gravity constraints on basin geometry and fault …earthsci.fullerton.edu/parmstrong/undergrad...
TRANSCRIPT
GRAVITY CONSTRAINTS ON BASIN GEOMETRY AND FAULT LOCATIONS IN SOUTHERN CADIZ VALLEY, EASTERN CALIFORNIA SHEAR ZONE
An Undergraduate Thesis
Presented to
The Faculty of
California State University, Fullerton
Department of Geological Sciences
In Partial Fulfillment
of the Requirements for the Degree
Bachelor of Science
in Geology
By
David Carpenter
2012
Phillip Armstrong, Faculty Advisor
GRAVITY CONSTRAINTS ON BASIN GEOMETRY AND FAULT LOCATIONS IN SOUTHERN CADIZ VALLEY,
EASTERN CAIFORNIA SHEAR ZONE
Undergraduate Thesis Presented to California State University, Fullerton
Written by David Carpenter
Advised by Phillip A. Armstrong
TABLE OF CONTENTS
1 ABSTRACT ......................................................................................................................... 2
2 INTRODUCTION ................................................................................................................. 3
3 GEOLOGIC BACKGROUND .............................................................................................. 4
4 EQUIPMENT AND FIELD PROCEDURES ......................................................................... 5
5 METHODOLOGY ................................................................................................................ 7
6 DATA PROCESSING, REDUCTION, AND MODELING ..................................................... 9
7 DISCUSSION AND INTERPRETATION OF RESULTS .....................................................11
7.1 GRAVITY LINE 1 ................................................................................................................................. 11 7.2 GRAVITY LINE 2 ................................................................................................................................. 12 7.3 INTERPRETATION OF GRAVITY LINES 1 AND 2 ...................................................................................... 12
8 CONCLUSION ...................................................................................................................14
9 REFERENCES ...................................................................................................................15
LIST OF FIGURES Figure 1 Site Locations Map Figure 2 Cross Section of Cadiz Valley Figure 3 Geology Map of Cadiz Valley Figure 4 Topography with Relative Isostatic Anomaly Graph Figure 5 Gravity Model of Line 1 Figure 6 Gravity Model of Line 2 Figure 7 Interpretations Map Figure 8 Regional Isostatic Anomaly Map
2
1 ABSTRACT
The southern Mojave Desert is a tectonically complex area that is largely affected by
deformation associated with the Eastern California Shear Zone. Cadiz Valley is a NW-SE –
oriented valley located east of the Sheephole Mountains that is bound by the Iron Mountains on
the east and Calumet Mountains on the west. Basement rocks include the Cretaceous Iron
Mountain and Coxcomb Mountain Intrusive Suites, which are mostly comprised of granodiorite
and granite. Although southern Cadiz Valley is located east of the main Eastern California Shear
Zone fault exposures, a poorly constrained NW-SE - striking fault is mapped in the valley below
Holocene alluvial deposits; the presence and location of this fault is presumably based on
projection of basement exposures. In order to analyze basin geometry and evaluate possible fault
locations, a gravity survey consisting of two transects was performed across southern Cadiz
Valley. From the west side of the valley, adjacent to the Calumet Mountains, isostatic anomalies
decrease approximately 8 to 12 mGals to the center of the valley. Eastward, isostatic anomalies
increase 18 to 20 mGal to the Iron Mountains range front. Two prominent changes in isostatic
gravity gradient values occur each of the surveyed transects. Evaluation of this isostatic anomaly
inflection suggests that the Cadiz Valley fault may be more complex than previously thought.
Models of the isostatic anomaly values show four interpreted faults and a basement bedrock
ridge present. The simplistic, non-unique models of the survey may more accurately reflect the
Cadiz Valley geometry if further research is conducted in the area.
3
2 INTRODUCTION
Cadiz Valley is a potentially valuable basin with poor constraints on basin geometry.
Cadiz Valley is located in the southern portion of the Mojave Desert in San Bernardino County
(Figure 1). The valley is potentially valuable as a groundwater resource, but may also be a
seismic hazard area. A geophysical gravity survey was conducted in order to help evaluate these
factors.
