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Difficulties in Seismically Imaging the Icelandic Hotspot



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Difficulties in Seismically Imaging the Icelandic Hotspot William R Keller Don L Anderson Robert W Clayton Department of Geological and Planetary Sciences California Institute of Technology Submitted to Science 02 28 00 Correspond with William R Keller Seismological Laboratory Caltech MC 252 21 Pasadena CA 91125 Ph 626 395 6932 Fax 626 564 0715 e mail keller gps caltech edu Keller et al IMAGING ICELAND HOTSPOT 03 08 00 page 1 of 12 Abstract The locations of volcanic islands may be controlled by thin or extending parts of the lithosphere over a partially molten asthenosphere 1 2 by edge effects near the boundaries of thick cratonic lithosphere 3 or by narrow jets of hot mantle rising from deep within the mantle 4 6 Many hotspots are found on or near ridges at lithospheric discontinuities or in extensional environments so high resolution seismic images are required to determine whether it is lithospheric structure stresses in the lithosphere or the deep mantle that is the controlling factor for the location of active volcanoes In this study we perform a simple experiment in which we use basic geometrical arguments and idealized experimental parameters in order to understand the resolution of tomographic images of the upper 400 km of the mantle under Iceland Our results indicate that a narrow deep seated mantle plume is not required in order to explain the observed travel time delays Results of tomographic inversions are often viewed as unique however recent seismic studies of the Icelandic Hotspot have illustrated the non unique nature of these models The geometry of plumes in laboratory and computer simulations is a narrow cylinder capped by a bulbous head that flattens beneath the lithosphere giving an overall mushroom shape to the upwelling 7 9 Deep mantle upwellings are also expected to broaden beneath the 650 km endothermic phase change On the other hand the geometry of upwellings driven by Keller et al IMAGING ICELAND HOTSPOT 03 08 00 page 2 of 12 plate divergence or by lateral changes in lithospheric thickness are expected to be focused at the surface toward the thin or extending regions Iceland is in a particularly complex region different from other volcanic islands because it is located on a very slowly spreading ridge in the youngest narrowest part of the Atlantic Ocean and is bounded by thick cratonic lithosphere The separation of thick cold cratonic lithosphere will generate a deep upwelling which focuses toward the surface to fill in the newly formed gap Passive steady state upwellings such as those found at mature ridges away from thick cratonic lithosphere will exhibit a similar geometry but will not have as deep of an expression Ribe et al 10 showed that a hot narrow rising plume underneath Iceland would produce a bathymmetric signature that is inconsistent with observations 11 and suggested that the anomaly must be cooler and wider than would be expected from a hot rising plume Using seismic methods it is theoretically possible to distinguish between a narrow plume upwelling passive effects due to plate divergence and dynamic upwelling between two cratons however distinguishing between these three scenarios is problematic with real data Using data from a regional broadband seismic experiment ICEMELT Wolfe et al 12 produced three dimensional tomographic images of the mantle beneath Iceland which show a cone shaped low velocity zone beneath the island that is approximately 150 km wide at the surface and is inferred to extend to at least 400 km depth They suggest that this low velocity zone is the expression of a plume that is rising from deep within Earth s mantle However this cone shaped geometry is not consistent with published images of plumes that suggest the existence of a cylindrical plume conduit which feeds a broadening plume head in the uppermost Keller et al IMAGING ICELAND HOTSPOT 03 08 00 page 3 of 12 mantle 7 9 The cone shaped tomographic appears to be defined by the cone of incoming rays and most of the rays are traveling nearly vertically in the upper 400 km beneath Iceland Because of the lack of crossing rays the structure described might be explained by the smearing out of a shallow 200 km depth low velocity anomaly instead of the effect of a deep mantle plume This is the well known parallax problem and is not unlike the problems encountered when a light is shone on an object and one attempts to reconstruct the shape of the object from the shadow it forms on the wall For instance a disc a sphere an ellipsoid a cone and a cylinder will all cast a circular shadow on the wall when oriented in the proper way The only way to determine the three dimensional shape of the object is to observe the shadow when the light source is shone on the object at many different angles We show that the uniqueness and resolution problem encountered when imaging the Icelandic mantle is due to the geometry of the experiment and the lack of crossing ray information Other tomographic studies in areas near hotspots have found that it is impossible to distinguish between a shallow anomaly in the upper 200 km of the mantle and a narrow deep seated plume In a recent study of the Yellowstone Hotspot Saltzer and Humphreys 13 found that both scenarios fit their tomographic inversion results equally well We perform simple tomographic resolution tests in which we calculate idealized synthetic delay times for S waves through a variety of velocity anomalies in the upper 400 km of the mantle and then invert these delays for structure in order to understand how well these anomalies may be resolved in a tomographic inversion Raypaths from sources and receivers Keller et al IMAGING ICELAND HOTSPOT 03 08 00 page 4 of 12 along the two dimensional profile discussed in Wolfe et al 12 were calculated to determine the ray coverage number of ray crossings and angle of ray crossings along this cross section of the model figure 2a The sources and receivers used in this calculation are shown in red in figure 1a and 1b The sources in light red are the earthquakes that occurred further than 90 degrees from Iceland This profile is a best case scenario because it contains the widest range of source to receiver offsets of any profile in the data set and thus it contains the best ray crossing information We expect this cross section to provide an upper bound on the resolution of the full 3D analysis because it has more constraints per degree of freedom than the full model In addition all of the possible


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