This supplement provides additional description, beyond that contained in the article text, of the gravity modeling techniques used to define the geometry of the crystalline basement surface beneath the Salton Trough sedimentary basin.
The crystalline basement surface, where not well defined by previous studies [Fuis and Kohler, 1984], was defined with free-air gravity anomaly data and constrained by a 2 dimensional cross-section along strike through the trough, from Biehler [1964]. Grav2DC, a freeware two-dimensional gravity modeling application, was used to establish a quantitative relation between the gravity anomaly and basin shape. The first step in this process involved verifying the correlation between the free-air gravity anomaly and basin shape and calibrating the regional density contrasts that best relate the known basin shape to the gravity data. This was done using transects 2 and 3 (locations shown in figure A1), which cross the basement surface defined by Fuis and Kohler [1984]. Topographic data from GTOPO30 [2000] and the Moho surface, extracted from the SCEC Community Velocity Model [Magistrale, et al., 2000], were imported to Grav2DC, along with the basin geometry. Density contrasts were then applied to each of these features based on average rock densities of 2.40 g/cm3 for sediment, 2.65 g/cm3 for crystalline basement, and 3.20 g/cm3 for the mantle. In order to calculate density contrasts, a background density of 2.65 was assumed below sea level to represent the crystalline basement. The effects of a local high in the Moho due to the extensional tectonic environment dominated the profiles with a signature that did not resemble the measured free-air anomaly. The measured gravity anomaly was best modeled by the effects of surface topography and the sedimentary basin, using density contrasts of 1.3 g/cm3 and -0.25 g/cm3 respectively. The effective density contrast of topographic variation is significantly less than would be expected from the crystalline rocks of which it is composed, perhaps because large topographic relief and isostatic compensation of the mountainous terrain east and west of the trough. Using these values, the influence of basin shape and topography on the gravity profile in the sedimentary basin could be readily distinguished, as illustrated in figure A2.
Using the available constraints, basin geometries were then determined on transects 1, 4, 5, 6, and 7 which best model the measured free-air gravity. Figure A3 shows transects illustrating the basin shape that produced the best fitting gravity profile, as well as the resulting synthetic gravity profile and the measured free-air anomaly.
Good fits between the model derived gravity data and the measured data were seen in all transects except transect 1, particularly within the lateral extent of the basin where we emphasized a best-fit. Beyond the extent of the sedimentary basin, the quality of the fits is poorer, probably a result of the complex effects of varying topography and basement structure, but an accurate fit between the synthetic and measured gravity profiles outside the basin is not important for our purposes. Basement geometry across transect 1 could not be modeled by gravity data because of the highly exaggerated gravity anomaly produced by greatly varying topography in this region. The depth of the basin on transect 1 was determined from the basin depth according to Biehler [1964] at the single point where profile A-B intersects transect 1. Otherwise, the sedimentary basin shape across transect 1 is quite poorly constrained.
The 2 dimensional basin geometries defined by our model were then assembled with the Biehler (1964) cross section to form a “rib and backbone” structure roughly defining basin geometry. A surface with a mesh resolution of approximately 10 km was then created by interpolation between the points defining this “rib” structure. This surface was then merged with the high resolution surface defined by Fuis and Kohler [1984] in order to produce a complete crystalline basement surface. The final step in the creation of this crystalline basement surface was to adjust the basin edges to match the topography and surface geology. It was assumed that, outside the trough, crystalline basement rocks reach the surface. The Fuis and Kohler [1984] basement surface suggests that this is true, as do crystalline rock exposures in the mountains on either side of the trough, as can be seen on regional geological maps [Jennings, 1977; Jennings, 1967]. The southeastern edge of the basin is left open to represent the southward continuation of the rift basin. The crystalline basement surface used for our model is presented in figure 4 of the publication.
Figure 1: Map view illustrating the locations of transects along which gravity profiles were used to calculate the two dimensional shape of the sedimentary basin and the “rib” structure which was then interpolated to produce a three-dimensional representation of the crystalline basement surface beyond the extent of that defined by Fuis and Kohler [1984]. The seven lines perpendicular to the strike of the basin represent transects along which free-air anomaly gravity data was used to calculate basin shape. These have been numbered, increasing from north to south. Transects 2 and 3 (green) cross the crystalline basement surface defined by Fuis and Kohler [1984]. These transects were used to calibrate the density contrast values. The narrow blue lines represent the other transects along which gravity was used to determine crystalline basement surface geometry. The thick yellow lines represent the map view of the “ribs” which were produced from gravity data to represent the basin shape. The thick pale-blue line represents the “backbone” derived from the crystalline basement surface defined in the along-strike transect provided by Biehler [1964]. The white box represents the final extent of the new Salton Trough pwave velocity model.
Figure 2: Measured (dashed) and synthetic (solid) free-air gravity profiles across transects 2 and 3, which intersect the crystalline basement surface defined by Fuis and Kohler [1984]. Topography is represented by the red polygon, with a positive density contrast of 1.3 g/cm3 and the sedimentary basin is represented by the blue polygon with a density contrast of -0.25 g/cm3. These density contrasts were selected because they produced the best fit between the synthetic and measured gravity profiles. These figures demonstrate that the combined gravitational effects of topography and the sedimentary basin create a synthetic gravity profile which mirrors the measured free-air anomaly quite well, particularly in the basin. Thus, it was confirmed that gravity data can be used to roughly model the sedimentary basin shape.
Figure 3: Measured free-air (dashed green) and synthetic (solid black) gravity anomalies along five transects perpendicular to the axis of the Salton Trough (transect locations shown in figure A1). Density contrasts of 1.3 g/cm3 for topography and -0.25 g/cm3 for the sedimentary basin were used to define the sedimentary basin shape producing a synthetic profile best fitting the measured free-air anomaly. As demonstrated in figure A2, this provides a reasonable, albeit rough, approximation of the actual basin shape. Note that the fit of the gravity profiles in regions of high topography, and thus outside of the sedimentary basin, are not important for our purpose. A strong effort was made to match the measured and synthetic gravity profiles only within the bounds of the sedimentary basin. Most transects show a good fit between modeled basin shape and the gravity data; however, transect 1 produces a very poor fit with the measured gravity profile. Thus, a reasonable basin geometry across this profile could not be produced by gravity modeling because of the highly exaggerated effects of topography. Due to this poor fit, the basin’s lateral dimensions across this transect were defined primarily by the topography and the depth was determined from Biehler [1964] transect A-B.
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