U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015–1129

Prepared in cooperation with the New Mexico Environment Department

Preliminary Geophysical Interpretations of Regional 

Subsurface Geology near the Questa Mine Tailing Facility 

and Guadalupe Mountain, Taos County, New Mexico
Cover. Guadalupe Mountain and Questa Mine Tailing Facility. View to the northwest. Photograph by Paul Bauer.
Preliminary Geophysical Interpretations 

of Regional Subsurface Geology near the 

Questa Mine Tailing Facility and Guadalupe 

Mountain, Taos County, New Mexico

By V.J.S. Grauch, Benjamin J. Drenth, Ren A. Thompson, and Paul W. Bauer

Prepared in cooperation with the New Mexico Environment Department

Open-File Report 2015–1129

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Grauch, V.J.S., Drenth, B.J., Thompson, R.A., and Bauer, P.W., 2015, Preliminary geophysical interpretations of regional 

subsurface geology near the Questa Mine Tailing Facility and Guadalupe Mountain, Taos County, New Mexico: 

U.S. Geological Survey Open-File Report 2015–1129, 35 p., http://dx.doi.org/10.3133/ofr20151129.

ISSN 2331-1258 (online)
iii

Acknowledgments

This report was completed under a technical assistance agreement between the U.S. Geological 

Survey and the New Mexico Environment Department. However, the analysis and data acquisition of geophysical data were also funded by several other projects working in similar geographic 

areas. Additional funding sources included the Village of Questa, New Mexico Bureau of Geology 

and Mineral Resources, and National Cooperative Geologic Mapping Program (Federal and 

STATEMAP funds).
iv

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic Setting.............................................................................................................................................4

Geophysical Data...........................................................................................................................................4

Magnetic Data.......................................................................................................................................4

Gravity Data ...........................................................................................................................................5

Methods.........................................................................................................................................................11

Magnetic Methods .............................................................................................................................11

Gravity Methods..................................................................................................................................12

Interpretations..............................................................................................................................................12

Volcanic Deposits of Hatchery Volcano—Tvh—and Andesite of Red River—Tvr.................12

Trachydacite of Guadalupe Mountain—Tagn, Tags, Tag.............................................................17

Faults and Other Contacts.................................................................................................................19

Basin Shape and Thickness..............................................................................................................19

Summary........................................................................................................................................................22

References Cited..........................................................................................................................................24

Figures

1. Map showing a satellite image of the study area from Google Earth, 2013........................2

2. Map of the regional geologic setting of the study area within the Rio Grande rift................3

3. Preliminary geologic map of the study area with unit descriptions.....................................6

4. Aeromagnetic map of the study area after application of the reduction-to-pole 

transformation...............................................................................................................................8

5. Data for four ground magnetic traverses acquired in 2014...................................................9

6. Gravity map of the study area after removal of an isostatic regional field.......................10

7. Interpretations overlain on the aeromagnetic map with an explanation of 

lines and symbols .......................................................................................................................12

8. Map of a computed magnetic-terrain field for the study area............................................14

9. Geologic map of study area overlain by interpretative lines and annotations 

of figure 7 .....................................................................................................................................15

10. Map showing horizontal-gradient magnitude of the reduced-to-pole 

aeromagnetic data .....................................................................................................................16

11. Gravity map overlain by interpretative lines and annotations of figure 7 .........................20

12. Alternative gravity models across the west-to-east profile A–A’ ......................................21

13. Map showing interpretative lines and annotations overlain on the satellite 

image of the area........................................................................................................................23
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Density

pound per cubic foot (lb/ft3

) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3

) 0.01602 gram per cubic centimeter (g/cm3)

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Abbreviations

A/m Amperes per meter

Ma Million years before present

mgal milligal

nT nanotesla

USGS U.S. Geological Survey
vi
Preliminary Geophysical Interpretations of Regional 

Subsurface Geology near the Questa Mine Tailing Facility 

and Guadalupe Mountain, Taos County, New Mexico

By V.J.S. Grauch,1 Benjamin J.Drenth,1 Ren A. Thompson,1 and Paul W. Bauer2

Introduction

The Tailing Facility of the Questa Mine is located on the 

southeast side of Guadalupe Mountain, just west of Questa, 

New Mexico (fig. 1). As part of site remediation and reclamation, Chevron Mining, Inc., began a drilling program to 

expand existing information about groundwater at and near the 

Tailing Facility in 2014. Because groundwater and geology 

form the basis for reclamation designs and remedial systems 

at hard rock mine sites with acid and neutral mine drainage, 

the New Mexico Environment Department was interested in 

assessing the subsurface geology in the area before siting the 

wells. To accomplish the assessment of subsurface geology, 

the New Mexico Environment Department requested assistance from the U.S. Geological Survey (USGS) to do a brief 

evaluation and interpretation of existing geophysical data. At 

the time of the request, the USGS was conducting geophysical and geologic work for several other projects in the area 

in collaboration with New Mexico Bureau of Geology and 

Mineral Resources. The other projects, which represent different geographic areas that all overlap with parts of Guadalupe 

Mountain, include (1) a hydrogeology, geology, and geophysical study for the Village of Questa; (2) geologic mapping of 

the Guadalupe Mountain and Questa 7.5-minute quadrangles; 

(3) investigation of Cenozoic landscape evolution of the southern Rocky Mountains, and (4) geologic training of National 

Aeronautics and Space Agency (NASA) astronauts.

Available geophysical data for the study area are limited 

to regional gravity data, aeromagnetic data, and data from 

four ground magnetic traverses. The data only have adequate 

resolution for a broad assessment of the regional setting rather 

than mapping detailed subsurface geology, especially considering the known complexity and variability of the geology 

of the area. Thus, the scope of the study was restricted to 

qualitative and semiquantitative geophysical map interpretation to understand the regional setting of subsurface geology. This report presents interpretations of the gravity and 

magnetic maps for the study area, augmented by available 

Abstract

This report presents geophysical interpretations of 

regional subsurface geology in the vicinity of the Tailing 

Facility of the Questa Mine near Guadalupe Mountain, Taos 

County, New Mexico, in cooperation with the New Mexico 

Environment Department. The interpretations were developed 

from aeromagnetic data, regional gravity data, data from 

four ground magnetic traverses, geologic mapping, a digital 

elevation model, and information from a few shallow wells. 

The resolution of the geophysical data is only appropriate for 

a broad assessment of the regional setting. Aeromagnetic data 

provided the most comprehensive information for interpretation. Qualitative and semiquantitative interpretations indicate 

the nature and extent of volcanic rocks, their relative depths, 

and inferred contacts between them, as well as conjectured 

locations of faults. In particular, the aeromagnetic data indicate 

places where volcanic rocks extend at shallow depths under 

sedimentary cover. Trachydacites of Guadalupe Mountain are 

magnetic, but their associated aeromagnetic anomalies are 

opposite in sign over the northern versus the southern parts 

of the mountain. The difference indicates that lavas erupted 

during different magnetic-polarity events in the north (reverse 

polarity) versus the south (normal polarity) and therefore 

have different ages. We postulate a buried volcano with 

reverse-polarity magnetization lies under the northeast side 

of Guadalupe Mountain, which likely predated the exposed 

trachydacites. Faults interpreted for the study area generally 

align with known fault zones. We interpret a northern extension to one of these faults that crosses northwesterly underneath the Tailing Facility. Gravity data indicate that Guadalupe 

Mountain straddles the western margin of a subbasin of the 

Rio Grande rift and that significant (>400 meters) thicknesses 

of both volcanic and sedimentary rocks underlie the mountain.

1

U.S. Geological Survey

2

New Mexico Bureau of Geology and Mineral Resources
2 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 1. A satellite image of the study area from Google Earth, 2013. Tailing ponds and workings (Tailing Facility) related to the 

storage of waste from the Questa Mine are near the center of the image (lighter colored region). Study area location is on figure 2. 

