THE FARALLON BASIN BASEMENT STRUCTURE, SOUTH CALIFORNIA GULF BY INTERPRETATION OF POTENTIAL AND SEISMIC DATA

Lieutenant Marco BARTENS

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Abstract
The Gulf of California is an active continental rift with an oblique and dextral opening displacement and is one of the few examples of an initiating continental rift where processes can be studied along a complete rift. Establishing the boundary between continental and oceanic basement holds the key to fully understanding the tectonic mechanisms acting on this initial rifting stage. Unfortunately, this is also a challenging task due to the presence of a newly formed oceanic crust. The aim of this work is to identify, delineate and estimate the thickness of the oceanic and continental crusts in the Farallon Basin. For this purpose, high-resolution 2D multichannel seismic reflection data from the Ulloa’s 2006 expedition along with gravity anomaly data from the global marine gravity database, obtained by repositioned radar altimetry from Geosat and ERS-1 satellites, was used. To assist with the interpretation, two-dimensional gravity modeling was carried out along each seismic profile, followed by 3D inversion of potential field data from the Farallon Basin area as constrained by the interpreted seismic profiles. The interpreted depths from the gravity models indicate that a thin oceanic crust exists at the center of the basin, with minimum thicknesses of 2.5 km in the abandoned south axis and 3 km in the dorsal axis. The continental crust gradually thickens towards the continent. Keywords: seismic reflection, gravity anomaly, Farallon Basin, Gulf of California.

1. Objective

The aim of this study was to identify, delimit and estimate the thickness of the oceanic and continental crusts in the Farallon Basin in order to understand the geological processes that occur during the continental lithosphere’s rupture and the deformation of passive margins. In order to achieve this objective, the following specific goals were defined:

  • Process, interpret and identify high resolution 2D multichannel seismic reflection data from the seismic profiles;
  • Create gravimetric models from the main measurements interpreted in the seismic profiles;
  • Estimate the depth of the Basement supported by gravimetric data;
  • Estimate the thickness of the crust in the area.

2. Seismic reflexion method

A marine linear array (Figure 1) was used for data acquisition. For this purpose, a 150 cubic inch (0.245m3) per shot airgun was used as the energy source (see Table 1).

Figure 1. Linear array
Table 1. Seismic data acquisition parameters

3. Seismic Data Processing

Data from four seismic lines from the Ulloa-6 campaign (Table 2 and Figure 2) were processed using the ProMAX software of the Landmark software platform. The processing sequence shown in Figure 3 used a typical reflexion seismic processing sequence described by Yilmaz (1987) and Sheriff and Geldart (1995).

Table 2. Seismic lines processed
Figure 2. Seismic line location
Figure 3. Typical seismic processing sequence

Once the seismic process sequence was finished the interpretation of the seismic line was performed (Figure 4).

Figure 4. Processed and interpreted seismic profile 14_3

After the four seismic profiles were processed and interpreted, a Basement’s type distribution map was generated using the seismic data collected in the area of study (Figure 5).

Figure 5. Distribution of the different types of Basement interpreted over the area: Plutonic: (lightblue); Volcanic from Farallon-Sur Massif (red); newly created oceanic crust (yellow) and transition between crusts (green). Seismic profiles are in blue.

4. Potential Field Methods (Gravimetric Method)

The gravimetric method of geophysical exploration is based on the measurement of small variations of the earth’s gravitational field on its surface and their interpretation in relation to possible mass distribution in the subsurface (Nettleton, 1976). The gravimetric observations can be used to interpret changes or differences of mass below different regions of the earth (Lillie, 1999).

A first step is to isolate the gravity field fraction originated by these mass differences. This is the reason calculating the Free-Air gravimetric anomaly (Figure 6 – left). Second, in order to remove the gravimetric effect generated by the sea and the land in the area of study, the gravimetric anomaly of Bouguer was calculated (Figure 6 – right).

Figure 6. Free-Air Anomaly (mGal) map. (right) Total Bouguer Anomaly (mGal) map.
Lines show the seismic profile locations.

5. 2D Modelling

The Free-Air gravimetric data was used to generate subsurface models. To compile these models, the depths of the main interpreted reflectors of the seismic sections, the general characteristics inferred from the qualitative interpretation of the gravimetric anomalies’ maps as well as the gravimetric data itself were used. A trial and error adjustment was used in the bi-dimensional case obtaining models of the four seismic profiles (Profile 18 is shown in Figure 7).

