Download No plume, no extension in the WARS

Frontiers and Opportunities in Antarctic Geosciences * Certosa di Pontignano * 29-31 July 2004
Tight Link between Cenozoic Magmatism and
Local-Regional Fault Activity in the West Antarctic Rift
ROCCHI S.1, ARMIENTI P. 1, DI VINCENZO G. 2, NARDINI I. 1, ROSSETTI F. 3, STORTI F. 3
1
Dipartimento di Scienze della Terra, Università di Pisa - Italy
2
Istituto Geoscienze e Georisorse, CNR, Pisa - Italy
3
Dipartimento di Scienze Geologiche, Università di Roma Tre - Italy
*Corresponding author ([email protected])
The West Antarctic rift is one of the largest areas of thinned crust on Earth and is characterized by late Cretaceous
amagmatic extension and Cenozoic alkaline magmatism with little or no extension. Thinning of the lithosphere and igneous
activity in major continental rifts are commonly considered in terms of the end-members models of active versus passive
rifting, and a genetic connection is acknowledged between lithospheric extension, magmatism and mantle plume activity.
Yet, the Ross Embayment-Victoria Land offers a new perspective on the link between plate dynamics, tectonic activity and
igneous activity in rift systems.
Recent studies of structural-chronological relationships between emplacement of plutons, dyke swarms and volcanic
edifices in the Victoria Land led to a critical re-examination of the plume models. Indeed, in Victoria Land, the linear
geometry of the uplift and the relative chronology of uplift and extension conflict with the traditional concepts of
lithospheric evolution above a mantle plume. The OIB-HIMU geochemical and Sr-Nd-Pb isotopic signature of mafic rocks
cannot be exclusively interpreted in terms of plume activity, particularly in this region, where magma source is located in
hydrous mantle. Also the low 3He/4He attests against the involvement of deep upwelling mantle in the magma source.
Moreover, magma production rates are low, and hot mantle beneath the Antarctic rift is not documented as deep as expected
for mantle plumes, with a regional shape of thermal anomaly which is linear instead of circular.
Conversely, the evidence for a tight link between regional tectonics and igneous activity stands out against this lack of key
evidence for plume activity. Actually, magmatism set off sequentially in adjacent crustal sectors, and the dyke swarms have
NNW or NS strikes, pointing to a geometric link with the NNW dextral strike-slip fault systems that dissect the rift shoulder
in Victoria Land. New pseudotachylyte ages show that faults were active at least in the early Oligocene, coeval with dyke
emplacement. On a larger scale, a tight chronologic-geometric link exists between dextral strike-slip systems in Victoria
Land and the Southern Ocean fracture zones. Moreover, the beginning of magmatism (48 Ma) is coeval with the increase in
differential movements on these fracture zones (43-47 Ma).
Such a plate-driven igneous activity suggests a model alternative to both passive and plume-driven rifting, with lithospheric
strike-slip deformation promoting transtension-related decompression melting of a sub-plate mantle previously
decompressed/veined during the late Cretaceous amagmatic extension.