NEW_Accomplishments.indd - IRIS

2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES
Small-Scale Variations in Seismic Anisotropy Near Kimberley, South Africa
Matthew J. Fouch, David R. Bell • Arizona State University
Paul G. Silver • Carnegie Institution of Washington
Jean N. Lee • Harvard University
Using broadband seismic data from the IRIS PASSCAL broadband telemetered
array installed near Kimberley, South Africa, we place new constraints on seismic
anisotropy in an area of extensive mantle modification within an Archean cratonic
setting (Fouch et al., 2004). The array was installed as part of the multidisciplinary
Kaapvaal Project, which extensively studied the region via petrologic, geochemical,
and seismic means, and provided a unique opportunity to meld the results of a broad
range of datasets to examine processes of cratonic evolution.
Splitting time (sec)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
A
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35
Projected distance from array center (km)
Figure 2. Stacked shear wave splitting time versus
distance along projection of array midline from
Figure 1a. Thin lines denote 95% confidence limits
for each station. Splitting times show a linear decrease
from NW to SE (A to A’), with the exception
of stations Kaapvaal stations SA18 and SA19 and
IRIS/GSN station BOSA (white circles).
We analyze the shear wave
splitting of SKS phases recorded
near Kimberley. These phases
exhibited consistent fast polarization
directions of ~NE SW (Figure
1a) and splitting times that ranged
from 0.15 s in the SE to nearly
0.75 s in the NW regions of the array
(Figures 1a, 2). Multi-channel
cross correlation of relative arrival
times of teleseismic SKS phases
across the array revealed clear
azimuthal variations and a relative
arrival time range comparable to
the shear wave splitting delay time
range (Figures 1b, 1c). A like analysis
of teleseismic PKPdf phases
did not exhibit significant differences
in relative arrival times across the array (Figure 1d). The combined relative
arrival time and splitting analyses indicate that a significant change in the strength
of anisotropic structure is required across the region. Our results are most consistent
with a model in which variations in seismic anisotropy near the Kimberley region are
constrained to the lithosphere with an average anisotropic strength of ~1.8%, extend
to depths no greater than 150 km, and are primarily controlled by a significant change
in the strength of anisotropy within the lateral bounds of the seismic array.
We hypothesize that the observed seismic anisotropy is due to strain induced
lattice preferred orientation of olivine caused by the amalgamation and deformation
of the eastern and western blocks of the Kaapvaal craton. This collisional process
mechanically weakened the region to generate the observed mantle fabric, which
was exploited by significant rifting events that may have subsequently enhanced
the regional fabric. Our results support the notion that seismic anisotropy beneath
Archean continental regions is not created (or alternatively not preserved) during
initial continental formation, but instead is generated during subsequent significant
mantle deforming events.
Fouch, M.J., P.G. Silver, D.R. Bell, and J.N. Lee, Small-scale variations in seismic anisotropy near Kimberley,
South Africa, Geophys. J. Int., 157, 764-774, doi: 10.1111/j.1365-246X.2004.02234.x, 2004.
Silver, P.G., S.S. Gao, K.H. Liu, and the Kaapvaal Seismic Group, Mantle deformation beneath southern
Africa. Geophys. Res. Lett., 28, 2493 2496, 2001.
A'
29S
29S
24.5E
24.5E
Figure 1. Results from shear wave splitting
and relative delay time analysis of
SKS and PKP phases recorded at the
Kimberley telemetered array. Proposed
boundary of strong/weak anisotropic
domain is delineated by dashed line.
a) Stacked shear wave splitting results.
Azimuth of black bars denote fast polarization
direction; thin black bars denote
95% confidence limits for fast polarization
directions; size of circles is scaled to
splitting time. For figures 1b-1d, crosses
(faster than average arrivals) and circles
(slower than average arrivals) are scaled
to magnitude of relative arrival time.
Triangles denote relative arrival times
of ± 0.025 s.
25E
25E
25.5E
28.5S 28.5S
29S
SKS delays; SE baz
FAST
0.1 s
0.2 s
SLOW
25.5E
28.5S 28.5S
29S
dt = 0.75 s?
A
SKS delays; NE baz
FAST
0.1 s
0.2 s
SLOW
24.5E 25E 25.5E
28.5S 28.5S
FAST
STRONG
ANISOTROPY
PKP delays
0.1 s
0.2 s
24.5E 25E 25.5E
28.5S ANISOTROPY
28.5S
SKS splitting
0.25s
s
0.50s
s
0.75s
SLOW
A'
WEAK
dt = 0.15 s?
24.5E 25E 25.5E
Colesberg Lineament
A
B
C
D
29S
29S
29S
29S
105