STRUCTURE OF THE TUCSON BASIN, ARIZONA 

FROM GRAVITY AND AEROMAGNETIC DATA 

By Victoria Louise Rystrom

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey 

editorial standards or with the North American Stratigraphic Code. 

Any use of trade, firm or product names is for descriptive purposes only and does not imply

endorsement by the U.S. Government. 

Open-File Report 03-116

U.S. Department of the Interior 

U.S. Geological Survey
Abstract 

Interpretation of gravity and high-resolution aeromagnetic data reveal the

three-dimensional geometry of the Tuscson Basin, Arizona and the lithology of its 

basement. Limited drill hole and seismic data indicate that the maximum depth to the 

crystalline basement is approximately 3600 meters and that the sedimentary 

sequences in the upper ~2000 m of the basin were deposited during the most recent 

extensional episode that commenced about 13 Ma. The negative density contrasts 

between these upper Neogene and Quaternary sedimentary sequences and the 

adjacent country rock produce a Bouguer residual gravity low, whose steep gradients 

clearly define the lateral extent of the upper ~2000m of the basin. The aeromagnetic 

maps show large positive anomalies associated with deeply buried, late Cretaceousearly Tertiary and mid-Tertiary igneous rocks at and below the surface of the basin. 

These magnetic anomalies provide insight into the older (>13 Ma) and deeper 

structures of the basin. Simultaneous 2.5-dimensional modeling of both gravity and 

magnetic anomalies constrained by geologic and seismic data delineates the thickness 

of the basin and the dips of the buried faults that bound the basin. This geologic-based 

forward modeling approach to using geophysical data is shown to result in more 

information about the geologic and tectonic history of the basin as well as more 

accurate depth to basement determinations than using generalized geophysical 

inversion techniques.
CONTENTS

Chapter Page

1. INTRODUCTION 1

Geologic and Tectonic History 1

 Proterozoic 1

 Paleozoic 5

 Mesozoic 5

 Cenozoic 8

2. PREVIOUS RESULTS 10 

 Geology 10 

 Seismic Studies 10 

 Gravity Studies 12 

 Aeromagnetic Studies 16 

3. GEOPHYSICAL DATA 19

 Gravity Data 19

 Aeromagnetic Data 23 

4. ANOMALY IDENTIFICATION 28 

 Gravity Map 28

 Aeromagnetic Map 30 

5. MODELING OF THE GRAVITY AND 

 AEROMAGNETIC DATA 36

Two Dimensional Modeling 36 

 Model Sensitivity 55

Areal Extent of the Basin 57 

Three-dimensional character of the basin 58

 Gravity Corrections 60 

 Areal Extent of the Igneous Basement 69 

6. DISCUSSION 72 

7. CONCLUSIONS 75 

REFERENCES 77
TABLES 

Table Page

1. Exxon well bulk densities and susceptibilities. 32 

2. Susceptibility measurements of Tucson Basin area rocks. 38 

3. Susceptibilities and densities of rock units used in modeling. 50 

4. Density contrasts of basin fill and basin depths. 56
FIGURES 

Figure Page

1.1 Reference Map 2 

1.2 Arizona Tectonic History 3 

1.3 Principle Strain 7 

2.1 Estimated Alluvial Thicknesses 11 

2.2 Davis and Litinsky Gravity Models 13 

2.3 Model of Saltus and Jachens 17 

3.1 Locations of Gravity Stations 20 

3.2 Bouguer Gravity Map 21 

3.3 Maximum Horizontal Gradient of Bouguer Gravity 22 

3.4 Body Edge Determination 23 

3.5 Reduced-to-the-Pole Aeromagnetic Map 24 

3.6 Pseudogravity Map 26 

3.7 Pseudogravity Maximum Horizontal Gradient Map 27 

4.1 Bouguer Gravity Anomaly Identification 29 

4.2 Reduced-to-Pole Aeromagnetic Anomaly Identification 31 

5.1 Stratigraphic Column of Exxon Well (32) 37 

5.2 Exxon Seismic Line Two 43 

5.3 Exxon Seismic Line Three 44 

5.4 Exxon Seismic Line Four 45 

5.5 Model Cross Section CS-1 46
5.6 Exxon Seismic Line CS-2 47 

5.7 Exxon Seismic Line CS-3 48 

5.8 Depth to Bottom of Units 59 

5.9 Regional Isostatic Gravity Field 61 

5.10 Residual Isostatic Gravity Anomaly Map 62 

5.11 Basement Gravity Map 63 

5.12 Basin Residual Gravity Map 64 

5.13 Basin Residual Gravity Map with Depth to Lower Fill 66 

5.14 Gravity and Pseudogravity Maximum Horizontal 

 Gradients Overlying Basin Residual Gravity Map 67 

5.15 Sources of Magnetization Map from Reduced-to-the 

 Pole Aeromagnetic data 71
Chapter 1. 

INTRODUCTION: 

The Tucson Basin, near Tucson in southeast Arizona, lies in the Southern 

Basin and Range Province and is surrounded by the Sierrita, Tumaccacori, Santa Rita 

and Rincon Mountains (Figure 1.1). A long history of volcanism, plutonism, 

sedimentation, erosion and tectonism has created a complex geology in the region. In 

order to help unravel the tectonic history of the Tucson Basin, I model existing 

gravity (Davis, 1971; Gettings and Houser, 1997; Sweeney and Hill, 2001) and new 

high-resolution aeromagnetic data (Sweeney and Hill, 2001) constrained by borehole 

(Davidson, 1973; Eberly and Stanley Jr., 1978; Houser and Gettings, 2000), seismic 

reflection (Johnson and Loy, 1992), and geologic map (Anderson, 1987; Drewes, 

1976; Drewes, 1980) data, thereby integrating available geophysical data with known 

geology in order to understand the structure of the Tucson Basin. The results are then 

compared to previous geophysical inversion models of the Tucson Basin. 

 

Geologic and Tectonic History: 

• Proterozoic: 

 The oldest rocks in the Tucson region are the Proterozoic metasedimentarymetavolcanic Pinal Schist (Keep, 1996), part of the Mazatzal province that was 

accreted onto the Yavapi province of Laurentia about 1.65 Ga (Dickinson, 1989) 

(Figure 1.2-A). Following accretion of the Mazatzal province, a period of quiescence 

ensued, suggesting a possible distancing from the plate margin. In the middle 

Poterozoic (1.4 Ga) the Continental Granodiorite and Rincon Valley Granodiorite 

1 
Figure 1.1: Reference Map, topography overlain by generalized geology. 

2 
Figure 1.2: Tectonic History of Arizona, explanations on next page.

3 
Figure 1.2 explanations: 

A - Proterozoic boundaries of Arizona after Karlstrom (1998) Mojave province 

contains Archean cust incorporated into Proterozoic crust. Yavapai province contains 

1.75 Ga volcanic-arc crust. The Mazatzal province contains 1.65 Ga crust. The 

stippled areas indicate transitions between provinces. 

B - 1.4Ga granitoids - stippled areas and dots. After Karlstrom and Humphreys 

(1998). 

C - North America was simultaneously squeezed (Grenville) and rifted (midcontinent rift) possibly the cause of the Butte and Canyone Creek fault trends (bold 

lines) and subsequent basins (dashed lines). 1.1 Ga mafic rocks were emplaced as 

diabase sheets (dots). After Karlstrom and Humphreys (1998). 

D – Cambrian sediments transgressed from west to east during thermal subsidence of 

the Cordilleran miogeocline. Upper Cambrian and Lower Paleozoic depositional 

patterns were influenced by the Transcontinental arch. After Karlstrom and 

Humphreys (1998). 

E - Early Jurassic magmatic arc plutonism related to oblique subduction off the coast 

of the North American plate was widespread throughout southeast Arizona. 

Continental Red Bed subaereal sediments were deposited in the north. After 

Dickinson, 1989. 

F- Late Mesozoic backarc sedimentary basins formed behind the magmatic arc to the 

west. Subsidence during the Early Cretaceous occurred as a result of backarc crustal 

extension related to sea-floor spreading within the Gulf of Mexico during the 

Jurassic. After Dickinson, 1989. 

G – Laramide magmatism and deformation. Gray denotes area of magmatism,

stippled area, Eocene two-mica granite formation. Arrows denote basement-cored 

Laramide uplifts. After Dickinson, 1989. 

H- Mid-Tertiary arc magmatism and associated extensional deformation. Dashed area 

denotes main belt of tectonically denuded core complexes. After Dickinson, 1989. 

I – Late Cenozoic block faulting and subsidence. Lined area denotes location of 

Neogene extensional tectonics, Colorado Plateau is relatively undeformed. After 

Dickinson, 1989. 

4
batholiths were emplaced (Figure 1.2-B) (Davis, 1981; Drewes, 1980). A rifting 

event characterized by high angle faulting and diabase dike, sill and sheet intrusions 

occurred in the Late Proterozoic (1.1 Ga) (Davis, 1981; Dickinson, 1989; Drewes, 

1981; Howard, 1991). In the period 1.1-0.6 Ga, episodic rifting oriented N23oW

(Figures 1.2-C, 1.3) culminated in the rifting of Laurentia from Rodinia (Karlstrom

and Humphreys, 1998; Stewart, 1976). The presence of Precambrian rocks in the 

Tucson Basin area is limited to small widely dispersed Pinal Schist outcrops mixed 

with outcrops of Continental Granodiorite and Rincon Valley Granodiorite. These 

outcrops are limited mainly to the eastern side of the basin (Figure 1.1, gray). 

• Paleozoic: 

The Paleozoic era was a time of stable platform development (Figure 1.2-D), 

for Arizona lay within the interior of the Laurentian supercontinent (Stewart, 1976). 

The Paleozoic sequences in southern Arizona are composed mainly of Cambrian 

strata overlain disconformably by a Devonian-Permian sequence of limestone, 

dolomite, and interbedded mudstone to sandstone (Butler, 1971), suggesting marine 

conditions transitioned to sub aerial (Drewes, 1981) during the Devonian to Permian. 

In the Tucson Basin most Paleozoic rocks were removed in the Mesozoic, but a few 

outcrops remain along the Sawmill Canyon Fault, notably in the Pima Mining District 

(Figure 1.1) and just off the southeast corner of the study area. 

