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2.4
2.2
2.0
1.8
1.6
VP / V
S
mod
ern
subd
uctio
n zo
nes*
rock
s &
min
eral
s*Lz
dQ
tz
isot
ropi
cca
lcul
ated
usi
ngA
bers
& H
acke
r[2
016]
Con
drey
Mou
ntai
n Sc
hist
She
ar Z
one
cum
ulat
ive
sign
uatu
re o
fm
iner
al a
niso
trop
y &
frac
ture
sca
lcul
ated
usi
ng M
SAT
**A
udet
& K
im, 2
016
Stage 1Stage 2Stage 3
1. Rapid underplating
downgoing plate
interfaceshear zone
increasingthicknessover time
overriding plate
up to4 km
ė ~ 10-13
2. Non-accretionary period
downgoing plate
interface shear zone
up to0.5 km
overriding plate
3. Onset of underplating
ė ~ 10-13
downgoing plate up to2 km
overriding plate
interfaceshear zone
Carolyn M. Tewksbury-Christle1*, Dr. Whitney M. Behr1, & Dr. Mark A. Helper2
1: Department of Earth Sciences, ETH Zürich, 2: Department of Geological Sciences, Jackson School of Geology, University of Texas at Austin
Rock record constraints on the seismic signature of subduction interface shear zones SGTETHZ
VP
/ VS
oceanic mantleov
errd
ing
plat
e
sediments
oceanic crust
viscous interface shear zone
MOHO
dow
ngoi
ngpl
ate
plate interface
Low Velocity Zone
LowVelocityZone3-5 km
SubductionInterfaceShear Zone0.5 - 4+ km
overriding platedowngoing plate
shear zone thickness & occupying lithologies vary temporallycombined detailed geochronology & structural mapping to track strain localization evolution
seismic signature is highly anisotropic & VP/VS are elevatedaccounted for mineral anisotropy & fracture porosity measured from the CMS rock record
The fossil subduction interface shear zone preserved in the CMS records:
1) distributed deformation comparable to or within modern LVZ thicknesses
2) comparable seismic signatures to modern LVZs
VP
/ VS
VP
/ VS
ė ~ 10-12
© Tewksbury-Christle, Behr, & Helper. All rights reserved. Work is in preparation for submission to Geophysical Research Letters. Please contact [email protected] with questions.
Used to characterize:
1) thickness & lithologies of the actively deforming interface
2) seismic signature for comparison to modern subduction zones
Condrey Mountain Schist (CMS) records Late J - Early K subduction and punctuated underplating at 350-450°C and 0.8-1 GPa
Graphitic Mica SchistEpidote Blueschist
CMSi (Epidote Blueschist Facies)
Serpentinite Dioritic to tonaliticorthogneiss
Greenschist
Graphite Phyllite
CMSo(Greenschist Facies)
X X’
0 5 km V.E. = 1x
fault
thrust fault
inferred fault
contact
map & cross-section after Helper, 1985additions as in Tewksbury-Christle et al., in prep
prograde ductile thrusts
mappedductilethrust
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.04
1.23
2.00
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.39
1.60
2.26
1.5
1.6
1.7
1.8
1.9
2.00
1.47
1.83
OR
CA
after Snoke &Barnes, 2006
42°00’
41°45’
X X’
123°00’ 123°10’
0 5 km
ORCA
124° 123°
43°
42°
41°
Medford
Redding0 30 km
KlamathTerranes
Franc
iscan
CondreyMountain
Schist
Compl
ex
