middle or early Late Ordovician time. If the Buchans Group is of Silurian age and correlative with the Roberts Arm Group then this lithostratigraphic unit is probably diachronous, younging to the south.