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Article Volume 11, Number 6 25 June 2010 Q06013, doi:10.1029/2010GC003048 ISSN: 1525‐2027

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Crustal structure beneath the Montserrat region of the Lesser Antilles island arc Winchelle Ian Sevilla, Charles J. Ammon, and Barry Voight Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA ([email protected])

Silvio De Angelis Montserrat Volcano Observatory, Montserrat, West Indies [1] We use receiver function analysis to estimate the first‐order subsurface seismic structure in the vicinity of the island of Montserrat, located in the northern Lesser Antilles arc and home of the active Soufrière Hills Volcano. Near‐surface complexity in the island structure inhibits our ability to resolve lateral variations in seismic structure, so we average the signals from eight stations in two directions to produce a first‐order regional seismic velocity model. Although we are unable to resolve a precise sharpness of the crust‐mantle transition, we can limit its thickness to less than ∼4 km. Lateral variations in shallow structure complicate a receiver function based estimate of Poisson’s ratio, but the data indicate that in both directions sampled by the receiver functions, the lower crustal Poisson’s ratio is greater than 0.27 and more likely in the range 0.29–0.30. We estimate a mean crustal thickness of ∼30 km, and the observations suggest that the crust may be slightly thinner northwest of the island (∼26–30 km) than it is to the south (∼30–34 km). The high P wave speed and Poisson’s ratio indicate a generally mafic lower crust, with rocks of intermediate composition not precluded in the upper part of the lower crust. We did not find evidence for a thick high‐speed lower crust (>7.4 km/s) as has been inferred in some other arcs. Components: 8000 words, 8 figures. Keywords: Montserrat Island; receiver function; crustal structure; Lesser Antilles. Index Terms: 7280 Seismology: Volcano seismology (8419); 8038 Structural Geology: Regional crustal structure; 8185 Tectonophysics: Volcanic arcs. Received 19 January 2010; Revised 27 April 2010; Accepted 19 May 2010; Published 25 June 2010. Sevilla, W. I., C. J. Ammon, B. Voight, and S. De Angelis (2010), Crustal structure beneath the Montserrat region of the Lesser Antilles island arc, Geochem. Geophys. Geosyst., 11, Q06013, doi:10.1029/2010GC003048.

1. Introduction [2] The island of Montserrat is located in the northern segment of the 700–800 km long Lesser Antilles Volcanic Arc, about 150 km east of the trench, and about 120–140 km above the subducting North American lithosphere (Figure 1). The Copyright 2010 by the American Geophysical Union

Lesser Antilles arc broadens north of Martinique and Dominica where the current volcanic arc (Guadeloupe to Saba), active since the early Miocene, is located west of an older Eocene to mid‐Oligocene arc (eastern Guadeloupe, Antigua, Barbuda, St. Maarten) [Macdonald et al., 2000]. The entire region has a relatively shallow seafloor 1 of 13

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SEVILLA ET AL.: CRUSTAL STRUCTURE IN MONTSERRAT REGION

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Figure 1. Eastern Caribbean region showing the main features of the Lesser Antilles arc, surrounding structures, and approximate plate boundary location (dashed line). Orange circles show USGS NEIC hypocenters for the time period from 1973 to 2008.

with depths in the range 0.5–1.0 km (Figure 1). Limestone outcrops on the eastern arc and observed as far west as northern Montserrat and St. Eustatius suggest that the region has experienced some uplift in the Quaternary [Macdonald et al., 2000]. [3] The crust of the Caribbean plate has a thickness intermediate between that of typical ocean and continental lithosphere [Officer et al., 1957, 1959]. Thus Montserrat’s volcanic complexes likely formed on modified oceanic crust thickened early in the history of the Caribbean plate, before ∼85 Ma and prior to the emplacement of the plate between North and South America [Donnelly, 1989; Pindell and Barrett, 1990]. The Montserrat crust may have been further modified during activity associated with the Eocene to mid‐Oligocene age arc now located east. The entire region from the trench to at least the Aves ridge (an older late Cretaceous to Paleogene age arc structure) appears to contain modified oceanic crust with thickness estimates ranging from 7.4 km/s at the base of the crust. [18] Although the variability in the seismic structure

at intraoceanic arcs complicates generalizations, a typical intraoceanic arc structure consists of an upper crust of intrusive and extrusive igneous rocks, sedimentary and volcaniclastic rocks that are altered, fractured, and porous and have substantially slow P wave speeds (≤5.0 km/s). The middle crust varies in thickness between and within arcs, and generally has wave speeds in the 6.0–6.5 km/s range. Slower speeds and thin (∼5 km thick) middle crusts have been interpreted as original oceanic crust on which the arc was built. Alternate views suggest that this layer is composed of intrusive rocks of mafic and intermediate composition developed by differentiation of deeper mafic magmas [e.g., Holbrook et al., 1999]. Thus in the vicinity of Montserrat, observed seismic velocities of ∼6.2 km/s at ∼5– 10 km depth seem compatible with an intermediate, dioritic composition [Paulatto et al., 2009; Christensen and Mooney, 1995], and this is consistent also with the observed eruptions of andesite at SHV. In most arcs the lower crust P wave speeds range from the high sixes to the high sevens of km/s. The thickness of this deeper layer ranges from a few kilometers beneath some arcs to 10–20 km beneath others. This layer is interpreted to represent mafic rocks, and mafic and ultramafic cumulates left behind by magmatic differentiation processes. [19] Our estimated range of crustal thickness, 26–

