Eastern United States Crust Characterization Melanie Crampton, TCNJ Mentor: Dr. Margaret Benoit, Physics Department

Introduction The lithosphere of the east coast of the United States has been modified dramatically by two different supercontinent cycles over the past 1.2 billion years. A supercontinent cycle involves continental collision and subduction as tectonic plates converge, as well as rifting as the continental plates break apart and diverge. The Northeast of the United States has been a site of numerous collisions and rifts during these supercontinent cycles. In order to learn more about the structure of the Earth in the past as well as the process of rifting, we can examine the crust of the Earth in the Northeast to determine exactly where rifting occurred and why it occurred. This can be done through analysis of gravity data collected by the United States Geologic Survey.

Inverse Model We used inverse modeling on the gravity data to estimate the crustal thickness values across the northeast. To inverse model the gravitational data, we used a Matlab program, 3DINVER.m designed by David Gomez-Ortiz and Bhrigu N.P. Agarwal. This program inverse models the gravity data in the frequency domain and then Fourier transforms the results to calculate a mathematical model, which is then used to estimate crustal thickness values.

Faults To look at the finer scale variations in the gravity data, we mapped the gradient of the gravity data. The gradient was able to show the variations that occurred, most often near major fault lines. Analysis of the gradient of the gravity data along with known fault lines suggests that there are likely small scale variations in crustal thickness that are unexplored along the region. This would then be an area of further study to better understand why there are such differences in gravity at the faults, and potentially help to uncover more failed rift basins.

Results The discrepancy between crustal thickness data from seismic data and the crustal thickness values estimated through inverse modeling confirmed our hypothesis that the Scranton Gravity High is the location of a failed rift basin from ~ 1 billion years ago. The magnitude of high density material here suggests that it was an area of rifting and therefore crustal thinning at one point in history. Analysis of the gravity data near the faults suggests that there may be more failed rift basins in other areas that are still in need of investigation.

Future Work More detailed seismic studies are still needed to confirm these results. Offshore seismic data would help stabilize the inversion data, making the conclusions more accurate. Also, forward modeling could be completed to compare to the conclusions found through inverse modeling. Further investigation at the locations of various faults with large gravity gradients could lead to further uncovering of more failed rift basins. Eventually all of this data and procedure could then be applied to the entire eastern United States, creating a bigger picture of the supercontinent cycle.

Acknowledgement This work was supported by the NASA –Impress-Ed grant to The College of New Jersey and Long Island University

We then mapped the estimated crustal thickness and compared it to the seismic analysis data of crustal thickness. When doing this, we saw that there was a lot of correlation with the crustal thickness values found using seismic analysis, except in the area of the Scranton gravity high. Here, 3DINVER.m estimated a thin crustal thickness based on the high gravity values, while seismic analysis predicted a much thicker crust.

Gravity Gravity can enact a different amount of force on certain areas on the surface of the Earth based on the density of the materials under the Earth’s surface. The thinner the crust, the shallower the high-density mantle is to the surface, creating a stronger gravity force in that particular area. Therefore, gravity data can be used to estimate the crustal thickness and therefore the crustal structure through the use of various methods.

Gravity Data Map from United States Geologic Survey Data

Map of Crustal Thickness generated from seismic analysis Gravity Data calculated from inverse model showing that the depth data we retrieved corresponded closely to the gravity data input.

Crustal Thickness calculated from inverse modeling and overlaid with the seismic crustal data to note discrepancies.

Crustal thickness depths that were compared with the inverse modeling depths.

Gradient of the gravity data overlaid with fault lines to note the areas where further investigation is needed.