• Mesozoic: 

Late Triassic(?) to Jurassic magmatic arc volcanism and plutonism (Figure 

1.2-E) related to oblique subduction of the oceanic Kula and (or) Farallon plates 

beneath the North American plate (Coney, 1979) was widespread throughout 

5
southeast Arizona (Dickinson, 1989). Early and Middle Jurassic plutons are thought 

to connect at depth to form a possible continuous batholithic belt comparable to the 

Sierra Nevada (Tosdal, Haxel, and Wright, 1989). Jurassic magmatism continued in 

southeastern Arizona from mid-Early Jurassic (c.195 Ma) to mid-Late Jurassic (c. 150 

Ma) (Dickinson, 1989) then shifted westward as the Pacific subduction system moved 

westward. In the Tucson Basin small outcrops of these plutons are associated with the 

southeastern Santa Rita, northern Tumacaccori, central Sierrita and western Tucson 

Mountains (Figure 1.1, green). 

Synorogenic high angle block faulting (Figure 1.3, shortening N71oE, Davis, 

1981) with differential uplift caused mass-wasting and removal of all or parts of the 

Paleozoic section throughout Arizona (Davis, 1981; Drewes, 1981) while newly 

created basins filled with volcanic and sedimentary rocks (Drewes, 1981) during the 

Jurassic-early Cretaceous. The Sawmill Canyon Fault Zone (Figure 1.1, red fault) 

defines the northern extent of the known great thickness of Jurassic plutonic rocks in 

Arizona (Titley, 1976). This fault zone is thought to have been active at various times 

throughout the Mesozoic (Titley, 1982). Drewes (1971) suggests that the fault is high 

angle dip-slip with the northeast block up. 

The Cretaceous period saw the development of thick sequences of 

sedimentary rocks in two different types of depositional basins (Figure 1.2-F) 

(Dickinson, 1989) as well as the return of arc magmatism during the Laramide 

orogeny. Early Cretaceous crustal extension and sediment deposition in southern 

Arizona (Figure 1.2-F) was possibly a product of the rifting event that opened the 

Gulf of Mexico (Dickinson, 1989). Thick sequences of marine and subareal 

6
Figure 1.3: Principle strain directions for Arizona. 

sedimentary rocks conformably overlie mid-Upper Jurassic volcanics and

volcanoclastic rocks and are exposed in the Tucson region (e.g. Bisbee group) 

(Dickinson, 1989; Drewes, 1971). In the Late Cretaceous, thrusting and plate flexure 

from the Sevier orogeny (~140Ma) to the northwest of the study area created a 

subareal-sediment-filled foreland basin in northern Arizona. 

7
The Laramide orogeny (~75-55 Ma) (Figure 1.2-G) was a period of widespread 

magmatism, compressional deformation, and crustal thickening (Dickinson, 1989) as 

a result of northeast/southwest compression (Figure 1.3, trending N46oW). Laramide 

magmatism included widespread volcanism (Figure 1.1, dark blue) and later granite 

batholith emplacement (Figure 1.1, light blue; Figure 1.2-G) (Dickinson, 1989). 

Large portions of the Santa Rita and Tucson Mountains and to a lesser extent the 

Sieritta Mountains (Figure 1.1) consist of Laramide-age igneous rocks. Reverse and 

thrust faulting mostly reactivated the Jurassic (and probably Precambrian) faults 

(Davis, 1981). Narrow bands of folded Cretaceous sedimentary rocks with axes 

striking northwest and west-northwest developed. The Sawmill Canyon Fault Zone 

parallels one such band. Crustal thickening across the southern half of Arizona 

induced regional uplift (Dickinson, 1989) and subsequent erosion. 

• Cenozoic: 

Closely following the Laramide orogeny, the crust was intruded by scattered 

batholiths of two-mica granite during Early to Middle Eocene time (~55-45 Ma) 

(Dickinson, 1989), the batholiths were produced by crustal melting. No volcanic 

equivalents are known. The Late Eocene was a time of tectonic quiescence with low 

topographic relief, followed by the Mid-Tertiary orogeny (Oligocene to middle 

Miocene), when arc magmatism and widespread extensional deformation returned to 

the region (Figure 1.2-H) (Dickinson, 1989) due to the evolving plate tectonic setting 

of the western continental margin. Important tectonic events include widespread 

rhyolitic to basaltic volcanism (Figure 1.1, dark orange), shallow crustal plutonism

(Figure 1.1, light orange), development of depositional basins and extension (Menges

8
and Pearthree, 1989). The youngest volcanic rocks in the Tucson Basin region are 

33-23 Ma andesitic, dacitic, and rhyolitic flows, tuffs, pyroclastic rocks, dikes, plugs 

and laccoliths (Spencer and Reynolds, 1989). Andesitic flows are found in the 

Tumacaccori Mountains (Figure 1.1, dark orange). Mid-Tertiary crustal extension 

was associated with east-northeast-west-southwest-directed low-angle normal faulting 

(Zoback and Zoback, 1991), and block rotation that accommodated large amounts of 

extension (Spencer and Reynolds, 1989). Detachment faults in Arizona, some

associated with metamorphic core complexes, have a few to several tens of 

kilometers of displacement and some think (Spencer and Reynolds, 1989) that they 

are one of the most important structural features of mid-Tertiary age. An important 

sedimentary sequence in the region, the Pantano Formation (Figure 1.1, purple), was 

deposited as a result of uplift of the core complexes. 

High-angle normal faulting associated with Basin and Range extension began 

in southeast Arizona around 13Ma and continued until 5Ma, resulting in northwest to

northeast trending ranges and alluvial valleys (Menges and Pearthree, 1989) (Figure 

1.2-I). Compared with the Great Basin portion of the Basin and Range province, there 

is more variety in the trends of the ranges in Arizona (Loring, 1976). Late Tertiary 

and Early Quaternary basaltic flows are associated with Basin and Range extension; 

however, the Tucson Basin area is devoid of these rocks.

9
Chapter 2 

PREVIOUS RESEARCH: 

Geology: 

The primary interest of geologic studies of the mountains surrounding the 

Tucson Basin has been mineral deposits; geologic studies in the basin itself have 

focused on water resource and quality issues. The USGS undertook a geologic study 

of the central, northern and eastern portion of the basin in 1959 (Pashley, 1966) to do 

a water recharge study, and from 1970 to 1973 produced detailed studies of the 

hydrology of the Tucson Basin (Anderson, 1972; Burkham, 1970; Davidson, 1973; de 

la Torre, 1970; Laney, 1972). Again in 1987, the USGS cooperated with the City of 

Tucson to combine the data from more than 500 wells to define the characteristics of 

the basin sediments (Anderson, 1987). An alluvial deposits thickness map of the 

Tucson area was produced by the USGS in 1973 (Cooley, 1973), however the 

contours are limited to a maximum depth of 365 meters (Figure 2.1). In 1972, Exxon 

drilled 3600m to crystalline basement in the north end of the basin. Eberly and 

Stanley (1978) interpreted the well logs. These were reinterpreted along with the 

stratigraphy of the basin sediments by Houser and Gettings (2000). These well logs

have been important in studying the Tucson Basin, for they are the only logs to reach 

crystalline basement (Houser and Gettings, 2000). Other examinations of localized 

geology have been covered in many University of Arizona thesis and dissertations. 

Seismic Studies: 

An east-west cross section of the northern portion of the Tucson Basin 

correlating the Exxon State (32) well log data and reflection seismic lines was created 

10
Figure 2.1 Alluvial thickness contours based on wells from Cooley 1973. Generalized 

geology from figure 1.1.Depth to bedrock of Exxon well (32) shown for reference. 

11
by Eberly and Stanley (1978). They suggest a late Miocene symmetric high angle 

faulted basin with a central graben surrounded by sets of homothetic faults. They 

further suggest that the basin was once a narrow valley between broad ranges that has 

subsequently eroded and widened (Eberly and Stanley Jr., 1978). 

Interpretation of a seismic reflection line across the Tucson Basin (Johnson 

and Loy, 1992) show a low-angle normal fault (the Santa Rita Fault) that crops out 

along the trend of late Quaternary fault scarps (pink, Figure 1.1). Velocityindependent dip analysis from shot records of the Santa Rita fault indicate that it has a 

true northwestdip of ~20o to a depth of at least 6km.

Limited seismic refraction and receiver function data have been collected in 

the region. Receiver function data (Myers and Beck, 1994) indicate that the crustal 

thickness increases from 30 km under the Tucson basin to 34 km under the Rincon 

Mountains. Refraction data, primarily along the Santa Rita Fault (indicated by fault 

scarps, Figure 1.1), show Quaternary normal faults offsetting piedmont surfaces of 

the Pliocene to late Pleistocene (Pearthree and Calvo, 1987; Rutledge, 1984). 

Gravity Studies:

Several previous studies have modeled gravity data in the Tucson Basin 

region. Davis (1971) modeled gravity data collected in the Tucson area for the 

purposes of identifying major hydraulic boundaries of the Tucson Basin (Figure 2.2, 

a, b). The study used a 2-dimensional modeling program to model the depth to 

basement using an average crustal density of 2670 kg/m3

. A density contrast of -400 

kg/m3

 represented the density contrast between the basin fill and the Pantano 

Formation and the pre-Tertiary crystalline basement rocks (Davis, 1971). The basin 

12
13 

a b c 

Figure 2.2: a. Residual Bouguer gravity from Davis (1971), 2 mGal contours, b. Basement depth contours in feet Basement depth contours in km of Litinsky, after Litinsky (1989). Maps are all approximately same scale.
fill consists of late Pliocene to Holocene sedimentary sequences (Figure 1.1). The 

bottom of the basin is assumed to be the bottom of the Pantano Formation (Figure 

1.1). Davis removed a regional gradient from the Bouguer gravity data. The regional 

gradient removal was based on representative values for Bouguer gravity in broad 

areas of granitic or metamorphic terrain. These values were then corrected for 

topographic effects. The resulting regional gradient was defined as a plane dipping 

S20oE at 1mGal/mile (0.62 mGal/km) (Davis, 1967). The resulting residual Bouguer 

gravity anomalies were used to model the 3-dimensional geometry of the Tucson 

Basin. The model shows the deepest portion of the basin to be south of and shallower 

than Exxon well (32) at 2.15 km (7000 feet, Figure 2.2, b). 

 Another study of the entire Arizona Basin and Range province including the 

Tucson Basin also used residual gravity data to obtain basin thickness. 

