Upper Palaeozoic subduction/accretion processes in the closure of Palaeotethys: Evidence from the Chios Melange (E Greece), the Karaburun Melange (W Turkey) and the Teke Dere Unit (SW Turkey)     

Alastair H.F. Robertson  Timur Ustamer 《Sedimentary Geology》2009,220(1-2):29-59

During Late Palaeozoic time a wide ocean, known as Palaeotethys, separated the future Eurasian and African continents. This ocean closed in Europe in the west during the Variscan orogeny, whereas in Asia further east it remained open and evolved into the Mesozoic Tethys, only finally closing during Late Cretaceous–Early Cenozoic.Three Upper Palaeozoic lithological assemblages, the Chios Melange (on the Aegean Greek island), the Karaburun Melange (westernmost Aegean Turkey) and the Teke Dere Unit (Lycian Nappes, SW Turkey) provide critical information concerning sedimentary and tectonic processes during closure of Palaeotethys. The Chios and Karaburun melanges in the west are mainly terrigenous turbidites with blocks and dismembered sheets of Silurian–Upper Carboniferous platform carbonate rocks (shallow-water and slope facies) and poorly dated volcanic rocks. The Teke Dere Unit to the southeast begins with alkaline, within-plate-type volcanics, depositionally overlain by Upper Carboniferous shallow-water carbonates. This intact succession is overlain by a tectonic slice complex comprising sandstone turbidites that are intersliced with shallow-water, slope and deep-sea sediments (locally dated as Early Carboniferous). Sandstone petrography and published detrital mineral dating imply derivation from units affected by the Panafrican (Cadomian) and Variscan orogenies.All three units are interpreted as parts of subduction complexes in which pervasive shear zones separate component parts. Silurian–Lower Carboniferous black cherts (lydites) and slope carbonates accreted in a subduction trench where sandstone turbidites accumulated. Some blocks retain primary depositional contacts, showing that gravitational processes contributed to formation of the melange. Detached blocks of Upper Palaeozoic shallow-water carbonates (e.g. Chios) are commonly mantled by conglomerates, which include water-worn clasts of black chert. The carbonate blocks are restored as one, or several, carbonate platforms that collided with an active margin, fragmenting into elongate blocks that slid into a subduction trench. This material was tectonically accreted at shallow levels within a subduction complex, resulting in layer-parallel extension, shearing and slicing. The accretion mainly took place during Late Carboniferous time.Alternative sedimentary-tectonic models are considered in which the timing and extent of closure of Palaeotethys differ, and in which subduction was either northwards towards Eurasia, or southwards towards Gondwana (or both). Terrane displacement is also an option. A similar (but metamorphosed) accretionary unit, the Konya Complex, occurs hundreds of kilometres further east. All of these units appear to have been assembled along the northern margin of Gondwana by Permian time, followed by deposition of overlying Tauride-type carbonate platforms. Northward subduction of Palaeotethys beneath Eurasia is commonly proposed. However, the accretionary units studied here are more easily explained by southward subduction towards Gondwana. Palaeotethys was possibly consumed by long-lived (Late Palaeozoic) northward subduction beneath Eurasia, coupled with more short-lived (Late Carboniferous) southward subduction near Gondwana, during or soon after closure of Palaeotethys in the Balkan region to the west.  相似文献   

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control     

Alastair G.C. Graham  Robert D. Larter  Karsten Gohl  Claus-Dieter Hillenbrand  James A. Smith  Gerhard Kuhn 《Quaternary Science Reviews》2009,28(25-26):2774-2793

The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.  相似文献   

Periglacial trimlines and nunataks of the Last Glacial Maximum: the Gap of Dunloe,southwest Ireland     

Alaric C. Rae  Stephan Harrison  Timothy Mighall  Alastair G. Dawson 《第四纪科学杂志》2004,19(1):87-97

During the last main phase of glaciation (26–13 ka) an ice‐cap developed in southwest Ireland and ice, from a dispersal centre in the vicinity of Kenmare, flowed north through the Gap of Dunloe in the Macgillycuddy's Reeks. On surrounding hillsides a weathering limit separates ice‐moulded bedrock, on low ground, from frost‐weathered terrain above. Assessment of bedrock dilation joint characteristics, Schmidt hammer R‐value data and clay‐sized mineral contents of basal soil samples, demonstrate significant contrasts in the degree of weathering above and below this limit. The weathering limit declines in altitude along former ice flow‐lines and is confluent with morainic deposits on the eastern side of the Gap. This supports the assertion that the high‐level weathering limit is a periglacial trimline that marks the former maximum upper limit of the body of ice which occupied the Gap of Dunloe during the Last Glacial Maximum (LGM). Reconstruction of the former ice‐surface profile from periglacial trimline limits on the eastern side of the Gap yields a mean estimate for basal shear stress of 106.5 kPa. This value suggests that the ice mass which occupied the Gap of Dunloe at the LGM was warm based and flowed on a bedrock substrate. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Quaternary environments. M. A. J. Williams,D. L. Dunkerley,P. de Deckker,A. P. Kershaw and T. W. Stokes. Publisher Edward Arnold,London 1993 £16.95     