The Village of Questa is just off the east side of the map along New Mexico Highway 522.

0 1 2 Miles

Kilometers

0 1 2

River Gorge

Guadalupe Mountain

Red

Fish

Hatchery

Dam No. 4

522

Facility

Tailing

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Abstract 3

Figure 2. Regional geologic setting of the study area within the Rio Grande rift. Geology modified from Lipman (1983) 

and Keller and Baldridge (1999).

36 o

38 o

38 o

39 o

36 o

106 o

106o

50

50

0

0

0

Miles

Kilometers

0

Miocene and younger

volcanic rocks

EXPLANATION

Sedimentary rocks in Rio

Grande Rift

Major rift fault

Middle Tertiary volcanic

rocks

Rio Grande

Taos

Santa

Fe

Pueblo

Leadville

Albuquerque

Alamosa

COLORADO

NEW MEXICO

 Taos Plateau

volcanic field

San

Luis

Basin

San Juan

volcanic

field

MOUNTAINS

CRISTO

DE

SANGRE 

Poncha

Springs

Questa

Española

Basin

Albuquerque

Basin

Study 

Area
4 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

geologic mapping and general information from a few water 

wells in the area provided by the New Mexico Environment 

Department. The geophysical interpretations are preliminary 

and somewhat subjective; they could be improved in the future 

with additional detailed ground-based geophysics, comprehensive physical-property studies, and deep drillholes. Readers 

should note also that the regional picture is an approximate 

representation of the subsurface geology. The results may not 

be accurate in representing details of what is encountered at a 

particular drill site.

Geologic Setting

The study area is located in the south-central part of the 

San Luis Basin at the eastern edge of the Taos Plateau volcanic 

field (fig. 2). The San Luis Basin is one of a series of northsouth elongated structural basins that compose the Cenozoic 

Rio Grande rift, which extends from Mexico to Colorado 

(Hudson and Grauch, 2013). As the basins developed in 

response to rifting of the crust, they filled with sediments 

and interbedded lava flows. These now poorly to moderately consolidated sediments, known as the Santa Fe Group, 

accumulated from 1 to 6 kilometers (km) thick within the 

San Luis Basin mainly during the period 26–2 million years 

before present (Ma) (Ingersoll and others, 1990; Lipman, 

1975; Lipman and Mehnert, 1979, Thompson and others, 

1991; Bauer and Kelson, 2004). Pliocene lava flows of the 

Taos Plateau volcanic field erupted late in rift history (6–2 Ma) 

(fig. 2; Lipman and Mehnert, 1979; Dungan and others, 1984; 

Read and others, 2004). The volcanism was characterized by 

multiple eruptions of basaltic- and intermediate-composition 

magma from scattered cones and shield volcanoes. The total 

volume of eruptive material is predominantly olivine tholeiitic 

basalt (Lipman and Mehnert, 1979; Dungan and others, 1984; 

Read and others, 2004). Basin tilting and uplift of surrounding rift borders continued into Quaternary time, resulting in 

the accumulation of coalescing alluvial fans near the eastern mountain fronts and deep incision associated with the 

Rio Grande and Red River Gorges (Dungan and others, 1984; 

Heffern, 1990).

Figure 3 is a preliminary geologic map for the study 

area and brief description of units. The Quaternary surficial 

deposits represent many different types of young materials, including landslide, eolian, alluvial-fan, stream-channel, 

valley-floor, and stream-terrace deposits. Quaternary deposits—except for the youngest unit Qal—are represented as one 

undivided unit (Qu) on figure 3 because they all have similar 

physical properties (low values of density and magnetization) 

for purposes of geophysical interpretation. The QuaternaryTertiary Lama Formation (QTl) is composed of alluvial-fan 

material of variable thickness that is widespread throughout 

the region. Maximum thickness is estimated from geologic 

mapping and well information to be 140 meters (m) near the 

mountain front east of Questa (P. Bauer, New Mexico Bureau 

of Geology and Mineral Resources, unpub. data, 2014). 

Although Servilleta Basalt forms much of the landscape of the 

Taos Plateau volcanic field, only small portions are exposed 

in the study area. Lower and upper Servilleta Basalt (Tsbl and 

Tsbu) occur in the southwest and northwest corners of the 

study area, respectively.

Most of the study area is covered by trachydacites of north 

and south Guadalupe Mountain (Tagn and Tags, respectively) 

and volcanic deposits of Red River (Tvr) and Hatchery Volcano 

(Tvh). Limited exposures of Guadalupe Mountain trachydacite 

(Tag) in Red River Gorge are stratigraphically beneath, but 

not in contact with, north or south Guadalupe Mountain lavas. 

The exposures are tentatively associated with a subsurface unit 

inferred from the geophysical interpretation, discussed in the 

section of this report entitled “Interpretations.” Tertiary Santa Fe 

Group sediments are not exposed in the study area but must be 

present in the subsurface based on drillhole information for the 

region (Winograd, 1959; New Mexico Office of State Engineer, 

unpub. well records, 2004–2015), and regional geophysical 

interpretations (Cordell and Keller, 1984; Keller and others, 

1984; Grauch and Keller, 2004).

Geophysical Data

Magnetic and gravity data are measurements that show 

variations in the Earth’s magnetic and gravity fields, respectively. The field variations, in turn, are produced by differences in the distribution and nature of the physical properties 

of rocks and sediments. Geophysicists use an understanding of 

the behavior of these fields combined with physical-property 

and geologic information to infer the nature of geology in the 

subsurface. For magnetic data, the applicable physical property is magnetization, which is determined by naturally occurring magnetic minerals in rocks and sediments. For gravity 

data, the applicable physical property is bulk density, which is 

the overall mass per unit volume of rocks, sediments, and their 

pore spaces.

Magnetic Data

Magnetic data, as considered in this report, are measurements of the Earth’s magnetic field strength collected 

from a magnetometer mounted in an aircraft (aeromagnetic 

data) or mounted on a pole raised above the ground as the 

operator walks (ground magnetic data). Differences in total 

magnetization of rocks and sediments cause subtle variations 

in these data. Total magnetization is the vector sum of two 

components: remanent and induced. The induced component 

is a function of the quantity of magnetic minerals (commonly 

magnetite) and is a vector that is always oriented parallel to 
Geophysical Data 5

the present-day Earth’s magnetic field. The remanent component represents the vector sum of all permanent magnetizations held by the magnetic minerals, which have fixed 

directions irrespective of applied magnetic fields (McElhinny, 

1973; Blakely, 1995). Remanent components that are aligned 

with or opposite to the present-day Earth’s field are considered 

to have normal or reverse polarity, respectively.

Total magnetizations of Tertiary volcanic rocks in the 

region of study are dominated by the remanent component and 

are typically orders of magnitude greater than those of sediments, 

which are dominated by the induced component (Reynolds and 

others, 1990; Grauch and Keller, 2004; M. Hudson, unpub. data, 

2013). Moreover, comparisons of ages and polarities of several 

volcanoes suggest that the dominant remanent magnetizations 

of the volcanic rocks represent polarities acquired at the time of 

cooling (Grauch and Keller, 2004). Thus, volcanic rocks of different ages having reverse-polarity or normal-polarity remanent 

components can be distinguished on the basis of patterns of 

strongly negative versus strongly positive magnetic-field values, 

respectively. These patterns can be used to aid in mapping volcanic rocks of different ages.

The primary magnetic data used for this study come from 

a fixed-wing aeromagnetic survey described in Bankey and 

others (2005). Over the study area, data were mainly acquired 

along parallel east-west lines spaced 400 m apart, flown 

200–300 m above ground. Several north-south lines were also 

flown 1,000 m apart. After flight-line data from the surveys 

were processed to remove Earth’s field variations and reduce 

ordinary data-acquisition errors, they were interpolated onto a 

50-m grid. The gridded data were analytically continued from 

the variable observation surface to a surface draped 100 m 

above ground. A standard reduction-to-pole transformation 

was then applied to the gridded data using the local Earth’s 

field declination of 10 degrees (°) and inclination of 64°. 