(Oppenheimer, 1980; Oppenheimer and Sumner, 1981) A regional was calculated 

using a double Fourier trend surface analysis of elevations. The trend surface 

elevations were calculated by averaging the elevations within 1x1’ squares on a 

topographic sheet. Oppenheimer related this removal to crustal thickening from the 

southwest to the northeast. To determine depth to bedrock from the residual gravity 

data a variable density scale was used and, for the Tucson region, the depth to 

crystalline basement determined from the Exxon well (32) was used as a constraint. 

Their 3-dimensional model shows a mostly north-south trending narrow basin whose 

thickest section is about 3800 m near the location of the Exxon well. 

14
Two later studies also used the Exxon well to constrain their models, with the 

assumption that the base of the sedimentary basin coincided with the top of the 

crystalline basement (3.66 km depth). Litinsky (1989) calculated the depths to 

basement using a hyperbolic density depth function with an effective density contrast 

of –137 kg/m3 and the Bouguer infinite-slab formula from residual gravity data 

(Davis, 1971) over the Exxon well (32) borehole A maximum depth of 10 km was 

obtained for the southern portion of the basin (Figure 2.2, c). The shape of the basin is 

similar to that of Davis (1971). Chakrarathi, et al. (2001) (also using the residual 

Bouguer gravity of Davis (1971)) inverted a gravity anomaly profile across the 

Tucson Basin using a parabolic density function with a surface density contrast of –

450 kg/m3

 and a depth-density relation of 280.6 g/m3 / km. Assuming the depth of 

3.6 km from the Exxon well (32) a density distribution was determined. This 

approach yielded a thickness of 3.8 km in the northern part of the Tucson Basin (near 

the Exxon drill hole). 

Another approach was used by Saltus and Jachens (1995) who removed 

regional gravity anomalies from the Bouguer gravity field for the western United 

States in order to isolate anomalies related to Cenozoic basins. They first removed 

the isostatic effect of the terrain (Simpson et al., 1986). They recognized that the 

basement exposed in ranges adjacent to the basins varied in density. In order to 

correct for this effect they gridded the isostatic residual gravity data using only the 

data over the basement outcrops to create a basement gravity field. This was 

subtracted from the isostatic residual anomaly to obtain the gravity anomaly 

associated with the basins, referred to here as the residual basin gravity anomaly. 

15
This regional-removal process will be discussed in detail in a later section. The 

thicknesses of the basins (figure 2.3) were then modeled using a density-depth 

function in which a stepped density contrast decreases with depth (0-0.2 km, -650 

kg/m3

; 0.2-0.6 km, -550 kg/m3; 0.6-1.2 km, -350 kg/m3 and >1.2 km, -250 kg/m3). 

The deepest part of the Tucson Basin in their model was at greater than 2.5 km near 

the location of the Exxon drill hole (Figure 2.3), shallowing to 500 m at the southern 

portion of the basin. In all of these models, no attempt was made to link density 

layers with stratigraphy, especially important in area with a complex tectonic history 

like the Tucson Basin. 

Aeromagnetic Studies: 

The first aeromagnetic survey in the region, in 1959, (Andreasen and Pitkin, 

1963) was over the Twin Buttes area in the Sierrita Mountains. A statewide survey 

was flown in 1968 (Summer and Sauck, 1971), at 2740m constant elevation and 5km 

line spacing. Only three flight lines covered this study area. A higher resolution 

survey (1830m constant elevation and 800m and 1600m line spacing) was flown later 

in 1968 for a northern portion of the Tucson Basin (Sauck, Sumner, and Christensen, 

1971). 

The 1968 survey profiles contained several major positive magnetic anomalies 

over the Tucson Basin area. The dominant feature is a positive anomaly with 

amplitudes up to 300nT and trending N45°W (Sauck, Sumner, and Christensen, 

1971), in the northeast corner of the Tucson Basin over the Rincon Mountains (A5, 

Figure 4.2) that they suggested is caused by a batholith with an age of either Late 

Cretaceous (not Laramide), Laramide, or mid-Tertiary. Depth to source estimates 

16
Figure 2.3: Depth to basement map derived from basement-corrected gravity 

anomalies (Saltus & Jachens, 1995). See text for explanation. Contours show depth in 

kilometers. Exxon well (32) shown for reference. 

17
ranged from 330m to 1130m and they suggested most of the source is considerably 

deeper. They also postulate a minimum susceptibility of 0.02 (SI) for the source and 

note that the gneiss (A5, Figure 4.2) overlying has susceptibility measured at 0.00082 

(SI) and is an unlikely source for the high positive anomaly. Other positive anomalies 

identified over the area are smaller and were thought to be dikes unrelated to the 

larger batholithic anomaly source (Sauck, Sumner, and Christensen, 1971). 

18
Chapter 3

GEOPHYSICAL DATA: 

Gravity Data 

This study uses 2349 gravity stations from a database maintained by the 

National Geophysical Data Center, augmented with data from the USGS and several 

university theses and dissertations (Sweeney and Hill, 2001) (Figure 3.1). Free-air, 

and Bouguer corrections, calculated using the 1967 gravity formula (International 

Association of Geodesy, 1971) and a reduction density of 2670 kg/m3

 were applied to 

the data. Terrain corrections were calculated radially outward 167 km from each 

station using a method developed by Plouff (1977). The irregularly-spaced data were 

converted to a regular grid with a 500 m spacing using minimum curvature (Sweeney 

and Hill, 2001) (Figure 3.2). 

Derivatives of gravity data can highlight features of interest. One of the goals 

of this study is to locate the edges of the Tucson basin. To aid in this, I used the 

magnitude of the maximum horizontal gradient of the Bouguer gravity to highlight 

the edges of rock units or faults (Blakely, 1995). The maximum horizontal gradient 

method can be applied to both magnetic and gravity data (Figure 3.3), and uses the 

amplitude of the horizontal component of the gravity or magnetic gradient (Cordell 

and Grauch, 1985). The position of the gravity inflection point is easily determined 

by locating the peak in the amplitude of the horizontal gradient of the gravity field 

(Figure 3.4). The interpretation of this map will be discussed in a later section.

19
Figure 3.1: Locations of gravity stations used in calculating Bouguer gravity 

anomalies. Yellow lines show locations of model cross sections. 

20
Figure 3.2: Bouguer gravity anomaly map, 2 mGal contours. Yellow lines show 

locations of model cross sections. 

21
Figure 3.3: Maximum horizontal gradient of the Bouguer gravity data. Blue lines 

locate traces of the maximum gradient ridges that indicate steep density boundaries, 

and possible edges of bodies. 

22
Figure 3.4: Cartoon of gravity and magnetic anomalies and the amplitude of the 

horizontal gradient over a contact between a dense, magnetic layer (shaded) and 

surrounding rocks, illustrating that the horizontal gradient is a maximum over the 

contact. 

Aeromagnetic data 

Three aeromagnetic surveys were flown from 1996 to 1999 in southeastern 

Arizona (Sweeney and Hill, 2001), two at a height of 150m and one at a height of 

230m above the terrain, along lines spaced 400m, 150m and 250m apart. The surveys 

were designed to have a flight height to line spacing ratio close to 2 to 1 to avoid 

aliasing due to 3-D sources at the surface (Reid, 1980). Aliasing occurs when 

interpolation of the profile data onto a grid produce lower frequency anomalies than 

what is actually produced by the source. Low height to line spacing ratios also 

increase the confidence in the type of derivative processing methods used in this work 

(Reid, 1980). The Definitive Geomagnetic Reference Field (DGRF) calculated for 

the date of the original survey was removed, and the data were microleveled, 

reducing artificial trends and anomalies caused by flight noise. All data were upward 

continued to 230m above the terrain and gridded at a 100 meter interval (Sweeney 

and Hill, 2001) (Figure 3.5). 

23
Figure 3.5: Reduced-to-the-Pole aeromagnetic map. Yellow lines show locations of

model cross sections. 

24
To enhance the magnetic map and aid in geologic and tectonic interpretation, we 

applied several derivative processing methods to the magnetic data. The reduction-tothe-pole transform corrects total field anomalies in the total field for magnetic 

inclination and shifts the magnetic anomaly over the source body. This transform

alters the shape of the anomaly such that symmetrical sources produce symmetrical 

anomalies (Baranov and Naudy, 1964). Induced magnetization is assumed for this 

transformation. This assumption is reasonable for most of the intrusive rocks in the 

region, large batholiths typically do not have large remanent magnetization because 

as a batholith cools the magnetite domains align themselves to magnetically cancel 

each other (Reynolds, Rosenbaum, and others, 1990). The volcanic rocks in the 

region, however, do possess significant remanent magnetization; the reduced-to-thepole transform, therefore, might not accurately shift these anomalies over their 

sources. 

The pseudogravity transformation converts the total-field magnetic anomaly 

into the gravity anomaly that would be observed if an identical density distribution 

replaced the magnetic distribution (Figure 3.6) (Blakely, 1995). The usefulness of 

this transformation is that gravity anomalies have properties that simplify resolving 

the shape of the body causing the anomaly. Maximum horizontal gradients can also 

be computed for pseudogravity, defining the edges of steep magnetic bodies (figure 

3.7). 

 

25
Figure 3.6: Pseudogravity from Reduced to the pole aeromagnetic data, calculation 

described in text. Yellow lines show location of model cross sections.

26
Figure 3.7: Map of the maximum horizontal gradient of the pseudogravity map. 

White lines are traces of the maximum horizontal gradients, to be discussed later. 

Yellow lines show locations of model cross sections.

27
Chapter 4 

ANOMALY IDENTIFICATION: 

Gravity Map 

In order to aid in interpretation of the sources of the Bouguer gravity anomaly 

map, I compare the anomalies to mapped geology. A relative Bouguer gravity high of 

25-35 mGals is associated with the areas surrounding the Tucson Basin from the east 

all the way counter-clockwise to the southwest. Major gravity highs are associated 

with mapped plutonic and volcanic outcrop. The oval shaped high (Figure 4.1, A) 

over the Tumacaccori Mountains corresponds to a Jurassic granite and quartz 

monzonite plutonic stock. To the north, a Late-Tertiary granitoid and quartz 

monzonite stock produces a gravity high over the Sierrita Mountains (Figure 4.1, B). 

