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Eastern United States Crust Characterization

Faculty Mentor: Maggie Benoit

Student: Melanie Crampton

The lithosphere of 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 summer, we analyzed gravity data collected by the United States Geologic Survey to help understand the crustal structure.  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.  In order to analyze the gravity data, we used two different techniques: inverse modeling to estimate crustal thickness values and analysis of the gravity gradient, which shows finer scale variations in the gravity data.  To inverse model the gravitational data, we used a Matlab program, 3DINVER.m.  This program inverse models the data in the frequency domain and then Fourier transforms the results to calculate crustal thickness values.  The inverse model results correlated very well with 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.  This discrepancy confirmed our hypothesis that the Scranton Gravity High is the location of a failed rift basin from ~ 1 billion years ago.  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 margin.  More detailed seismic studies are needed to confirm these results, but these studies would help elucidate the location and scale of known and unknown rift basins along the eastern margin of the US.