Imperial College London

DrRobertAllen

Faculty of EngineeringDepartment of Earth Science & Engineering

Teaching Fellow in Field and Near-Surface Geophysics
 
 
 
//

Contact

 

r.allen16

 
 
//

Location

 

2.41Royal School of MinesSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

11 results found

Schlaphorst D, Harmon N, Kendall JM, Rychert CA, Collier J, Rietbrock A, Goes Set al., 2021, Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles From Ambient Noise Tomography, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 22

Journal article

Allen RW, 2021, Shedding New Light on an Enigmatic End Member of Back-Arc Spreading: The Structure of the Grenada Basin in the Lesser Antilles, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 126, ISSN: 2169-9313

Journal article

Chichester B, Rychert C, Harmon N, Allen R, Collier J, Henstock T, Rietbrock Aet al., 2020, Seafloor sediment thickness beneath the VoiLA broad-band ocean-bottom seismometer deployment in the Lesser Antilles from P-to-S delay times, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 223, Pages: 1758-1768, ISSN: 0956-540X

Journal article

Cooper GF, Macpherson CG, Blundy JD, Maunder B, Allen RW, Goes S, Collier JS, Bie L, Harmon N, Hicks SP, Iveson AA, Prytulak J, Rietbrock A, Rychert CA, Davidson JP, Kendall MJ, Schlaphorst Det al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc (vol 87, pg 931, 2020), Nature, Vol: 584, Pages: E36-E36, ISSN: 0028-0836

Journal article

Cooper GF, Macpherson CG, Blundy JD, Maunder B, Allen RW, Goes S, Collier JS, Bie L, Harmon N, Hicks SP, Iveson AA, Prytulak J, Rietbrock A, Rychert CA, Davidson JPet al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc, Nature, Vol: 582, Pages: 525-529, ISSN: 0028-0836

Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.

Journal article

Davy R, Collier JS, Henstock TJ, the VoiLA consortiumet al., 2020, Wide‐angle seismic imaging of two modes of crustal accretion in mature Atlantic Ocean crust, Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-21, ISSN: 2169-9313

We present a high‐resolution 2‐D P‐wave velocity model from a 225 km long active seismic profile, collected over ~60‐75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three non‐transform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient layer 2 (~0.9 s‐1) above a lower gradient layer 3 (0.2 s‐1). Here PmP coincides with the 7.5 km s‐1 contour and velocity increases to >7.8 km s‐1 within 1 km below. We interpret these segments as magmatically‐robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between upper and lower crust, and PmP shallower than the 7.5 km s‐1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. Whilst velocity depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (> 20 Ma) Atlantic Ocean crust. They have a ~20 km wide “domal” morphology with shallow basement and increased upper‐crustal velocities. We interpret their mid‐crustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults.

Journal article

Bie L, Rietbrock A, Hicks S, Allen R, Blundy J, Clouard V, Collier J, Davidson J, Garth T, Goes S, Harmon N, Henstock T, van Hunen J, Kendall M, Kruger F, Lynch L, Macpherson C, Robertson R, Rychert K, Tait S, Wilkinson J, Wilson Met al., 2020, Along‐arc heterogeneity in local seismicity across the lesser antilles subduction zone from a dense ocean‐bottom seismometer network, Seismological Research Letters, Vol: 91, Pages: 237-247, ISSN: 0895-0695

The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65km∼65  km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

Journal article

Allen R, Collier J, Stewart A, Henstock T, Goes S, Rietbrock Aet al., 2019, The role of arc migration in the development of the Lesser Antilles: a new tectonic model for the Cenozoic evolution of the eastern Caribbean, Geology, Vol: 47, Pages: 891-895, ISSN: 0091-7613

Continental arc systems often show evidence of large-scale migration both towards and away from the incoming plate. In oceanic arc systems however, whilst slab roll-back and the associated processes of back-arc spreading and arc migration towards the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesise new evidence to show two phases of back-arc spreading and eastward arc migration towards the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier back-arc basin on either side of the present-day Lesser Antilles Arc. This is the first example of regional multi-directional arc migration in an intra-oceanic setting and has implications for along-arc structural and geochemical variations. The back and forth arc migrations are probably due to the constraints the neighbouring American plates impose on this isolated subduction system rather than variations in subducting slab buoyancy.

