Shillington
Donna J.
Shillington
Donna J.
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ArticleSeismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada(American Geophysical Union, 2006-11-17) Van Avendonk, Harm J. A. ; Holbrook, W. Steven ; Nunes, Gregory T. ; Shillington, Donna J. ; Tucholke, Brian E. ; Louden, Keith E. ; Larsen, Hans Christian ; Hopper, John R.We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide-angle data recorded on a linear array of 24 ocean-bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35-km-thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ~6 km thickness. In the distal margin we find a 60-km-wide zone with seismic velocities between 5.0 and 6.5 km/s that thins to the southeast from 6 km to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km/s and higher beneath this thin crustal layer in the continent-ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater, or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km-wide zone where seismic velocities increase gradually from 6.3 km/s just below the top of acoustic basement to 7.7 km/s at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seawards, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km/s in basement ridges to 7.0 km/s at its base, overlying mantle velocities between 7.8 and 8.2 km/s. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magmastarved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent-ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.
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ArticleCharacterization of the in situ magnetic architecture of oceanic crust (Hess Deep) using near-source vector magnetic data(John Wiley & Sons, 2016-06-16) Tominaga, Masako ; Tivey, Maurice A. ; MacLeod, Christopher J. ; Morris, Antony ; Lissenberg, C. Johan ; Shillington, Donna J. ; Ferrini, Vicki L.Marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals, lithological boundaries, topographic contrasts, and alteration fronts in the oceanic lithosphere. Our aim here is to detect lithological contacts in fast-spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins. We conducted a high-resolution, near-bottom, vector magnetic survey of crust exposed in the Hess Deep “tectonic window” using the remotely operated vehicle (ROV) Isis during RRS James Cook cruise JC21 in 2008. Hess Deep is located at the western tip of the propagating rift of the Cocos-Nazca plate boundary near the East Pacific Rise (EPR) (2°15′N, 101°30′W). ROV Isis collected high-resolution bathymetry and near-bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of EPR lower crust and mantle exposed on the steep (~20°dipping) south facing slope just north of the Hess Deep nadir. Ten magnetic profiles were collected up the slope using a three-axis fluxgate magnetometer mounted on ROV Isis. We develop and extend the vertical magnetic profile (VMP) approach of Tivey (1996) by incorporating, for the first time, a three-dimensional vector analysis, leading to what we here termed as “vector vertical magnetic profiling” approach. We calculate the source magnetization distribution, the deviation from two dimensionality, and the strike of magnetic boundaries using both the total field Fourier-transform inversion approach and a modified differential vector magnetic analysis. Overall, coherent, long-wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope. The total field anomalies indicate a coherently magnetized source at depth. The upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology, including a “bench” and lobe-shaped steps, imaged by microbathymetry. The lower part of the slope is strongly magnetized, with a gradual reduction in amplitude from east to west across the slope. Surface morphology and recent drilling results indicate that the slope has been affected by mass wasting, but the observation of internally coherent magnetization distributions within the upper and lower slopes suggest that the disturbance is surficial. We attribute the spatial differences in magnetization distribution to the combination of changes in in situ lithology and depth to the source. These survey lines document the first magnetic profiles that capture the gabbro-ultramafic and possibly dike-gabbro boundaries in fast-spreading lower crust.
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ArticleEvidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin(American Geophysical Union, 2006-09-22) Shillington, Donna J. ; Holbrook, W. Steven ; Van Avendonk, Harm J. A. ; Tucholke, Brian E. ; Hopper, John R. ; Louden, Keith E. ; Larsen, Hans Christian ; Nunes, Gregory T.Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: (1) extended continental crust, (2) transitional basement, and (3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first-order differences between this margin and its well-studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: (1) slow spreading oceanic basement produced by unstable early seafloor spreading, (2) exhumed, serpentinized mantle with different properties from that off Iberia, and (3) thinned continental crust, likely emplaced by one or more detachment or rolling-hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide-angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.
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ArticleCorrection to “Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin”(American Geophysical Union, 2006-12-09) Shillington, Donna J. ; Holbrook, W. Steven ; Van Avendonk, Harm J. A. ; Tucholke, Brian E. ; Hopper, John R. ; Louden, Keith E. ; Larsen, Hans Christian ; Nunes, Gregory T.
