| 1. |
- Alwmark, Carl, et al.
(author)
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The mid-Ordovician Osmussaar breccia in Estonia linked to the disruption of the L-chondrite parent body in the asteroid belt
- 2010
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In: Geological Society of America Bulletin. - 0016-7606. ; 122:7-8, s. 1039-1046
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Journal article (peer-reviewed)abstract
- The Middle Ordovician (466 Ma) Osmussaar breccia, situated along the northwestern coast of Estonia, is rich in angular chromite grains of extraterrestrial origin (>13 grains kg(-1)) and shocked quartz. The angularity of the chromite grains implies that they have not been transported or reworked to any large extent, connoting that the brecciation is the result of a contemporary impactor, either as a direct consequence of the impact or as a result of an earthquake triggered by the impact, and thus is not, as previously suggested, redeposited material from the nearby similar to 70 m.y. older Neugrund impact structure. The chemical composition of the chromite indicates that the impactor was an ordinary chondrite of L-type, which concurs well with the hypothesis that the influx of large bodies to Earth increased during this period due to the breakup of the L-chondrite parent body. This in turn gives support to the recent suggestion that abundant coeval mega-breccias worldwide are impact triggered. The presence of extraterrestrial chromite also strengthens the theory that physical pieces of a large celestial body can survive upon impact with Earth.
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| 2. |
- Burchardt, Steffi, et al.
(author)
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The Slaufrudar pluton, southeast Iceland : An example of shallow magma emplacement by coupled cauldron subsidence and magmatic stoping
- 2011
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In: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 124:1-2, s. 213-227
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Journal article (peer-reviewed)abstract
- The Tertiary Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland. Five glacial valleys cut through the uppermost 900 m of the pluton, exposing spectacular sections through its roof, walls, and interior. The wall contacts are subvertical and sharp. Only in the northeast and southwest is the wall contact characterized by brittle faulting. The pluton roof is smooth at map scale, so that the overall cross-sectional shape of the pluton and its internal layering indicate emplacement by incremental floor sinking through cauldron subsidence. A pronounced elongation of the pluton, parallel to the trend of regional fissure swarms, and its angular shape in map view indicate strong tectonic control on horizontal ring-fault propagation, whereas faulted wall contacts represent step-over structures between the earlier-formed ring faults. On outcrop scale, the roof contact exhibits numerous steps, faults, and apophyses associated with conjugate fracture sets that are parallel and perpendicular to the strike of the length of the pluton. These structures were presumably formed by sequential inflation and deflation of the pluton during episodic magma intrusion and therefore are closely coupled to cauldron subsidence. As a result of roof fracturing and magma injection along the fractures, roof material is found partly or completely detached within the granite. The Slaufrudalur pluton therefore provides new insight into the coupling of the emplacement mechanisms of cauldron subsidence and magmatic stoping in the upper crust.
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| 3. |
- Carracedo, J. C., et al.
(author)
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Evolution of ocean-island rifts : The northeast rift zone of Tenerife, Canary Islands
- 2011
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In: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 123:3-4, s. 562-584
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Journal article (peer-reviewed)abstract
- The northeast rift zone of Tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone. Field geology, isotopic dating, and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the NE rift zone, which includes a period of very fast growth toward instability (between ca. 1.1 and 0.83 Ma) followed by three successive large landslides: the Micheque and Guimar collapses, which occurred approximately contemporaneously at ca. 830 ka and on either side of the rift, and the La Orotava landslide (between 690 +/- 10 and 566 +/- 13 ka). Our observations suggest that Canarian rift zones show similar patterns of development, which often includes overgrowth, instability, and lateral collapses. Collapses of the rift flanks disrupt established fissural feeding systems, favoring magma ascent and shallow emplacement, which in turn leads to magma differentiation and intermediate to felsic nested eruptions. Rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes. Conversely, the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars, even if concealed by postcollapse volcanism. Comparative analysis of the main rifts in the Canary Islands outlines this general evolutionary pattern: (1) growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions; (2) flank collapse and catastrophic disruption of the established feeder system of the rift; (3) postcollapse centralized nested volcanism, commonly evolving from initially ultramafic-mafic to terminal felsic compositions (trachytes, phonolites); and (4) progressive decline of nested eruptive activity.
