Transcript Document

LA RISTRA: Lithospheric and Upper Mantle Structure of the Rio Grande Rift
Abstract
Rick Aster, New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, NM
Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment show a
pure shear extension mechanism for the Rio Grande rift (RGR) at the lithosphere scale. Receiver function results show
crustal thickness ranging from 45 to 50 km beneath both the Colorado Plateau and the Great Plains, with crustal
thinning to a minimum of approximately 37 km centered beneath the RGR axis. The centering of the thinnest crust on
the rift axis indicates that the deep crust has undergone primarily pure shear extension. Inversion of LA RISTRA
surface wave data and tomographic inversion of teleseismic body-wave delay times for upper-mantle structure show a
broad low velocity region, also centered beneath the rift axis. This low-velocity region is interpreted as rift-centered
lithospheric extension, indicating that lithospheric deformation, like that of the deep crust, is also primarily pure shear.
A pure shear extensional mechanism for the RGR is consistent with geochemical evidence and regional gravity data
which both suggest lithospheric thinning, with the greatest thinning centered beneath the rift axis. We find distributed
lithospheric extension that is roughly twice the width of the surface expression of the rift. This geometry suggests less
concentrated vertical mantle upwelling than would arise from more localized deformation, which may have led to
decreased partial melting, magmatism, and less vigorous convection that might have otherwise occurred. While the
upper crust along the RGR has undergone brittle deformation expressed as a series of asymmetric grabens, the lower
crust and mantle lithosphere of the RGR have undergone distributed ductile deformation, symmetric about the rift axis,
with a broad mantle signature that was likely controlled by pre-rift structure and thermal conditions. New data is
currently being collected along an 18-station overlapping extension, RISTRA 1.5, that traverses the northwestern edge
of the Colorado Plateau and extends into the easternmost Great Basin.
Receiver Function Deconvolution
•We use traditional receiver functions calculated from teleseismic body waves
(assumed to be nearly planar) by deconvolving the vertical seismogram from the
radial horizontal seismogram. In the Fourier domain, this can be characterized by a
spectral division:
R(w )
H(w ) =
V(w )
source * path * horizontal Earth response near receiver
=
source * path * vertical Earth response near receiver
•This produces a seismic trace Fourier transform that is ideally solely a function of the
Earth’s structure near the receiver, a “Receiver Function”.
•Receiver functions isolate near surface (crust and upper mantle) structure from
teleseismic body waves.
Ray paths
LA RISTRA
Earth’s Surface
•Colorado Plateau, Rio Grande rift, Great
Plains, Seismic Transect Experiment
Raypaths for commonly observed receiver
function phases.
Jim Ni, New Mexico State University, Department of Physics (MSC 3D), Las Cruces, NM
Dave Wilson, Wei Gao and Steve Grand, University of Texas, Austin, Department of Geological Sciences, Austin, TX
W. Scott Baldridge, Los Alamos National Laboratory, Earth and Environmental Sciences Division, MS D462, Los Alamos, NM
Steve Semken, Arizona State University, Department of Geological Sciences, Tempe, AZ
Receiver
function
migration
Pure Shear
End member
modes of
lithospheric
extensional
deformation
•Kirchhoff pre-stack
depth migration.
•Each of the three typically observed phases is migrated onto a separate 2-d output grid.
•Output grids can then be compared to validate the velocity model, and/or stacked to reduce
imaging artifacts. Where the different receiver function phases coincide in space, they will
stack constructively, thus suppressing multiple reverberation and other imaging artifacts.
Timing of receiver function phases depends on:
•18.1 ± 3.6 km station spacing.
•P-wave velocity
Delaware
Basin
•S-wave velocity
Moho
meters
•57 seismic station, 950 km-long,
broadband passive PASSCAL experiment
(54 stations on main transect).
•Currently being extended across the
Colorado Plateau and into the Great Basin
(RISTRA 1.5)
Receiver Function
Colorado Plateau
Station
RGR #
•Hand dug vaults successfully coordinated with
diverse landowners and agencies.
•Continuous recording at 20 samples/second.
RISTRA 1.5 stations (2004-2006)
extending from the central Colorado
Plateau into the Great Basin.
•Data openly available from the IRIS Data
Management Center.
A subtlety in calculating receiver functions is proper regularization of the commonly
ill-posed deconvolution problem. We employ a receiver function estimation technique
which takes advantage of characteristic moveout to enhance receiver function arrivals,
while filtering out features attributable to noise and deconvolution instability. We do
this by filtering receiver function gathers in the frequency-pseudowavenumber domain
to remove spurious apparent high--moveout receiver function features while preserving
true receiver function phases (Wilson and Aster, 2005).
Tectonic Setting
•RISTRA is oriented approximately along
the age gradient of continental accretion.
(4x vert. Exagg.)
RGR
Upper mantle Swave velocity
model from
surface wave
inversion
This, along with RISTRA seismic imaging results, favors a pure shear rifting
mechanism for the Rio Grande rift.
The observation that RGR deformation in the mantle
lithosphere is over twice the width of the surface expression of
the rift may have played an important role in rifting.
A narrow zone of concentrated lithospheric stretching leads
to increased vertical transport of asthenospheric material.