Cadiz Valley is an elongate valley trending northwest-southeast. The valley is defined by
the Calumet Mountains on the western side and the Iron Mountains on the eastern side.
The valley has the potential to be a groundwater retention basin. The Colorado River
aqueduct flows along the southeastern side of the valley and the Iron Mountains pumping station
is located on the eastern side of the Iron Mountains. As the population of Southern California
continues to grow, ground water resources such as retention basins become more valuable.
While few groundwater retention basins are present in the California, water districts are
continually exploring new possibilities to retain this valuable resource.
Cadiz Valley is located within the Eastern California shear zone and contains inferred
faults that may present a seismic hazard to the area. A northwest-southeast trending inferred
fault has been mapped on Keith Howard’s geologic map of the Sheep Hole Mountains
Quadrangle (Figure 2).
The following sections include a discussion of site geology, equipment and field
procedures, methodology, data processing and modeling, and results of the geophysical gravity
survey conducted across the southern Cadiz Valley.
4
3 GEOLOGIC BACKGROUND
Cadiz Valley contains crystalline basement bedrock overlain by younger sedimentary
rocks. The valley is comprised of hypothesized Neogene sedimentary deposits underlying
Quaternary alluvium according to Howard’s cross-section (Figure 2)(Howard, 2002). The Iron
Mountains Intrusive Suite and the Coxcomb Intrusives Suite make up both of the mountain
ranges and together are known as the Cadiz Valley Batholith (Figure 3). The Iron Mountains
Intrusive Suite is comprised of late Cretaceous porphyritic gneiss, granites, and granodiorites.
The Coxcomb Intrusive Suite is comprised of granites and granodiorites similar in composition
to the Iron Mountains Intrusive Suite (Howard, 2002).
The southern Mojave Desert is a tectonically complex area that is largely affected by the
deformation associated with the Eastern California Shear Zone. The effect of the Eastern
California Shear Zone on the Mojave block has a dextral slip of approximately 14mm/yr (Miller
et al, 2001). Although southern Cadiz Valley is located east of the main Eastern California Shear
Zone fault exposures, a poorly constrained northwest-southeast striking fault is mapped in the
valley. Characteristics of the valley such as the faulting and geometry show similarities to the
Eastern California Shear Zone.
5
4 EQUIPMENT AND FIELD PROCEDURES
A geophysical gravity survey was performed across the southern portion of the Cadiz
Valley from February to April, 2010. The survey consisted of two gravity transects across the
valley starting on bedrock in the Iron Mountains and ending on bedrock in the Calumet
Mountains.
A regional gravity base station was located and multiple gravity measurements were
taken in order to bring absolute gravity values to the survey. A local base station was set up on a
rock outcrop on the eastern side of Cadiz Valley. Measurements were recorded at the local base
station at the beginning and end of each day in which gravity data were collected. After the last
gravity reading was taken at the local base station, the gravimeter was taken directly to the
regional base station and measurements were recorded.
Gravity measurements were recorded using a Scintrex CG-5 gravimeter. The gravimeter
was provided by the Geologic Department of California State University of Fullerton. The
Scintrex CG-5 gravimeter has accuracy on the order of 0.001 mGals through a range of 8,000
mGals (Scintrex, 2004). A study to evaluate the accuracy and repeatability of the C.S.U.F. CG-5
gravimeter was conducted by Tammy Surko in 2005. It was determined that the gravimeter is
accurate within 0.05 mGals and repeatable less than 0.01 mGals (Surko, 2006). The accuracy
and repeatability of the instrument are well within the ranges needed for this survey.
Locations of gravity stations were recorded by a Topcon GPS system and a handheld gps.
A GPS base station was set up near the local gravity base station and recorded data throughout
the day that gravity data was being collected. A portable Topcon gps unit was used to record the
locations of each gravity station. The Topcon GPS system was post-processed with Topcon
software to increase accuracy. The gps systems were provided for use by the Geologic
Department of California State University of Fullerton.