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| 4. |
- Cawood, P. A., et al.
(author)
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Orogenesis without collision : Stabilizing the Terra Australis accretionary orogen, eastern Australia
- 2011
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In: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 123:11-12, s. 2240-2255
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Journal article (peer-reviewed)abstract
- The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis.At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting.The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis.Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.
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| 5. |
- Chew, David M., et al.
(author)
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Timing of ophiolite obduction in the Grampian orogen
- 2010
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In: Geological Society of America Bulletin. - 0016-7606. ; 122:11-12, s. 1787-1799
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Journal article (peer-reviewed)abstract
- This study addresses the timing and pressure-temperature (P-T) conditions of ophiolite obduction, one of the proposed causes of the ca. 470 Ma Grampian orogeny of Scotland and Ireland. This event gave rise to the main structural and metamorphic characteristics of the Grampian terrane-the type area for Barrovian metamorphism, the cause of which remains enigmatic despite a century of research. Zircons from the Highland Border ophiolite, Scotland, define a 499 +/- 8 Ma U-Pb concordia age, which is interpreted as dating magmatism. Its metamorphism is dated by a 490 +/- 4 Ma Ar-40-Ar-39 hornblende age, and a 488 +/- 1 Ma Ar-40-Ar-39 muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 degrees C relate to ophiolite obduction. Metamorphism of the Deerpark complex ophiolitic melange (Irish correlative of the Highland Border ophiolite) is constrained by a 514 +/- 3 Ma Ar-40-Ar-39 hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and Ar-40-Ar-39 muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 degrees C for the mica schist constrain the conditions of ophiolite obduction. Metamorphic mineral ages from the Grampian terrane (Dalradian Supergroup) are substantially younger (ca. 475-465 Ma) than those from the ophiolites. If conductive heating in overthickened crust was the cause of Barrovian metamorphism, then collisional thickening must have started soon after ophiolite obduction at ca. 490 Ma in order to generate the ca. 470 Ma metamorphic peak in the Grampian terrane.
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| 6. |
- Cramer, Bradley D., et al.
(author)
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Testing the limits of Paleozoic chronostratigraphic correlation via high-resolution (<500 k.y.) integrated conodont, graptolite, and carbon isotope (delta C-13(carb)) biochemostratigraphy across the Llandovery-Wenlock (Silurian) boundary: Is a unified Phanerozoic time scale achievable?
- 2010
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In: Geological Society of America Bulletin. - 0016-7606. ; 122:9-10, s. 1700-1716
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Journal article (peer-reviewed)abstract
- The resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration. By virtue of deep-ocean cores and astrochronology, the Cenozoic and Mesozoic time scales carry error bars of a few thousand years (k.y.) to a few hundred k. y. In contrast, most of the Paleozoic time scale carries error bars of plus or minus a few million years (m. y.), and chronostratigraphic control better than +/- 1 m. y. is considered "high resolution." The general lack of Paleozoic abyssal sediments and paucity of orbitally tuned Paleozoic data series combined with the relative incompleteness of the Paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the Paleozoic time scale to the level achieved during the Mesozoic and Cenozoic was viewed as impractical, impossible, or both. Here, we utilize integrated graptolite, conodont, and carbonate carbon isotope (delta C-13 carb) data from three paleocontinents (Baltica, Avalonia, and Laurentia) to demonstrate chronostratigraphic control for upper Llandovery through middle Wenlock (TelychianSheinwoodian, similar to 436-426 Ma) strata with a resolution of a few hundred k.y. The interval surrounding the base of the Wenlock Series can now be correlated globally with precision approaching 100 k.y., but some intervals (e. g., uppermost Telychian and upper Shein-woodian) are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and/or extinction or carbon isotopic features to delineate such small time slices. Although producing such resolution during the Paleozoic presents an array of challenges unique to the era, we have begun to demonstrate that erecting a Paleozoic time scale comparable to that of younger eras is achievable.
