The Iceland hotspot fundamentally controls the geophysical and geochemical
character of the adjacent Mid-Atlantic Ridge (MAR). This hotspot-ridge system offers
an excellent opportunity to study the effect of an anomalous mantle source on melt
generation and upper-mantle flow beneath a spreading center. Although the uppermantle character of the Iceland hotspot is now relatively well studied [e.g., Wolfe et al.,
1997, Allen et al., 1999], long-standing and fundamental questions remain about the
nature of mantle flow and melt generation beneath the Reykjanes and Kolbeinsey Ridges
and how they are affected by the hotspot. Models of ridge-hotspot interaction differ on
how hotspot material is transported along the ridge. In one view, it is thought that the
hotspot material is channeled down the ridge at shallow depth. In an opposing view, the
plume expands at deeper levels in a radial manner from its source, without channeling.
The importance of passive versus buoyant flow within the melt zone is also uncertain.
These models predict distinct differences in the character of the partial-melt zone beneath
the ridge, including its width, depth, and magnitude, and how these properties change
with distance from the Iceland. They also predict differences in flow-induced mineral
fabric within and beneath the Atlantic lithosphere. These models can be differentiated
using accurate estimates of seismic velocities and anisotropy in the upper-mantle beneath
and adjacent to the Reykjanes and other portions of the MAR.
This project had three overlapping seismological objectives that were motivated by
our principle scientific goal to characterize the nature of mantle flow and melt supply
beneath the Reykjanes and Kolbeinsey Ridges and to determine the influence of the
Icelandic hotspot on the seafloor-spreading system. We sought to:
1) Determine lithospheric thickness, thermal structure and melt distribution in the
mantle and crust beneath the Reykjanes and Kolbeinsey Ridges through
surface-wave analyses.
2) Determine the nature of mantle upwelling beneath the Reykjanes and
Kolbeinsey Ridges through surface-wave studies of seismic anisotropy.
3) Evaluate the influence of Iceland on upwelling beneath these ridges by
estimating fabric-induced anisotropy elsewhere in the Atlantic.
Summary of Activities at LDEO:
Analyses at LDEO and Hawaii proceeded in parallel, with substantial participation and
interaction between the groups to ensure a coherent analysis. The Hawaii group took the
lead on an innovative surface-wave tomography study of the upper-mantle beneath
Iceland and the adjoining Reykjanes ridge, in an effort to address goals 1 and 2 (above).
The LDEO group led a companion analysis of upper-mantle structure beneath the
Kolbeinsey ridge, the MAR adjacent to the Azores, and the MAR adjacent to Ascension
island, thereby addressing goal 3. Gaherty’s efforts at Lamont in support of these efforts
included:
1) For the 3D Reykjanes analysis, completed initial processing of the HOTSPOT and
ICEMELT recordings of Reykjanes events, and provided these data in a useful
format to co-PI Dunn and his student (Andrew Delorey) at Hawaii. Assisted Delorey
in processing and preparation of the seismic data, including providing SAC macros
and sorting out problems with instrument responses. Provided feedback on choice of
modeling parameterization, model uncertainty, scientific focus, and subsequent
implications. Advised a summer intern (Katie Kirsch) on a side project, modeling Pwave triplications of Reykjanes events recorded on Iceland to assess the thermal
impact of hotspot on the 410 km discontinuity. Assisted with preparation of JGR
manuscript on 3D structure south of Iceland.
2) Collected a large dataset of broadband recordings of earthquakes located within the
Atlantic basin, recorded at seismic stations within the basin (islands) or along the
Atlantic margins. Calculated synthetic seismograms for these event-station pairs, and
selected a subset of these data that indicate purely oceanic paths for further analysis.
Chose a data set focused on structure near ridge-centered hotspots of Iceland, Azores,
and Ascension. Utilized a cross-correlation procedure to measure frequencydependent phase delays (travel times) of surface waves traversing these regions.
Inverted these data for path-average models as a function of seafloor age, thereby
providing a means to evaluate the variable impact of the hotspots on near-ridge
thermal and anisotropic structure. Presented results in two AGU talks, posters at two
IRIS workshops, and a poster at a MARGINS workshop. Lead the writing of Gcubed manuscript on comparative study of hotspot-ridge interaction in the Atlantic.
