Two-Dimensional Geophysical Modeling Over Bathymetrists Seamounts

Alexa Fernandez Author

04/05/2021 Added

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Two-Dimensional modeling of crustal architectures beneath the Bathymetrists Seamounts using integrated geophysical data. This is a continuation from my Summer 2020 UCARE project.

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[00:00:00.000]Hello, my name is Alexa Fernández
[00:00:02.057]and my research is based on
[00:00:03.581]creating a two-dimensional model
[00:00:05.110]over the Bathymetrists Seamounts
[00:00:06.637]using integrated geophysical data.
[00:00:09.691]I will be starting off with some
[00:00:11.212]background information on the study area.
[00:00:13.225]I took this image from google earth
[00:00:14.942]and mapped some important features.
[00:00:16.640]This is an overview of the Atlantic Ocean.
[00:00:18.972]The Mid-Atlantic ridge is the actively
[00:00:20.853]spreading center where new oceanic crust
[00:00:22.952]is being formed, its spreading rate
[00:00:24.727]averages 2.5 cm per year.
[00:00:27.063]The spreading pushes the South American
[00:00:29.306]plate to the West, and the
[00:00:30.603]African plate to the East.
[00:00:32.777]The study area is outlined with this
[00:00:34.507]orange box. The area, located on the
[00:00:36.589]African plate, includes the Bathymetrists
[00:00:38.584]Seamounts, outlined in yellow,
[00:00:40.120]the Sierra Leone Rise, outlined in red,
[00:00:42.020]and the Sierra Leone Basin.
[00:00:44.333]The Sierra Leone Rise has a
[00:00:45.623]conjugate feature, the Cearà Rise,
[00:00:47.280]located on the South American plate.
[00:00:49.397]As conjugate features, both
[00:00:51.072]the Sierra Leone Rise and Cearà Rise
[00:00:52.812]had a common origin. Going back
[00:00:54.826]to approximately 80 million years ago,
[00:00:56.983]a mantle plume beneath the spreading ridge
[00:00:58.980]added magmatic material, thickening the
[00:01:00.888]crust and giving rise to both features.
[00:01:03.057]Then, the two features were pushed away
[00:01:04.912]from each other following the spread of
[00:01:06.766]each plate. As a result, the Cearà Rise
[00:01:09.185]was moved towards the West
[00:01:10.482](on the South American plate),
[00:01:12.189]and the Sierra Leone Rise was moved
[00:01:13.943]to the East (on the African plate).
[00:01:16.608]From drilling in the Cearà Rise that
[00:01:18.438]penetrated crustal rocks,
[00:01:19.679]the age was approximated.
[00:01:21.660]On the other hand, drilling done
[00:01:23.310]in the Sierra Leone Rise did not
[00:01:24.882]penetrate crustal rocks.
[00:01:26.302]However, given they are conjugate features
[00:01:28.422]originating at the same time, both
[00:01:30.311]features have the same approximated age.
[00:01:33.047]Now, the Sierra Leone Basin is a region
[00:01:35.217]of normal oceanic crust formed before
[00:01:37.364]the Sierra Leone Rise and Cearà Rise
[00:01:39.404]structures. The basin has a normal thin
[00:01:41.699]oceanic crust without magmatic additions,
[00:01:44.192]unlike the two Rises.
[00:01:46.235]The Bathymetrists Seamounts are located
[00:01:48.143]on the African tectonic plate,
[00:01:49.690]north of the Sierra Leone Rise.
[00:01:51.469]Although no drilling has been done over
[00:01:53.332]the seamounts, and their age is yet to be
[00:01:55.404]determined, dredging of the seafloor rocks
[00:01:57.601]suggests these rocks are younger than
[00:01:59.501]80 million years.
[00:02:00.856]Additionally, the Bathymetrists Seamounts
[00:02:02.936]have a northeast to southwest trend,
[00:02:05.013]contradicting the current eastward
[00:02:06.778]direction of the African plate.
[00:02:11.327]Due to the contradicting pattern of the
[00:02:13.337]Bathymetrists Seamounts, I wanted to
[00:02:15.105]understand more about what was happening
[00:02:17.015]beneath the surface. These are the results
[00:02:19.014]from my previous UCARE research,
[00:02:20.536]done over the Summer of 2020.
[00:02:22.751]I performed isostatic modeling to
[00:02:24.611]determine possible crustal architectures
[00:02:26.543]based on a study with seismic evidence
[00:02:28.725]for crustal underplating,
[00:02:30.100]published by Jones, et al. 2015.
[00:02:33.306]In my previous findings,
[00:02:34.496]the Bathymetrists Seamounts crustal
[00:02:36.166]architecture was derived from the
[00:02:38.004]assumption that isostacy occurs.
[00:02:40.146]This means that pressure exerted by the
[00:02:42.238]crust is equal throughout the area.
[00:02:44.864]This was derived by using each layer’s
[00:02:46.775]density, gravitational constant, and
[00:02:48.737]thickness. Using this method,
[00:02:50.681]it was possible to model the approximate
[00:02:52.718]depths for each layer at different
[00:02:54.354]locations, using the Jones experiment as
[00:02:56.494]a baseline, and further creating
[00:02:58.353]my own two models beneath the
[00:02:59.764]Bathymetrists Seamounts.
[00:03:01.492]The results show depths for each
[00:03:03.052]of the layers: water, sediments,
[00:03:05.209]extrusive and intrusive rocks, and
[00:03:08.184]magmatic underplating, also