Further north, small outcrops of Tertiary quartz latite porphory (Figure 4.1, C, pink) 

Cretaceous quartz monzonite (Figure 4.1, C, blue) and a small Jurassic rhyolitic 

porphory pluton outcrop (Figure 4.1, C, green) all probably contribute to the largeamplitude gravity high over the Tucson Mountains. To the east a Precambrian twomica gneiss is associated with a gravity low (Figure 4.1, D1, gray), and a Precambrian 

granodiorite is associated with a high (Figure 4.1, D2, gray). To the south highs can 

be correlated with Laramide quartz monzonite and granodiorite (Figure 4.1, E, blue). 

The central and southeastern portions of the map have relatively low (20 

mGals) gravity anomalies. The central north-northeast trending 15-20 mGal low is 

associated with the relatively low density sedimentary rocks of the Tucson Basin 

(Figure 4.1, F). The northeast-trending ~10 mGal low south of the Tucson Basin low 

(Figure 4.1, G) is related to the Amado subbasin (Gettings and Houser, 1997). To the 

southwest (Figure 4.1, E2, gray) Precambrian Continental Granodiorite is associated 

28
Figure 4.1: Bouguer gravity anomalies overlain by polygons representing selected 

geologic units. Letters are discussed in the text. Contours are 2 mGals. 

29
with gravity lows as well as the (Figure 4.1, H, blue) Laramide diorite and quartz 

monzonite. Of the two lows in the southeast portion of the map one is associated 

with Triassic and Cretaceous volcaniclastic sedimentary rocks (Figure 4.1, I) and the 

other is covered with Quaternary sediment (Figure 4.1, J) and the source is unknown. 

Aeromagnetic Map 

In order to determine the sources of magnetic anomalies, polygons delineating 

geologic units with magnetizations high enough to produce magnetic anomalies at the 

height of the surveys (Telford, Geldart, and others, 1976) are overlain on the reduced 

to the pole magnetic map (Figure 4.2). Magnetic susceptibility measurements from

the region and values from the literature (Table 1) aid in the identification of 

magnetic units. Susceptibilities of normally and reversely magnetized mid-Tertiary 

volcanic rocks range from 0.13 to 0.15 SI (0.17 SI average andesite referenced from

Telford, Geldart, and others, 1976). The susceptibilities of Early Cretaceous-Late 

Tertiary granitoids vary from the western side of the basin to the eastern side. The 

Ruby Star Granodiorite of the Sierrita Mountains has a susceptibility of 0.002-0.0035 

SI while the Piper Gulch Monzonite of the Santa Rita Mountains is an order of 

magnitude more susceptible with 0.0219-0.031 SI. Jurassic granites, which are mostly 

reversed, are found to have low susceptibilities ranging from 0.00666 SI to 0.1288 SI. 

Precambrian Continental Granodiorite is estimated to have a low susceptibility 

around 0.00035 SI (Baldyga, 2001). 

30
Figure 4.2: Reduced-to-Pole Aeromagnetic anomaly identification. Letters are 

discussed in the text. Human generated anomalies are labeled. 

31
Map Unit Lithology Susceptibility Remnant 

Polarity Age 

Tv 30-20 Ma 1

Tva 

Black 

Mountain 

andesites 

0.17 SI 7 R 5 31-24 Ma 1

Tlv 

Tucson 

mountain 

rhyolite 

61 Ma 1

Trv 

rhyodacite 

laccoliths & 

dikes 

R 27, 27 Ma 1

Tgdu & 

Tgd rhyodacite R

Tlg Ruby Star 

Granodiorite 0.0035 SI 6 60-52 Ma 1

Tgr rhyolite N 26 Ma 1

Klp quartz latite 

porphyry 

0.0002 SI 

(SD: 0.00004) N 

Kj 

Josephine 

Canyon 

diorite 

0.0302 SI 

(SD: 0.0028) 

0.002 SI 

(SD 0.001) 

N ~ 63 Ma 2

Keq 

Elephant 

Head quartz 

monzonite 

0.0358 SI 

(SD: 0.0056) 

0.0049 SI 

(SD: 0.0007) 

R 68 Ma 1

Trp Piper Gulch 

monzonite 

0.0263 SI 

(SD: 0.0044) N 188 ± 2 Ma 3

Js 

Squaw 

Gulch 

granite 

0.0072 SI 

(SD: 0.00054) 

0.0116 SI 

(SD: 0.00128) 

R 141 Ma 1

Yc Continental 

Granodiorite 0.00035 SI 4 1450 Ma1

Table 1: Measured susceptibilities, remnant magnetization and ages of 

plutonic and volcanic rocks. Data from Gettings, (2002)unless otherwise noted. 1

KAr, Drewes, (1971), 2 Pb-α, Drewes, (1971), 3

 Asmerom (1988), this date reassigns 

the unit to the Early Jurassic, 4

 Baldyaga, (2001), 5 Percious, (1968), 6 Walker, 

(1982), 7 Average Andesite, Telford, et al, 1976.. Units from Drewes, (1971), Mt. 

Wrightson quadrangle 1:48000. SD-Standard Deviation, average number of samples 

5. 

32
Some features on the map are not caused by geological sources. Linear 

northwest striking anomalies correspond to railroad tracks, pipelines and Highway 89 

(figure 4.2). Buildings and other city features cause short-wavelength dimples. 

Trends, wavelengths, and amplitudes of magnetic anomalies can help 

determine their sources. Positive magnetic anomalies (250 to 450 nT) represent 

normally magnetized sources, such as dikes, granitic intrusions, volcanic flows, tuffs, 

and volcano-clastic units; low anomalies (-100 to -250nT) characterize non-magnetic 

sedimentary and metamorphic rocks in the region reverse-polarity magnetic sources, 

or the dipole of a positive anomaly. Where positive anomalies correlate with high 

topography, the rocks are probably normally magnetized. As well, low anomalies in 

high terrain suggest reversed magnetic rocks (Grauch, Sawyer, and others, 1997). 

When there is lack of correlation with topography or mapped geology, the possibility 

of buried sources arises. 

Broad magnetic highs in the region primarily correspond to intrusive rocks of 

various ages. The high-amplitude, northeast-trending positive magnetic anomalies on 

the east side of the Tucson Basin primarily correlate with magnetic (Table 2)

Laramide-age (Late Cretaceous-Early Tertiary) granodiorites/diorite intrusions as 

described in detail below. Laramide quartz monzonite and granodiorite outcrops 

correspond to nearly 300nT highs in the Santa Rita Mountains (Figure 4.2, A1). Just 

to the south, extrusive andesitic to dacitic volcanic breccias (Figure 4.2, B1, white) 

are associated with highs ranging from 50-150 nT. Quartz latite porphyry intrusions 

(Figure 4.2, C1, pink) also correspond to magnetic highs. Further south, highs on 

either side of the Sawmill Canyon Fault seem similar in amplitude and texture; 

33
however, the mapped geology is very different. Precambrian Continental 

Granodiorite (Figure 4.2, A2) is associated with the northern anomaly, while 

Cretaceous-Tertiary Josephine Canyon Diorite, is linked to the southern anomaly 

(Figure 4.2, A3). To the west (Figure 4.2, A4) the reversely magnetized (Gettings, 

2002) Cretaceous-Tertiary Elephant Head Quartz Monzonite is associated with a 

moderate amplitude negative magnetic anomaly. The similarity of the magnetic 

anomalies A2-A3, the low magnetic susceptibilities of the Precambrian granodiorite 

(Baldyga, 2001) and their lack of correlation with magnetic anomalies elsewhere 

suggest that the source of anomaly A2 is a buried Laramide-age granodiorite/diorite 

and not the surficial Precambrian rocks. The similar north-west trending long 

wavelength positive anomaly to the north (Figure 4.2, A5) is partially associated with

a low susceptibility Precambrian granodiorites and gneiss. No Laramide-age intrusive 

rocks crop out in this area, however there are two small outcrops of mid-Tertiary 

andesites (Figure 4.2, B2), but the wavelengths of the positive anomaly are too long 

to be related to these shallow volcanic rocks. This is the same high previously 

identified as a possible Laramide-age intrusion. (Sauck, Sumner, and Christensen, 

1971). 

In contrast to the Laramide-age intrusions, the Mid-Tertiary Ruby Star 

Granodiorites (Figure 4.2, D) in the Sierrita Mountains do not seem to be associated 

with high-amplitude positive anomalies like those of the Santa Rita Mountains, but 

with moderate amplitude anomalies ranging from 150nT to –250 nT. A reversely 

magnetized (Gettings, 2002) Jurassic intrusion in the northern Tumacacori Mountains

(Figure 4.2, E) is associated with moderate- frequency and amplitude magnetic lows 

34
(-50 to 50 nT). Over the Tucson Mountains a broad positive anomaly (Figure 4.2, F) 

correlates with a small outcrop of a Jurassic-Triassic rhyolitic porphyritic dike. 

However this is probably not the source, because a dike is not wide enough to 

produce such a broad anomaly. 

Volcanic rocks in the region often produce high-frequency positive and 

negative magnetic anomalies. Andesitic volcanic fields in the Tumacaccori 

Mountains (Figure 4.2, B3) are associated with positive and negative magnetic 

anomalies, reflecting normally and reversely magnetized volcanic rocks (Gettings, 

2002) erupted during multiple episodes. The reversely magnetized CretaceousTertiary (Laramide) rhyodacite tuff, breccia, and volcanics (Figure 4.2, B4) (Gettings, 

2002) correspond to magnetic lows (-250 nT to –50 nT), and younger Oligocene 

volcanic rocks correspond to positive anomalies (150nT to 0 nT, (Figure 4.2, B5)). 

The positive anomaly over Black Mountain (B6) is associated with Oligocene 

andesites. At the north edge of this anomaly is a small outcrop of reversely 

magnetized Turkey Track porphyry (Percious, 1968) dated at 46 Ma, and therefore 

probably associated with the reversed volcanic rocks to the north in the Tucson 

Mountains (Figure 4.2, B4). 

35
Chapter 5 

MODELING OF THE GRAVITY AND MAGNETIC DATA 

Two Dimensional Modeling 

To determine the geometry of the Tucson Basin, the dip of the bounding 

faults, the density distribution of sedimentary basin fill, and the lithology of the 

basement, I modeled gravity and magnetic profiles with a 2-1⁄2 dimensional forward 

and inverse gravity and magnetic modeling program (after Webring, 1985). The 

program uses profiles of gravity and magnetic observations (here taken from the grids 

shown in Figures. 4.1 and 4.2) and starting models consisting of body corners and 

density and susceptibility contrasts. I then adjusted the starting model so that its 

gravitational and magnetic anomalies fit the profiles of the observed data. In order to 

reduce edge effects, the bodies at the ends of the model sections were continued 3000 

km. Perpendicular to the profiles, the bodies were extended distances determined by 

their mapped geologic extents. 