Reduction-to-pole transformations correct for shifts of anomalies away from the centers of their magnetic sources; these 

shifts are an effect of the oblique orientation of the measured 

magnetic field at high latitudes with respect to Earth’s surface (Blakely, 1995). If not corrected, positive aeromagnetic 

anomalies for the study area would be offset to the southwest 

of their causative sources, and partial rings of negative anomalies would occur on the northeast.

Figure 4 is a color shaded-relief image of the final, 

reduced-to-pole aeromagnetic data. This image uses color to 

display the broad variations in the data and shading (analogous 

to illumination by the sun) to enhance many subtle details 

obliquely oriented to the direction of illumination.

In addition to aeromagnetic data, several ground magnetic traverses were acquired within the study area (white 

lines on fig. 4). These data were acquired from a magnetometer mounted on a pole held at head height as the operator 

traversed the ground on foot. The measurements of Earth’s 

field strength were corrected for spikes and time breaks only 

and displayed in profile form on figure 5. Some interference 

from nearby metallic objects, such as barbed-wire fences and 

metal debris on the ground, as well as overhead low-voltage 

electrical transmission wires may have introduced some data 

problems, noted on the figure. The ground traverses are important for detecting relative depths to volcanic rocks within the 

top 25 m of the ground surface. As discussed in the section of 

the report entitled “Magnetic Methods,” ground measurements 

can resolve the magnetic effects of much thinner, near-surface 

volcanic rocks than can be resolved from the air.

Gravity Data

Gravity data are particularly effective in determining the subsurface configuration of structural basins in the 

Rio Grande rift of New Mexico, owing to the generally large 

contrasts between low-density sedimentary fill and surrounding high-density crystalline bedrock (for example, Drenth and 

others, 2013; Grauch and Connell, 2013). Thus, large areas 

of measured gravity values that are lower than surrounding 

regions (gravity lows) can serve as proxies for understanding basin shape and volume of sediment. However, highdensity volcanic rocks that overlie or are interbedded with 

sediments in basins may substantially increase measured 

gravity values. This effect builds with increasing volume of 

volcanic rock but is difficult to predict without constrained 

geophysical modeling.

Gravity data for the study area were collected in 2009 

and 2014 at locations (stations) shown in figure 6. The station 

locations were mapped using differential global positioning system (GPS) navigation. Field measurements were tied 

to a base station established at the post office in El Prado, 

New Mexico. Standard Bouguer corrections (Blakely, 1995) 

were applied for each gravity station using a reduction density 

of 2,670 kilograms per cubic meter (kg/m3

). Terrain corrections were applied within a 167-km radius to the data at each 

station using digital terrain data with a resolution as fine as 

10 m. The station data were then interpolated onto a grid at a 

500-m interval. In order to focus on density variations within 

the upper crust, an isostatic regional field was removed from 

the terrain-corrected Bouguer gravity grid using parameters 

established for New Mexico by Heywood (1992). The resulting “isostatic residual gravity map” (fig. 6) generally isolates 

the gravity effects produced by sources within the upper 

10 km of the crust (Simpson and others, 1986).

Reliability of the features depicted on the gravity map is 

highly dependent on station spacing, which is unevenly distributed across the study area (fig. 6). Spacing averages approximately 750 m for most of the area, providing only regional 

definition of the gravity field in the area. Larger gaps in coverage over the western side of Guadalupe Mountain span more 

than 2 km. In these areas, the variations in the gravity values 

depicted by color contours on figure 6 are poorly constrained.
6 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 3. Preliminary geologic map of the study area with unit descriptions (modified from K. Kelson, U.S. Army Corps of Engineers, 

R. Thompson, U.S. Geological Survey, and P. Bauer, New Mexico Bureau of Geology and Mineral Resources, unpub. data, 2015.)

Tsbu

Qal

Qu Qu

Qu

Qu

Qu

Qu

Qu

Qu Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

0 1 2 Miles

Kilometers

0 1 2

Facility

River Gorge

Tailing

Guadalupe Mountain

Red

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Topographic base from U.S. Geological Survey, 1963

Original polyconic projection, North American Datum of 1927,

reprojected to Universal Transverse Mercator (UTM) projection, Zone 13,

North American Datum of 1983

1,000-meter UTM grid shown in blue
Geophysical Data 7

Tagn

Tag

QTl

Tsbu

Tsbl

Tvr

Tvh

Tvhc

Tags

Qal

Qu

DESCRIPTION OF MAP UNITS

Normal fault, dashed where inferred, dotted where concealed; ball on downthrown side

Disturbed ground

Surficial Deposits

Qal Alluvium (Holocene)—Poorly to moderately sorted sand, pebbles, and boulders in stream channels, valley 

floors, and active floodplains

Qu Undifferentiated alluvium, colluvium, and alluvial-fan and stream-terrace deposits (middle? Pleistocene 

to Holocene)—Poorly sorted silt, sand, pebbles, and cobbles, and boulders

 Older Sedimentary Deposits

QTl Lama Formation (Pliocene to middle Pleistocene)—Poorly sorted basin-fill deposits of the Santa Fe Group

 Volcanic Deposits of the Taos Plateau Volcanic Field

Tsbu Servilleta Basalt, upper (Pliocene)—Flows of dark-gray tholeiitic basalt, 40Ar/39Ar age of 3.78±0.08 Ma 

(Cosca and others, 2014)

Tsbl Servilleta Basalt, lower (Pliocene)—Flows of dark-gray tholeiitic basalt, 40Ar/39Ar age of 5.22±0.11 Ma 

(Cosca and others, 2014)

Tvr Volcanic deposits of Red River volcano (Pliocene)—Dacite lava flow and near vent pyroclastic deposits, 

40Ar/39Ar age determination of 4.67±0.06 Ma (Thompson and others, 2011)

Tvhc Volcanic deposits of Hatchery volcano, near vent (Pliocene)—Near-vent deposits associated with lava 

flows of map unit Tvh, predominantly cinder, spatter and agglutinate, isolated exposures on south side of Red 

River gorge

Tvh Volcanic deposits of Hatchery volcano (Pliocene)—Includes a sequence of dominantly andesitic lava flow, 

intercalated volcanic breccia, and locally, near vent pyroclastic deposits, 40Ar/39Ar age determination of 

4.82±0.07 Ma (Thompson and others, 2011) 

Tagn Trachydacite of Guadalupe Mountain, north (Pliocene)—Predominantly trachydacite lava flows and 

associated near-vent pyroclastic deposits; a 40Ar/39Ar age of 5.04±0.04 Ma was obtained from a lava flow at 

the north end of Guadalupe Mountain (Thompson and others, 2011)

Tags Trachydacite of Guadalupe Mountain, south (Pliocene)—Predominantly trachydacite lava flows and 

associated near-vent pyroclastic deposits, Appelt (1998) reported 40Ar/39Ar ages of 5.11±0.08 and 5.34±0.06 

Ma for groundmass separates from the south side of Guadalupe Mountain; a similar 40Ar/39Ar age (5.27±0.05) 

was obtained from the base of the volcanic section in the Red River gorge (Cosca and others, 2012)

Tag Trachydacite of Guadalupe Mountain, undifferentiated (Pliocene)—Predominantly trachydacite lava flows 

exposed at the base of volcanic section in Red River gorge

Figure 3. Preliminary geologic map of the study area with unit descriptions (modified from K. Kelson, U.S. Army Corps of Engineers, 

R. Thompson, U.S. Geological Survey, and P. Bauer, New Mexico Bureau of Geology and Mineral Resources, unpub. data, 2015.)—Continued
8 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 4. Aeromagnetic map of the study area after application of the reduction-to-pole transformation. Note the nonlinear color display 

to enhance variations. Image is illuminated from the northeast. Locations of ground magnetic traverses are shown by white lines.

