Salton Seismic Imaging Project

Salton Seismic Imaging Project (SSIP)

Collaborative Research: John Hole (Virginia Tech), Joann Stock (Caltech), Gary Fuis (USGS Menlo Park)

Project funded by NSF through both the MARGINS Program and the EarthScope Program and funded by the U. S. Geological Survey.

We encourage and invite potential collaborations.

Since seismic data acquisition is currently scheduled for January 2010, there may be time for you to arrange a piggyback survey. You may also suggest minor changes to our field survey design. Our survey is shown in the figure at bottom, which will be kept up to date. Please contact the people listed above for more information.

Project Summary from NSF Proposal:

Seismic Imaging of New Transitional Crust in the Salton Trough Oblique Rift

This proposal addresses the processes of rupturing a continent through a seismic reflection and refraction survey at the Salton Trough in southern California. The Salton Trough is the northernmost part of the Gulf of California extensional province that rifted the North American continent and transferred Baja California and the Peninsular Ranges to the Pacific plate. Despite similar total extension along the province, very different extensional structures have been produced, from seafloor spreading in the southern Gulf to 14-20 km thick continental or transitional crust in the northern Gulf and Salton Trough. The southern and central Gulf were the target of a recent MARGINS project, and significant differences in rifted continental margins were observed in adjacent rift segments. Seismic refraction and reflection studies in the northern Gulf indicate that stretched continental crust still underlies the rift. In the Salton Trough, however, the 20-22 km thick crust is composed entirely of new material added by magmatism from below and Colorado River sedimentation from above.

A combination of driving forces, thermal regime, inherited structure, magmatism, and sedimentation produce a range of rheological responses to extension. At rifted continental margins, narrow or broad areas of new crust, transitional between continental and oceanic, are created through lithospheric stretching (normal faulting, detachment faulting, and/or lower crustal flow), magmatism, and sedimentation. In the Salton Trough, low-angle faulting has been observed at the surface, but the central rift appears to have been completely ruptured and new, thick crust has been created. It is an excellent place to study the effects of magmatism and sedimentation on continental extension, and the partitioning of strain vertically in the lithosphere. As a highly oblique rift, it also provides the opportunity to study the lateral partitioning of strain into rifting, transform faults, and zones of oblique extension.

Our scientific goals are to investigate: i) the nature of transitional crust at rifted continental margins, ii) the role and mode of magmatism in the final stages of continental breakup, iii) the effect of rapid syn-rift sedimentation on magmatism and extension mechanism, iv) the partitioning of displacement in highly oblique continental rifting, and v) 3-D structure for earthquake hazards evaluation. These will be constrained by densely sampled seismic refraction/wide-angle reflection and by seismic reflection imaging of the crust and upper mantle.

Recent MARGINS studies in the Gulf of California, the current EarthScope-USArray deployment, and a new USGS earthquake hazards initiative on the southern San Andreas Fault (see below) currently provide a synergy for new studies in the Salton Trough.

Broader Impacts

The proposed work addresses the goals of the Rupturing Continental Lithosphere initiative of MARGINS and lies in an identified focus site. It also addresses the goals of EarthScope to study the evolution of the North American continent and lies within an identified focus region of the integrative GeoSwath initiative. It uses EarthScope instrumentation.

The southernmost San Andreas Fault is considered at high risk of producing a damaging earthquake. For this reason, the U. S. Geological Survey is a partner on this proposal and will contribute a substantial portion of the funds for fieldwork. Structure of the faults, sedimentary basins, and underlying crust will be incorporated into community models for simulation of strong ground motion and earthquake hazards in southern California.

Collaboration will be created with earthquake networks and with scientists in Mexico. The proposed work would be the subject of graduate student research, undergraduate student internships, and a formal graduate/undergraduate field and data analysis course.

Click on a figure for a larger version.

Figure 1. Models of rifted continental margins and transitional crust. Top: "volcanic" margin; Center: moderately stretched margin; Bottom: magma-starved margin. "MUB" is the mafic-ultramafic boundary.

Figure 2a. Tectonic map of the Gulf of California and Salton Trough, from the MARGINS Science Plan [2004]. Thick black lines are the seismic lines of Lizarralde et al. [2007] and Gonzalez-Fernandez et al. [2005].

Figure 2b. Map of the Salton Trough showing major active tectonic features. Triangles are the existing seismic network.

Figure 6. Model of magmatic addition to the crust from Schmitt & Vazquez [2006]. Mafic magma reaches neutral buoyancy in the lower to middle crust. Hydrothermal circulation transmits the heat, preventing massive melting of overlying sediment but also allowing re-melting of altered basalts.

seismic map
Figure 7. Map of proposed seismic survey.
Black lines are receiver lines, and some are hiding under the red lines.
Red lines are small (circa 250 lb) shot lines at 2-4 km spacing.
Thicker lines mean denser receiver / shot spacing.
Triangles are large (1000-3000 lb) shots that would be recorded on all of the receiver lines.
Note that this map is schematic: actual field plans will depend upon logistics and scientific re-tuning, which are underway. Your input is welcome!
[January 2009 version of this figure]

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