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Pearce J.A.,University of Cardiff | Hastie A.R.,University of Cardiff | Leat P.T.,British Antarctic Survey | Dalziel I.W.,University of Texas at Austin | And 5 more authors.
Global and Planetary Change | Year: 2015

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c. 20. m.y. (early Oligocene to middle-late Miocene) before slab retreat and subsequent 'resurrection' as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf-Nd-Sr-Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic-acidic, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, middle-late Miocene period produced basic-acidic, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early-middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich Island Arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge-trench collisions are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17-15. Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (<. 11. Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific-South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because the northward flow of the South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA. © 2014. Source


PHILLIPS B.A.,University of Cardiff | KERR A.C.,University of Cardiff | BEVINS R.,National Museum Cardiff
Geological Magazine | Year: 2015

The Fishguard Volcanic Group represents an excellently preserved example of a volcanic sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis and proposed tectonic setting for the Llanvirn (467–458 Ma) Fishguard Volcanic Group, South Wales, UK. New major and trace element geochemical data and petrographic observations are used to re-evaluate the magma chamber processes, mantle melting and source region. The new data reveal that the Fishguard Volcanic Group represents a closely related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel lherzolite source which had been modified by subduction components. The rocks of the Fishguard Volcanic Group are co-genetic and the felsic members are related to the more primitive basalts mainly by low-pressure fractional crystallization. The geochemistry of the lavas was significantly influenced by subduction processes associated with a coeval arc, while significant amounts of assimilation of continental crust along with fractional crystallization appear to have contributed to the compositions of the most evolved lavas. The Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting but no true seafloor spreading had occurred. Copyright © Cambridge University Press 2015 Source


Ghavidel-Syooki M.,University of Tehran | Popov L.E.,National Museum Cardiff | Pour M.G.,Golestan University | Alvaro J.J.,CSIC - National Institute of Aerospace Technology | Ehsani M.H.,University of Tehran
Earth and Environmental Science Transactions of the Royal Society of Edinburgh | Year: 2015

Brachiopods are among the most common components of the Late Ordovician benthic faunal assemblages recorded in the Faraghan Mountains, Zagros Ranges, Iran. A total of 19 species referable to 16 genera are identified, including seven new species; namely Drabovia elegans, Hibernodonta bonehensis, Hedstroemina zakeenensis, Jezercia faraghani, Protomendacella multicostata, Tafilaltia dargazensis and Tafilaltia seyahouensis. Reported brachiopod assemblages are oligotaxic (2-3 species per assemblage) to monotaxic. The only exception is the medium diversity Aegiromena-Hedstroemina Association, with up to ten species, which occurs at the lower part of the A. nigerica Zone. Low diversity patterns were probably controlled by the palaeogeographical position of the region in high southern latitudes throughout the Ordovician. Paterula sp. and Iranospirifer sp. are the only brachiopods documented from the Silurian (Llandovery) Sarchahan Formation. This paper gives the first detailed report of shelly fossils from the Llandovery 'hot shale' of the Arabian margin of Gondwana. Most of the Katian genera, and many of their species, are shared with neighbouring high- to mid-latitude peri-Gondwanan margins, including Morocco, Spain, France, Sardinia and Bohemia. Copyright © The Royal Society of Edinburgh 2015. Source


Knox R.W.O'B.,British Geological Survey | Pearson P.N.,University of Cardiff | Barry T.L.,Open University Milton Keynes | Condon D.J.,NERC Isotope Geosciences Laboratory | And 11 more authors.
Proceedings of the Geologists' Association | Year: 2012

The 'Tertiary', omitted from IUGS-approved timescales since 1989, is still in common use. With the recent re-instatement of the Quaternary as a formal unit, the question arises as to whether the Tertiary too should be reinstated as a formal period, with the 'Paleogene' and 'Neogene' being downgraded to sub-periods. This paper presents arguments for and against this proposal, stemming from discussions by members of the Geological Society Stratigraphy Commission. It is intended to stimulate discussion of the topic in the wider community. © 2012. Source


Pearce J.A.,University of Cardiff | Hastie A.R.,University of Cardiff | Hastie A.R.,University of Birmingham | Leat P.T.,British Antarctic Survey | And 8 more authors.
Global and Planetary Change | Year: 2014

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c. 20. m.y. (early Oligocene to middle-late Miocene) before slab retreat and subsequent 'resurrection' as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf-Nd-Sr-Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic-acidic, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, middle-late Miocene period produced basic-acidic, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early-middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich Island Arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge-trench collisions are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17-15. Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (<. 11. Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific-South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because the northward flow of the South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA. © 2014. Source

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