Journal article

Allen RW, Berry C, Henstock T, Collier JS, Dondin FJ-Y, Rietbrock A, Latchman JL, Robertson REAet al., 2018, 30 years in the life of an active submarine volcano: A time-lapse bathymetry study of the Kick-‘em-Jenny Volcano, Lesser Antilles, G3: Geochemistry, Geophysics, Geosystems, Vol: 19, Pages: 715-731, ISSN: 1525-2027

Effective monitoring is an essential part of identifying and mitigating volcanic hazards. In the submarine environment this is more difficult than onshore because observations are typically limited to land-based seismic networks and infrequent shipboard surveys. Since the first recorded eruption in 1939, the Kick-‘em-Jenny (KeJ) volcano, located 8km off northern Grenada, has been the source of 13 episodes of T-phase signals. These distinctive seismic signals, often coincident with heightened body-wave seismicity, are interpreted as extrusive eruptions. They have occurred with a recurrence interval of around a decade, yet direct confirmation of volcanism has been rare. By conducting new bathymetric surveys in 2016 and 2017 and reprocessing 4 legacy datasets spanning 30 years we present a clearer picture of the development of KeJ through time. Processed grids with a cell size of 5m and vertical precision on the order of 1-4m allow us to correlate T-phase episodes with morphological changes at the volcano's edifice. In the time-period of observation 7.09x106 m3 of material has been added through constructive volcanism – yet 5 times this amount has been lost through landslides. Limited recent magma production suggests that KeJ may be susceptible to larger eruptions with longer repeat times than have occurred during the study interval, behavior more similar to sub-aerial volcanism in the arc than previously thought. T-phase signals at KeJ have a varied origin and are unlikely to be solely the result of extrusive submarine eruptions. Our results confirm the value of repeat swath bathymetry surveys in assessing submarine volcanic hazards.

Journal article

Allen R, Braszus B, Goes S, Rietbrock A, Collier Jet al., Evolution of Caribbean subduction from P-wave tomography and plate reconstruction, Publisher: Copernicus GmbH

<jats:p> &amp;lt;p&amp;gt;The Caribbean plate has a complex tectonic history, which makes it&amp;amp;#160; particularly challenging to establish the evolution of the subduction zones at its margins. Here we present a new teleseismic P-wave tomographic model under the Antillean arc that benefits from ocean-bottom seismometer data collected in our recent VoiLA (Volatile Recycling in the Lesser Antilles) project. We combine this imagery with a new plate reconstruction that we use to predict possible slab positions in the mantle today. We find that upper mantle anomalies below the eastern Caribbean correspond to a stack of material that was subducted at different trenches at different times, but ended up in a similar part of the mantle due to the large northwestward motion of the Americas. This stack comprises: in the mantle transition zone, slab fragments that were subducted between 70 and 55 Ma below the Cuban and Aves segments of the Greater Arc of the Caribbean; at 450-250 km depth, material subducted between 55 and 35 Ma below the older Lesser Antilles (including the Limestone Caribees and Virgin Islands);&amp;amp;#160; and above 250 km, slab from subduction between 30 and 0 Ma below the present Lesser Antilles to Hispaniola Arc. Subdued high velocity anomalies in the slab above 200 km depth coincide with where the boundary between the equatorial Atlantic and proto-Caribbean subducted, rather than as previously proposed, with the North-South American plate boundary. The different phases of subduction can be linked to changes in the age, and hence buoyancy structure, of the subducting plate.&amp;lt;/p&amp;gt; </jats:p>

Conference paper

Schlaphorst D, Harmon N, Kendall J-M, Rychert CA, Collier JS, Rietbrock A, Goes S, Allen RW, Bie L, Blundy JD, Chichester B, Cooper G, Davy RG, Henstock TJ, Hicks SP, Macpherson CG, Maunder B, Prytulak J, van Hunen J, Wilkinson JJ, Wilson M, Davidson JPet al., Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles from Ambient Noise Tomography

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00596842&limit=30&person=true