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ArticleConstraints on Appalachian orogenesis and continental rifting in the Southeastern United States from wide-angle seismic data(American Geophysical Union, 2019-06-24) Marzen, Rachel E. ; Shillington, Donna J. ; Lizarralde, Daniel ; Harder, Steven H.The Southeastern United States is an ideal location to understand the interactions between mountain building, rifting, and magmatism. Line 2 of the Suwannee suture and Georgia Rift basin refraction seismic experiment in eastern Georgia extends 420 km from the Inner Piedmont to the Georgia coast. We model crustal and upper mantle VP and upper crustal VS. The most dramatic model transition occurs at the Higgins‐Zietz magnetic boundary, north of which we observe higher upper crustal VP and VS and lower VP/VS. These observations support the interpretation of the Higgins‐Zietz boundary as the Alleghanian suture. North of this boundary, we observe a low‐velocity zone less than 2 km thick at ~5‐km depth, consistent with a layer of sheared metasedimentary rocks that forms the Appalachian detachment. To the southeast, we interpret synrift sediments and decreasing crustal thickness to represent crustal thinning associated with the South Georgia Rift Basin and subsequent continental breakup. The correspondence of the northern limit of thinning with the interpreted suture location suggests that the orogenic suture zone and/or the Gondwanan crust to the south of the suture helped localize subsequent extension. Lower crustal VP and VP/VS preclude volumetrically significant mafic magmatic addition during rifting or associated with the Central Atlantic Magmatic Province. Structures formed during orogenesis and/or extension appear to influence seismicity in Georgia today; earthquakes localize along a steeply dipping zone that coincides with the northern edge of the South Georgia Basin and the change in upper crustal velocities at the Higgins‐Zietz boundary.
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ArticleComposition and structure of the central Aleutian island arc from arc-parallel wide-angle seismic data(American Geophysical Union, 2004-10-21) Shillington, Donna J. ; Van Avendonk, Harm J. A. ; Holbrook, W. Steven ; Kelemen, Peter B. ; Hornbach, Matthew J.New results from wide-angle seismic data collected parallel to the central Aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry. The 3-D ray tracing/2.5-D inversion of this sparse wide-angle data set, which incorporates independent phase interpretations and new constraints on shallow velocity structure, produces a faster and smoother result than a previously published velocity model. Middle-crustal velocities of 6.5–7.3 km/s over depths of ∼10–20 km indicate an andesitic to basaltic composition. High lower-crustal velocities of 7.3–7.7 km/s over depths of ∼20–35 km are interpreted as ultramafic-mafic cumulates and/or garnet granulites. The total crustal thickness is 35–37 km. This result indicates that the Aleutian island arc has higher velocities, and thus more mafic compositions, than average continental crust, implying that significant modifications would be required for this arc to be a suitable building block for continental crust. Lateral variations in average crustal velocity (below 10 km) roughly correspond to trends in major element geochemistry of primitive (Mg # > 0.6) lavas. The highest lower-crustal velocities (and presumably most mafic material) are detected in the center of an arc segment, between Unmak and Unalaska Islands, implying that arc segmentation exerts control over crustal composition.
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ArticleCharacterization of sills associated with the U reflection on the Newfoundland margin : evidence for widespread early post-rift magmatism on a magma-poor rifted margin(John Wiley & Sons, 2010-05-25) Peron-Pinvidic, Gwenn ; Shillington, Donna J. ; Tucholke, Brian E.Drilling during ODP Leg 210 penetrated two post-rift sills (dated as ∼105.3 and ∼97.8 Ma) in the deep sediments overlying basement of the continent–ocean transition zone on the magma-poor Newfoundland margin. The sill emplacement post-dated the onset of seafloor spreading by at least 7–15 Myr. The shallower of the two sills coincides with the high-amplitude U reflection observed throughout the deep Newfoundland Basin, and strong reflectivity in the sub-U sequence suggests that a number of other sills are present there. In this paper, we use multichannel seismic reflection data and synthetic seismograms to investigate the nature, magnitude and extent of this post-rift magmatism in the deep basin. Features observed in seismic profiles that we attribute to sill injection include high-amplitude reflections with geometries characteristic of intrusions such as step-like aspect; abrupt endings, disruptions and junctions of reflections; finger-like forms; differential compaction around possible loci of magma injection and disruption of overlying sediments by apparent fluid venting. Interpreted sills occur only over transitional basement that probably consists of a mixture of serpentinized peridotite and highly thinned continental crust, and they cover an area of ∼80 000 km2. From analysis of synthetic seismograms, we estimate that sill intrusions may comprise ∼26 per cent of the sub-U high-reflectivity sequence, which yields a crude estimate of ∼5800 km3 for the total volume of sills emplaced by post-rift magmatism. This is significant for a margin usually described as 'non-volcanic'. We discuss competing hypotheses about the source of the magmatism, which is still uncertain.
