Paradoxical Gravity Anomalies of the Ross Sea, Antarctica

Garry D. Karner, Michael Studinger & Robin E. Bell

Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, 
Palisades, NY 10964, USA

Tectonic reconstructions of Antarctica and Australia suggest that the Ross Sea 
basins were formed in the late Cretaceous.  The adjacent Transantarctic 
Mountains are often explained in terms of the uplift and flexural adjustment of 
the lithosphere to extension in the Ross Sea.  As such, the high average elevation 
of the mountains requires significant extension, presumably the late Cretaceous 
event.  However, the exhumation of the mountains occurred significantly after 
late Cretaceous rifting.  Fission-track data suggest that the main period of 
denudation is in the mid-Tertiary, but geological and geophysical evidence for 
significant mid-Tertiary deformation does not exist.  Gravity anomalies (with 
wavelengths of 100-200 km in the Ross Sea are paradoxical in that positive 
gravity anomalies generally exist over the basins and negative anomalies over 
the basement highs.  Density inversions between basement and sediment or 
volcanic intrusions are unlikely to be the cause of the observed gravity 
relationship.  Measured basement densities from DSDP cores gives values 
between 2600-2800 kg/m3.  In contrast, measured sediment bulk densities from 
both DSDP sites range in value from 1600-2200 kg/m3 and shallow coring (1-2 m) 
across the Victoria Land Basin indicate sediment densities of 1210-2000 kg/m3, 
with the average being about 1800 kg/m3.  While volcanic intrusions are 
sometimes associated with positive gravity anomalies, they tend to be randomly 
distributed and are spatially restricted (5-10 km wavelengths).  Analysis of the 
broad gravity anomalies provides new constraints on the evolution of the Ross 
Sea basins and the dilemma of Tertiary denudation in the absence of significant 
Tertiary tectonics.  We demonstrate that the observed gravity relationship is the 
consequence of a relatively low flexural strength of the lithosphere during late 
Cretaceous rifting and higher flexural strengths significantly later during 
sediment delivery to the basin.  This gravity relationship requires a delay in the 
infilling of the basins relative to late Cretaceous extension, as evidenced by the 
almost complete absence of Paleogene sediments within the Ross Sea basins.  The 
sedimentary succession appears to be dominated by Oligocene-Holocene glacial 
and possible Late Cretaceous syn-rift sediments.  Large amplitude 
progradational systems beginning in the Oligocene indicate that significant 
accommodation space was generated prior to the Oligocene.  The time delay 
between late Cretaceous uplift of the Transantarctic Mountains, mid-Tertiary 
exhumation and the generation of significant Paleogene paleo-bathymetry 
requires the Paleogene climate to be ineffective in producing clastics until the 
early Oligocene.