Gravity stations were recorded along two east-west profiles across the southern portion of
Cadiz Valley. The gravity line 1 consisted of 27 gravity stations including the local base station
and gravity line 2 is comprised of 28 gravity stations, totaling 55 gravity stations for this survey
(Figure ). Gravity stations were occupied at approximately a 0.4 km interval along the profiles
across the valley. Gravity station locations were decided in the field by using distance from the
previous gravity station, changes in local topography, and changes in gravity readings. In areas
6
where gravity measurements differed from the expected change of gravity, the station spacing
was decreased to approximately a 0.2 km interval. This was decided by in field analysis of the
measurements and was conducted to better constrain possible anomalous areas.
The procedure for a new gravity station was repeated at all gravity sations in the field
used in this survey. The CG-5 gravimeter was set-up and leveled at the new gravity station. The
gravimeter recorded measurements for thirty seconds and used internal processing to average the
readings. The averaged measurement was recorded by the CG-5 and recorded in a field
notebook. A log of the plot of gravity reading, standard deviation, time, and an approximate
angle of slope of local terrain were recorded in a field notebook. Following the recording of the
gravity data a GPS location was recorded for each gravity station.
7
5 METHODOLOGY In order to model basin geometry a geophysical high precision gravity survey was
conducted. A gravity survey is done by taking precise gravity measurements and looking for
slight variations within the data. These slight variations are due to the gravitational field which
is dependent upon the density of subsurface material. By calculating the variations of the
gravitational force acting on a gravimeter and assuming the density of the bedrock and overlying
basin fill material of a region, one can predict the depths to bedrock. Therefore, the more data
collected allows for better modeling of a region. All data must be collected the same way and
reduced for a relative measurement to each other.
An important part of the survey is reducing and correcting the gravity measurements. In
order for the data to be relative from one measurement to another, many variables must be taken
into consideration. A tidal correction due to the earth moon tidal effect of the gravitational field
of Earth must be taken into consideration. The drift of the springs of the instrument throughout
the day must be accounted for. The latitude correction takes into effect the difference of the
Earth’s gravity at different latitudes. A standard correction for this is to use the “Geodetic
reference system formula of 1967”:
G = 978.03185 (1 + 0.005278895sin2φ + .000023492sin4φ) cm/s2
The free-air correction compensates for the variation of gravity at different elevations
from sea-level: 0.3086 mGal/m. Bouger and terrain correction take into account the amount of
additional mass of material due to the elevations of topography above sea-level along the studied
area. The standard equation in order to correct for this is:
∆g = 2πρGh.
The isostatic correction compensates for lower-density material that sits atop of higher
density mantle material, which normally corresponds to the topography of the area. These
corrections are standard corrections for gravity measurements (Blakely, 1995).
For the corrections to be calculated some assumptions must be made. It must be assumed
that the bedrock is an infinite slab where the slab height is the elevation above sea level. Density
8
measurements of the bedrock must be calculated as well. The data reduction follows a simple
formula of:
Gravity Anomaly = Gravity Observed – Standard Gravity – Gravity Corrections
Once the gravity data has been reduced, the modeling of the data for possible depths to
crystalline bedrock and gravity highs or lows due to possible subsurface structures can begin.
9
6 DATA PROCESSING, REDUCTION, AND MODELING Once the field data collection was completed, gravity data is reduced and processed
before modeling of the data can be done. The reducing and processing of gravity data is
applying the gravity corrections to the measured field values. A general outline of the reducing
and processing of gravity data was discussed in the previous section.
The measured field values were input into a Microsoft Excel worksheet to apply the
corrections. The first step applied to the gravity data was assigning true gravity values to the
field measured values. This was done by comparing the gravimeter measurement taken at the
end of data collection at the regional base station (PB0825) to the true gravity value of the
regional base station (979,478.859 mGal) (Roberts and Jachens, 1986). The correction for true
gravity values were then applied to the rest of gravity measurements taken that day.
