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Analysis of flexural evolution of the lithosphere over the past 4.6 Ma around Ross Island, West Antarctica




Jha, Sumant, author
Harry, Dennis L., advisor
Aster, Rick C., committee member
Schutt, Derek L., committee member
Amberg, Gregory C., committee member

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Ross Island is in the southern Victoria Land Basin along the western margin of the West Antarctic Rift System. Episodic volcanism since ca. 4.6 Ma produced a discontinuous sedimentary moat around the island coeval with ongoing extension. The moat is a composite of four smaller flexural sub-basins created during four distinct phases of volcanism on Ross Island. In this research we determine the flexural rigidity of lithosphere under Ross Island, to understand the load partitioning between surface (relief) and subsurface (e.g. the density anomaly in crust/mantle) and to test the hypothesis that the strength of the lithosphere around the island varied with time over last 4.6 Ma. An interactive toolbox called Toolbox for Analysis of Flexural Isostasy (TAFI) was developed in MATLAB to model the flexure around Ross Island. TAFI supports two-dimensional (2-D) and three-dimensional (3-D) modeling of flexural subsidence and uplift of the lithosphere in response to vertical tectonic loading. Flexural deformation is approximated as bending of a thin elastic plate overlying an inviscid fluid asthenosphere. The associated gravity anomaly is calculated by summing the anomalies produced by flexure of each density interface within the lithosphere, using Parker's algorithm. TAFI includes MATLAB functions provided as m-files (also called script files) to calculate the Green's functions for flexure of an elastic plate subjected to point or line loads, and functions to calculate the analytical solution for harmonic loads. Numerical solutions for flexure due to non-impulsive two-dimensional (2-D) or three-dimensional (3-D) loads are computed by convolving the appropriate Green's function with a spatially discretized load function read from a user-supplied file. TAFI uses MATLAB's intrinsic functions for all computations and does not require any other specialized toolbox, functions, or libraries except those distributed with TAFI. The modeling functions within TAFI can be called from the MATLAB command line, from within user-written programs, or from a graphical user interface (GUI) provided with TAFI. The GUI facilitates interactive flexural modeling and easy comparison of the model to gravity observations and to data constraining flexural subsidence and uplift. Flexural subsidence within each of the four flexural sub-basins around Ross Island is modeled by using a continuous elastic plate in TAFI. Models of subsidence are constrained by thickness of strata that accumulated in each sub-basin during the time interval in which the associated volcanic center was active. Flexure models were created along profiles trending radially away from volcanic centers. The load due to each volcanic center is represented as a point load at the location of volcano. These models yield best-fit flexural rigidities ranging from 6-36 x 10¹⁸ N-m, with the lowest values on the south and southwest sides of the island and associated with the youngest volcanoes. This systematic variation in flexural rigidity may be attributed to progressive weakening of the plate with time, or to spatial variations in plate strength. A second group of models constrained by the thickness of the entire stratigraphic interval filling the moat since volcanism began on Ross Island yield a flexural rigidity up to twenty times greater than models that consider only strata deposited within each sub-basin when the associated volcano was active. The second group of models overestimates the strength of the lithosphere due to the inclusion of strata deposited during periods in which regional extension rather than local flexure was the dominant control on subsidence in each sub-basin. In addition to plate flexure, the models indicate a small buoyant load with a magnitude of 7-10 x 10¹⁶ N beneath Ross Island, equivalent to a volume of 1.4 – 6.9 x 10¹³ m³ assuming a density contrast range of 3010 – 3160 kg/m³, is needed to maintain isostatic equilibrium with a moderately low-density upper mantle. 3-D flexural models are constructed to determine whether variations in the shape of the flexural moat around Ross Island are a result of variations in the strength of the lithosphere with space or with time. These models were constrained by the width and depth of flexure sub-basins around associated loading centers derived from the thickness of entire stratigraphic interval filling the moat since volcanism began on Ross Island. The models with constant flexural rigidity through time are unable to fit the width and depth of flexural subsidence in all parts of flexural moats around Ross Island. Models with time-varying flexural rigidities resulted in an improved fit for different parts of the moat with flexural rigidities varying between 3.0 x 10¹⁹ N-m to 2.6 x 10¹⁹ N-m, which is not a resolvable difference.


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