1. An E-W gravity profile across the La Bajada fault Zone in the Rio Grande Rift, North Central New Mexico Rajesh Goteti University of Rochester SAGE 2007

2. Outline Introduction Introduction Gravity Survey Gravity Survey Data Analysis and Interpretation Data Analysis and Interpretation Conclusions Conclusions

3. Introduction La Bajada Fault Zone Profile Line

4. Lacoste Romberg analog gravity meter Scintrex digital gravity meter Equipment

5. Data Reduction  Tidal and Drift Correction  Latitude Correction  Free-air Correction  Bouguer correction  Terrain Correction

6. Drift & Tidal Corrections Inner Zone Terrain Correction Complete Bouguer Anomaly Contouring the Anomaly Gravity Method Flowchart

7. La Bajada Fault Zone Complete Bouguer Anomaly Map on the geologic map ∆ρ = 0.47 g/cc C. I. = 1 mGal  Steep gradient across the anomaly contours coincides with the location of mapped fault zone trace 0 24 km

8. Complete Bouguer Anomaly Map C. I. = 1 m. Gal ∆ρ = 0.47 gm/cc HighLow Possible Location of the La Bajada Fault Zone + Gravity Stations

9. SFF LBF TF LBF HighLow Differences Between CBA and RBA SFF

10. Regional Aeromagnetic Survey Map Gravity Profile Line nT High Low  Steep gradient supports the possibility for a fault zone

11. La Bajada Fault Zone Profile Line Geological Map showing the location of the profile in the gravity survey

12. Residual Gravity Anomaly Profile Easting (m) RA (mGal)

13. So what do we have so far ?  Complete and Residual Bouguer Anomalies maps to help identify the La Bajada fault surface trace  Aeromagnetic map which predicts a fault trace that agrees with the fault trace above  Residual Gravity anomaly profile for the line of interest  Model Density contrast of 0.47 gm/cc

14. In addition  Transocean Oil Company seismic lines to (1) constrain the depth to ‘basement’ in both the footwall and hanging wall of the La Bajada fault and (2) throw on both the fault segments  Velocity estimates from Baldridge et al (1994) Transocean Seismic Line 78-7 SP 90 Transocean Seismic Line 79-1 SP 70 N 12 km

15. 2-D Forward Model using GM-SYS TO lines Model Derived Observed Misfit 78/7 Sp 90 79/1 Sp 70 ρ = 2.2 gm/cc ρ = 2.67 gm/cc “Basement” 3.6 Km 1.4 Km 0.5X (km)

16. TO lines Model Derived Observed Misfit 78/7 Sp 90 79/1 Sp 70 ρ = 2.2 gm/cc ρ = 2.67 gm/cc “Basement” 3.8 Km 1.4 Km 0.5X ρ = 2.4 gm/cc ρ = 2.67 gm/cc “Basement” Non- uniqueness

17. Conclusions Model Derived Observed Misfit 78/7 Sp 90 79/1 Sp 70 ρ = 2.2 gm/cc ρ = 2.67 gm/cc “Basement” 3.6 Km 1.4 Km 0.5X

18. Conclusions (contd..)  Gravity surveys can be used to locate faults (e.g. La Bajada Fault) and estimate approximate depths of layers based on density contrast.  Gravity surveying is a relatively inexpensive, fast technique for a first order insight into the subsurface geology.  Subsurface models based on gravity cannot yield unique solutions. Other geophysical techniques (e.g., seismics) can complement and constrain the gravity model better.

19. Acknowledgements -Shawn, George, Darcy -Scott for the wonderful field trips -Team members -SAGE faculty/staff for giving this enriching experience

20. Group Conclusions

21. 1D Forward Model 10 1 10 2 10 3 Depth(m) 10 1 10 2 10 0 Rho(ohm m) App Rho(ohm m) Phase Period(s) 45 o 10 -4 10 -3 10 -2 90 o 0o0o 10 1 10 2 10 0

22. 1D Occam Inversion, 8 layers forward model Station 1 Station 2 Station 3 10 4 10 3 10 2 10 1 10 0 10 1 10 2 (m) 10 4 10 3 10 2 10 1 10 0 10 1 10 2 (m) 10 4 10 3 10 2 10 1 10 0 10 1 10 2 (m)

23.  Gravity modeling can be used to locate faults and other geophysical techniques can be used to constrain a gravity model and generate a reasonable subsurface geological interpretation  Seismic reflection shows no evidence of Tanos fault. Creating a synthetic seismogram for Hawley’s geological cross-section may hint about the signatures of Tanos fault  The shallow depths along the survey line modeled with refraction seismics does not reveal any faults Thanks!!!!