A geophysical investigation of crustal structure and segmentation of the central Antarctic Peninsula

Johnson, Ashley Charles (1998). A geophysical investigation of crustal structure and segmentation of the central Antarctic Peninsula. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001021f


Investigations of the crustal structure and segmentation of the Antarctic Peninsula Mesozoic-Cenozoic magmatic arc system are limited by the 98% ice cover and sparse rock outcrop. Airborne geophysical surveys provide an ideal method of obtaining evenly spaced geological information over such an area. This thesis describes an aeromagnetic survey of the central Antarctic Peninsula, covering a transect 650 km long and 240 km wide at a 3 km line spacing. This is the most detailed large-scale aeromagnetic survey ever undertaken in Antarctica. Approximately 50 000 km of aeromagnetic data were acquired, which after standard processing and network adjustment show a mean absolute intersection mis-tie vale of 0.83 nT, with and RMS error of 5.92 nT. The data were gridded with a 750 m cell size and contoured at 25 nT intervals.

Interpretation of the new aeromagnetic data was constrained where possible by existing geophysical data, and supplemented with new measurements of magnetic susceptibility. A distinct magnetic signature for each of the common tectonic components of an evolving magmatic arc is identified. The presence of the large positive magnetic anomaly belt along the west coast of the Antarctic Peninsula (the Pacific Margin Anomaly) is shown to be related to a predominantly magnetite-series suite of granitoids, forming a sub-province of the Antarctic Peninsula Batholith. Pseudogravity and terrace maps show major variations over the central and eastern Antarctic Peninsula, which are interpreted as distinct basement provinces. Long-wavelength anomalies are present over arc-marginal basins and accretionary complexes, and short-wavelength, high-amplitude anomalies occur over post-subduction volcanic centres. A new geological sketch map is presented, based on interpretation of the geophysical data, showing new extents for the magmatic arc, basement and accretionary prism elements of the Antarctic Peninsula. The diagnostic magnetic signatures for each component of the arc system can be used for the identification of arcs and palaeo-arcs worldwide.

Four discrete segments of oceanic crust are interpreted from satellite altimetry-derived gravity data, and five segments are recognised in the continental crust. The correspondence between continental and oceanic segments, together with the new interpretation of Antarctic Peninsula crust, indicates that pre-existing continental structure of the Antarctic Peninsula may have had an important effect on segment development. A model of segmented and variable extension is presented, which links the oceanic tectonism to the onshore segmentation. The emphasis in the model is placed on changes in plate convergence rates rather than the arrival of ridge segments at the subduction trench as the major cause of the apparent segmentation.

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