34 km, for the Montserrat region is consistent with earlier estimates of the thickness beneath the northern Lesser Antilles arc [Boynton et al., 1979; Whitmarsh et al., 1983] and slightly thicker than the Christeson et al. [2008] estimate for the southern Lesser Antilles. The assumed P wave speed for the lower crust is compatible with the older refraction profile estimates [e.g., Boynton et al., 1979; Whitmarsh et al., 1983] and with the average speed in the model of Christeson et al. [2008]. We have also shown that the Poisson’s ratio for the lower crust is relatively high (>0.27). The receiver function observations allow values as high as 0.33–0.34, but we think that it is more likely that the value is in the range of 0.28–0.30 since few common rocks have ratios higher than 0.30 under laboratory conditions [Christensen, 1996]. The high Vp value and the relatively high Poisson’s ratio are consistent with a mafic composition such as gabbroic rocks [Christensen, 1996; Brocher, 2005; Behn and Kelemen, 2006]. We interpret the lower crust (greater than ∼10 km thick) in our model to be modified Caribbean crust (originally similar to the

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Venezuelan basement) that has been intruded and thickened with differentiated mafic material added to the crust as part of the subduction process. We see little evidence for a deep layer of mafic to ultramafic cumulates (Vp > 7.4 km/s) although our modeling of the Ps and PpPmS amplitudes suggests that a layer a few kilometers thick could exist undetected near the crust‐mantle boundary. [20] One possible explanation for the absence of

cumulates is that they are removed by subduction processes as they form. This may be typical of most arcs, since the Aleutians is the only arc with a region containing rocks with Vp > 7.4 km/s. An alternate view is that what we commonly identify as the crust‐mantle transition is in fact a seismic velocity increase associated with an increase in garnet within the cumulates beneath the arc. However, Montserrat andesite displays no geochemical signature of a garnet influence in crystal fractionation (R. S. J. Sparks, personal communication, 2008). This observation argues strongly against a role of garnet in explaining the 30 km deep velocity increase, and is at the same time consistent with our crustal thickness estimate of ∼30 km, rather than ∼35 km. Other supportive arguments include the models of Behn and Kelemen [2006], that suggest garnet would form in gabbronorite at >35–40 km depths or deeper, and the models of Furlong and Fountain [1986], that predict lower seismic wave speed increases from garnet additions to crustal rocks than required by our data to match the amplitude of the receiver function converted phases. Thus we prefer to interpret the Ps and PpPmS arrivals as originating from the crust‐mantle boundary beneath the Montserrat region. [21] Wadge [1984] used the volume of observed

surface volcanic rocks to estimate that during the last 0.1 Ma, the Lesser Antilles arc volcanic production rate per unit kilometer of arc length was ∼4.0 km3 Ma−1 km−1. If we assume that the original Lesser Antilles crust resembled the Venezuelan Basin crust, then the thickness has tripled from roughly 10 km to roughly 30 km. To produce the observed thickening of 20 km in 50 Ma across a 50 to 100 km wide arc (Figure 1) requires a growth rate of 20 to 40 km3 Ma−1 km−1, which is lower than Reymer and Schubert’s [1986] estimate of 70 km3 Ma−1 km−1 for the long‐term growth rate of the Lesser Antilles, but comparable to the range (30–50 km3 Ma−1 km−1) of more recent results from numerical modeling of subduction processes [Nikolaeva et al., 2008]. Finally, we note that to convert a 30 km thick Lesser Antilles arc structure to “average” continental crust from the Lesser Antilles 10 of 13


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arc, would require thickening the crust by about 10 km and slightly reducing the mean speed of the lower crust [e.g., Christensen and Mooney, 1995]. The point here is not that crust evolves that way in any specific case, but appears to do so in bulk. The average Poisson’s ratio for our Montserrat region crust is 0.28, which is slightly above continental average of 0.27, but well within the ranges observed on the continents [Zandt and Ammon, 1995].

5. Conclusions [22] The observations from this work include the

timing and amplitude of P to S converted phases propagating between a relatively strong and less than 4 km thick crust‐mantle transition located roughly 26–34 km beneath the region surrounding Montserrat. We estimate that the average thickness of the crust beneath the Montserrat region is ∼30 km, and slightly thinner to the northwest than the south. If we assume that the Lesser Antilles arc formed on top of modified oceanic crust similar to that found in the Venezuelan basement, then the crustal thickness has increased by at least 10–20 km in the last 40 Ma. This value is reduced if the current arc formed on a thick accretionary prism of the older Aves Ridge arc or on a Mesozoic protoarc structure [Bouysse et al., 1990; Macdonald et al., 2000]. Tomography results [Shalev et al., 2008, 2010; Paulatto et al., 2009] show that the upper crust has a strong gradient in P wave speed starting from very slow velocities. Our resolution of the seismic wave speeds in the middle and lower crust speed is limited. We assumed that the mean speed in the lower crust is roughly 6.9 km/s, and that for reasonable perturbations around that value, the lower crust has a Poisson’s ratio greater than about 0.27 (Vp/Vs > 1.78) and more likely is closer to 0.29–0.30. The high wave speed and Poisson’s ratio strongly favor a mafic (gabbro) composition for the lower crust, but intermediate compositions in the upper part are not precluded, especially under SHV and the extinct volcanic centers. We see little evidence of a substantial cumulate layer (Vp > 7.4 km/s) in the lower crust, suggesting that cumulates remaining from deep fractionation were removed earlier (or perhaps continuously), consistent with other observations of active intraoceanic arcs.

Acknowledgments [23] We thank the Montserrat Volcano Observatory for sharing the teleseismic P waveforms with us and for their help in this

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study. We also thank Eylon Shalev for sharing his seismic velocity model with us. We thank three anonymous reviewers whose comments helped improve this paper. This work was partially supported by the SEA‐CALIPSO project, supported by the National Science Foundation Continental Dynamics, Geophysics, and Instrumentation and Facilities Programs.

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