Geology of the Salton Trough region of Southern California

Geology of the Salton Trough
region of Southern California
Erik Olson
The Salton Trough region
Landward portion of the rifting seen in the
Gulf of California
Highest strain rates in the entire U.S.
Some of the highest seismicity rates in all
of California
Several major faults : Ellsinore, San
Jacinto, San Andreas and Imperial
Several recent earthquakes on non-major
faults : Big Bear, Landers, Joshua Tree,
and Superstition Hills
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Map of the Salton Trough region
Sylvester, 1991
Durmid Hill Region Geology
Two late Cenozoic units: Shavers Well
Formation, and the Borrego Formation
Shavers Well: 1100m thick in this region,
arkosic sandstones and conglomerates
Borrego: ~1600m thick in this region,
siltstones and mudstones, and contains
Bishop ash deposit
These sedimentary units are underlain by
Tertiary metasedimentary basement and
mafic intrusive rocks
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Durmid Hill Region Tectonics
Located at a right step in the San Andreas
Fault, which is oriented 7˚ oblique to the
inferred plate slip vector
Numerous folds and faults occur in the
sedimentary units around this region of the
San Andreas fault
Estimated 23-25 mm/yr of slip, 2mm/yr of
creep observed
Last major earthquake on SAF ~1663
Geometry of the SAF in the Salton
Trough region
Sylvester, 1991
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Durmid Hill Region Shortening
A wide variety of fold sizes exist,
predominately thin limbs with thick hinges
Fold axes are less parallel to the SAF with
increasing distance, to ~900m
As much as 40%-60% shortening
recorded primarily in folds, but also in
small scale faults and folds.
Fold Geometry
Burgmann, 1991
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Durmid Hill Region Extension
Extension parallel to fold axes calculated
from boudinaged Bishop Ash bed
Initial calculations suggest 58% average
extension
With the inclusion of inhomogeneous
matrix flow the average extension value is
68.6%
Durmid Hill Region Uplift
Uplift in the region has been estimated by
many methods:
– 1mm/yr : Leveling array 1985 – 1987
– 3mm/yr : Shoreline uplift from ~1676
– 1-2mm/yr : Stream cutting over 2200 years
– .27-1.76mm/yr : Stratigraphic uplift analysis
over 740,000 years
Average rate of uplift estimated to be .52mm/yr over the last 740,000 years
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Durmid Hill Region Transpression
Does a model of two rigid blocks
converging obliquely fit the system?
Using 18 steps of incremental strain, the
model does approximate much of the
observed deformation
The model predicts 50.2% compression
perpendicular to fold axes, 64.4%
extension, and .5mm/yr of uplift
Transpressional Model for Durmid
Hill
Burgmann, 1991
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Displacement accommodated by
Durmid Hill
By calculating the shear strain based on
fold orientation, it is possible to calculate
the displacement across the shear zone
In the Durmid Hill region this displacement
was calculated to be 2.23km over the last
740,000 years
This displacement is 8.6-13.2% of the
deformation that occurred in the area
Mecca Hills Region
Located near a portion of the SAF that
oriented is oblique to plate motion (similar
to Durmid hill)
Total elevation ~200m above sea level
Abundant lizards, occasional snakes,
bighorn sheep, ravens hawks and owls
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Mecca Hills Geology and Tectonics
Transpression similar to that explained
above is the primary cause of uplift of the
hills in this region
The large anticline is formed from
deformed Cenozoic sedimentary rocks
once flat lying in the region
Mecca Hills Cross section
Burgmann, 1991
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The San Jacinto Fault; yet another
piece of the Salton Trough puzzle
Using GPS and a USGS trilateration
network in the Salton Trough, Anderson et
al. 2003 examined the regional
deformation of the Salton Trough
These researchers found the strain rate on
the San Jacinto Fault is at least as high as
that on the San Andreas fault in this
region, which was attributed to a higher
slip rate on the San Jacinto fault
Survey Site of Anderson et al.
Anderson et al., 2003
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Larger strain rate near the San
Jacinto fault
Anderson et al., 2003
Conclusions
Transpression along the Southern San
Andreas, caused by a 7˚ divergence of
fault strike from plate motion has resulted
in the uplift of several hills, including the
Durmid Hills and Mecca Hills
Shear zones around the fault may
accommodate a significant portion of the
plate boundary deformation
The Salton trough as a whole is a complex
structural region experiencing active
deformation
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References
Anderson, G., Agnew, D.C., Johnson, H.O. Salton
Trough Regional Deformation Estimated from Combined
Trilateration and SurveySurvey-Mode GPS Data. BSSA, 93 6,
24022402-2414 2003
Burgmann,
Burgmann, R. Transpression Along the Southern San
Andreas Fault, Durmid Hill, California. Tectonics, 10, 6
11521152-1163, 1991
Sylvester, A.G. Geologic Structure, Transpression,
Transpression, and
Neotectonics of the San Andreas Fault in the Salton
Trough, California. IN Geological excursions in Southern
California and Mexico. Walawender,
Walawender, M.J. (ed) 378378-387,
1991
Discussion Questions
Is the San Jacinto fault taking over the role
as the primary fault in this region of
California? What about the Mojave?
What do the observations at Durmid and
Mecca Hills suggest is occurring at the
“Big Bend” in the SAF?
Where are other regions on the San
Andreas fault where the surrounding shear
zone may be accumulating a significant
portion of the deformation?
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