The geometry of the starting model was constrained by seismic reflection data 

(Roy Johnson, pers. comm, 2000) and well-logs where available. For the Tucson 

Basin, the starting model consisted of four sedimentary layers consisting of Pliocene 

and Pleistocene upper fill; Late/Mid-Miocene lower fill; the Early Miocene Pantano 

Formation; and denser pre-Miocene sedimentary rocks (Figure 5.1—well log). 

The densities used in the models were obtained from outcrop and well-log 

data (Table 2). The Bouguer reduction density of 2670 kg/m3

 as measured in 

outcrops of granitoids throughout the area (Gettings, 1997) was assumed as the 

average for the models. Based on well log bulk density data (Figure 5.1) (Houser and 

36
Figure 5.1: Exxon well (32) stratigraphic column after Houser and Gettings (2000). 

Densities averaged from well log, susceptibilities determined from modeling and 

samples (see text for more detail). Eberly and Stanley (1978) layer divisions for 

comparison. 

37
Formation Unit Map Unit Davidson, 1973 

Map 

Unit 

Drewes, 

1980 

Density 

range kg/m3

Average 

Density 

kg/m3

Density 

Contrast 

kg/m3

Upper fill Unit A Unit B Q Ts Fort Lowell Formation 2000-2300 2170 -500 

Lower Fill Unit C Unit D Tos Tinaja beds 2250-2450 2350 -320 

Pantano Formation Tc 2450 - 2550 2500 -170 

Middle Tertiary 

volcanic/sedimentar

y rocks 

Tvs 2650 - 2750 2700 30 

Other sedimentary 

rocks 2670 0 

Granitoid 2550 – 2700 2670 0 

38

Table 2: Exxon well (32) log bulk densities and susceptibilities: Densities from Houser and Gettings, 2000.
Gettings, 2000), the density contrast between the sedimentary units below the 

Pantano Formation and the basement is zero. Therefore, I assume that the upper three 

bodies consisting of Miocene and younger sedimentary rocks produce the observed 

Bouguer gravity low (Figure 4.2). I assume that the magnetization of the sedimentary 

basin fill is zero because the possible magnetic anomaly from sediment is very small 

compared to underlying or nearby volcanic and igneous rock. 

The separation into upper and lower basin fill was based on age, degree of 

consolidation, and the amount of deformation (Houser and Gettings, 2000). In 

southeastern Arizona’s Basin and Range province, upper basin fill deposits are 

generally poorly to moderately consolidated, flat lying and only disturbed by a few 

small displacement faults (Houser and Gettings, 2000). The change from upper fill to

lower fill is noted in the Exxon log as a change in bulk density from 2150 kg/m3

(density contrast of –520 kg/m3 in the models) to 2350 kg/m3 (density contrast of –

320 kg/m3

) (Figure 5.1). The contact between the lower basin fill and the Pantano 

Formation is sharp, characterized by an increase in bulk density from 2350 kg/m3 

(density contrast of –320 kg/m3) to 2550 kg/m3 (density contrast of –120 kg/m3) and 

an increase in seismic velocity from 3.3 km/s to 4 km/s (Houser and Gettings, 2000). 

This division is considered to mark the beginning of the Basin and Range extensional 

event at about 13-12 Ma (Houser and Gettings, 2000) (Roy Johnson, pers. comm., 

2002). 

Although the major portion of the fill of the basin consists of sedimentary 

rock, well logs reveal non-sedimentary rocks distributed within the sedimentary fill. 

Several wells hit layers of Tertiary volcanic rocks, welded tuff, and gypsum/anhydrite 

39
pockets. The gravity and magnetic perturbations caused by these other rocks are small 

because their thickness and lateral extent is limited. Therefore, in modeling the 

gravity and magnetic data, we have assumed that the density and magnetization 

contrasts between the sedimentary layers and the country rock are responsible for the 

first order shapes and amplitudes of the gravity and magnetic anomalies except in the 

case of magnetic anomalies where a body is close to the surface. 

Combining known geologic data with seismic data helps to constrain the 

geologically feasible models derived from gravity and magnetic analysis. Well log 

data (Anderson, 1987; Davidson, 1973; Drewes, 1980; Houser and Gettings, 2000) 

and rock outcrops are mostly limited to the eastern edges of the three east-west model 

cross section lines. However, combining these data with the seismic cross sections 

constrains the interpretation of the model cross-sections. For example, a few well logs 

at the edges of the basin reach the Pantano Formation (Figure. 1.1) and crystalline 

basement. Correlation between the well logs (especially the Exxon (32)) and the 

seismic sections indicates that the top of the Pantano Formation is represented on the 

seismic sections as a sharp reflection that represents the transition between flat lying 

sedimentary units (< 13 Ma), and underlying faulted Pantano Formation (Figure. 1.1, 

purple). This correlation agrees with the division of Houser and Gettings (2000) and 

Johnson (oral communication, 2002). Eberly and Stanley (1978), however, 

interpreted that the strong reflection corresponded to the top of a lava flow below the 

Pantano Formation at a depth of 2.195 km (Figure 5.1), roughly 300 meters deeper 

than the interpreted depth from Houser and Gettings (2000). Houser and Gettings 

(2000) note that the sharpness of the increase of seismic velocities and densities 

40
logged in the Exxon well (32) suggest that the contact is probably an erosional 

unconformity representing a considerable hiatus and not the top of a lava flow. 

Although it is difficult to determine the exact source of the reflection, geologic lines 

of evidence support the interpretation as the top of the Pantano Formation. The 

Pantano Formation crops out near Black Mountain in the western portion of the basin 

(Figure 1.1) and in the Cienega Gap (Figure 1.1) at the eastern edge of the basin. 

Because the formation is at the surface on both sides of the basin, and under the lower 

fill in the basin, the formation would need to be faulted. Therefore the Pantano 

Formation must lie below the flat lying sedimentary structures represented on the 

seismic lines. This suggests that the strong seismic reflector represents the top of the 

Pantano Formation at 1.88 km depth at the location of Exxon well (32). 

 In addition to the difficulty in correlating the prominent seismic reflector with 

a specific rock unit, problems exist with correlating the depths determined from the 

well log with those that can be inferred from the seismic reflection profiles by 

applying reasonable seismic velocities. The two-way travel time to the prominent 

reflector inferred to represent the top of the Pantano formation is 1.8 s at the location 

of Exxon well (32). If the depth to the top of the Pantano Formation from the Exxon 

well (32) is ~1880m, an average p-wave velocity of 2 km/s is required for the basin 

fill units. While this seems low, the sonic logs from the Exxon well (32) show a 

highly variable range of 2 km/s to 3 km/s, with an average of 2.4 km/s for the upper 

fill and a range of 2.75 to 3.2 km/s, with an average of 3.15 km/s for the lower fill. 

For the upper fill average velocity of 2.4 km/s this places the bottom of the upper fill 

at 1.08 km. The stratigraphic column (Figure 5.1) shows a depth to the bottom of the 

41
upper fill to be 0.8 km or 74% of the seismically estimated depth. The estimated 

depth to the top of the Pantano Formation using an average velocity of 2.78 km/s 

results in an estimated depth of 2.5 km. The Exxon well (32) depth estimate is 1.88 

km or 75% of the seismic travel time estimate. The sources of error in correlating the 

depth estimates between the seismic sections and the Exxon well (32) can arise from

uncertainties in parameters used in the seismic data processing. A 25% error is not 

uncommon when using a constant velocity in the migration process (Lapinski, pers. 

comm., 2003). A velocity of 3 km/s was used in this migration. Therefore, the 

seismic lines are used primarily to delineate the shape of the sedimentary layers for 

modeling and to identify the top of the Pantano Formation. In the following, I assume

that the depth to the top of the Pantano is indeed 1.88 km (Gettings and Houser, 

1997). 

By assuming that the thickness of each sedimentary layer in the central part of 

the basin is a constant percentage of the total basin thickness as represented in the 

Exxon well (32), I can correlate the stratigraphic column from seismic line two 

(Figure 5.2) to seismic line three (Figures 5.3 and 5.4). By correlating the known 

depth of the top of the Pantano Formation with the strong reflection in the seismic 

line, we can show the approximate depth of the other layer divisions. The seismic 

lines were placed behind the cross section during modeling, and layer reflections

were traced to obtain the shape of the interface between the top of the Pantano and 

lower fill and adjacent bedrock in the starting model. The location of the boundary 

between the upper and lower basin fill was inferred from the well logs. 

42
43

Figure 5.2: Exxon seismic line two. Green lines are modeled layer divisions shown in figure 5.5. Column is taken Exxon well (32), figure 5.1. Seismic line two is coincident with cross section CS-1. The location is shown in red 

CS-1 in figure 1.1. The top of the Pantano Formation (blue) is correlated with the strong reflector (discussed mor
44 

Figure 5.3: Exxon seismic line three. Green lines are modeled layer divisions shown in figure 5.7. Column is tak

Exxon well (32), figure 5.1. Seismic line three is coincident with cross section CS-2. The location is shown in red 

CS-2 in figure 1.1. The top of the Pantano Formation (blue) is correlated with the strong reflector (discussed mor
45

Figure 5.4: Seismic line four. The location is shown in red along north-south line in figure 1.1. Columns are prop

well (32) column in figure 5.1 with the top of the Pantano Formation aligned with the strong reflector (discussed 
W E 

Figure 5.5: Cross section CS-1, location shown in Figure 1.1. A. Magnetic anomalies, 

solid lines respond to magnetic response from individual colored bodies in C, dashed 

blue line is pseudogravity acquired from figure 3.7. C. Gravity anomalies, solid lines 

respond to gravitational response from individual colored bodies in C. C. modeled 

cross section. Colors correspond to layers derived from Exxon well (32) (figure 5.1). 

If not labeled, susceptibilities of bodies are zero (discussed more in text). 