Ground traverse

-776 -495 -400 -319 -267 -214 -155 -84 14 227 Nanoteslas

0 1 2 Miles

Kilometers

0 1 2

Facility

Dam No. 4

River Gorge

Tailing

Guadalupe Mountain

Red

Fish

Hatchery

522

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Geophysical Data 9

Figure 5. Data for four ground magnetic traverses acquired in 2014 (located on fig. 4). The relative frequency and amplitudes of the 

data curves are categorized. These features are a general indicator of the relative depths to volcanic rocks along the lines. Very highfrequency data indicate that magnetic rocks are at or very near the surface, whereas very low-frequency magnetic data indicate the 

rocks are much deeper (>25 meters [m]) or not present at all.

49,000

50,000

51,000

Nanoteslas

WEST EAST

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800

Distance in meters

LINE 4 Ditch

Inferred fault

Line3 

49,000

50,000

51,000

Nanoteslas

SOUTH NORTH

0 200 400 600 800 1,000 1,200 1,400

Distance in meters

LINE 3

Possible electric line interference

Inferred fault

Line 4 Line 2

49,000

50,000

51,000

Nanoteslas

WEST EAST

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800

Distance in meters

LINE 2

Metal

Interference

Magnetic contact;

deeper fault Depth gradually increasing

Line 3

49,000

50,000

51,000

Nanoteslas

WEST EAST

0 200 400 600 800 1,000 1,200 1,400

Distance in meters

Metal LINE 1

Interference

Eastern limit

of study area

Metal

Interference

Magnetic contact;

deeper fault

INDEX OF

LINES Line 1

Line 2

Line 3

Line 4

Very high-frequency magnetic data

High-frequency magnetic data

Low-frequency magnetic data

Very low-frequency magnetic data

EXPLANATION
10 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 6. Gravity map of the study area after removal of an isostatic regional field showing the locations of gravity stations (X).

-16 -18 -20 -22 -26 -30 -24 -28 -32 -34 Milligals

0 1 2 Miles

Kilometers

0 1 2

Gravity station

Facility

River Gorge

Tailing

Guadalupe Mountain

Red

Fish

Hatchery

Dam No. 4

522

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Methods 11

Methods

Magnetic Methods

The reduced-to-pole aeromagnetic anomaly map (fig. 4) 

was the primary focus for qualitative and semiquantitative 

map interpretation. For the qualitative approach, patterns on 

the map corresponding to different volcanic units and their 

relative depths were recognized through comparisons to 

topography and to the geologic map. Volcanic rocks commonly exhibit characteristic anomaly patterns that are a 

function of their total magnetizations, their depth below the 

magnetometer, and their volume. Patterns associated with 

particular geologic units are recognized by comparing contacts 

on the geologic map and by comparing anomaly shapes to 

the topography. After volcanic units are recognized by certain 

anomaly patterns, their presence under cover can be mapped 

by following similar anomaly patterns into areas of cover.

Topographic features composed of magnetic rocks, such 

as hills and canyons, produce aeromagnetic patterns that 

reflect the form of the topography. These anomalies, called 

magnetic terrain effects, provide clues to the total magnetization and extents of volcanic rocks that underlie the topography 

(Grauch, 1987). Positive or negative correlations indicate 

normal- or reverse-polarity magnetizations, respectively. For 

example, areas of high and low values that correspond respectively to hills and valleys in the area (a positive correlation) 

indicate that the rocks in the near surface have normal-polarity 

total magnetizations. Similarly, areas on the aeromagnetic map 

that take forms that are inverted to topography (in other words, 

high values correspond to valleys and low values to hills) have 

a negative correlation, which indicates the rocks have reversepolarity magnetizations.

A way to assess magnetic terrain effects is to compute 

a theoretical magnetic field by modeling a digital elevation 

model of the area using a uniform magnetization. For this 

study, the magnetic-terrain field was computed using the 

program MBOXES (Phillips, 1997), which sums the magnetic effects of vertical prisms assigned to each grid interval 

of a digital elevation model (50-m grid interval for the study 

area). The top of each prism represents the elevation at each 

grid node of the digital elevation model. For this study, a total 

magnetization of 2.8 amperes per meter (A/m) in the direction of the Earth’s present-day magnetic pole (inclination of 

90°) was assigned to all prisms. This direction simulates the 

field that would be produced by rocks of uniform, normalpolarity magnetization after application of the reduction-tothe-pole transformation. In addition, the magnetic-terrain 

field is computed on the same 100-m draped surface to which 

the observed aeromagnetic data were continued. Thus, the 

reduced-to-pole aeromagnetic and magnetic-terrain maps can 

be directly compared (discussed in the Interpretations section 

of this report). Moreover, the total magnetizations of the rocks 

in areas that are correlated are proportional to the magneticterrain field so that relative magnetic intensities of the rocks 

can be estimated (Grauch, 1987). However, care must be 

taken in interpreting aeromagnetic lows that correspond to 

topographic lows because the lows may be caused by a lack 

of magnetization rather than reversed-polarity magnetic rocks. 

In these situations, an experienced interpreter may be able to 

resolve the ambiguity by inspecting amplitudes and details of 

the anomaly patterns.

Because aeromagnetic anomalies become broader and 

lower in amplitude as distance between the magnetometer 

and the source increases (Blakely, 1995), relative depths 

of magnetic rocks are recognized by the differing detail 

and amplitude expressed in the anomaly patterns. Anomaly 

patterns over shallow magnetic rocks can be recognized by 

patterns that look noisy with high-amplitude variations that 

occur over short distances. Patterns over deep magnetic rocks 

are recognized by patterns that look smooth with broad, 

low-amplitude variations. As a rule of thumb, amplitudes of 

anomalies decrease proportionally with respect to the inverse 

of the square of the distance d (1/d 2

) from the magnetic 

source. The sampling interval, as well as differences in the 

distances between the magnetometer and the sources, must 

also be considered in comparing high-frequency anomalies 

between observations on the ground versus from aircraft. 

High-frequency anomalies with widths of tens of meters and 

very large amplitudes may be observed from data collected in 

ground traverses but are not detected in aeromagnetic maps 

constructed with a 50-meter grid interval because the sampling 

interval in the latter is too large.

For the semiquantitative approach, interpretations of 

contacts and faults were guided by the horizontal-gradient 

method (Cordell and Grauch, 1985; Blakely, 1995; Grauch and 

Hudson, 2007). This method relies on the principle that abrupt 

lateral contrasts in rock properties, such as those that occur at 

faults or steeply dipping contacts, correspond with the steepest gradients, or slopes, of their associated anomalies. Thus, 

mapping the steepest gradients in the aeromagnetic map show 

locations where these abrupt magnetic contrasts, or magnetic 

edges, occur. The steepest gradients are located by computing the local maxima of the horizontal-gradient magnitudes 

(HGM) of reduced-to-pole magnetic data. The procedure is 

analogous to taking a first derivative to find the inflection 

point of a curve. The HGM is computed as a separate grid 

(discussed in the Interpretations section of this report). Corresponding magnetic edges are located along HGM maxima 

that appear as ridges, guided by anomaly shapes observed in 

the aeromagnetic map in comparison to expected magnetic 

terrain effects.

Although many magnetic edges detected from HGM 

grids correspond directly with underlying faults, the interpretation is not always straightforward. First, steep gradients can 
12 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

be caused by steeply dipping surfaces other than faults, such 

as dipping beds truncated by erosion, lithologic rather than 

fault-related contacts, and high-relief topography. Although 

the nature of magnetic edges usually becomes apparent during interpretation, the origins of some may remain ambiguous. Second, mapping the surface projections of faults from 

magnetic edges is affected by the dip of the interface and the 

location of the magnetic contrast at depth along the interface. 

