B. Biagini1, K. Bloor1, B. Castro2, J. Chang3, D. Feucht4, D. Friedman5, B. Hollingshaus6, M. Kennedy7, B. Lee8, F. Le Pape9, J. Overacker10, B. Phrampus7, D. Tang3, E. Tursack11, C. Wilson12, P. Zablowski5, G. Jiracek1, L. Pellerin13, D. Hasterok14, P. Bedrosian15, N. Carlson16, S. Biehler17, D. Mcphee15, and J. Ferguson16

1SDSU, 2U Rochester, 3UC Berkeley, 4U Colorado, 5Boston U, 6U Utah, 7SMU, 8Binghamton U, 9DIAS, 10U Nevada Reno, 11MIT, 12James Madison U, 13Green Eng,14UC San Diego,15USGS,16Zonge Inter, 17UCRiverside, 16UT Dallas

1. SAGE Program http://www.sage.lanl.gov/

• A four-week-long, field-based program in the central Rio Grande rift (RGR), near Santa Fe, New Mexico featuring “hands-on” land-based geophysics for 25-30 students per year.

2. Central RGR Geothermal Indicators• High heat flow (100 to >300 mW/m2).

• Young volcanism (e, g., Valles caldera (0.13-1.75 Ma) and Cerros del Rio volcanic field (1.14-2.8 Ma) with >50 exposed vents).

• High 3He/4He fluids (numerous, rift-related faults and volcanic vents argue for high-temperature magmatic/mantle fluid component).

• Española Basin aquifer >2 km deep with underlying, low-porosity, impermeable basement rocks.

3. SAGE Geothermal Examples Using Magnetotelluric (MT) Data

• Crustal low resistivity (high conductivity) zones are: 1) electrical conductors , e.g., graphite or sulfide minerals, 2) H2O brines ,or 3) magma.

• Depth to midcrustal conductor (MC) is a proxy for the depth of ~500oC isotherm: 27 to 7 km from west to east under the Valles caldera (Fig. 3) and 17 km deep north of Santa Fe (Fig 4).

• Defining basin freshwater, brine, and clay zones and low-permeability, resistive basement depth (B) are critical for conventional geothermal and EGS (Fig. 6).

Fig. 2. Schematic interpretation of prominent midcrustal conductor (MC) across Eastern Great Basin-Transition Zone-Colorado Plateau (EGB-TZ-CP). MC is physically interpreted to be high temperature H20 brine above basaltic and silicic melts with MC top at 500oC.1

6. Conclusions

• 1Wannamaker et al., 2008, in Geochemistry Geophysics Geosystems, AGU Electronic Journal, 38 p.

• 2Johnson et al., 2011, in GSA Special Paper 2021, in press.

7. References

Fig. 1.“Hands-on”geophysics at SAGE.

MC 17 km

4. Sample SAGE MT Results in Central RGR

5.Caja del Rio Geothermal Prospect ( CR in Fig. 5 red area)

Fig. 4: 1-D TE inversion.

Fig. 3: 2-D MT TM inversion.

Fig. 6. 2-D interpreted MT-derived geoelectric section across Santo Domingo Basin.

Fig. 5. Simplified geologic map of central Rio Grande rift.2

• The combination of a thick hydrologically permeable, Santa Fe Group aquifer and a relatively shallow basement, both at ~ 2 km with temperatures projected to be >150oC, makes this region the most attractive geothermal prospect in the central RGR outside of the Valles caldera.

Fig. 7. Cenozoic stratigraphy in central RGR. Santa Fe Group above Espinaso Formation is key aquifer.

Fig. 8. S-N industry seismic line (Fig. 5, SL) time-section. N-end ~2 s TWTT yields ~2 km-thick Santa Fe Group aquifer above top of Espinaso Formation.

• Recently published borehole temperature gradients exceed central RGR background of ~30oC/km .

Fig. 9. a) Map of shallow temperature gradients (oC/km) and b) Selected temperature/depth profiles of 24 to 58oC/km.2

Fig. 11. Proposed hydrothermal settings: a) Constriction model and b) Up-flow through deep faults or volcanic pipes and along shallow fault barriers.2

Fig. 12. SAGE 2011 gravity (Fig. 5,GM) and MT do not support Fig. 11a constriction model. a) Gravity high is too far west and b) MT-estimated basin depths (B=2.1 to 2.3 km) are nearly uniform over E-W region of proposed basement horst .

• Depth to midcrustal conductor potentially a very valuable regional geothermal assessment tool in active regions.

• Caja de Rio area has viable geothermal direct use and EGS potential. The thermal source is likely of deep origin.

CR

B 1.6 km

MC 7 km

MC 27 km

B

B

B

SL

Santa Fe Group

S-N SL

58oC/km

40oC/km

24oC/km

-220

-240

-260

Density (Mg/m3)

Ele

v(m

)

GM

Fig. 10 Caja de Rio.

MT

a

b

a b

a bTE B (km) MC (km)

2.3 142.1 122.1 122.3 10

E

W

W E