46
47

W E 

Figure 5.6: Cross section CS-2, location shown in Figure 1.1. A. Magnetic anomalies, 

solid lines respond to magnetic response from individual colored bodies in C, dashed 

blue line is pseudogravity acquired from figure 3.7. C. Gravity anomalies, solid lines 

respond to gravitational response from individual colored bodies in C. C. modeled 

cross section. Colors correspond to layers assigned in Exxon well (32) (figure 5.1). If 

not labeled susceptibilities of bodies are zero (discussed more in text). 
48

W E 

Figure 5.7: Cross section CS-3, location shown in Figure 1.1. A. Magnetic 

anomalies, solid lines respond to magnetic response from individual colored bodies in 

C, dashed blue line is pseudogravity acquired from figure 3.7. C. Gravity anomalies, 

solid lines respond to gravitational response from individual colored bodies in C. C. 

modeled cross section. Colors correspond to layers assigned in Exxon well (32) 

(figure 5.1). If not labeled susceptibilities of bodies are zero (discussed more in text).
For the magnetic modeling, several assumptions were made for the rock 

properties. In all cases, the magnetizations of the modeled bodies were assumed to be 

in the direction of the present Earth’s field (inclination of 59° and declination of 12°

with an intensity of 49000 nT). This assumption is reasonable for most of the 

intrusive rocks because their magnetization is largely induced (Reynolds,

Rosenbaum, and others, 1990). This assumption however is not true for volcanic 

bodies as we can clearly see negative anomalies due to reversed rocks in the 

Tumacaccori Volcanic Field (B3, Figure 4.2). 

Correlation of magnetic anomalies with outcrops of igneous and volcanic 

rocks as described above guided the selection of the rock types in the region that

produce magnetic anomalies. Susceptibility measurements of dozens of igneous rocks 

(Gettings, 2002) suggest a range within which to restrict the modeling. Table 1 shows 

susceptibilities of some of the rocks in the study area. Magnetic bodies were placed 

below the modeled sedimentary layers and the susceptibilities were varied in order to 

fit the observed anomaly. 

Figures 5.5-5.7 show the three model cross sections (c) and observed and 

calculated magnetic (a) and gravity profiles (b). The difference between the observed 

and calculated is given by the mean error on the figure. Figures 5.5 and 5.6 are 

modeled across the two east-west seismic lines and Figure 5.7 is to the south. For all 

cross sections, the lower crust (from 12km to a depth of 30km (Myers and Beck, 

1994) is modeled with a density contrast of 280 kg/m3

, and a susceptibility of zero. 

The susceptibilities and density contrasts used for the crustal section above 12 km

depth are shown in table 3. 

49
Lithology Susceptibility Density 

Contrast 

Tertiary 

reversely 

magnetized 

volcanics 

-0.15 SI -270 k/m3

Tertiary 

dikes 0.13 SI -270 k/m3

Ruby Star 

Granodiorite 0.0035 SI -30 k/m3

Rincon and 

Continental 

Granodiorite

0.0035 SI 0 k/m3

TertiaryCretaceous

quartz 

granodiorite 

0.035 SI 0 k/m3

other 

sedimentary 

rocks 

0 -150 k/m3

 to 

+200 k/m3

Table 3: Susceptibilities and densities of rock units used in modeling excluding units 

from the Exxon well (32) log that are listed on Fig. 5.1. 

Cross section CS-1 (Figure 5.5) partially coincides with Exxon seismic line 2 

(Eberly and Stanley Jr., 1978). Outcrop of Precambrian Rincon Valley Granodiorite 

bounds the eastern edge of the model at the surface and has an assigned density 

contrast of zero and susceptibility of 0.003 SI. The body is on the upper plate of the 

detachment fault related to the Santa Catalina Metamorphic Core Complex and can be

modeled as very thin (500m). A sedimentary rock body (density contrast, –150 

kg/m3

; susceptibility, zero) mostly Cretaceous in age is modeled to the east. The -13 

mGal main gravity low over the Tucson Basin can be modeled as due to the negative 

density contrast between the intra-basin late Cenozoic sedimentary rocks and the 

adjacent denser country rock. The thicknesses and density contrasts taken from the 

Exxon well (32) (Figure 5.1) are: upper fill, 0.908 km, -500 kg/m3; lower fill, 0.972 

50
km, -320 kg/m3

; Pantano Formation, 0.636 km, -120 kg/m3; other sedimentary rocks, 

1.141 km, 0 kg/m3. The effect of each layer’s density contrast can be seen when the 

gravity response of each block is calculated. It is shown that the primary body 

responsible for the basin gravity anomaly is the upper fill (Figure 5.5b, yellow line), 

and that the deeper layers have decreasing affect on the gravity anomaly (Figure 5.5 

b, pink, and purple lines), with the pre-Oligocene rocks having no effect due to their 

lack of assigned density contrast with the adjacent rocks. 

Increases in the dips of the interfaces at the edges of the basin are modeled at 

17 km and 27 km along the profile. These changes in the dips are marked by peaks in 

the amplitude of the gravity maximum horizontal gradients (Figure 5.5b, dashed blue 

line; Figure 3.3). The third maximum horizontal gradient peak at 38 km along the 

cross section represents a steep contact between the sedimentary layers and the 

granitic outcrop with a dip of 65o. The basin is deepest from 18 to 26 km along the 

profile and the layers thin towards the edges. However, a thicker layer of the Pantano 

Formation is seen towards the east from 26 to 32 km along the profile. 

The basement beneath the basin is modeled with two bodies. On the west, the 

body has zero density and magnetization contrasts. Based on the lack of high positive 

magnetic anomalies and outcrop distribution, a combination of Precambrian or 

Jurassic granitoid rocks and perhaps Paleozoic sedimentary rocks most likely 

correspond to this body, but the lithology is unconstrained by the modeling. In 

contrast, the eastern body produces a large (150 nT) positive magnetic anomaly 

(anomaly A5, Figure 4.2) that can be modeled with 0 density contrast and 

susceptibility 0.035 SI (Figure 5.5,a, light blue line). The match of the modeled with 

51
measured susceptibilities in the region, corroborates the interpretation above that this 

body represents a Laramide diorite. This body begins at a depth of 1 km and thins as 

it dips to the west under the basin. In the eastern portion of the cross section it reaches 

a maximum thickness of 11 km. The Exxon well (32) hits granitoid rock at 3.66 km

depth (Figure 5.1) that is interpreted to be part of the Precambrian Oracle Granite 

exposed 15 km to the east in the Cienega Gap (Houser, pers. comm., 2002). Therefore 

in my model I connect the Precambrian granite outcrop at the surface to granite below

the drill hole. It was shown previously that the Precambrian granites of this region 

have very low susceptibilities compared to the underlying Cretaceous/Tertiary 

igneous bodies, so it is modeled as zero density contrast, 0.0035 susceptibility. The 

sharp dip of the top of this body that begins at 32km along the profile and bottoms out 

at 31 km along the profile is coincident with a peak in the amplitude of the 

pseudogravity maximum horizontal gradient (Figure 5.5, a, dashed blue line; Figure. 

3.7). On the west side of the profile, the sedimentary section thins to 1 km. At the 

surface, a thin reversely magnetized body is placed in the western edge of the basin 

that is 0.1 km thick with a density contrast of -270 kg/m3

 and susceptibility, -0.2 SI 

(Figure 5.5, a, dark blue line). 

Cross section CS-2 (figure 5.6) was modeled along Exxon seismic line three. 

To estimate the depths of this cross section’s sedimentary rock layers, I correlated the

sedimentary rock layers logged in the Exxon well (32) with the reflective horizons in 

Exxon seismic line three (Figure 5.3). This procedure is approximate, but gives a 

starting point for the modeling of the layer thicknesses. A mixture of Pinal Schist, 

Precambrian granite, Lower Cretaceous sedimentary rocks and small intrusions of 

52
Tertiary quartz latite porphyry bound the eastern edge of the cross section. I assume

that the sedimentary rocks and the porphyry intrusions are thin compared to the 

thicknesses of the granitoids, based on the high amplitude positive gravity anomaly 

and lack of correlation between the porphyry and the magnetic signature. Therefore, I 

modeled the mix of rock types as one body with zero density contrast, zero 

susceptibility and a thickness of 1 km. The magnetic profile above this area shows 

that a short wavelength magnetic body would help the profile fit, but my main interest 

was to model the main anomaly features. 

The body primarily responsible for the negative 20 mGal gravity anomaly is 

the upper fill (yellow line). However, the effect is much less than in cross section CS1 (Figure 5.5, b). Each lower layer is responsible for less of the anomaly (pink and 

purple lines) and the pre-Oligocene rocks have no effect (light green line). The layers 

are thinner in cross section CS-2 (where seismic line four intersects the cross section) 

than in cross section CS-1 (upper fill, 0.716 km, -500 kg/m3

; lower fill, 0.744 km, -

320 kg/m3; Pantano Formation, 0.48 km, -120 kg/m3; other sedimentary rocks, 0.99 

km, 0 kg/m3

). The western side of the basin has an apparent steeper dip than the 

eastern side; this roughly correlates with the peak amplitude in the gravity maximum 

horizontal gradient (Figure 5.6, b, dashed blue line; Figure. 3.3). The layers thin 

towards the edges of the model. 

The positive magnetic anomaly on the east end of the profile can be modeled 

by a large body beneath the surface similar to that in cross section one (density 

contrast 0, susceptibility 0.035 SI, light blue line line). This body crops out and the 

base of the unit is shallower than in cross section one. The amplitude of the anomaly 

53
in cross section CS-2 is 100 nT versus 130 nT in cross section CS-1. Therefore the 

body is modeled as thinner. Several normally magnetized units can be modeled in the 

basement; while there effect is small (dark pink lines) they were included because 

they are very evident on the magnetic map (Figure 4.2,G). 

Cross section CS-3 (Figure 5.7) was not constrained by a seismic line. The 

densities and susceptibilities used in the previous two cross sections were applied to 

this model in order to determine the layer geometry. The body primarily responsible 

for the negative gravity anomaly is the upper fill (density contrast –500 kg/m3

, 

susceptibility 0, 0.48 km thick, yellow line), however, it has less of a response than in 

the first two cross sections, primarily because it is thinner. Each lower layer is 

responsible for less of the anomaly (upper fill, density contrast –320 kg/m3

, 

susceptibility 0, 0.75 km thick, pink line; Pantano Formation density contrast –120 

kg/m3, susceptibility 0, 0.64 km thick, purple line; other sedimentary rocks, density 

contrast +100 kg/m3

, susceptibility 0, 0.65 km thick, light green line). The preMiocene sedimentary rocks are modeled here as dense Paleozoic limestone and 

marble based on analysis of the gravity and magnetic maps above. A change in dip of

the western edge is shown in the peak in amplitude of the gravity maximum

horizontal gradient (Figure 1.14,b, dashed blue line; Figure. 3.3). This apparent steep 

dip on the western side of the basin is similar to the western side of cross section CS2. The eastern dip is slightly steeper than the western. The outcrop on the eastern 

edge of the cross section is a thin layer (~1 km) of Precambrian granite (density 

contrast 200, susceptibility 0.0035 SI) underlain by the large magnetic body modeled 

in the previous two cross sections (density contrast 0, susceptibility 0.035 SI, light 

54
blue line). This body is responsible for almost the entire magnetic anomaly along the 

profile (light blue line line). Outcrop of the Ruby Star Granodiorite on the western 

side contributes very little to the magnetic anomaly (susceptibility 0.0035, Figure 5.7, 

c, black/white line) but is responsible for the gravity high on the western edge of the 

basin (density contrast +30 kg/m3

). A thin reversely magnetized volcanic layer on the 

western side is included in the model (density contrast –270 kg/m3

, susceptibility -

0.15, 20m thick, dark blue line). 