Where dip is less than vertical, the maximum of the HGM 

migrates downdip farther away from the surface projection as 

depth to the magnetic contrast increases (Grauch and Cordell, 

1987; Grauch and Hudson, 2007). The offset increases as a 

function of shallowing dip and increasing depth to the magnetic contrast. For example, at depths of 200–500 m and a dip 

of 60°, maximum offsets can be approximately 200–450 m 

(Grauch and Hudson, 2007).

Gravity Methods

The large spacing between gravity stations relative to 

the size of the study area allows for regional interpretation 

only. Regional interpretations rely on recognizing large gravity gradients that slope into gravity lows to recognize basins 

filled with low-density sediments, typical of the Rio Grande 

rift (Cordell, 1978). Relatively steep gravity gradients indicate 

basin margins, and the gravity minima indicate the deepest 

parts of the basins.

Two-dimensional gravity modeling is used in this study 

to address simple questions where there is ambiguity about the 

composition and thickness of basin fill. The modeling employs 

simple polygonal shapes assigned with densities estimated 

for different rock types to test various end-member scenarios. 

Densities assigned to the model for this study area are estimated differently for Precambrian basement rocks, basin-fill 

sediments, and volcanic rocks of Guadalupe Mountain. For 

simplicity, the density of modeled basement rock is assumed 

to be 2,670 kg/m3

, which is an average crustal density for the 

Earth, a common density used in modeling and data reduction, and is measured by density logs in boreholes in the 

Albuquerque Basin to the south (Grauch and Connell, 2013). 

Densities for basin-fill sediments follow a density-depth 

function derived from borehole density logs in the rift in the 

vicinity of Albuquerque (Grauch and Connell, 2013). In this 

function, densities of the basin-fill sediments are estimated as 

2,170 kg/m3

 for the top 1.25 km and 2,300 kg/m3 to 2.75 km 

depth. Nettleton profiling was used to estimate the bulk density of rocks contained within Guadalupe Mountain. Nettleton 

profiling is a standard technique (Blakely, 1995) wherein 

different densities are tested during processing of gravity data 

collected in a traverse over hilly topography. The density that 

results in the least correlation between gravity and topographic 

profile data is considered to be the average bulk density for 

the rocks composing the hills. This approach gave a bulk 

density of 2,450 kg/m3

 for rocks underlying the main part of 

the mountain.

Interpretations

Figure 7 shows the reduced-to-pole aeromagnetic map 

overlain by interpretative lines and annotations regarding 

the nature and extent of volcanic rocks, their relative depths, 

inferred contacts between them, and conjectured locations of 

faults. The interpretations were guided by comparisons among 

the aeromagnetic map, a map of the computed magneticterrain field (fig. 8), the geologic map (fig. 9), and the HGM of 

the reduced-to-pole aeromagnetic map (fig. 10). For comparison, the same interpretative lines and annotations are overlain 

on all four maps (figs. 7–10).

General inspection and comparisons of the maps indicate 

that the Quaternary deposits are not significant sources of 

aeromagnetic anomalies at the detection limit of the current aeromagnetic survey. Instead, observed aeromagnetic 

anomalies arise primarily from the volcanic units mapped 

throughout the area: andesitic volcanic deposits of Hatchery 

volcano (Tvh), dacite of Red River (Tvr), and predominantly 

trachydacite of Guadalupe Mountain (Tagn, Tags, and Tag). 

Aeromagnetic interpretations regarding these volcanic rocks 

and contacts and faults that juxtapose them are discussed in 

the following three sections. Discussion of the aeromagnetic 

interpretations in relation to gravity interpretations of the 

regional rift-basin underlying Guadalupe Mountain is included 

in the fourth section.

Volcanic Deposits of Hatchery Volcano—Tvh—

and Andesite of Red River—Tvr

Exposed areas mapped as Tvh generally correspond to 

strong positive aeromagnetic anomalies along the Red River 

(figs. 7 and 9), indicating that Tvh is strongly magnetic. 

Anomaly highs and lows positively correlate with the topographic highs and lows across the Red River Gorge southwest of the Red River fault zone (figs. 7 and 8), indicating 

normal-polarity magnetization. Strong positive anomalies also 

correspond to exposed areas of Tvr, but because Tvh underlies Tvr in these areas, it is ambiguous whether anomalies are 

caused by Tvh alone or a combination of Tvh and Tvr. Strong, 

normal-polarity magnetization for Tvh is supported by rock 

magnetic-property measurements of exposed andesite near the 

fish hatchery (M. Hudson, USGS, unpub. data, 2013). Similar 

measurements for Tvr in the same area show an induced component similar in magnitude to that of Tvh, but measurements 

of remanent magnetization were obscured by local remagnetization caused by lightning strikes.

Figure 7 (following page). Interpretations overlain on the 

aeromagnetic map of figure 4 with an explanation of lines and 

symbols. Tailing Facility, highway, and fault zones are depicted 

as in figure 4. Locations of ground magnetic traverses are coded 

according to inferred relative depths based on anomaly patterns 

(fig. 5). Models for profile A–A’ are shown in figure 12.
Interpretations 13

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pprox

mi ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

EXPLANATION

Magnetic contact inferred to be a lithologic or fault 

contact between different volcanic rocks, dashed 

where uncertain

Inferred normal fault, dashed where uncertain, dotted 

where deeply buried

Inferred major boundary between different rock 

types at depth, approximately located

Mapped fault from geologic map, only where 

confirmed; ball on downthrown side

Location of geophysical profile model

Location of well used to constrain geophysical 

profile model

Tags Geologic unit (from fig. 3) interpreted to represent 

aeromagnetic anomalies between contacts or faults

A A'

0 1 2 Miles

Kilometers

0 1 2

Red

River Gorge

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
14 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 8. Computed magnetic-terrain field for the study area using a uniform, normal-polarity (vertical inclination) total magnetization 

of 2.8 amperes per meter (A/m). Color display is the same as figure 4. Interpretative lines and annotations are from figure 7.

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pproxim

ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

A A'

-776 -495 -400 -319 -267 -214 -155 -84 14 227 Nanoteslas

0 1 2 Miles

Kilometers

0 1 2

Red

River Gorge

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Interpretations 15

Figure 9. Geologic map of study area (fig. 3) overlain by interpretative lines and annotations of figure 7. 

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pprox

mi ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

A A'

0 1 2 Miles

Kilometers

0 1 2

Red

Fish

Hatchery

Dam No. 4

River Gorge

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Topographic base from U.S. Geological Survey, 1963

Original polyconic projection, North American Datum of 1927,

reprojected to Universal Transverse Mercator (UTM) projection, Zone 13,

North American Datum of 1983

1,000-meter UTM grid shown in blue
16 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 10. Horizontal-gradient magnitude (HGM) of the reduced-to-pole aeromagnetic data. Magnetic edges that may correspond to 

faults or lithologic contacts are inferred along and extrapolated between ridges. Interpretative lines and annotations are from figure 7.

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pprox

mi ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

A A'

0.034 0.086 0.138 0.192 0.251 0.318 0.400 0.511 0.690 1.121 Nanoteslas/meter

0 1 2 Miles

Kilometers

0 1 2

Red

River Gorge

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Interpretations 17

East of the Red River fault zone, the area of strong 

positive aeromagnetic anomalies continues southward of the 

exposures of Tvh that are on the south side of the Red River, 

suggesting that Tvh extensively underlies the Lama Formation 

(QTl) in the southern part of the study area. Considering that 

the anomaly patterns where Tvh is exposed and extrapolated 

under cover are very similar in character, we conclude that the 

Lama Formation is thin in this area.

Relatively low magnetic field values extend over a wide 

area within and north of the Red River Gorge on either side 

of an unnamed fault zone where it crosses the Red River 

Gorge south of Dam No. 4 (fig. 7). Volcanic units Tvh, Tvr, 

and Tags are exposed in the area of the low values (fig. 9), 

which contradict their associations with high values elsewhere. 

