Seismics of the Rio Grande
Seismic Reflection Study
of North Western Espanola Basin at Abiquiu
This model is a combination of seismic reflection,
seismic refraction and gravity
from the western boundary of the Rio Grande Rift. It is located in the
Abiquiu embayment in the Espanola Basin in the Northern Rio Grande Rift.
This data was taken along a 16 km long line during the 1990 and 1991 SAGE
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The rift in this area is bounded by northeast striking
faults that offset the Paleozoic/Mesozoic rocks of the Colorado Plateau
from the rift sediments. Go to Stratigraphic section.
The above figure represents the interpretive geologic
cross section at Abiquiu. In B, the cross section has been restored to
reset faults to make the Madera Formation smooth. The amount of horizontal
extension has been calculated to be about 1.1 km and the vertical displacement
is about 0.6 km.
(Figure from Baldridge et al 1994)
Model for Rio Grande Rift
The low velocity zone is representative
of the transition zone and not the rift graben trend. The pattern that
results seems consistent with a clockwise rotation of the Colorado Plateau.
The low velocity zone that strikes NNE to NE currently, is roughly perpendicular
to the WNW-ESE current regional extension trend. This suggests that the
lithosphere could be extending due to modern regional stress, thus the
surface indication of stress at the surface.
Vertically averaged variations
of velocity were contoured at an interval of 1%. It was assumed that the
velcity variation under the stable Great Plains, is zero. The velocity
map is overlaid on a tectonic map
of the Rio Grande Rift.
(Figure from Slack et al 1996)
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SKS Splitting Beneath
the Rio Grande Rift
SKS splitting is caused
by an anisotropic upper mantle. Anisotropy in the upper mantle is caused
by the LPO (Lattice Preferred Orientation) of olivine and other ellastically
anisotropic minerals in the upper mantle (orthopyroxene). With strain the
LPO of olivine (a-axis) aligns perpendicular with the maximum compressional
direction ( uniaxial compression), perpendicular to shortening direction
(simple shear), or aligns to flow direction (simple shear) (Gao et al 1997).
Passive rifting is expected
to have a LPO to align with the direction of extension. Active rifting
involves asthenospheric upwelling and any fabric that would develop the
LPO would depend on the convection (Gao et al 1997).
SKS Splitting results show
a fast direction in the north-northeast direction, perpendicular to the
regional stress field and parallel to subparallel to the rift trend. Splits
ranged from 0.9 to 1.5 seconds. The interpretation of the rift-parallel
fast polarization direction is consistent with a rift-parallel flow in
a convection cell within the mantle. The Rio Grande Rift is above a low
velocity upper mantle that has been determined from teleseismic tomography
(Gao et al 1997).
Another possible model involves
the SKS splitting to be from fossil anisotropy from past tectonic disturbances
(Laramide deformation). Though it is unlikely that any anisotropy could
be preserved with high heat flow associated from upwelling asthenosphere.
Tomography suggests that the 9000C isotherm is about 50 km beneath
rift zones (Gao et al 1997).
(Figure from Gao et al 1997)
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