Findings to date:
For our 3D analysis of hotspot-ridge interaction along the Reykjanes ridge, we modeled
broadband records of fundamental mode Love and Rayleigh waves that were generated
by regional earthquakes occurring in the North Atlantic to the south of Iceland, and were
recorded by the HOTSPOT and ICEMELT arrays and the GSN station BORG, located on
Iceland. The phase, group arrival time, and amplitude information were measured for
narrow-pass filtered waveforms over the period range of 9.5-100s. Over 12,000 such
measurements were included in a two-part inversion for mantle and crustal shear wave
velocity structure and seismic anisotropy. In a vertical plane oriented normal to the ridge
axis, the shear wave velocity structure contains a broad and deep low velocity zone in the
upper mantle beneath the Reykjanes Ridge. A joint analysis of the seismic structure with
gravity data reveals that the low velocities are consistent with elevated temperatures (5075˚) and only a very small amount of melt (<1%). This study shows that plume material
spreading outward beneath the Reykjanes Ridge from Iceland is not confined to a
lithospheric channel beneath the ridge. Away from the ridge, shear wave anisotropy
indicates a predominant horizontal alignment of the fast-axes of anisotropic crystals
above 20-30 km depth, which can be interpreted as horizontal, ridge-perpendicular flow.
However, within ±200 km of the ridge, the anisotropy indicates a general vertical
alignment of the fast axes or an alignment such that the fast axes point along the ridge.
The transition to this type of anisotropy coincides with the appearance of increased
hotspot-ridge interaction approximately 20 Myr ago, indicating that plume flow outward
from the hotspot has largely disrupted mantle corner-flow beneath the ridge.
The parallel analysis of other stretches of the MAR suggests that this strong hotspot
influence is largely unique to Iceland and the Reykjanes, and that the hotspot influences
not only temperature and fabric along the RR, but a unique compositional structure as
well. We invert frequency-dependent phase delays from MAR events recorded on
Iceland, the Azores, and Ascension Island to estimate 1-D mean shear velocity and radial
shear anisotropy profiles in the upper 200 km of the mantle within two seafloor age
intervals: 5-10 Ma and 15-20 Ma. Mean shear velocity profiles correlate with apparent
hotspot flux: lithosphere formed near the low-flux Ascension hotspot is characterized by
high mantle velocities, while the MAR near the higher-flux Azores hotspot has lower
velocities. The impact of the high-flux Iceland hotspot on mantle velocities along the
nearby MAR is strongly asymmetric: the lithospheric velocities near the Kolbeinsey ridge
are moderately slow, while velocities near the Reykjanes ridge are much slower. Within
each region, the increase in shear velocity with age is consistent with a half-space cooling
model, and the velocity variation observed between Ascension, the Azores, and
Kolbeinsey are consistent with approximately ±75o potential-temperature variation along
axis. In comparison, the Reykjanes lithosphere is too slow to result purely from halfspace cooling of a high temperature mantle source. We speculate that the anomalously
low shear-velocities within the lithosphere produced at the Reykjanes ridge result from
high asthenospheric temperatures of +50-75K (consistent with the 3D tomography
results) combined with ~12% (by volume) gabbro retained in the mantle due to the
imbalance between high hotspot-influenced melt production and relatively inefficient
melt extraction along the slow-spreading Reykjanes. Radial shear anisotropy in the upper
150 km also indicates an apparent hotspot influence: mantle fabric near Ascension is
quite weak, consistent with previous models of anisotropy produced by corner flow
during slow seafloor spreading. The fabric near the Azores and the Kolbeinsey ridge is
stronger, suggesting that the hotspot increases mantle deformation beyond that produced
by slow-seafloor spreading in these regions. In none of these regions do we find
evidence of the apparent vertical fabric observed beneath the Reykjanes. The asymmetry
of both isotropic (thermal) and anisotropic structure between Reykjanes and Kobeinsey
ridges is particularly striking – dynamic modelsof hotspot-ridge interaction must account
for this asymmetry.
Contributions:
1) Discipline (seismology and geodynamics): Extension of technique of modeling
2D surface-wave propagation to Rayleigh waves. Evidence that Iceland hotspot
spreads broadly beneath the North Atlantic south of Iceland, with little suggestion
of channeling down the ridge. Quantification of thermal anomaly in the
asthenosphere beneath the Reykjanes at ~50K. Suggestion that this temperature
anomaly produces excess melt that is trapped below the Moho, effectively
impregnating the oceanic lithosphere with ~10% gabbro in this region.
Anisotropic character along the Reykjanes suggests a strong hotspot influence, at
least over the last 20 Myr. In contrast, the other stretches of hotspot influenced
stretches of the MAR show only modest velocity variations that are consistent
with along-axis temperature variations.
2) Other science: mantle petrologists and geochemists often assume 100%
efficiency of melt extraction near ridges, despite some evidence to the contrary
(e.g. Cannat and others). This study provides further evidence that the process of
melt extraction is complex, and can be significantly perturbed by non-steady state
behavior such as hotspot-ridge interaction. They also will be interested in the
results demonstrating strong asymmetry of the seismic character of hotspot-ridge
interaction north and south of Iceland; the spatial distribution of temperature
variations beneath the Reykjanes; and the temperature variations inferred along
the 6 stretches of the MAR.
3) Human resources: furthered the research and teaching careers of two productive
scientists; provided training for a graduate student (at Hawaii) and an
undergraduate student (at LDEO)