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ArticleConstraints on the composition of the Aleutian arc lower crust from VP/VS(John Wiley & Sons, 2013-06-07) Shillington, Donna J. ; Van Avendonk, Harm J. A. ; Behn, Mark D. ; Kelemen, Peter B. ; Jagoutz, OliverDetermining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the composition of high P-wave velocity (VP = 7.3–7.6 km/s) lower crust of the Aleutian arc from best-fitting average lower crustal VP/VS ratio using sparse converted S-waves from an along-arc refraction profile. We find a low VP/VS of ~1.7–1.75. Using petrologic modeling, we show that no single composition is likely to explain the combination of high VP and low VP/VS. Our preferred explanation is a combination of clinopyroxenite (~50–70%) and alpha-quartz bearing gabbros (~30–50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that ~30–40% of the full Aleutian crust comprises ultramafic cumulates. These results also suggest that small amounts of quartz can exert a strong influence on VP/VS in arc crust.
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ArticleLimited and localized magmatism in the Central Atlantic Magmatic Province(Nature Research, 2020-07-07) Marzen, Rachel E. ; Shillington, Donna J. ; Lizarralde, Daniel ; Knapp, James H. ; Heffner, David M. ; Davis, Joshua K. ; Harder, Steven H.The Central Atlantic Magmatic Province (CAMP) is the most aerially extensive magmatic event in Earth’s history, but many questions remain about its origin, volume, and distribution. Despite many observations of CAMP magmatism near Earth’s surface, few constraints exist on CAMP intrusions at depth. Here we present detailed constraints on crustal and upper mantle structure from wide-angle seismic data across the Triassic South Georgia Rift that formed shortly before CAMP. Lower crustal magmatism is concentrated where synrift sedimentary fill is thickest and the crust is thinnest, suggesting that lithospheric thinning influenced the locus and volume of magmatism. The limited distribution of lower crustal intrusions implies modest total CAMP volumes of 85,000 to 169,000 km3 beneath the South Georgia Rift, consistent with moderately elevated mantle potential temperatures (<1500 °C). These results suggest that CAMP magmatism in the South Georgia Rift is caused by syn-rift decompression melting of a warm, enriched mantle.
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ArticleEpisodic intraplate magmatism fed by a long-lived melt channel of distal plume origin(American Association for the Advancement of Science, 2023-06-09) Naif, Samer ; Miller, Nathaniel C. ; Shillington, Donna J. ; Becel, Anne ; Lizarralde, Daniel ; Bassett, Dan ; Hemming, Sidney R.In the past decade, marine geophysical observations have led to the discovery of thin channels at the base of oceanic plates with anomalous physical properties that indicate the presence of low-degree partial melts. However, mantle melts are buoyant and should migrate toward the surface. We show abundant observations of widespread intraplate magmatism on the Cocos Plate where a thin partial melt channel was imaged at the lithosphere-asthenosphere boundary. We combine existing geophysical, geochemical, and seafloor drilling results with seismic reflection data and radiometric dating of drill cores to constrain the origin, distribution, and timing of this magmatism. Our synthesis indicates that the sublithospheric channel is a regionally extensive (>100,000 km2) and long-lived feature that originated from the Galápagos Plume more than 20 Ma ago, supplying melt for multiple magmatic events and persisting today. Plume-fed melt channels may be widespread and long-lived sources for intraplate magmatism and mantle metasomatism.