The Scintrex CG-5 applies the tidal correction and instrument drift correction internally
during field data collection. Measurements were taken at the local base station at the beginning
and end of data collection to monitor the effects of instrument drift. After review of these
readings it was decided that the instrument drift during the survey time was negligible.
The latitude, free-air, and Bouger corrections were applied to the gravity data by using
the methods and equations outlined in the previous section. The terrain and isostatic corrections
were applied to the gravity data by using in house software of the United States Geological
Survey by Vicki Langenhiem. This data was added to a database of pervious gravity stations in
the area and an isostatic anomaly map of the region was exported by V. Langenhiem. The
finalized gravity data that will be discussed in the rest of this report and used for modeling is the
isostatic anomaly.
The isostatic anomaly data for the two gravity transects across southern Cadiz Valley
were modeled using the software GRAVMAG. GRAVMAG is a simplistic 2-D forward
modeling software in which assumes that solid prisms extend indefinitely in and out of the cross
section. The observed gravity values, elevation, and a horizontal distance are input into the
software. By creating expected subsurface conditions the software creates calculated gravity
values. Best fit models are made when calculated and observed gravity values differ by the least
amount possible and realistic subsurface basin features are created.
10
The gravitational field is dependent upon the subsurface material densities as mentioned
in the previous section. Therefore more accurate densities of the subsurface materials of the
survey area equal a more accurate model. For this survey, density samples were not acquired
and a simplistic density model was used. The density for basement bedrock of the granites and
granodiorites of the survey area used in modeling was 2,620 3mkg and the density for the
overlying basin fill was 2,220 3mkg . The difference in densities of the basement rock and basin
fill for this gravity survey was 400 3mkg .
11
7 DISCUSSION AND INTERPRETATION OF RESULTS
7.1 Gravity Line 1
Gravity line 1 trends west to east in the southern portion of the Cadiz Valley survey area,
and is comprised of twenty-six gravity stations. The relative isostatic gravity values for line 1
change approximately 18.18 mGals across the valley. From the west side of the vally, adjacent
to the Calumet Mountains, isostatic anomalies decrease approximately 7.27 mGals to the center
of the valley (Figure 4). Eastward, isostatic anomaly values increase rapidly 12.86 mGals across
a distance of 3.4 km which is approximately 3.8 mGals/km. At roughly 2 km from the Iron
Mountains range front, a prominent inflection point in anomaly values occurs and the relative
isostatic anomalies decrease 0.3 mGals across a distance of 0.5 km. Farther east, isostatic
anomaly values again increase at approximately the same rate as that west of the anomaly
inflection point.
The model of line 1 from the isostatic anomaly values is presented as Figure 5. The
model is non-unique and represents realistic subsurface conditions to approximately fit the
calculated relative isostatic anomaly values. From the west the model shows a drastic drop in
bedrock approximately 1 km from the Calumet Mountains range front. The bedrock gradually
increases in depth from 0.7 km to 0.8 km over approximately 4 km. Bedrock is shown to have a
rapid increase in depth over the next 1.5 km to a depth of approximately 1.7km in the middle of
the valley. Bedrock rapidly decreases in depth to approximately 0.15km over the next 3km. At
this location the model shows a potential subsurface bedrock ridge. Basement rock then
decreases to a depth of 0.3 km and gradually returns to the surface, outcropping as the Iron
Mountains.
Four potential faults have been interpreted in areas along the profile in which there are
drastic changes in bedrock topography. The four possible faults are poorly constrained but may
be splays of a master fault at depth. The faults correspond well to areas in which there is a
change in the gravity gradient.