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| 7. |
- Cramer, Bradley D., et al.
(author)
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U-Pb (zircon) age constraints on the timing and duration of Wen lock (Silurian) paleocommunity collapse and recovery during the "Big Crisis"
- 2012
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In: Geological Society of America Bulletin. - 0016-7606. ; 124:11-12, s. 1841-1857
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Journal article (peer-reviewed)abstract
- High-precision isotope-dilution U-Pb (zircon) dating was conducted on three volcanic ash fall (bentonite) samples from the Swedish island of Gotland, and on a fourth bentonite from the West Midlands, England. Zircons from the Ireviken, Grotlingbo, Djupvik (Gotland), and Wren's Nest Hill-15 (West Midlands) bentonites yielded weighted mean Pb-206/U-238 ages of 431.83 +/- 0.23/0.67 Ma, 428.45 +/- 035/0.73 Ma, 428.06 +/- 0.2110.66 Ma, and 427.86 +/- 032/0.71 Ma, respectively (analytical/total uncertainties). These biostratigraphically well-controlled age dates effectively bracket the Wenlock Epoch of the Silurian Period and provide control for the duration of one of the major Paleozoic biotic events and associated perturbations to the global carbon cycle (the "Big Crisis" or lundgreni event- graptolites; the NIulde Event-conodonts; the Mulde excursion-carbon isotopes). These new data suggest an older and shorter duration for the recalibration of the Wenlock Series and demonstrate that the cascade of biological and chemical events that took place during the Big Crisis happened on time scales of tens to hundreds of thousands of years.
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| 8. |
- Greenwood, Sarah L., et al.
(author)
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Ice-flow switching and East/West Antarctic Ice Sheet roles in glaciation of the western Ross Sea
- 2012
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In: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 124:11-12, s. 1736-1749
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Journal article (peer-reviewed)abstract
- The long-term behavior of the East and West Antarctic Ice Sheets, and their respective responses to forcing provide essential context for assessment of modern dynamic changes in ice-flow regimes and ice-sheet and shelf margins. The western Ross Sea discharges ice from both the East and West Antarctic Ice Sheets, and the paleoglacial record from this region is therefore valuable in unraveling their long-term behavior. New, high-resolution multibeam bathymetric data reveal snapshots of well-preserved glacial landforms on the seafloor around Ross Island and McMurdo Sound. Glacial lineations, grounding zone wedges, draped recessional moraines, and meltwater channels record a series of different ice-flow events in the region, contradictions between which require major phases of ice-flow reorganization. From the glacial geomorphology, we reconstruct a four-stage model of ice-flow evolution for the last glacial cycle, consisting of: (1) northeastward flow into the Ross Sea from McMurdo Sound; (2) westward flow from the Ross Sea, around Ross Island, and onto the Victoria Land coast and coastal seafloor trough; (3) a deglacial phase of ice-sheet thinning, minor shifts in flow, and grounding line retreat into McMurdo Sound; and (4) grounding line pinning on Ross Island during regional retreat, uncoupling of a remnant Ross Island ice cap, and local oscillation of Victoria Land outlet glaciers. We find that East Antarctic Ice Sheet ice discharge had a strong influence on ice-flow geometry in this part of the Ross Sea during the last glacial stage, but that it was not necessarily in phase with the behavior of the West Antarctic Ice Sheet. It is similarly evident that the ice streams that drained the Ross Sea over the continental shelf at the Last Glacial Maximum did not all operate synchronously, and exerted different drawdown power at different times. Finally, we conclude that Ross Island acts as an important pinning point in the Ross Sea ice-sheet-shelf system, stabilizing grounding line retreat and encouraging lasting ice-shelf development.
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