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Lithospheric structure of the Labrador Sea from constrained 3-D gravity inversion (2013)

Welford, J. K., Hall, J.

Oxford University Press

In: Geophysical Journal International

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Publication Date: 2013-10-09

Description: Regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the Labrador Sea and its margins to generate 3-D density anomaly models to investigate broad-scale crustal structural variations across the extinct spreading centre. Benchmarked against independent seismic Moho depth constraints, a density anomaly isosurface within the inverted volumes was selected as a Moho-proxy and regional maps of Moho structure were developed. Inversions using two different sources for depth to basement constraints revealed similar Moho structures with a depth to Moho of 12 km beneath the Labrador Sea which deepens to 20 km and greater towards Davis Strait and beneath the offshore extension of the Grenville Province. Density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted Moho-proxy and the seismic Moho, with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide-angle reflection/refraction profiling studies. The inverted depth to Moho estimates were combined with depth to basement constraints to investigate crustal thickness, both for the full crust and for individual crustal layers, revealing that the crust of the Labrador Sea is generally 5–10 km thick but thickens to 20–25 km towards Davis Strait and beneath the offshore extension of the Grenville Province, not taking into account high density underplates or anomalously high density lower crust. Sediment and crustal thickness variations were investigated to compute stretching factors, β, across the Labrador Sea and to identify zones which deviate from local isostatic compensation. Assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model, much of the Labrador Sea has experienced 70–90 per cent thinning. The derived β values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern Labrador Sea, inboard of known oceanic crust. Isostatic considerations reveal sediment excess and deficiency on the Labrador and Greenland margins, respectively, possibly reflecting fundamental rheological asymmetry, with the Greenland margin appearing weaker than the Labrador margin. A strong gradient from sediment deficiency to excess, exclusive to the Labrador margin, may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments.

Print ISSN: 0956-540X

Electronic ISSN: 1365-246X

Topics: Geosciences

Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).

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Crustal seismic velocity structure from Eratosthenes Seamount to Hecataeus Rise across the Cyprus Arc, eastern Mediterranean (2014)

Welford, J. K., Hall, J., Hubscher, C., Reiche, S., Louden, K.

Oxford University Press

In: Geophysical Journal International

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Publication Date: 2014-12-21

Description: Wide-angle reflection/refraction seismic profiles were recorded across the Cyprus Arc, the plate boundary between the African Plate and the Aegean–Anatolian microplate, from the Eratosthenes Seamount to the Hecataeus Rise immediately south of Cyprus. The resultant models were able to resolve detail of significant lateral velocity variations, though the deepest crust and Moho are not well resolved from the seismic data alone. Conclusions from the modelling suggest that (i) Eratosthenes Seamount consists of continental crust but exhibits a laterally variable velocity structure with a thicker middle crust and thinner lower crust to the northeast; (ii) the Hecataeus Rise has a thick sedimentary rock cover on an indeterminate crust (likely continental) and the crust is significantly thinner than Eratosthenes Seamount based on gravity modelling; (iii) high velocity basement blocks, coincident with highs in the magnetic field, occur in the deep water between Eratosthenes and Hecataeus, and are separated and bounded by deep low-velocity troughs and (iv) one of the high velocity blocks runs parallel to the Cyprus Arc, while the other two appear linked based on the magnetic data and run NW–SE, parallel to the margin of the Hecataeus Rise. The high velocity block beneath the edge of Eratosthenes Seamount is interpreted as an older magmatic intrusion while the linked high velocity blocks along Hecataeus Rise are interpreted as deformed remnant Tethyan oceanic crust or mafic intrusives from the NNW–SSE oriented transform margin marking the northern boundary of Eratosthenes Seamount. Eratosthenes Seamount, the northwestern limit of rifted continental crust from the Levant Margin, is part of a jagged rifted margin transected by transform faults on the northern edge of the lower African Plate that is being obliquely subducted under the Aegean–Anatolian upper plate. The thicker crust of Eratosthenes Seamount may be acting as an asperity on the subducting slab, locally locking up subduction of the Cyprus Arc on its northern margin, while deformed Tethyan oceanic crust remains trapped between its northeastern margin and the Hecataeus Rise.

Keywords: Geodynamics and Tectonics

Print ISSN: 0956-540X

Electronic ISSN: 1365-246X

Topics: Geosciences

Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).

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Crustal structure from the Hecataeus Rise to the Levantine Basin, eastern Mediterranean, from seismic refraction and gravity modelling (2015)

Welford, J. K., Hall, J., Rahimi, A., Reiche, S., Hübscher, C., Louden, K.