12
7.2 Gravity Line 2
Gravity line 2 trends west to east in the northern portion of the Cadiz Valley survey area,
and is comprised of twenty-eight gravity stations. The relative isostatic gravity values for line 1
change approximately 19.8 mGals across the valley. From the west side of the vally, adjacent to
the Calumet Mountains, isostatic anomalies decrease approximately 12 mGals over a distance of
approximately 4 km (Figure 4). Eastward, slight isostatic anomaly values change over the 3 km
in the center of the valley. Isostatic anomaly values then rapidly increase 17.8 mGals across a
distance of approximately 4 km which is approximately 4.4 mGals/km. Along line 2 no
prominent inflections points are shown in the data however at approximately 6.8 km and 10 km
from the east, there exist anomalous gravity gradient changes in the observed relative isostatic
gravity data.
The model of line 2 from the isostatic anomaly values is presented as Figure 6. The model
is non-unique and represents the realistic subsurface conditions to approximately fit the
calculated relative isostatic anomaly values. From the west the model shows a rapid drop in
depth of bedrock to approximately 1.5 km over a distance of about 3 km from the Calumet
Mountains range front. Over the next 2 km bedrock shows a gradual change in depth to
approximately 1.6 km below the surface. At approximately 6 km from Calumet Mountains range
front a subsurface bedrock ridge is shown, where basement rock is 1.1 km deep. At
approximately 4 km from the Iron Mountains Range front basement rock rapidly decreases in
depth from 1.5 km to the surface.
Three potential faults have been interpreted in areas along the profile in which there are
drastic changes in bedrock topography. The three possible faults are poorly constrained and may
be splays of a master fault at depth. The faults correspond well to areas in which there is a
change in the gravity gradient.
7.3 Interpretation of Gravity Lines 1 and 2
The isostatic anomaly data from lines 1 and 2 show in the Cadiz Valley survey area show
good agreement in gravity values. Both of the models have similar interpreted faults and display
a possible subsurface bedrock ridge due to faulting (Figures 5 and 6). The approximate locations
13
of the interpreted faults with the gravity lines are shown on Figure 7. The fault system of Cadiz
Valley may be more complex than previously thought.
A map of the regional isostatic anomaly values is presented as Figure 8. This map shows
good agreement between the isostatic anomaly values between previous surveys and this survey.
This survey has helped better confine the isostatic anomaly in the southern portion of the Cadiz
Valley, an area where gravity data was lacking. Both lines show a rapid decrease in the isostatic
anomaly values from the east side of the Iron Mountains range front with the lowest anomaly in
the middle of the valley. A more rapid increase of isostatic anomaly values is shown on line 2
than line 1 as it approaches the Calumet Mountains from the middle of the valley.
14
8 CONCLUSION
Cadiz Valley is located in the southern portion of the Mojave Desert in San Bernardino
County (Figure 1). Cadiz Valley is an elongate valley trending northwest-southeast. The valley
is defined by the Calumet Mountains on the western side and the Iron Mountains on the eastern
side. Cadiz Valley contains Cretaceous crystalline basement bedrock overlain by Quaternary
sedimentary rocks and alluvium (Figure 2 and 3). Characteristics of the valley such as the
faulting and geometry show similarities to the Eastern California Shear Zone although the valley
is located east of the main fault exposures of the Eastern California Shear Zone.
A geophysical gravity survey was conducted across the southern portion of Cadiz Valley
from February to April, 2010 in order to evaluate basin geometry which could be used for
possible ground water resources or seismic hazard investigation. The gravity survey consisted of
two transects trending west to east from the Calumet Mountains to the Iron Mountains, and was
comprised of 55 gravity stations. The gravity data was reduced, processed, and modeled by
standard practices outlined in previous sections of this report. The models of the isostatic
anomaly values of the two transects are presented as Figures 5 and 6. The models show an
approximate basin depth of 1.7 km near the middle of the valley. Four potential faults and a
basement bedrock ridge have been interpreted from the models (Figure 7).
The fault system and basin geometry of Cadiz Valley may be more complex than previously
thought. The interpreted faults are poorly constrained due to the simplistic, non-unique
geophysical modeling used in this survey. In order to truly evaluate Cadiz Valley as
groundwater resource or potential seismic hazard, further research of the area would need to be
conducted. It is recommended that density samples of the rock and alluvium from the survey
area be collected, at least one other gravity transect north of line 2, and a more complex
modeling software be used in order to evaluate these factors of Cadiz Valley.