Great Plains
•Increased partial melting
•Increased convection to further drive rifting
West et al.
A laterally- distributed zone of lithospheric stretching leads to
lesser vertical transport of asthenospheric material.
(after West et al.)
•Less heat produced, reduced partial melting, and less
volcanism.
Ps Moho
•RISTRA encompasses the southern Great
Plains, the Rio Grande Rift, the Colorado
Plateau, and the easternmost Great Basin.
(from Mutter et al., 1988)
Low velocity region is symmetric about the rift axis
•Laramide tectonics immediately prior to
rifting caused widespread deformation
including the uplift of the Rocky
Mountains
Low velocity region is over twice the width of the surface expression of the rift.
(Compiled from Woodward, 1984; Baldridge et al., 1995; Reed and others, 1993)
RISTRA Data
PpPs Moho
•Deployed from July, 1999 through May, 2001.
•401 earthquakes recorded with mb ≥ 5.6 and 129
earthquakes with mb ≥ 6.0.
•RISTRA is approximately on-azimuth for
Alaskan/South American earthquakes.
•Decreased small-scale convection
Summary:
Crustal thinning and upper mantle lithospheric deformation beneath the Rio Grande rift is
highly symmetric about the rift axis, indicating a pure shear rifting mechanism at the
lithosphere scale. The broad lateral distribution of lithospheric deformation may have led
to decreased partial melting, magmatism, and reduced convection that otherwise could
have further driven the rifting process. This geometry is most likely the result of a
combination of a regionally elevated geotherm at the time of rift initiation and low strain
rates. Also, pre-rift tectonism in the RGR region may have weakened the lithosphere
allowing more laterally distributed ductile deformation. This may explain why the RGR
has only experienced relatively small volumes of rift related volcanism when compared to
other rift systems (5-10% of the Kenya rift).
•The RGR has been opening for the last 30
Ma, along with widespread western U.S.
Cenozoic extension.
mb ≥ 6.0
•This transition occurred between 8 and 4 Myr ago in the central RGR, and was relatively
symmetric about the rift axis (Baldridge et al., 1991).
•The decompensative gravity anomaly (Bouger gravity corrected for changes in crustal thickness
and composition) shows approximately symmetric thinning of the lithosphere which tracks the
axis of the rift (Cordell et al., 1991).
Crustal thinning is centered on, and is roughly
symmetric about the rift axis, favoring a pure
shear deformation model. The Moho shows
complex impedance structure and topography on
the several km scale.
Colorado Plateau
Greatest lithospheric thinning
offset laterally from greatest
crustal thinning
•Isotopic geochemical data (eNd values) showing an evolution from magmatism with an
“enriched” source region (lithospheric source) to a “depleted” source region (asthenospheric
source) (Perry et al, 1987; 1988).
RGR
Receiver Function Amplitude
(after Buck, 1988)
Other evidence supporting symmetric thinning of the lithosphere:
Great Plains
•Distance to earthquake (incidence angle of the
incoming wavefield).
Simple Shear
Symmetric lithospheric
cross-section
Migrated cross section with imaging artifacts suppressed via multimode stacking (Wilson et al., 2005)
•Depth to the discontinuity
•All stations equipped with IRIS PASSCAL STS-2
(120 s) seismometers.
www.ees.nmt.edu/Geop/Ristra/ristra.html
Michael West, Alaska Volcano Observatory, Geophysical Institute, University of Alaska, Fairbanks, AK.
S-wave velocity model from body wave tomography (after Gao et al.)
pseudowavenumber
Acknowledgements: Essential field and data handling assistance was provided by the IRIS PASSCAL Instrument
Center at the New Mexico Institute of Mining and Technology (NMT). This research supported by NSF grants EAR
9707190 and EAR 9706094, Los Alamos National Laboratory Institute for Geophysics and Planetary Physics, the
New Mexico State University Arts and Sciences Research Center, and the NMT Geophysical Research Center.
Persons wishing to conduct geological investigations on the Navajo Nation must first apply for and receive a permit
from the Navajo Nation Minerals Department, PO Box 1910, Window Rock, Arizona, 86515, 928-871-6587.
References:
•Regularized receiver function deconvolution decreases sensitivity to instabilities in individual
receiver function deconvolution operations.
Baldridge, W.S., F.V. Perry, D.T. Vaniman, L.D. Nealey, B.D. Leavy, A.W. Laughlin, P. Kyle, Y. Bartov, G. Steinitz, and E.S. Gladney, Middle to Late Cenozoic Magmatism of the Southeastern Colorado
Plateau and Central Rio-Grande Rift (New-Mexico and Arizona, USA) - A Model for Continental Rifting, Tectonophysics, v. 197, no. 2-4, pp. 327-354, 1991.
•Phases which are consistent between adjacent receiver functions are enhanced.
Cordell, L., Y.A. Zorin, and G.R. Keller, The Decompensative Gravity Anomaly and Deep Structure of the Region of the Rio-Grande Rift, J. Geophys. Res., v. 96, n. B4, pp. 6557-6568, 1991.
Buck, R.W., F. Martinez, M. S. Steckler, J.R. Cochran, Thermal Consequences of Lithospheric Extension: Pure and Simple, Tectonics, v. 7, no. 2, pp. 213-234, 1988.
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