Oxford University Press

In: Geophysical Journal International

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Publication Date: 2015-11-05

Description: In 2010, a wide-angle seismic reflection/refraction profile was acquired along the Hecataeus Rise, an area of shallow seabed immediately south of Cyprus in the eastern Mediterranean. The profile crossed from the Hecataeus Rise, through the Cyprus Arc to the Levantine Basin beyond. Due to the short length of the profile and the corresponding lack of deep ray coverage, velocity modelling was complemented by gravity modelling to gain constraints on deep crustal structure. The resultant model reveals velocities for the Hecataeus Rise that show no evidence of shallow ophiolites like those seen on mainland Cyprus, and the velocities are not diagnostic of a unique crustal affinity. Low-velocity sediments make up at least 7 km of the upper structure of Hecataeus Rise and these sediments overlie a two-layered crust. From the gravity modelling, the combined sediments and crust of Hecataeus Rise appear to be thinner than the Eratosthenes Seamount block to the southwest. A high-velocity lower crustal block is modelled under the seaward edge of Hecataeus Rise and, based on the gravity modelling, is inferred to extend landwards beneath the Rise. Similar high-velocity blocks were identified on the southwestern edge of Hecataeus Rise along nearby refraction lines and were interpreted as remnant Tethyan oceanic crust, foundered in the Cyprus Arc, along which subduction has ceased in this area. Given the thin two-layered crust beneath a thick accumulation of sediments modelled for Hecataeus Rise, we interpret that Hecataeus Rise represents a collage of oceanic fragments, accreted together within the failed subduction zone. Outboard of the crust of Hecataeus Rise, a 5-km deep low-velocity basin, possibly an accretionary wedge, is imaged that appears to correspond with the Cyprus Arc deformation zone imaged on both coincident and along-strike seismic reflection lines. A similar and wider feature is observed on seismic refraction lines to the west and combined, these may be revealing an eastward tapering zone of crustal deformation. To the south of the profile, the Levantine Basin appears undisturbed by the collision to the north and exhibits a uniform and homogeneous velocity structure.

Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism

Print ISSN: 0956-540X

Electronic ISSN: 1365-246X

Topics: Geosciences

Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).

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Distinct relict fringing reefs in the northern shelf of the Gulf of Elat/Aqaba: Markers of Quaternary eustatic and climatic episodes (2014)

G. Hartman, T.M. Niemi, Z. Ben-Avraham, G. Tibor, A. Al-Zoubi, R. A. Sade, J. K. Hall, E. Akawi, A. Abueladas, R. Al-Ruzouq, Y. Makovsky

Wiley

In: Sedimentology

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Publication Date: 2014-11-16

Description: Quaternary reef development and demise have been shown to correspond to worldwide sea-level fluctuations and related environmental changes, yet the mechanisms and rates affecting this relationship are not well resolved. A set of high-resolution seismic reflection and multibeam bathymetric data was interpreted. Six distinct systems of relict fringing reefs embedded within the sedimentary cover of the northern shelf of the Gulf of Elat/Aqaba were identified. The two uppermost relict fringing reefs are also exposed on the sea floor in water depths of ca 20 m and ca 60 m, along the north-western corner of the northern Gulf of Elat/Aqaba and sub-parallel to the current northern coast, respectively. Two other relict fringing reefs are laterally correlated to each of the last two transgressive cycles, and are inferred to have formed during decelerations in sea-level rise during the last two deglaciations. These reefs and the units to which they are laterally correlated portray a repeating stratigraphic pattern of reef development during deposition of seismically homogeneous sediment and an ensuing demise during deposition of heterogeneous sediment. Correlation of the reef stratigraphy with rates of Late Quaternary sea-level rise and a sediment accumulation rate from one shelf core provide age estimations for the seismic stratigraphy. Two phases of fringing reef generation occurred during the last deglaciation (since ca 18 ka); the older reef probably developed between 12.8 and 11.5 ka at ca 60 m below present sea-level (m bpsl) and the younger reef after 8.4 to 8 ka at ca 20 m bpsl. These last two phases of reef generation are separated by a lobate seismic unit that is interpreted as fluvial-deltaic deposits that backstepped across the shelf during the Early Holocene transgression. Results suggest that fringing reefs evolved along the northern shelf of the Gulf of Elat/Aqaba only during relative decelerations in sea-level rise, contemporaneous with low input of terrigenous sediment and probably during a period of aridity. At present, no fringing reef grows along the northern coast of the Gulf of Elat/Aqaba, suggesting that these relatively arid periods may have been drier than Recent. The pattern of slowdown in sea-level rise contemporaneous with conditions of increased aridity seems to have repeated during the last two sea-level rises, approximately at the same sea-levels, suggesting a common mechanism of short (millennium-scale) phases of eustatic and climatic alteration during deglaciations. This article is protected by copyright. All rights reserved.

Print ISSN: 0037-0746

Electronic ISSN: 1365-3091

Topics: Geosciences

Published by Wiley

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Continental breakup by a leaky transform: the gulf of elat (aqaba) (1979)

Z. Ben-Avraham ; Z. Garfunkel ; G. Almagor ; [et al.]

American Association for the Advancement of Science (AAAS)

In: Science. 206(4415): 214-6. doi: 10.1126/science.206.4415.214.

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Publication Date: 1979-10-12

Description: The floor of the Gulf of Elat consists of five distinct deeps. Its structure is controlled by faulting which has produced rhomb-shaped grabens. The gulf is a newly formed plate boundary between Arabia and Sinai.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Ben-Avraham, Z -- Garfunkel, Z -- Almagor, G -- Hall, J K -- New York, N.Y. -- Science. 1979 Oct 12;206(4415):214-6.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17801787" target="_blank"〉PubMed〈/a〉

Print ISSN: 0036-8075

Electronic ISSN: 1095-9203

Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics

Published by American Association for the Advancement of Science (AAAS)

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