Model sensitivity 

In order to test the sensitivity of the model to the input density and 

susceptibility contrasts, I varied the density contrasts of the basin units and the 

country rock but left the depth to the top of the Pantano Formation fixed (at 1.88 km

depth in cross section CS-1). For cross section CS-1, increasing the density contrast 

of the upper fill by 50% (-500 kg/m3

 to –750 kg/m3) and allowing the lower fill 

thickness to vary, decreases the modeled upper fill thickness from 0.908km to 0.46 

km, or roughly 50%. The dip on the eastern side of the basin shallowed from roughly 

9 degrees to 4.5 degrees. The dips on the western edge of the basin were not affected. 

Increasing the density contrast of the lower fill by 25%, from –320 kg/m3 to –

400 kg/m3

, holding the density of the upper fill and Pantano Formation constant, 

decreased the thickness of the upper fill by 50% from 0.972 km to 0.54 km and 

decreased the upper fill. The dips in the lower fill shallowed on the western side from

21.5 degrees to roughly 10 degrees. Some of the dips on the eastern side of the basin 

increased and some decreased, but most remained unchanged at 20 degrees. 

Increasing the lower fill density contrast by 50% from –320 kg/m3

 to –480 kg/m3 and 

55
not moving the fixed top of the Pantano Formation, the upper fill cannot be made thin 

enough to fit the gravity profile alone. 

The modeled upper and lower fill represent rocks deposited during and after 

the basin and range extension event beginning ~13 Ma , and lying unconformably on 

top of the Pantano Formation. However, the interface between the upper and lower 

fill is not evident in the seismic reflection data. With this in mind I decided to 

determine if the fill above the Pantano Formation could be modeled with just one 

density contrast and if this assumption would affect the location of the top of the 

Pantano Formation in the models. Table 4 shows the results of changing the density 

Fill Description Density

kg/m3

Density 

Contrast 

kg/m3

Depth to top 

of Pantano 

Formation 

one fill, inverted density 2289 -381 1.88 km 

one fill, lowest reasonable density 2175 -495 1.76 km 

one fill, highest reasonable density 2335 -335 2.52 km 

one fill, Top of Pantano at Eberly & Stanley depth 2308 -362 2.19 km 

one fill, weighted average density 2252 -418 

Table 4: The results of changing the density contrast and the depth to the top of the 

Pantano Formation. 

contrast and the depth to the top of the Pantano Formation. Modeling with the 

highest (-495 kg/m3

) and lowest (-335 kg/m3) geologically feasible density contrasts 

(a weighted average of the highest or lowest densities recorded in the Exxon well (32) 

density logs) results in a single basin fill thickness of 1.76 km and 2.52 km

respectively on cross section CS-1. The 2.52 km depth calculated with the lowest 

density contrast resulted in a zero thickness of the Pantano Formation in the deepest 

portion of the model, violating the constraints. Inverting for density contrast while 

56
holding the top of the Pantano Formation fixed to a depth of 1.88 km (the depth 

recorded in the Exxon well (32)) resulted in a density contrast of –381 kg/m3

. This 

value is less than the weighted average density contrast of –418 kg/m3 from the 

Exxon well (32) density log. To try to ascertain whether the depth at which Eberly 

and Stanley (1978) placed the bottom of the basin fill (at the top of the lava flow in 

the Exxon well (32)) is a better estimate than that obtained from Houser and Gettings 

(2000), I fixed the bottom of the basin fill at 2.194 km depth. The resulting density 

contrast is –362 kg/m3

, lower than would be expected from the weighted average 

density contrast of the well log, suggesting that the shallower depth determined by 

Houser and Gettings (2000) is more accurate. 

Changing the susceptibilities of the main inferred Laramide diorite body on 

the eastern sides of the models shows that the upper interface is not very sensitive to a 

change in susceptibility (i.e., the interface stays the same). Decreasing the 

susceptibility from 0.035 (SI) to 0.03 (SI) can be accommodated in the model, by 

increasing the thickness of the body down to the Curie depth of 12 km (Hong, Aiken, 

and others, 1982). Decreasing the susceptibility further to 0.025 (SI), the magnetic 

body’s upper interface cannot be made shallow enough to fit the magnetic profile. 

Increasing the susceptibility results in a thinner magnetic body. Without changing the 

magnetic/non-magnetic interface I can raise the susceptibility to 0.08 (SI), which is an 

unreasonable value for a pluton. 

Areal Extent of the Basin:

Combining the results of the modeling, maximum horizontal gradients of the 

gravity and pseudogravity and the map view of the gravity anomaly allows 

57
delineation of the 3-dimensional extent of the basin. The results of the modeling of 

the three cross sections can be gridded (linear interpolation using MATLAB) to create 

contour maps of the depths to the bottom of the upper (Figure, 5.8, a) and lower fill 

(Figure 5.8, b) and the bottom of the Pantano Formation (Figure 5.8, c). These 

contour maps depict the variations in the lateral and vertical extents of the lower fill 

and the Pantano Formation. For both lithologic units, the depths to the bottom of the 

units shallow and narrow to the south. The lower fill unit has roughly the same lateral 

extent in the south as the Pantano Formation, but the lower fill deepens towards the 

north and central portion of the basin. The depth to the bottom of the Pantano 

Formation however seems deepest between cross section two and at the eastern edge 

of cross section one. 

Three-dimensional character of the basin 

These results suggest that the deepest portion of the basin is towards the 

northern end, however, the Bouguer gravity anomaly values (Figure 4.1) decrease 

towards the north suggesting the basin shallows. Therefore, further correction of the 

gravity data is needed to isolate the gravity anomaly associated with the basin fill. 

Removing the gravity effect of changes in crustal thickness and variations in the 

density of the bedrock (Saltus and Jachens, 1995) produces a residual basin gravity 

anomaly with lowest values over the area identified as the deepest part of the basin by

my modeling. Therefore, I decided to use their residual basin gravity anomaly to help 

extend the 2 1⁄2 dimensional modeling to three-dimensions. The regional gravity 

corrections were made to isolate the gravity anomaly associated with the postOligocene sedimentary units. 

58
A. Depth to bottom of upper 

fill 

B. Depth to bottom of lower 

fill 

C. Depth to bottom of Pantano 

Formation 

Figure 5.8: Depth to bottom of units contoured from model profiles (figures 5.5-5.7). 

Model cross sections shown. Linear interpolation. Contours are 0.2 km. 

59
Gravity Corrections 

The Bouguer reduction process removes the gravitational attraction of 

topographic masses down to sea level, but the compensating mass underneath the 

topographic highs is not accounted for and can produce Bouguer gravity anomaly 

lows (Simpson, Jachens, and others, 1986). Therefore in order to remove the 

attraction of compensating masses an isostatic reduction was applied. This regional 

field is the gravitational effect of topographic roots calculated using the AiryHeiskanen model with a density contrast across the root of 350 kg/m3, topographic 

load density of 2670 kg/m3, depth to root at sea level of 30 km and topographical 

data (Simpson, Jachens, and others, 1986) (Figure 5.9). The removal of the isostatic 

anomaly results in the residual isostatic gravity anomaly map (Figure 5.10). 

The residual isostatic gravity anomaly map still indicates that the deepest 

portion of the basin is just south of cross section CS-2, suggesting that the basement 

rocks in the northern end of the basin have a higher density than the rocks to the 

south. This would result in a higher density contrast with the basin fill and an 

underestimation of the thickness of the fill in the northern portion of the basin. 

Therefore, the basin gravity anomaly still needs to be separated from the basement 

signature. The basement gravity map, from Saltus and Jachens (1995), was produced 

by gridding only the gravity stations that are on basement outcrop (Figure 5.11). The 

map shows that higher density rocks are in the north, and lower density rocks are in 

the south, in accord with geologic map (Figure 1.1), that shows dense, plutonic rocks

in the north and lower density volcaniclastic and sedimentary rocks to the south. This 

60
Figure 5.9: Regional isostatic gravity field, 1 mGal contours (discussed more 

in text). From Saltus and Jachens (1995).

61
Figure 5.10: Residual isostatic gravity anomaly map. Bouguer gravity (Figure 3.2) 

with isostatic compensation (Figure 5.9) removed. 2 mGal contour interval. 

62
Figure 5.11: Basement gravity map from Saltus and Jachens [1995]. Lateral density 

variations in the basement are delineated. Contour interval is 2 mGals. 

63
Figure 5.12: Basin residual gravity map, Residual isostatic gravity anomaly map 

(Figure 5.10) with basement gravity removed (Figure 5.11). The contour interval is 5 

mGals. 

64
basement gravity anomaly was removed from the isostatic residual gravity anomaly 

to produce the basin residual gravity map (Figure 5.12). This map shows the extent of 

the gravity anomaly due primarily to the density contrast of the upper and lower basin 

fill with the surrounding country rock. Now the lowest gravity anomalies lie over the 

northern portion of the basin and agree with the depth contour maps for the southern 

half (figure 5.13).

The peak amplitudes of the maximum horizontal gradient of the gravity 

(Figure 3.3) and pseudogravity (Figure 3.7) that delineate the edges of the basins in 

the cross sections (dashed blue lines, Figure 5.5-5.7, a and b) are overlain on the 

residual basin gravity anomaly map (Figure 5.14) to show the locations of density or 

magnetic boundaries. The edges of the basin, where low-density fill juxtaposes higher 

density basement, if steep, should appear as a maximum horizontal gradient in the 

gravity. Where steep magnetic boundaries exist, the pseudogravity maximum

horizontal gradient will delineate the edge. Since the gravity anomalies represent the 

density contrast of the basin fill, and the magnetic anomalies represent the igneous 

and volcanic rocks on the basin edges and beneath the fill, it is logical that the 

maximum horizontal gradients of the gravity would better represent the edges of the 

basin. Where the gradients coincide the fill juxtaposes a high density, high

susceptibility rock. 

