Plume driven plumbing and crustal formation in Iceland
Richard M. Allen (1),
Guust Nolet (1),
W. Jason Morgan (1),
Kristin Vogfjord (2),
Meredith Nettles (3),
Goran Ekstrom (3),
Bergur H. Bergsson (2),
Palmi Erlendsson (2),
Gillian R. Foulger (4),
Steinunn Jakobsdottir (2),
Bruce R. Julian (5),
Matt Pritchard (4),
Sturla Ragnarsson (2),
Ragnar Stefansson (2).
(1) Dept. Geosciences, Princeton University, USA.
(2) Vedurstofa Islands, Reykjavik, Iceland.
(3) Dept. Earth and Planetary Sciences, Harvard University, USA.
(4) Dept. Geological Sciences, University of Durham, UK.
(5) U.S. Geological Survey, Menlo Park, CA, USA.
J. Geophys. Res. 107 (B8) 10.1029/2001JB000584, 2002
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Through combination of surface wave and body wave constraints we derive
a 3-D crustal S-velocity model and Moho map for Iceland.
It reveals a vast plumbing system feeding mantle plume melt into upper
crustal magma chambers where crustal formation takes place.
The method is based on the Partitioned Waveform Inversion to which we
add additional observations. Love waves from six local events recorded
on the PASSCAL-HOTSPOT and SIL networks are fitted, Sn travel times from
the same events measured, previous observations of crustal thickness are
added, and all three sets of constraints simultaneously inverted for our
3-D model. In the upper crust (0-15 km) an elongated low-velocity region
extends along the length of the Northern, Eastern and Western Neovolcanic
Zones. The lowest velocities (-7%) are found at 5-10 km below the two
most active volcanic complexes: Hekla and Bardarbunga-Grimsvotn. In the
lower crust (> 15 km) the low-velocity region can be represented as a
vertical cylinder beneath central Iceland. Low velocities are not found
along the length of the neovolcanic zones. The low-velocity structure is
interpreted as the thermal halo of pipe work which connects the region of
melt generation in the uppermost mantle beneath central Iceland to active
volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies
from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct
Snaefellsnes rift zone, to 46 km beneath central Iceland. The average
crustal thickness is 29 km. The variations in thickness can be explained in terms
of the temporal variation in plume productivity over the last ~20 Ma, the
Snaefellsnes rift zone being active during a minimum in plume productivity.
Variations in crustal thickness do not depart significantly from an
isostatically predicted crustal thickness. The best fit linear isostatic
relation implies an average density jump of 4% across the Moho. Rare Earth
Element inversions of basalt compositions on Iceland suggest a melt thickness
(i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed
crustal thickness of up to 46 km implies active fluxing of source material
through the melt zone by the mantle plume at up to three times the passive rate.
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© Richard M Allen