Tectonic Evolution of the Weddell Sea
by Michael Studinger, Lamont-Doherty Earth Observatory

Free-Air and Bouguer Gravity Maps of the Weddell Sea Region
Figure 1 from Studinger and Miller, 1999, JGR. (a) Free-air anomaly map. The map is illuminated from the northwest to reveal smaller relief features. (b) Bouguer anomaly map. Contour interval is 40 mGal. Abbreviations are CL. Coats Land; DML, Dronning Maud Land; DPA, dipolar anomaly; FIS, Filchner Ice Shelf; FS, Filchner shelf; FT, Filchner Trough; RIS, Ronne Ice Shelf. Click here for a map of the Weddell Sea.

During my doctoral research at the Alfred Wegener Institute for Polar and Marine Research at Bremerhaven, Germany, my work focused on the geodynamic evolution of the Weddell Sea embayment and adjacent areas. I used mostly gravity data (see above) and magnetic data (see map), but also marine seismic data for this work.

Understanding the tectonic evolution of the Weddell Sea embayment (click here for a map) provides valuable insights on the breakup of Gondwana. Existing geodynamic models for this region differ mainly in the nature of the crust beneath the Filchner-Ronne shelf and the movement of the Ellsworth-Whitmore Mountains (EWM) crustal block. New gravity data presented here constrain the area and timing of this EWM movement from its prebreakup position to its present-day position. The crustal thickness estimates of 29 ± 5 km and 27 ± 4 km derived from power spectral analysis as well as forward gravity modeling make the existence of oceanic crust beneath the Filchner-Ronne shelf unlikely. A new crustal unit consisting of Precambrian crust has been identified with gravity and magnetic data and extends ~175 km off the coast of Coats Land. This crustal unit conflicts with the late Mesozoic movement of the EWM in this area. The effective elastic thickness of Te = 35 ± 5 km has been estimated from coherence spectra for the Ronne shelf. Together with the crustal thickness of 27 km from refraction seismic studies and comparison with theoretical models, the thermal age is estimated to be between 165 and 230 Ma for the Ronne shelf. This age marks the last significant tectonic event in this area and is in good agreement with the breakup of Gondwana. It further makes later movements of microplates like the EWM in this region unlikely. Thus a Filchner block consisting of continental crust should be considered in any geodynamic model of the early Weddell Sea evolution.

Huge Crack in the Ronne Ice Shelf, AntarcticaAntarctica formed the central part of the late Proterozoic supercontinent Rodinia and the Paleozoic/Mesozoic supercontinent Gondwana. The initial breakup of Gondwana started about 180 Myr ago in the present-day Weddell Sea. However, the Weddell Sea represents probably the least known and least accessible sea in the world. Its remoteness and harsh climatic conditions have prevented comprehensive geophysical and geological investigations as carried out in other Antarctic regions like the Ross Sea. Marine geophysical mapping in this area is restricted to a narrow coastal polynya opened by katabatic wind systems in austral summers. Ice-breaking research vessels can operate in this seasonal open water region to acquire marine geophysical data. The data collected in this region are unevenly distributed due to sea ice cover [e.g., Jokat et al., 1997a]. Furthermore, the Antarctic ice sheet restricts outcrop in surrounding regions to only a few nunataks located in Coats Land, Dronning Maud Land, and along the east coast of the Antarctic Peninsula (AP) which precludes onshore geological mapping of large areas. Nevertheless, the Weddell Sea is one of the most important areas for understanding the West Antarctic reconstructions of Gondwana. Conflicting geodynamic models for this area during the initial breakup history [e.g., Grunow, 1993; Storey et al., 1996; Jokat et al., 1997a] reflect the sparsity of geological and geophysical constraints for reconstruction models.

The most detailed models are constrained primarily by paleomagnetic and geologic data [e.g., Grunow et al., 1991; DiVenere et al., 1996]. They describe a movement of the Ellsworth-Whitmore Mountains (EWM) crustal block, presently situated south of the Filchner-Ronne shelf, from its prebreakup position off Coats Land through the present-day Filchner-Ronne shelf 125 Myr ago. In contrast, other geodynamic models [e.g., Storey et al., 1996; Jokat et al., 1997a] take into account an additional Filchner block, composed of stretched continental crust, which makes the movement of the EWM crustal block in this area unlikely.
One of the primary tools for studying geodynamic and global tectonic processes is regional mapping of potential field anomalies. In ice-covered regions, gravity and magnetic imaging is the optimum approach to survey large areas and to resolve the geological structures beneath the ice. During this work, a compilation of almost all existing gravity data in the Weddell Sea produced a homogenous database of gravity data in this area. The comprehensive study and interpretation of this data set includes power spectral analysis, coherence spectra, and forward modeling of gravity anomalies. The original position of the EWM (based on paleomagnetic data) and the nature of the crust of the Weddell Sea embayment (based on geophysical data [e.g., Jokat et al., 1997a]) lead to conflicting geodynamic models. Our main goal is to contribute new geophysical constraints to solve this apparent conflict.

Publications related to the Tectonic Evolution of the Weddell Sea and Gondwana Breakup:

Studinger, M., and Miller H., Crustal structure of the Filchner-Ronne Shelf and Coats Land, Antarctica from gravity and magnetic data: implications for the breakup of Gondwana, J. Geophys. Res., 104(B9), 20379-20394,1999. Download PDF file (4380 Kbyte).

Studinger, M., Compilation and analysis of potential field data from the Weddell Sea, Antarctica: implications for the breakup of Gondwana, Rep. Polar Res., 276, 134 pp, 1998. Download PDF file (6630 Kbyte).

Studinger, M., Kurinin R.G., Aleshkova N., Miller H., Power spectra analysis of gravity data from the Weddell Sea embayment and adjacent areas, Terra Antartica, 4(1), 23-26, 1997. Download PDF file (285 Kbyte)

Jokat W., Fechner N., Studinger M., Geodynamic models of the Weddell Sea embayment in view of new geophysical data, In: Ricci, C.A. (ed.) The Antarctic Region: Geological Evolution and Processes, Siena, 453-459, 1997. Download PDF file (740 Kbyte).