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

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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.

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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.

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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.

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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

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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.

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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

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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.

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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.

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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 .

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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.

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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

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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.

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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.

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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.

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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