Possible reasons for the area of low aeromagnetic values may 

be (1) primary differences in magnetizations of the rocks 

from one area to another; (2) hydrothermal alteration that 

destroyed the magnetizations of the rocks; or (3) an underlying 

unit that has strong, reversed-polarity magnetization with thin 

Tvh and Tvr on top. Differences in primary magnetizations 

of the volcanic units are possible but seem unlikely that all 

three units would show a difference in magnetization in this 

one area. Evidence for secondary hydrothermal alteration in 

the area is lacking, although primary oxidation of magnetic 

minerals associated with vent areas is common across the 

area mapped as Tags. Alternatively, local post-emplacement 

alteration of the volcanic sequence may simply be concealed. 

A candidate for a reversed-polarity, magnetic volcanic unit 

underlying thin Tvh and Tvr is evident from a small exposure 

of the trachydacite of Guadalupe Mountain (Tag) near the fault 

zone. This unit is inferred from aeromagnetic data to the north 

(discussed in the next section) as an older deposit, compositionally similar to erupted lavas of Guadalupe Mountain.

Trachydacite of Guadalupe Mountain—Tagn, 

Tags, Tag

Comparisons of the aeromagnetic map (fig. 7) to the 

magnetic-terrain model field (fig. 8) over Guadalupe Mountain 

show a general positive correlation on the southwest and negative correlation on the northeast. Where patterns in the observed 

aeromagnetic map correspond to those of the magnetic-terrain 

field, we infer that rocks composing the topography are the 

sources of the aeromagnetic anomalies. The positive and negative correlations indicate the rocks have different polarities, 

and thus different ages, despite the similarity in rock type. As a 

consequence, the trachydacites are mapped separately on north 

(Tagn) versus south (Tags) Guadalupe Mountain.

Positive correlation with the magnetic-terrain model 

field extends across most of south Guadalupe Mountain, 

most notably on the southwest side where steep gradients in 

both maps are oriented similarly. A general positive correlation is also evident along the northerly trending ridges that 

correspond to the high ridges of the mountain. Within the area 

of positive correlation, the extremely high magnetic values 

suggest that the rocks mapped as Tags are very magnetic with 

normal-polarity magnetization. This inference is supported by 

measurements at one site on the northwest side of the mountain (M. Hudson, USGS, unpub. data, 2013). Therefore, these 

magnetic rocks are the bulk of the volume of rock underlying 

south Guadalupe Mountain.

Positive correlations between the aeromagnetic and 

magnetic-terrain field maps are not present everywhere over 

south Guadalupe Mountain. Strong, positive aeromagnetic 

anomalies extend farther out from the mountain than predicted 

by the magnetic-terrain model on the southeast and northwest 

sides of the mountain. In these extended areas, the outer limits 

of the positive aeromagnetic anomalies generally locate the 

perimeter of the large volume of magnetic rocks underlying 

south Guadalupe Mountain in the subsurface (fig. 7). Except 

for one area, positive anomalies in the extended areas generally show broad shapes with low amplitudes, consistent with 

the expression of the contact of a large volume of rocks at 

depth. Where this aeromagnetic expression extends under 

areas of mapped Quaternary units on the east side of south 

Guadalupe Mountain (labeled as buried Tags), the top of the 

subsurface volume of Tags may be at some depth (>100 m) 

below the surface. The one exceptional area surrounds the 

high peak at the southernmost part of Guadalupe Mountain, 

labeled “area of high magnetization.” In this area, positive 

observed anomalies show higher amplitudes than predicted 

by the magnetic-terrain field model. The area of higher values 

abuts the area of low magnetic values discussed in the previous section, attesting to a large magnetic contrast between 

rocks at the contact between these two areas.

The wide area of low magnetic field values on the aeromagnetic map on the northeastern part of Guadalupe Mountain 

(blue colors on fig. 7) shows general negative correlation 

between the aeromagnetic and magnetic-terrain model map 

(fig. 8). In this area and extending out of the study area to the 

north and northeast, high-amplitude negative anomalies in the 

observed data indicate that rocks of Tagn are magnetic and 

reversely polarized. In detail, the inverse correlations between 

anomaly shapes in the two maps are only fair, suggesting that 

magnetic intensities of the rocks are actually quite variable.

Analogous to observations for Tags to the south, the 

negative aeromagnetic anomalies interpreted as Tagn extend 

farther east than predicted from the magnetic-terrain field 

model, extending east of the northeasterly dashed interpretative line following the east side of the mountain (near longitude 105°37′30′′) on figures 7 and 8. East of this interpretative 

line, a large-amplitude, broad negative anomaly extends to 

the eastern boundary of the study area. The broad negative 

anomaly is relatively featureless, without any obvious correlation to the magnetic terrain field (fig. 8), suggesting a large 

volume of magnetic, reversed-polarity rocks is located at 
18 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

depth in the subsurface (>100 m). Given the somewhat circular shape of the broad anomaly and that the anomaly minima 

occurs away from the vent areas mapped near the peaks of 

Guadalupe Mountain, we speculate that this inferred volume 

of rock represents an older volcanic edifice that predates the 

formation of Tags and Tagn. If so, eruptions of the trachydacites at Guadalupe Mountain must have spanned two reversals 

of the Earth’s magnetic field, such that Tag erupted during 

a reversal, and then Tags erupted during a time of normal 

polarity, and finally Tagn erupted during another reversal. 

Preliminary 40Ar/39Ar geochronology suggests that these lavas 

have eruption ages near 5 Ma (Thompson and others, 2011), 

compatible with a reverse-normal-reverse sequence of events 

in the magnetic polarity time scale between 5.3 and 5.1 Ma 

(Ogg and Smith, 2004).

If this hypothesized, older reversed-polarity unit (Tag) 

is widespread in the subsurface, it could clarify some unexplained areas of low magnetic values such as the one south of 

Dam No. 4, which was already discussed. It could also explain 

a generally elongated area of moderately low magnetic values 

that extends westward from Dam No. 4 and wraps around 

the southern and southwestern edges of Guadalupe Mountain 

(labeled Tag?). A roughly similar but much more subdued area 

of low values is apparent in the magnetic terrain field (fig. 8), 

suggesting a very general correlation with terrain. However, 

the observed aeromagnetic low is larger in amplitude than 

that of the magnetic terrain field, indicating that the rocks in 

this area have either normal-polarity total magnetization less 

than 2.8 A/m (the magnetization used to compute the magnetic terrain field) or have a reverse polarity. The shape of this 

elongated low is reminiscent of a channel, suggesting that it 

can be explained by an underlying paleochannel filled with 

weakly magnetic sediments or an old lava flow that was channeled around the southern limit of the volcano. Alternatively, 

the low is explained by underlying, reversely polarized Tag 

that is blanketed by a thin cover of normal-polarity Tags that 

correspond to the margins of the Tags volcano.

In the southern part of Guadalupe Mountain, a fault 

interpreted from the aeromagnetic data forms a triangular area 

with the northern extension of the fault zone near Dam No. 4, 

north of Red River. Within the triangular area, a small area of 

lower magnetic values (labeled “area of low magnetization”) 

does not correspond to the shapes expected from the magneticterrain field model (fig. 8). The low values suggest the presence of a volume of rock that is much lower in magnetization 

than surrounding rocks or rocks that have reverse polarity. The 

location of the area near the apex of the triangle formed by 

the mapped and inferred faults suggests that the explanation 

has a relation to faulting. Fault-related possibilities to explain 

the lower magnetizations are (1) differences in thickness of 

overlying Tags on top of a horst of rocks of differing magnetization, (2) hydrothermal alteration guided by the faults that 

decreased the magnetization of a significant volume of the 

rocks, and (3) an area that is significantly disrupted by chaotic 

fault blocks or composed of landslide material related to faulting. The possibilities remain speculative because evidence for 

them may be covered.