South of profile CS-2, the western edge of the sedimentary basin is delineated 

by a gravity maximum horizontal gradient (Figure 5.14, A, blue line; Figures. 5.5-5.7 

b). This trend shows that the edge of the basin is a steep contact, for moderate to 

shallow dipping contacts of rock of differing densities or susceptibility are not picked 

65
Figure 5.13: Basin residual gravity map (Figure 5.12) with contours denoting depth to 

bottom of lower fill (white lines) (from Figure 5.8). Gravity contour interval (black) 

is 5 mGal. 

66
Figure 5.14: Gravity and pseudogravity maximum horizontal gradient line overlain on 

the basin residual gravity map with a contour interval of 5 mGals (Figure 5.12). 

Letters are referred to in the text. White square is estimated deepest portion of the 

basin, white dashed lines are estimated basin extent. Yellow lines locations of

modeled cross sections. 

67
up well in the maximum horizontal gradient method (Blakely, 1995). This finding 

agrees with the modeling of the cross sections (Figures, 5.5-5.7b). Further to the north 

(Figure 5.14, B1, B3, B4, blue lines) additional gravity maximum horizontal gradients 

delineate the edges of the basin. 

 Several clear maximum horizontal gradient gravity trends define the eastern 

edge of the basin (Figure 5.14, C1-C5, blue lines). C2 is apparent on cross section 

CS-1 (Figure 5.5, b, km 35). Both gravity and pseudogravity maximum horizontal 

gradients coincide (Figure 5.14, D, white and blue lines) and correlate with the steep 

edge of the basin modeled in cross section CS-2 (km 30, Figure 5.6, c). This steep 

edge is associated with the location of the Santa Rita fault determined from seismic 

refraction (Rutledge, 1984) and reflection (Johnson and Loy, 1992) data. Two 

horizontal gravity gradient trends (Figure 5.14, E1, E2, blue lines) lie within the 

basin. These gradients probably delineate a subbasin, representing edges of the deeper 

and older portions of the basin. 

Combining the modeling results with the gradient analysis above allows me to 

characterize the 3-dimensional geometry of the basin. Using the gradients to 

delineate the edges of the basin (Figure 5.14, dotted white line) shows that the width 

of the upper portion of the basin decreases from about 27 km in the north to 10 km in 

the south. The length of the basin is ~ 63 km. The basin (that is, the depth to the 

bottom of the Pantano Formation) shallows from 2.5 km just north of the Exxon well 

(32) (Figure 5.14, white box) to 1.88 km at the southern-most end. 

68
Areal Extent of the Igneous Basement: 

 Modeling and correlation of anomalies with outcrop and drillhole data has 

shown that the magnetic anomalies can be used to determine the basement geology 

for the east side of the study area and the distribution of volcanic rocks on the west 

side (Figure 5.15). Much of the discussion on the rationale for delineating the various 

units is described in the anomaly identification section. From the modeling and 

correlation with outcrop, we determine that the large positive anomaly in the northern 

portion of the basin (Figure 4.2, A5) is related to the Laramide-age 

diorite/granodiorite plutons that underlie the Rincon and Santa Rita Mountains. While 

several outcrops of Precambrian granodiorite correlate with these magnetic highs, 

measurements show them to be approximately non-magnetic and modeling shows 

thin bodies. The basement geologic map shows that the much of the Tucson Basin is 

underlain by Laramide-age diorite/granodiorite intrusions and that the basement high 

that bounds the basin on the south is composed of these diorites/granodiorites. The 

southern portion of the basin (just north of cross section CS-3) is underlain by nonmagnetic rocks. The amplitude and wavelength of the anomalies over this portion of

the basin are similar to Paleozoic sedimentary sequences that crop out to the 

southeast, suggesting a similar source. To the west, the magnetic signature of the 

shallow, reversely and normally magnetized mid-Tertiary volcanic rocks (Figure 

5.15) obscure the magnetic signature of the basement. From cross section modeling, I 

show that the western basement is non-magnetic, and may correspond in the south to 

the Paleozoic sedimentary sequences inferred to underlie the basin. The lower half of 

69
the central-west portions of the Tucson Basin appear to have been intruded by 

Tertiary andesite and diorite dikes (Figure 5.15). 

70
Figure 5.15: Sources of magnetic anomalies as determined from reduced-to-the-pole 

aeromagnetic data. Generalized geology from figure 1.1 without Quaternary 

sediments and Pantano Formation. Black outlines delimit edges of sedimentary rocks. 

71
Chapter 6 

DISCUSSION: 

This 3-dimensional view of the Tucson Basin was determined by using many 

geologic and geophysical data sets. This relatively well constrained result can be 

compared to the several other modeling approaches used on the Tucson Basin. 

Several key points differentiate my model from previous ones. From a gravity 

modeling standpoint, I assume that the bottom of the basin is at the bottom of the 

Pantano Formation, below which the density contrast is zero. Geologically, the 

bottom of the basin is the top of the Pantano (the Pantano crops out on either side of 

the basin). The Pantano contributes little to the gravity signature (Figs. 5.5-5.7); our 

modeling reflects this. I constrained the densities used in the model from well logs 

and surface density measurements instead of assuming a density function. 

Davis (1971) used residual Bouguer gravity to model the depth to basement 

defined as pre-Tertiary basement rocks; therefore his depth estimates mark the bottom 

of the Pantano Formation. His depth to basement approximates the shape of the 

Bouguer gravity anomaly, with the deepest portion of the basement in the south at a 

depth of 2150 m. The density contrast of –400 kg/m3

 is a fair estimate for the fill 

above the Pantano Formation, but is a bit high when including the Pantano 

Formation. Also, failure to remove the basement gravity anomalies resulted in 

determining that the northern portion of the basin was much shallower than the Exxon 

well (32) records (1.5 km vs. 2.5 km depth to the bottom of the Pantano Formation, 

Figure 5.1). 

Litinsky (1989) modeled the same data set as Davis (1971) with a different 

density/depth model, that had an effective density contrast of –137 kg/m3

 and yielded 

72
a maximum depth of 10 km in the southern portion of the basin. His formula for 

determining the effective density contrast used the gravity anomaly and depth to 

bedrock over the Exxon well (32). The bulk density log from the Exxon well (32) 

(Houser and Gettings, 2000), shows the density contrast below the bottom of the 

Pantano Formation is zero, however Litinsky calculated an effective density contrast 

based on a basin depth of 3.66 km. Since the thickness of the basin is really ~2.5 km

at the location of Exxon well (32), the effective density contrast calculated by 

Litinsky is underestimated, ranging from 281-244 kg/ m3

. In comparison, inverting 

for one fill I calculate the density contrast for cross section CS-1 to be ~-300kg/m3. 

An underestimate of the density contrast, coupled with the failure to remove the 

basement gravity anomalies led Litinsky to estimate a depth of 10 km in the southern 

portion of the basin compared to my estimates of 1.93 km. Ignoring the bulk density 

information on the layered units in the Tucson Basin in this case has lead to excessive 

depth estimates based on the erroneous assumption that the bottom of the basin is at 

the top of the granitoid rather than near the bottom of the Pantano Formation 

Chakravarthi, et al., (2001) modeled a residual Bouguer gravity profile similar 

to my cross section CS-1. Using a surface density contrast of –450 kg/m3

 and a 

parabolic density function, they created a depth section. This profile has the 

approximate shape of my modeled cross section CS-1, but contains no geologic or dip 

information and has exaggerated depth estimates. The authors do not state whether 

this is the surface of the igneous basement or bottom of the Pantano Formation. 

Therefore, any conclusions about the tectonic history are left unanswered. 

73
Saltus and Jachens (1995) modeled the residual basin gravity over the western 

US and calculated maximum depths to Pre-Cenozoic basement to be ~2 km. This 

estimate is too shallow because of their assumption that the density contrast is a 

constant -250 kg/m3

 below 1.2 km (too high for the lower portion of the basin). This 

causes an overestimate of the total density contrast and therefore, underestimates the 

basin thickness. Like our model, they show the Tucson Basin shallowing to the 

south, however, their model shallows to 1 km in the location of my cross section CS2, versus 2.2 km in my model and 0.5 km along cross section CS-3, versus 1.8 km in 

my model. Their depth density contrasts were an average of the entire western US 

and not taken from local density measurements. 

Using all available data sets results in a much more accurate model of the 

Tucson Basin. Identifying what defines the basement, using density contrasts from

well logs, gives a much better model of the basin. 

74
Chapter 7 

CONCLUSIONS: 

By modeling of gravity and magnetic data, I show the 3-D extent of the 

Tucson Basin. The cross sections and the contour maps generated from them show 

the lateral extent and thickness of basin fill; the magnetic and gravity maps show the 

character of the basin away from the cross-sections. These two pieces of information 

together generate a 3-D picture of the Tucson Basin. The gravity effects of the upper 

and lower basin fill produce most of the Bouguer gravity low over the basin. As these 

rocks relate to the most recent extensional event in the region (Basin and Range 

extension) my model will help determine the amount of extension required to form

the basin. The seismic reflection and well data show that the upper basin fill is 

unfaulted while the lower portion is faulted, suggesting a different tectonic history. 

The lower fill was deposited during active extension while the upper fill represents 

deposition due to changes in location of rivers as well as erosion (e.g. Houser and 

Gettings, 2000). 

Modeling also shows that the magnetic anomalies primarily emanate from the 

basement, that is below the sedimentary layers that comprise the basin, except in the 

northwest where Tertiary volcanics lie close to the surface. The large amplitude, 

moderate wavelength magnetic anomalies are related to Laramide-age diorites and 

granodiorites that dip west under the basin. A few Early Tertiary andesite or diorite 

intrusions penetrate the sedimentary basin. Reversely magnetized Early Tertiary 

volcanics are shallowly buried along the western edge of the basin, and Paleozoic?

75
sedimentary rocks are thought to lie beneath the basin just north of the Sawmill 

Canyon Fault, connecting with similar rocks to the southeast. 

 

76
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