[00:03:10.123]determining the depth to Moho
[00:03:11.736](the crust-mantle boundary).
[00:03:13.584]As the isostatic model shows,
[00:03:15.201]the crust beneath the seamounts
[00:03:16.696]requires magmatic underplating,
[00:03:18.467]similar to the Sierra Leone Rise.
[00:03:20.499]My previous findings were the base
[00:03:22.364]for two-dimensional modeling.
[00:03:25.249]The purpose of this study was to
[00:03:27.089]develop a two-dimensional model
[00:03:28.618]of the crustal structures beneath the
[00:03:30.380]Bathymetrists Seamounts and
[00:03:31.666]Sierra Leone Rise to further analyze
[00:03:33.438]their crustal architecture.
[00:03:35.143]I used the isostatic analysis results
[00:03:37.053]from my UCARE over the Summer of 2020
[00:03:39.473]as a starting point for the
[00:03:40.862]two-dimensional model.
[00:03:42.350]I developed bathymetry, gravity,
[00:03:44.145]and magnetic maps
[00:03:45.047]using Geosoft, a geophysical software.
[00:03:49.572]This seismic reflection data, collected in
[00:03:52.222]1966 from expedition V2206
[00:03:55.171]was the framework for the 2-D model,
[00:03:57.441]along with my previous findings over the
[00:03:59.344]Summer. The profile’s vertical scale is
[00:04:01.630]a two-way travel time. And this outline
[00:04:04.714]is the sediment layer.
[00:04:06.299]From the maps shown previously,
[00:04:08.245]I extracted data from each
[00:04:09.682]over the location of the seismic line
[00:04:11.718]V2206 to develop a model.
[00:04:14.565]The goal is to find a model
[00:04:16.153]that agrees with the data altogether.
[00:04:18.434]To begin modeling I used the seismic
[00:04:20.429]profile, and assigned seismic velocity to
[00:04:22.805]each subsurface layer, in order to convert
[00:04:24.947]from time to depth in kilometers.
[00:04:27.077]The seismic data was crucial for
[00:04:28.993]accurate sediment thickness determination.
[00:04:31.680]Furthermore, densities were assigned
[00:04:33.759]in order to calculate gravity field
[00:04:35.630]and compare with the extracted
[00:04:37.177]data from the gravity map.
[00:04:38.436]Once the parameters were set,
[00:04:40.246]each layer was adjusted
[00:04:41.541]to agree with the data.
[00:04:44.761]After several adjustments,
[00:04:46.449]this is the resulting model.
[00:04:48.041]As you can see, each individual layer
[00:04:50.346]water, sediment,
[00:04:51.550]extrusive and intrusive rock,
[00:04:52.936]and magmatic underplating
[00:04:54.287]was adjusted to match the gravity data.
[00:04:56.547]The thicker line is data extracted
[00:04:58.537]from the gravity map,
[00:04:59.760]and the solid thin line
[00:05:01.210]is what I was able to match
[00:05:02.772]from adjusting my model’s layers.
[00:05:04.348]In addition,
[00:05:05.385]the red crosses are the depths extracted
[00:05:07.558]from the Jones, et al. 2015 study.
[00:05:10.835]Notice the resulting model is consistent
[00:05:13.025]with the refraction data from
[00:05:14.421]Jones et al. 2015 and gravity data.
[00:05:19.393]In conclusion,
[00:05:20.473]the resulting model is consistent
[00:05:22.050]with my previous research
[00:05:23.305]the isostatic modeling, and the
[00:05:25.083]Jones et al. 2015 study.
[00:05:27.282]The crustal architecture of
[00:05:28.852]the Bathymetrists Seamounts
[00:05:30.138]and Sierra Leone Rise
[00:05:31.137]are strikingly similar.
[00:05:32.302]The hypothesis remains the same from my
[00:05:34.540]summer research, the Sierra Leone Rise
[00:05:36.349]and Bathymetrists Seamounts
[00:05:37.635]crustal blocks originated
[00:05:38.830]at the same time.
[00:05:39.752]If this is the case,
[00:05:40.893]the seamounts crustal block
[00:05:42.195]was likely displaced
[00:05:43.178]at least 100 kilometers
[00:05:44.429]by a hypothesized right-lateral fault
[00:05:46.555]striking northeast to southwest,
[00:05:48.427]between the two crustal blocks.
[00:05:50.071]The strike of this hypothesized fault
[00:05:51.833]is similar to the overall
[00:05:53.081]trend of the seamounts.
[00:05:54.186]The model suggests
[00:05:55.158]the Bathymetrists Seamounts
[00:05:56.477]and Sierra Leone Rise crustal blocks
[00:05:58.266]were potentially formed at the same time
[00:06:00.356]and later separated
[00:06:01.530]by this hypothesized fault.
[00:06:03.382]It is important to note
[00:06:04.485]that the isostatic modeling research
[00:06:06.235]was based mainly on seismic refraction
[00:06:08.372]and bathymetry data.
[00:06:09.686]While the modeling
[00:06:10.712]involved seismic reflection
[00:06:12.071]and gravity data,
[00:06:13.110]in addition to the bathymetry
[00:06:14.496]and seismic refraction data.
[00:06:16.033]The next step is
[00:06:17.106]to bring magnetics into the model
[00:06:18.880]to better locate the hypothesized fault.
[00:06:22.702]To the UCARE Department
[00:06:24.062]I would like to thank the UCARE program
[00:06:25.926]for providing the funding
[00:06:27.131]to do my research and its continuation.
[00:06:29.100]I am very grateful for this opportunity.

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Two-Dimensional Geophysical Modeling Over Bathymetrists Seamounts

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