Exposures of Tagn are mapped within the broad negative 

anomaly hypothesized as a buried Tag volcano (figs. 7 and 9) 

and just east of the interpretative line near 105°37′30′′. These 

rocks apparently have no expression in the aeromagnetic 

data. They may be too thin to be detected by the aeromagnetic survey or have much lower magnetizations than those of 

Tagn to the west or those inferred for Tag in the subsurface. 

Because the ground magnetic data are much more sensitive to shallow magnetic sources than he aeromagnetic data, 

observations of the frequency content of the ground magnetic 

data from figure 5 can be used to evaluate the presence of 

very shallow volcanic rocks throughout the area of the broad 

negative anomaly. The different frequency contents from the 

ground magnetic traverses are color coded (fig. 5) and plotted 

on the aeromagnetic and geologic maps (fig. 7 and 9). The 

areas of high-amplitude, high-frequency anomaly patterns 

indicate very shallow volcanic rocks, whereas the broader, 

lower amplitude anomalies indicate deeper volcanic rocks. 

Maximum amplitudes of the anomalies range to approximately 

2,000 nanoteslas. Considering the aeromagnetic observation 

surface is almost 100 m higher than the ground magnetic 

traverse, the amplitudes of these anomalies observed from the 

air would be reduced by four orders of magnitude, suggesting that these volcanic rocks are not as magnetic as the bulk 

of Tagn to the west. Consistent with the earlier observation 

of variable correlation between the magnetic terrain model 

and observed aeromagnetic data to the west of 105°37′30′′, 

the rocks east of this line may represent a volcanic flow that 

formed in conditions unfavorable for acquiring a strong remanent magnetization.

Abrupt differences in frequency contents of the groundmagnetic data curves from high-frequency to low-frequency 

data (fig. 5) indicate places where shallow volcanic rocks 

terminate or are fault offset to lower depths. Such differences 

are observed at 420-m distance on line 3 and inferred at 740-m 

distance on line 4, coincident with a wide ditch related to the 

Tailing Facility. Faults are inferred at these locations because 

the difference in frequency content is large. Differences in 

frequency content on the west sides of lines 1 and 2 are not 

quite as large but coincide with the locations of more regional, 

presumably deeper, faults inferred from the aeromagnetic 

data (labeled as “Magnetic contact; deeper fault”). A gradual 

change from very high frequency to low frequency is observed 

from approximately 550-m to 1,200-m distance on line 2. The 

gradual decrease in frequency suggests a gradual deepening of 

the top of these volcanic rocks to the east. They probably reach 

at least 25-m depth (or are no longer present) underneath deposits of QTl east of 1,220-m distance. The faults inferred from 

the ground magnetic traverses correlate well with deeper faults 

inferred from the aeromagnetic data (dotted lines on fig. 7).
Interpretations 19

Faults and Other Contacts

The magnitude of the horizontal gradient (HGM) of the 

reduced-to-pole aeromagnetic grid was computed in a moving 

3-point window across the grid, shown in figure 10. Many of 

the ridges on the HGM map indicate contacts between rocks 

of differing magnetizations (magnetic contacts), which commonly occur at intrusive or depositional contacts or at normal 

faults. Faults are inferred where the contacts are linear and 

supported by differences in anomaly patterns and plausible 

geologic relations. In some cases, gradients evident in the 

aeromagnetic map have indefinite correspondence to HGM 

ridges or are difficult to connect. These uncertain contacts are 

shown as dashed lines on the interpretative overlay. Dotted 

lines are drawn across very wide or subdued ridges, which 

indicate the contacts are likely deeper than other magnetic 

contacts in the area.

Some HGM ridges correspond to gradients in the 

magnetic-terrain field model. Because they correspond to 

topography, these HGM ridges simply may reflect steep 

topographic slopes especially if they are curved, or reflect 

faults if they are linear. For example, HGM ridges surrounding the southwest side of Guadalupe Mountain likely represent 

steep topographic slopes because they are curved and match 

gradients in the magnetic-terrain field map (figs. 7 and 8). 

However, the west-northwest trending gradient on both maps 

on the west side of Guadalupe Mountain is linear and may be 

a fault. If so, the mapped fault that crosses this inferred fault 

just north of the area of high magnetization must be younger 

because it is evident at the surface but the geophysically 

inferred fault is not. Other HGM ridges align with mapped 

faults or cross the trends of topographic features. These ridges 

represent sharp juxtaposition of rock types having differing 

magnetizations as opposed to interfingering relations. Where 

these HGM ridges are linear, they likely represent faults. 

Examples include several northwest-trending ridges that align 

with the southeastern extension of the Red River fault zone. 

An inferred northerly to northwesterly extension of the fault 

zone near Dam No. 4 crosses through the middle of the Tailing 

Facility. It is inferred on the basis of an alignment of linear 

gradients that all indicate differences in magnetization, but 

the magnitude of the differences vary along strike. Therefore, it is better expressed in the aeromagnetic data (fig. 7) 

than in the HGM map (fig. 10). The depth to this fault is not 

well constrained.

A prominent HGM ridge marks the boundary between 

the magnetic highs associated with Tags and the magnetic 

lows associated with Tagn in the southern and northern parts 

of Guadalupe Mountain, respectively. The boundary is an 

expression of where the bulk difference in magnetizations 

occurs at contacts between these two rock types. In detail, 

Tagn may have some flows that extend across the boundary to the southwest on top of Tags, or parts of the older 

Tags volcano may have been left uncovered by the younger 

Tagn flows.

Basin Shape and Thickness

In figure 11, the gravity map of figure 6 is overlain by 

the interpretative lines and annotations from figure 7. The 

figure shows a large, oval-shaped region of low-gravity values 

elongated in a northeasterly direction and centered over the 

eastern side of Guadalupe Mountain. The large amplitude of 

the gravity low (approximately 15 milligals) and its location 

within the Rio Grande rift (fig. 2) suggests the gravity low is 

an expression of a subbasin of the rift. The color contours that 

define gravity gradients surrounding the lowest gravity values 

also locate the subbasin boundaries, which may represent a 

series of major normal faults that drop the basin floor down 

toward the gravity minima. Data coverage for the gravity 

map (fig. 6) is not dense enough to define the locations of 

these faults. However, mapped and inferred faults from the 

aeromagnetic data, such as the Red River fault zone and the 

unnamed fault zones to the east, have similar northwesterly 

orientations as do the gravity contours on the southwestern 

side of the gravity low. These faults may be expressions of the 

subbasin boundaries.

Because Guadalupe Mountain straddles the western side 

of the gravity-defined subbasin, it is unclear how much of the 

subbasin is filled with volcanic rocks and how much is filled 

with rift sediments of the older Santa Fe Group. To address 

this question, we constructed simple two-dimensional gravity 

models (fig. 12) using the geologic map (fig. 3) and several 

wells for constraints. In addition, we know that Guadalupe 

Mountain volcanic rocks are present as far east as 12-km 

distance along the profile (figs. 7 and 12). Densities for the 

model were assigned on the basis of estimations described in 

the Gravity Methods section.

Two alternate models of the thickness of Santa Fe Group 

(Tsf) and trachydacites of Guadalupe Mountain (Tags/Tag) 

are shown in figure 12. In figure 12A, the volcanic rocks 

were modeled with minimum thickness, as thin as the well 

constraints allowed, then the thickness of Tsf below it was 

adjusted so that the modeled curve generally matched the 

observed curve. In figure 12B, the volcanic rocks were modeled with much greater thickness by several orders of magnitude. To accommodate this increased mass and still fit the 

observed gravity curve, the basin had to be deepened significantly, reaching almost to sea level. Although the thickness 

of Tsf is not increased significantly from the first scenario, 

the second scenario requires a geologic history that includes 

(1) almost 2.4 km of basin subsidence before 5 Ma and 

(2) eruptions over a period of less than 2 Ma of sufficient volume to deposit over 1-km thickness of lavas in the subbasin. 

