Modeling Gravity Anomalies for the Medicine Lake

Highland Volcano, CaliforniaD. D. Torres

Sponsor: J. S. McClain

Department of Earth and Planetary Sciences | University of California, Davis

IntroductionThe Medicine Lake Highland is a shield volcano of varying

compositions in northeastern California - active as recently as

950 years ago[1].

The highland is situated between two geologic provinces: The

extensional province of the Basin and Range, and the volcanic

arc of the Cascade range[2].

Figure 2: Modified from Donnelly-Nolan et al, 2008

Figure 1: Modified from Donnelly-Nolan et al, 1996

Figure 9: Array 2 – S-N profile across Medicine Lake Highland. Basalt magma

chamber to the south not optimal. RMS Misfit = 0.9382929 mgal

Figure 8: Array 1 - E-W profile across Medicine Lake Highland. Finished

product after utilizing the previous models derived from seismic velocities,

magnetotellurics data and surficial geology. Resistivity soundings

(magnetotellurics) report presence of smectite clay cap at 1700 masl.

RMS misfit 0.9074531 mgal.

Figure 6: Adaptation of East-West (array 1) profile of MLV, based on rock densities

derived from seismic velocities (Figure 5). Dark blue line represents calculated

gravity. Light blue line represents observed gravity. Different body colors indicate

bodies of different densities. Calculated gravity profile does not fit the observed line;

therefore, model does not work. RMS Misfit=7.89107 mgal

The Purpose of…This study is to use regional gravity anomalies to identify

structures that influence the delivery of magma and thermal

waters to the Earth’s surface during volcanic eruptions.

The mechanics of this system could explain why the Medicine

Lake Highland exhibits anomalous distributions of gravitational

highs and lows. Regional anomalies could be attributed to the

location of magmatic intrusions beneath the Medicine Lake

volcano.

Results

Conclusions

1. Positive gravity anomalies correspond to rocks that are

denser than the assumed background density. While

negative gravity anomalies correspond to rocks that are

less dense than the assumed background density.

2. Presence of a subsolidus rhyolite chamber is supported

by the gravity models, which is consistent with the

seismic velocity model (Fig. 5).

3. The model based on surficial geology, magnetotellurics

and seismic velocity data fits the observed gravity best.

4. Eruptions over the past ~500 kya show lava beds of

varying compositions[2], which supports the conclusion

that there are several magma chambers of differing

compositions (Fig. 8, 9).

5. Presence of a dome-shaped smectite clay cap is

confirmed (Fig. 8, 9). Inclusion of a “clay cap” is

necessary to provide the best fit for the calculated CBA

to the observed values. This smectite cap is important

because the impermeable layer prevents hydrothermal

fluids from rising to the surface.

6. There is an unusually high anomaly to the south of

Medicine Lake, which could be representative of a large

basaltic magma chamber; however, the authors think

this unlikely.

– basalt, andesite, and rhyolite.

Figure 4: Oblique view of the Complete Bouguer Anomaly (CBA)

gridded and interpolated using MatlabTM. The CBA is the residual

gravity anomaly after the correction of gravity measurements for

instrument drift, latitude of measurement, elevation, and surrounding

topography. Black crosses mark the location of gravity stations.

High red areas correspond to regions of positive anomaly values. Low

blue areas correspond to regions of negative anomaly values.

Co

rrect

ed

Bo

ug

uer

Ano

maly

(m

gal)

Figure 3: Graph of Complete Bouguer Anomaly (CBA) versus position.

Red dashed circle highlights approximate location of volcanic basin.

Black lines indicate locations of modelled gravity profiles.

Easting (m)

No

rthin

g (m

)

Co

mp

lete

Bo

ug

uer A

no

maly (m

gal)

Complete Bouguer Anomaly of Medicine Lake Highland

Array 1

Array 2

References Cited1. Finn, C., & Williams, D. L. (1982). Gravity evidence for a shallow intrusion under

Medicine Lake volcano, California. Geology, 10(10), 503-507

2. Donnelly-Nolan, J. M., Grove, T. L., Lanphere, M. A., Champion, D. E., & Ramsey, D. W.

(2008). Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc

volcano in the southern Cascades. Journal of Volcanology and Geothermal Research,

177(2), 313-328.

3. Cumming, W., & Mackie, R. (2010, April). Resistivity imaging of geothermal resources

using 1D, 2D and 3D MT inversion and TDEM static shift correction illustrated by a glass

mountain case history. In Proceedings World Geothermal Congress 2010 (pp. 1-10).

4. Evans, J. R., & Zucca, J. J. (1988). Active high‐resolution seismic tomography of

compressional wave velocity and attenuation structure at Medicine Lake Volcano,

Northern California Cascade Range. Journal of Geophysical Research: Solid Earth (1978–

2012), 93(B12), 15016-15036.

5. Gardner, G. H. F., Gardner, L. W., & Gregory, A. R. (1974). Formation velocity and

density-the diagnostic basics for stratigraphic traps. Geophysics, 39(6), 770-780.

6. Lowenstern, J. B., Killgore, M., Mariner, R., Blakely, R. J., Smith, J. G., & Donnelly-

Nolan, J. M. (1998). 3-D dimensional visualization of the Medicine Lake Highland, CA;

topography, geology, geophysics, and hydrology (No. 98-777).

Acknowledgements

We would like to extend our gratitude to the Department of

Earth and Planetary Sciences, Colin Ferguson, and Samuel

Hawkes.

Figure 7: After addition of two low density alluvial layers the calculated gravity profile

better reflects the observed gravity. RMS misfit=2.567582 mgal

Easting (m)Northing (m)

Position v. CBA

Co

mp

lete

Bo

ug

uer

Ano

maly

(m

gal)

Figure 5: Seismic velocity model modified from Evans and Zucca (1988). For this study the seismic velocities

were converted to density that was used to model the gravity anomalies. A key result of Evans and Zucca (1988)

was the presence of a magma chamber below the volcano at a depth of ~500 meters above sea level (masl).