Alastair G. Dawson 《第四纪科学杂志》1995,10(2):187-187

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Lateglacial and Holocene relative sea‐level changes and first evidence for the Storegga tsunami in Sutherland,Scotland          下载免费PDF全文

Antony J. Long  Natasha L. M. Barlow  Sue Dawson  Jon Hill  James B. Innes  Catherine Kelham  Fraser D. Milne  Alastair Dawson 《第四纪科学杂志》2016,31(3):239-255

We reconstruct one of the longest relative sea‐level (RSL) records in north‐west Europe from the north coast of mainland Scotland, using data collected from three sites in Loch Eriboll (Sutherland) that we combine with other studies from the region. Following deglaciation, RSL fell from a Lateglacial highstand of +6?8 m OD (Ordnance Datum = ca. mean sea level) at ca. 15 k cal a BP to below present, then rose to an early Holocene highstand and remained at ca. +1 m OD between ca. 7 and 3 k cal a BP, before falling to present. We find no evidence for significant differential Holocene glacio‐isostatic adjustment between sites on the north‐west (Lochinver, Loch Laxford), north (Loch Eriboll) and north‐east (Wick) coast of mainland Scotland. This suggests that the region was rapidly deglaciated and there was little difference in ice loads across the region. From one site at the head of Loch Eriboll we report the most westerly sedimentary evidence for the early Holocene Storegga tsunami on the Scottish mainland. The presence of the Storegga tsunami in Loch Eriboll is predicted by a tsunami wave model, which suggests that the tsunami impacted the entire north coast of Scotland and probably also the Atlantic coastline of north‐west Scotland.

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Mineralogy and origin of rhizoliths on the margins of saline,alkaline Lake Bogoria,Kenya Rift Valley     

Richard Alastair Owen  Richard Bernhart Owen  Robin W. Renaut  Jennifer J. Scott  Brian Jones  Gail M. Ashley 《Sedimentary Geology》2008,203(1-2):143-163

A wide range of rhizoliths occurs around the margins of Lake Bogoria, Kenya. These include root casts, moulds, tubules, rhizocretions, and permineralised root systems. These rhizoliths are variably composed of opaline silica, calcite, zeolites (mainly analcime), fluorite, and possibly fluorapatite, either alone or in combinations. Some rhizoliths are infilled moulds with detrital silicate grains. Most rhizoliths are in situ, showing both vertical and horizontal orientations. Reworked rhizoliths have been concentrated locally to form dense rhizolites.Hot-spring fluids, concentrated by evapotranspiration and capillary evaporation, have provided most of the silica for the permineralisation of the plant tissues. Precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses. Calcite rhizoliths formed following evaporative concentration, evapotranspiration, and (or) CO₂ degassing of Ca-bearing runoff water that infiltrated the sediment, or by mixing of runoff with saline, alkaline groundwater. Fluorite precipitated in areas where mixing of hot-spring and meteoric waters occurred, or possibly where hot-spring fluids came into contact with pre-existing calcite. Zeolitic rhizoliths formed during a prolonged period of aridity, when capillary rise and evaporative pumping brought saline, alkaline waters into contact with detrital silicate minerals around roots.  相似文献   

Did the Kyrenia Range of northern Cyprus rotate with the Troodos–Hatay microplate during the tectonic evolution of the eastern Mediterranean?     

Antony Morris  Alastair H. F. Robertson  Mark W. Anderson  Emma Hodgson 《International Journal of Earth Sciences》2016,105(1):399-415

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Emplacement of the Jurassic Mirdita ophiolites (southern Albania): evidence from associated clastic and carbonate sediments     

Alastair H. F. Robertson  Corina Ionescu  Volker Hoeck  Friedrich Koller  Kujtim Onuzi  Ioan I. Bucur  Dashamir Ghega 《International Journal of Earth Sciences》2012,101(6):1535-1558

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.  相似文献   

Balkan ophiolite-related geology     

Annie Rassios  Alastair Robertson 《International Journal of Earth Sciences》2012,101(6):1465-1466

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Response to Stannard     

Alastair Bonnett 《Area》2003,35(3):323-324

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