These rates seem high for the region but not unreasonable. The 

exact ratio of volcanic-to-sediment thickness under Guadalupe 

Mountain is probably somewhere between the two geophysical models. The important conclusion is that both rock types 

must be present.
20 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Figure 11. Gravity map from figure 6 overlain by interpretative lines and annotations of figure 7.

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pprox

mi ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

A A'

-16 -18 -20 -22 -26 -30 -24 -28 -32 -34 Milligals

0 1 2 Miles

Kilometers

0 1 2

River Gorge

Red

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
Interpretations 21

Tags/Tag 

Sedimentary basin fill (Tsf) 

Density = 2,450 kg/m3

Density = 2,170 kg/m3

Basement 

Density = 2,670 kg/m3

Tags/Tag 

Sedimentary basin fill (Tsf) 

Density = 2,450 kg/m3

Basement Density = 2,170 kg/m3

Density = 2,670 kg/m3

Gravity (mGal) Gravity (mGal) 

-51 

-48 

-45 

-42 

-39 

-51 

-48 

-45 

-42 

-39 

8 9 10 11 12 13 

Vertical Exaggeration =1 Distance (km) 

Elevation (feet) 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

9,000 

Elevation (meters) 

200 

400 

600 

800 

1,000 

1,200 

1,400 

1,600 

1,800 

2,000 

2,200 

2,400 

2,600 

2,800 

Gravity (mGal) Gravity (mGal) 

-51 

-48 

-45 

-42 

-39 

-51 

-48 

-45 

-42 

-39 

8 9 10 11 12 13 

Vertical Exaggeration =1 Distance (km) 

Elevation (feet) 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

9,000 

Elevation (meters) 

200 

400 

600 

800 

1,000 

1,200 

1,400 

1,600 

1,800 

2,000 

2,200 

2,400 

2,600 

2,800 

EXPLANATION

Observed gravity data; extracted

 from grid

Gravity field computed from model

Regional gravity field that was 

 removed from the observed curve

Well used to constrain the model

(B)

(A)

West

West East

East

A A'

A A'

Figure 12. Alternative gravity models across the west-to-east profile A–A’ (located on figures 7–11) to test alternate scenarios of the 

thickness of Guadalupe Mountain volcanic rocks overlying Tertiary Santa Fe Group sediments (Tsf). Geologic units Tags and Tag are 

described in figure 3B. Quaternary deposits are not shown. km, kilometer; mGal, milligal; kg/m3

, kilogram per cubic meter.
22 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

Summary

Aeromagnetic data, regional gravity data, and data from 

four ground magnetic traverses were used along with geologic mapping and a few well constraints to interpret regional 

subsurface features in the vicinity of the Tailing Facility of the 

Questa Mine near Guadalupe Mountain. Aeromagnetic data 

provided the most comprehensive dataset for interpretation 

of the regional setting of the subsurface. Aeromagnetic data 

were first transformed using a standard reduction-to-the-pole 

operation. Interpretations were accomplished by comparing the reduced-to-pole data to a hypothetical model of the 

effects of uniformly magnetized terrain, geologic contacts, and 

computation of the horizontal gradient magnitude (HGM) of 

the data.

The interpretations focused on mapping rock types 

and their relative volumes in the subsurface and on locating 

the contacts, including faults, between them. To help visualize the interpretations in context of siting drill holes in the 

vicinity of the Tailing Facility, the interpretative overlay of 

figure 7 is overlain on the satellite image of the study area 

in figure 13. The main points from the interpretation are 

as follows.

• The primary sources of the aeromagnetic anomalies are 

the volcanic rocks of the study area: andesitic volcanic deposits of Hatchery volcano (Tvh), dacite of Red 

River (Tvr), and trachydacite of Guadalupe Mountain 

(Tagn, Tags, and Tag).

• Strong aeromagnetic expression of Tvh continues to 

the south of the Red River Gorge, suggesting that Tvh 

is shallow below sedimentary deposits of the Lama 

Formation (QTl) that are mapped at the surface.

• Trachydacites of Guadalupe Mountain are very 

magnetic but have magnetizations of normal polarity 

on the south and reverse polarity on the north, suggesting the rocks in these two areas generally have 

different ages. Strong aeromagnetic highs with positive correlation to terrain over southern Guadalupe 

Mountain are mapped as Tags, whereas strong aeromagnetic lows with generally negative correlation to 

terrain over northern Guadalupe Mountain are mapped 

as Tagn. The boundary between these two rock types is 

marked by a prominent HGM ridge that generally follows a topographic saddle that crosses the mountain.

• Aeromagnetic anomalies associated with both Tags and 

Tagn extend farther out from the mountain front than 

predicted by the magnetic-terrain model, suggesting 

that they extend out under cover in these areas.

• A large broad aeromagnetic low that extends east 

of the area mapped as Tagn suggests that additional 

Guadalupe Mountain lava flow deposits with reversepolarity magnetization lie at depth (Tag). If one infers 

that the circular nature of the anomaly and its minima 

located east of Guadalupe Mountain are indicative of a 

buried volcano, the additional deposits are likely to be 

older than both Tagn and Tags. Preliminary age dating 

compared to magnetic-polarity events near 5 Ma suggests this relation is geologically feasible.

• Ground magnetic traverses over the area suggest that 

exposed volcanic rocks at the surface in the area of the 

broad anomaly associated with Tag are not detected in 

the aeromagnetic data because they have much lower 

magnetization than the deeper rocks. The ground 

magnetic data also suggest these near-surface flows 

are fault-bounded on the west and deepen gradually to 

the east.

• Several aeromagnetic lows that do not correspond to 

geologic mapping or to the expected effects of terrain 

indicate areas where rocks of lower magnetic intensity or reverse-polarity magnetization (or possibly 

sediments) are present underneath Tags. The areas 

are located near the southern margin of Guadalupe 

Mountain, and are labeled as “area of low magnetization,” and “Tag?”.

• Several faults are inferred from the aeromagnetic 

data, coded by uncertainty or by depth on figure 13 

(explained on figure 7). Several of the inferred faults 

align with or extend known fault zones. An inferred 

northerly to northwesterly extension of the unnamed 

fault zone near Dam No. 4 crosses through the middle 

of the Tailing Facility. It is inferred on the basis of an 

alignment of linear gradients that all indicate differences in magnetization but the magnitudes of the differences vary along strike.

• Examination of the aeromagnetic interpretations in 

comparison to the gravity map shows that Guadalupe 

Mountain straddles the western margin of a subbasin 

of the Rio Grande rift. Two alternate profile models of 

the gravity data indicate that both volcanic rocks and 

underlying sediments of the Tertiary Santa Fe Group 

must be present under Guadalupe Mountain. However, 

their thicknesses are poorly constrained.
Summary 23

Figure 13. Interpretative lines and annotations of figure 7 overlain on the satellite image of the area from figure 1.

Tvh

Area of low 

magnetization

Buried

Tags

Tag?

Tag?

Tvr/Tvh

Tvr/Tvh

Tvh

Tvh Tvh

Tvh

Tags Tags

Tags

Tags

Tags

Tags Tags

Tagn

Tagn deepening to east;

Tag underneath

Tag?

A

pprox

mi ate

e

a

stern limit of

volc

a

nic rocks

Approximate

Tags/Tagn boundary

Area of high 

magnetization

A A'

0 1 2 Miles

Kilometers

0 1 2

River Gorge

Red

Fish

Hatchery

Dam No. 4

522

Red River fault zone

105°40’ 105°37’30”

36°42’30”

440000 441000 442000 443000 445000 446000

4066000

4065000

4064000

4063000

4062000

4061000

4060000

Universal Transverse Mercator (UTM) projection, Zone 13 

North American Datum of 1983

1,000-meter UTM grid shown in blue
24 Preliminary Geophysical Interpretations of Subsurface Geology—Questa Mine Tailing Facility and Guadalupe Mountain

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