Figure 7. Bi-dimensional gravimetric model (Profile 18)

6. 3D Inversion

To perform the 3D gravimetric inversion of the data the GMLayers software (Gallardo et al., 2005), as amended in October 2010, was used. To guide the inversion process using the GMLayers software, it is necessary to set up a number of parameters grouped as:

  • initial model;
  • precision of the initial model and the geophysical data;
  • acceptable smoothness level of each surface; and
  • the feasibility limits of the thickness and depth of each layer of the model (as shown in Figure 8).

The appropriate setting of theses parameters is key to obtaining an optimum model that not only justifies the data, but retains its geological meaning.

In Figure 8, the s2 surface (black line) corresponds to the bathymetric relief which is always located above the s4 depth ranges. In this case, the upper surface is fixed at 0 km (sea level) and the effect of the sea level is suppressed, hence the minimum and maximum thickness of the overlying layer match the minimum and maximum depths in the area. In both cases it is possible to observe the seismic profile’s restrictions given by depth.

After the program was run and the results checked, the anomaly obtained in the field was compared against the one calculated by the program. It was observed that their differences were minimal (Figure 9) which can be explained by the limitations of the seismic pro- files.

Figure 8. Minimum and maximum depths imposed over surface 4 (top of the dense sediments).
Figure 9. (top) Observed gravimetric anomaly. (bottom) Gravimetric anomaly calculated using the optimum 3D model selected. Both figures show a vertical scale in mGals. The pink seismic profiles represent the continental crust and the blue ones the oceanic crust.

7. Layer Structures

Figures 10 to 13 show the thickness of the various layer structures (km).

a. Sediments

Figure 10. (left) Depth of the surface which bounds the two proposed layers of sediments (km) (right) Thickness of the upper layer of sediments (km)
Figure 11. (left) Depth to the top of the oceanic crust (km) (right) Thickness of the continental crust (km)

b. Continental crust

Figure 12. (left) Basement’s depth (top of the continental crust (km). (right) Thickness of the lower layer of the densest sediments (km)

c. Oceanic crust

Figure 13. (left) Depth to the mantle (km). (right) Thickness of the oceanic crust (km)

8. Conclusions

The integration of seismic data, potential fields and 3D gravimetric inversion identified the type and structure of the crust in the study area. The different thicknesses and distribution of each identified layer were compatible with the tectonic scenario as follows:

  • In the uppermost part it was observed that the active axis of the Farallon Basin, at a depth of 3.2 km, does not present sediments; whereas, the abandoned axis, at a depth of 3.8 km, it is buried by a sedimentary package with a total thick- ness of 2.9 km.
  • Evidence suggests that the sedimentary material which overlays the recently created oceanic crust is affected by magmatic intrusions. This was not detected in the sediments deposited on the continental crust.
  • On the western zone, it was observed that a continental crust with a thickness of 15 to 17 kms thins towards the centre of the basin. At that point it is almost entirely replaced by a thin oceanic crust of recent creation which has a thickness of 2.7 to 6 km.
  • Finally, the rise of the mantle layer in the rift areas is clearly observed, reaching 5.8 km on the active axis and 7.3 km on the abandoned axis. A deepening was observed towards the continental zone which corresponds to the margins of the Gulf of California.

9. References

Gallardo, L.A., Perez, M.A., Gomez, E. (2005). Refinement of three-dimensional multilayer models of basins and crustal environments by inversion of gravity and magnetic data. Tectonophysics, 397, 37-54, doi:10.1016/ j.tecto.2004.10.010.

Lillie, R. J. (1999). Whole earth geophysics: an introductory textbook for geologists and geophysicists. Upper Saddle River, Prentice-Hall, New York.

Nettleton, L. (1976). Gravity and Magnetics in Oil Prospecting. McGraw-Hill, New York.

Sheriff, R.E., Geldart, L.P. (1995). Exploration Seismology. Cambridge University Press. 2nd Edition. New York, NY.

Yilmaz, O. (1987). Seismic data processing: Society of Exploration Geophysicists. Tulsa, OK.

10. Biography

Lieutenant Marco Bartens graduated from the Naval School of Peru in 2006 and completed his IHO Category B certification in 2008 whilst working in the various divisions of the Hydrography Department. In 2011, he was promoted to Second Lieutenant and commenced his specialization studies at the Cientific Investigation Center for Superior Studies of Ensenada (CICESE), Mexico. He graduated in 2013 with a Master of Science, specialization in Applied Geophysics. In 2015, he was promoted to First Lieutenant and is now working as the Deputy Chief of the Oceanography Department of the Peruvian Directorate of Hydrography and Navigation.

mbartens@dhn.mil.pe
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