by Michael Studinger, Lamont-Doherty Earth Observatory

Gravity anomalies of sedimentary basins and their mechanical implications: Application to the Ross Sea basins, West Antarctica

Background:

This work was motivated by the unusual gravity anomalies of some of the extensional basins in the Ross Sea, Antarctica (see map of the region). In general, sedimentary basins are characterized by negative gravity anomalies, however, the larger Ross Sea basins are overlain by positive gravity anomalies. The work demonstrates how gravity data together with isostatic models can be used to constrain the tectonic evolution of a region. The work was done by Garry Karner, who lead the effort, myself and Robin Bell. The results have been described in detail in Karner et al., (2005) and are summarized below.


Karner, G.D., Studinger, M., Bell, R.E., Gravity anomalies of sedimentary basins and their mechanical implications: Application to the Ross Sea basins, West Antarctica, Earth Planet. Sci. Lett., 235, p 577 - 596, 2005. doi:10.1016/j.epsl.2005.04.016.


In general, sedimentary basins are characterized by negative free-air and Bouguer gravity anomalies. However, the extensional basins of the Ross Sea are paradoxical in that positive gravity anomalies overlay the Victoria Land Basin, Northern Basin, Central Trough and Northern Central Trough while basement highs are associated with negative gravity anomalies. Measured basement densities from DSDP basement cores give values between 2600–2800 kg/m3 while bulk sediment densities range from 1210–2200 kg/m3, indicating a normal density relationship between basement and sediment infill. In contrast, the relatively young and narrow Terror Rift is associated with negative free-air and Bouguer gravity anomalies, but has a different geological history as compared to the larger Ross Sea basins. Process-oriented gravity modeling indicates that magmatic underplating and crustal intrusions are inconsistent with the observed gravity and basement geometry of the Ross Sea basins. The magma volume necessary to account for the distribution and amplitude of the positive gravity anomaly of the Central Basin and be isostatically balanced would need to be comparable to the tholeiitic flood basalt volume of the Columbia River province—it is thus unlikely that the volume of Neogene volcanics of the Ross Sea region is sufficient to explain the observed gravity relationship by modifying the bulk density of the crust.

Positive gravity anomalies over extensional basins can be a consequence of a relatively low flexural strength of the lithosphere during rifting being contrasted by higher flexural strengths later during sedimentation. As the difference between the rigidity of the lithosphere during sedimentation increases relative to the rigidity of the rifted lithosphere, the gravity over the basin becomes progressively more positive but only for a limited range of wavelengths (see figure below). The narrow width of the Terror Rift precludes it from having a positive gravity anomaly while the opposite is true for the large Ross Sea basins. For the Ross Sea region, such a loading scenario requires a significant delay between extension and the timing of sediment infilling of the basins, consistent with the late Cretaceous extension of the Ross Sea region and the sedimentary succession being dominated by large-scale late Eocene–Neogene glaciogenic progradational sequences. Sediment source was presumably from the denudation of the Transantarctic Mountains, which commenced in the late Eocene. The time delay between the late Cretaceous formation of the Transantarctic Mountains, late Eocene exhumation, and the generation of significant paleobathymetry requires either the Ross Sea region to be sub-aerial and sediment starved for most of the Paleogene and/or the Paleogene climate was ineffective in producing clastics until the onset of glaciation in the late Eocene–early Oligocene.


Figure 1: Admittance function Z(k) between the Moho and basin gravity as a function of varying rift and sedimentation flexural rigidities (modified from Karner et al., 2005). Two families of admittance functions are shown depending on the effective elastic thickness of the extended lithosphere; Ter of 5 km (red curves), Ter of 30 km (blue curves). For Ter of 5 km, sediment loading effective elastic thickness, Tes, ranges from 5–30 km while for a Ter of 30 km, sediment loading effective elastic thickness ranges from 30–80 km. For Z(k) < 1, the gravity effect over the basin is dominated by the density variation of the basin. For Z(k) > 1, the gravity is dominated by the Moho and the basin is associated with a positive gravity effect. When Ter and Tes are equal, the gravity is always negative across the basin (Z(k) < 1). As the difference between rift and sediment loading effective elastic thickness increases, the gravity over the basin becomes progressively more positive but only for a limited range of wavelengths. The wide late Cretaceous basins of the Ross Sea should be characterized by a positive gravity anomaly while the narrow late Paleogene–Neogene Terror Rift should be associated with a negative gravity anomaly.


Published Papers related to Gravity Anomalies of Sedimentary Basins and the Ross Sea embayment:

Karner, G.D., Studinger, M., Bell, R.E., Gravity anomalies of sedimentary basins and their mechanical implications: Application to the Ross Sea basins, West Antarctica, Earth Planet. Sci. Lett., 235, p 577 - 596, 2005. doi:10.1016/j.epsl.2005.04.016.

Bell, R.E., Studinger M., Karner, G.D., Finn, C.A., and Blankenship, D.D., Identifying major sedimentary basins beneath the West Antarctic Ice Sheet from aeromagnetic data analysis, in: D.K. Fütterer, D. Damaske, G. Kleinschmidt, H. Miller, F. Tessensohn, (Eds), Antarctica: Contributions to global earth sciences, Springer-Verlag, Berlin Heidelberg New York, pp. 117 - 122, 2006.

Studinger, M., Bell, R.E., Blankenship, D.D., Finn, C.A., Arko, R.A., Morse, D.L., and Joughin, I., Subglacial Sediments: A Regional Geological Template for Ice Flow in West Antarctica, Geophys. Res. Lett., 28(18), 3493-3496, 2001 doi:10.1029/2000GL011788.

Studinger, M., Bell, R.E., Finn, C.A., and Blankenship, D.D., Mesozoic and Cenozoic extensional tectonics of the West Antarctic rift system from high-resolution airborne geophysical mapping, In: J.A. Gamble, D.N.B. Skinner, and S. Henrys, Antarctica at the close of a millenium, R. Soc. of NZ Bull., 35, pp 563-569, 2002. Download PDF file (376 Kbyte).


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