Download Crustal Thickness of the Western U.S. (NEW!!! Brought

Crustal Thickness
and Geodynamics
of the Western U.S.
(NEW!!! Brought to
You By EarthScope)
Topography
Gravity
Seismic (Moho)
Anthony R. Lowry
Department of Geology, Utah State University
[email protected]
Heat Flow
Thanks Also To:
Philip Crotwell, University of South Carolina
Jon Kirby, Curtin University, Perth, Australia
Marta Pérez-Gussinyé, CSIC Barcelona, Spain
Christine Puskas, University of Utah
Joel Rackham, Utah State University
Bob Smith, University of Utah
Chris Swain, Curtin University, Perth, Australia
3D Viscosity
• Just a bit on Isostatic Analysis
• Where Geodynamics (& Rheology)
come into play…
• Crustal Thickness estimation
from EarthScope Transportable
Array (TA) seismometers
• Some likely geodynamical
implications of western U.S.
crustal thickness…
Isostasy
Local
Isostasy
Flexural
Isostasy
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 gdz   gdz
column 1
column 2
Balance of vertical stress
in a fluid “asthenosphere”
Equilibrium of horizontal and
vertical stress for an elastic
plate (or “lithosphere”) over a
fluid “asthenosphere”
Isostatic Analysis:
 Uses easily available
data (gravity,
topography)
Topography &/or Bathymetry
Gravity or Geoid
 Parameterized by
flexural rigidity
( rheology)
Effective Elastic Thickness (Te)
 Provides information
about loads (
mass flux processes)
Loads  Mass Flux Processes:
Surface Loads
 Erosion
 Deposition
 Fault
Displacements
 Volcanic
Construction
Subsurface Loads
 Thermal
Variations
 Lithologic
Variations
 Crustal Thickness
(Lower Crustal
Flow)
Gravity & Topography reflect a complicated mix of all mass
flux processes… But if we can separate the loads from their
isostatic response, it narrows the field of candidate processes.
METHOD:
Using equations for observed
topography h and geoid N
including:
 the definition of surface load
 finite amplitude geoid calculation
 flexure of a thin elastic shell over a
self-gravitating, viscous sphere
Then search for Te (& perhaps
other parameters) that minimize
the difference between observed
& predicted coherence
Or equivalently, that minimize
coherence of the load fields
EXAMPLE  The Tharsis Rise, Mars:
Martian Topography:
• Hemispheric
“crustal dichotomy”
• Tharsis rise
~ 5000 m elevation
20% of surface area
Martian Geoid:
• 2000 m Tharsis anomaly
(largest in solar system!)
Controversial Nature of Tharsis Rise:
Volcanic Construction?
( Surface Load!)
Thermal/chemical
buoyancy of a single
mantle plume?
[e.g., Willemann & Turcotte, 1982;
Solomon & Head, 1982]
[e.g., Sleep & Phillips, 1979; Harder
& Christensen, 1996; Harder, 2000]
Probably some combination of both!
“BEST ESTIMATE”
(i.e., minimum load
correlation) 
• ~ 17 km volcanic extrusives
• ~ 12 km flexural deflection
• Small (~ 5%) internal load
Flexural
Rigidity
reflects
rheology,
and hence
distributions
of intraplate
seismicity
Te and Flow Rheology
Reflects both the geotherm and composition…
A Geodynamical Application:
Flat Slab Subduction
Many studies
emphasize
buoyancy of
the down-going
slab and/or
velocity of the
over-riding upper
plate as controls
on subduction
geometry
But these are
poorly correlated
with slab dip in
South America
Isostasy illuminates geodynamics…
Correlation of
South American
flat slab
subduction with
high Te near the
trench suggests
thickness of
viscous upper
plate may control
subduction
geometry!
The Problem:
Recovery from synthetic data, from Pérez-Gussinyé et al. [2007]:
Kirby & Swain [G3, 2008] found poor recovery where
sub-sampled wavelet load fields exhibit chance correlation
Possible Solution:
Use seismic data
to independently
constrain internal
mass fields
E.g., EARS receiver function
estimates of crustal thickness
[Crotwell & Owens, SRL, 2005]
http://www.seis.sc.edu/ears/index.html
EarthScope sampling is promising, but
Receiver function compilations such as EARS
are prone to outliers & errors
Receiver Function Estimates of
Crustal Thickness:
Delay Time
P Ps
Crust
Mantle
P
Ps
• Deconvolve source-time function to get impulse response of
phases converted at impedance boundaries
• Delay time between phase arrivals depends on crustal
thickness and P- & S-velocity
• EARS uses iterative time-domain deconvolution [Ligorria &
Ammon, BSSA, 1999]: well-suited to automation
Contribution of crustal thickness H
versus Vp/Vs ratio K to delay time is
ambiguous…
P Ps
PpPs
PpSs
PsPs
PpSs
PsPs
P
Ps
PpPs
Resolve using reverberations, which
have differing sensitivity to H and K
H–K Stacking:
[Zhu & Kanamori, JGR, 2000]
Ps
PpSs
&
H-K parameters
that predict the
observed phase
delay times
intersect at
a point in
parameter
space
PsPs
PpPs
PpSs
PsPs
P
Ps
PpPs
H–K Stacking:
[Zhu & Kanamori, JGR, 2000]
(EARS H–K stack for station COR)
Ps
PpSs
&
PsPs
PpPs
P
Ps
PpPs
Method stacks
observed
amplitudes at
delay times
predicted for
each phase, for
all earthquakes.
Max stack
amplitude
should reveal
PpSs
true crustal
PsPs
thickness &
Vp/Vs ratio.
The Problem:
The Moho is
not the only
lithospheric
impedance
contrast…
And crustal
thickness is
not constant
(EARS H–K stack for station Y35)
EARS can yield
extreme crustal
thickness or
unlikely changes
over short
distances.
Crustal
Thickness
H
Vp/Vs
Ratio
K
Despite outliers, H & K have properties consistent with a
fractal surface…
Station TA.P10A (Central Nevada)
2
1
The semivariance
properties can be
used to estimate
a “most likely”
crustal thickness
and Vp/Vs ratio by
optimal interpolation
from nearby sites.
Station TA.P10A (Central Nevada)
Can also model
gravity predicted
by estimates…
2
1
1
2
And find a “most likely”
model with uncertainties.
Station TA.S16A (Central Utah)
Gravity Model
Likelihood Filter
Optimal Interp.
Likelihood Filter
Combined
Station TA.P10A (Central Nevada)
 Unlikely stack amplitude maxima are
downweighted using likelihood statistics
Method removes outliers, provides a map
that makes sense (with some exceptions)
The reliable (i.e.
western) parts of
the map confirm
some alreadyknown aspects
of western U.S.
geodynamics &
also suggest
a few new
interpretations…
EARS-derived
Old refraction data
The reliable (i.e.
western) parts of
the map confirm
some alreadyknown aspects
of western U.S.
geodynamics &
also suggest
a few new
interpretations…
Raw EARS estimates
Weighted stack estimates
And interesting patterns show up in Vp/Vs
Perhaps the
most intriguing
implications
arise from the
gravity modeling
however…