MY POSTER!!! 

I know it's unreadable, so I will include the text below in no particular order

 Title:  It's Still Downhill From Tonopah to Las Vegas but the Crust Doesn't Ride for Free 

Abstract  We investigate the crustal thickness in the central Basin and Range province of the western US.  There is a gravity anomaly at 37˚ N latitude at which the gravity increases ~100mgal from North to South over a distance of ~100 km. The majority of recent publications ascribe the gravity signal to a mantle influence based on observations of near constant crustal thickness in the area. However, Moho depth estimates are sparse in the area, and therefore higher gravity due to a thinner crust in the south is still a possible explanation to date. In order to determine Moho depths, we examined teleseismic receiver functions from broadband and short-period stations from 1993 to 2008 located within the region, including stations from the recent Earthscope Transportable Array deployment. We used a total of 11,751 high-quality receiver functions at 80 stations and picked arrival times of the Moho converted phase from backazimuthal and moveout stacks. Moho depths were determined from these arrival times using a fixed velocity model, as well as from forward modeling of moveout curves of the direct conversion as well as multiples. Our results confirm the presence of thinner crust south of 37˚ N latitude, indicating that a least part of the gravity signal is of crustal origin.

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Study area

The data used here are from 80 stations in the Basin and Range area in Southern Nevada.

Figure 1 - Location and seismic station maps with topography.

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Why is the gravity lower North of 37˚ latitude?

A dramatic North-South variation in gravity and topography has been observed in the Basin and Range Province (e.g. Jones et al., 1992; Saltus and Thompson, 1995).  We seek to determine whether there is a corresponding variation in crustal thickness in the region.  A variety of hypotheses have been suggested to explain the gravity anomaly.

Figure 2 - North - South gravity (top) and topography (bottom) profiles across the Basin and Range province (Saltus and Thompson, 1995).

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Gravity

The study area has a significant gravity anomaly at 37˚ N latitude.

Figure 3 - Gravity map of Nevada and seismic station locations as in Fig. 4.  The gravity has a sharp change at 37˚ N.  With a gravity variation such as this it is expected that the crust to the south is thinner if the signal is of crustal origin.

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Moho depths

Figure 11 - We picked an average Moho arrival time  at each station using the backazimuth and moveout plots.  By using an assumed Vp value and Vp/Vs ratio value of 6.3 km/s and √3 respectively, the arrival time values were converted into depth.  By looking at the map, one can see that areas of deeper crust match up with areas of lower gravity and areas of shallower crust match up with areas of high gravity, which is what was expected.

Figure 4 - Crustal Thickness map of Nevada (this study).  The crust thins dramatically at 36.5˚N, approximately 50 km South of the large gravity gradient.

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Receiver Functions are an attempt to deconvolve the seismic source and far field signal  from near-receiver conversions. In practice, we assume the vertical component to be representative of the source and far field signal, and remove it from the radial and transverse components to obtain the local P-SV and P-SH conversions under the station. The iterative deconvolution method by Ligorría and Ammon seeks waveforms on the radial and transverse components that match the vertical component with a time delay.  When a match is found, a spike is added to the receiver function at the corresponding time (negative spike if there is a polarity flip relative to the vertical component) and the next closest match is sought. A spike represents a phase conversion from local structure (mostly a velocity contrast). We use a Gaussian filter width of 5, corresponding to an upper cutoff frequency of around 2.4 Hz.

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Figure 12 - Elevation vs. Moho depth plot.  Good quality Moho times are plotted with the Northern stations in red and the Southern stations in blue.

This plot of elevation vs. Moho depth shows a positive correlation, with thicker crust beneath areas of higher elevation.  This trend suggests that isostatic compensation is accommodated, at least partially, in the crust. There is a lot of scatter, but a possible steeper slope north of 37˚N may imply a smaller density contrast between crust and mantle (less dense mantle, or denser crust). Low density mantle may therefore contribute to the higher elevations in the North, as suggested by Saltus and Thomson (1995).

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Individual station azimuthal stacks and moveout plots

Each panel shows a moveout plot, slowness and backazimuth histograms, and binned backazimuth sections for radial and tangential receiver functions for one station. Moveout and backazimuth plots are elevation corrected, and backazimuth plots are also corrected for slowness.

(Note:  I actually have 6 different examples of these in my poster, but I don't find it necessary to post them all here.  Here is one example.)

Figure 6 - Station LRL in the SW of the study area. This is an example of a simple backazimuth and moveout plot.  The number of receiver functions per back azimuth bin appears on the right. Moho conversion near 3.5 s. 

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Conclusions and further work

We have found evidence for crustal thinning associated with the gravity increase and lower topography from North to South at 37˚ N. We have not yet performed a quantitative evaluation of how much of the gravity signal is accommodated by the crust, and whether a mantle component is still necessary.

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Acknowledgments:

Thanks to IRIS for sponsoring M. Pettit’s summer internship and AGU presentation,

Jesse Lawrence for his Interactive Receiver Function Program, Chuck Ammon for the receiver

function deconvolution algorithm, IRIS and Regional Networks for seismic data, and the

National Geophysical Data Center (NGDC) for gravity data.

References:

Receiver Function information available at http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html

Jones, C. H., B. P. Wernicke, G. Lang Farmer, J. D. Walker, D. S. Coleman, L. W. McKenna, and F. V.

                Perry (1992), Variations across and along a major continental rift.  An interdisciplinary study of the

                Basin and Range Province, western USA, Tectonophysics, 213, 57-96.

Saltus, R.W. and Thompson, G.A. (1995), Why is it downhill from Tonopah to Las Vegas?: A case for

                mantle plume support of the high northern Basin and Range, Tectonics, 14, 1235-1242

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So, there you have it!