15
9 REFERENCES
Howard, K.A., 2002, Geologic map of the Sheep Hole Mountains 30 by 60 quadrangle, San Bernardino and Riverside counties, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-2344
Langenheim, V.E., Biehler, S., Negrini, R., Mickus, K., Miller, D.M., and Miller, R.J. 2009,
Gravity and magnetic investigations of the Mojave National Preserve and adjacent areas, California and Nevada: U.S. Geological Survey Open-File Report 09-1117.
Miller, M.M., Johnson, D.J., and Dokka, R.,K., 2001, Refined kinematics of Eastern California
Shear Zone from GPS observations, 1993-1998: Journal of Geophysical Research B: Solid Earth, v. 106, no. 2, p. 2245-2263.
Roberts, C.W., and Jachens, R.C., 1986, High-precision gravity stations for monitoring vertical
crustal motion in southern California: U.S. Geological Survey Open-File Report 86-44. Surko, S.L., 2006, Gravity Survey of the Lucerne Valley Groundwater Basin: Implications for
Basin Structure and Geometry [M.S. Thesis]: California State University, Fullerton, 21p.
16
FIGURES
on Line 2
Figure 1Cadiz Valley is an elongate valley trending northwest-southeast located in the southern portion of the Mojave Desert in SanBernardino County. The gravity stations were collected for the survey arcoss the valley trending relatively east-west.
Figure 2.Keith Howard’s cross section drawn through the Southern portion of Cadiz Valley. The valley
is comprised of hypothesized Neogene sedimentary deposits underlying Quaternary alluvium
with the Cadiz Valley Batholith as the crystalline basement bedrock. (Howard, 2002)
LEGEND
Gravity Line 1
Gravity Line 2
Potential fault locations
Quaternary Alluvium: gravels and sands
Cretaceous Coxcomb Intrusive Suite: granites and
granodiorites
Cretaceous Iron Mountains Intrusive Suite: granite, granodiorites
and gniess
N
Figure 3.Cadiz Valley contains crystalline basement bedrock overlain by younger sedimentary rocks. The Iron Mountains
Intrusive Suite and the Coxcomb Intrusives Suite make up both of the mountain ranges and together are known
as the Cadiz Valley Batholith (Howard, 2002).
Figure 4. Profiles of gravity lines 1 and 2 with isostatic anomaly values and topographic profiles.
Isostatic gravity anomaly values should decrease as elevation decreases at a constant rate. Both
profiles show two areas that have anomalous gravity gradients.
Figure 5.Model produced using GravMag from gravity data collected along Line 1. The model
shows an approximate basin depth of 1.7 km near the center of Cadiz Valley. Four
interpreted faults are shown where there are rapid changes in bedrock topography. On
the western portion of the profile an interpreted subsurface bedrock ridge is shown.
Figure 6.Model produced using GravMag from gravity data collected along Line 2. The model
shows an approximate basin depth of 1.6 km near the center of Cadiz Valley. Three
interpreted faults are shown where there are rapid changes in bedrock topography. On
the western portion of the profile an interpreted subsurface bedrock ridge is shown.
LEGEND
Gravity Line 1
Gravity Line 2
Potential fault locations
Interpreted Fault from Gravity Data
Quaternary Alluvium: gravels and sands
Cretaceous Coxcomb Intrusive Suite: granites and
granodiorites
Cretaceous Iron Mountains Intrusive Suite: granite, granodiorites
and gniess
N
Figure 7.The fault system of Cadiz Valley may be more complex than previously thought. The approximate location of
the four interpreted faults from the gravity models are shown above.
Figure 8.Isostatic anomaly map of the Cadiz Valley region with new and previously recorded gravity stations. The
white contours represent the previous isostatic anomaly contours while the black contours represent the
updated isostatic anomaly values from the data collected in during this survey.
Previous gravity stations
New gravity stations
Previously mapped faults