GRAVITY SURVEY AND ASSOCIATED DENSITY MODELS OF THE WARRAWOONA SYNCLINE REGION, EASTERN PILBARA CRATON, AUSTRALIA: A TEST OF EARLY ARCHEAN CONVECTIVE OVERTURN

One model for Early Archean lithospheric flow is convective overturn. An example often cited to support convective overturn is a localized “zone of sinking” in the greenstones of the Warrawoona syncline, east Pilbara craton, Australia. Field measurements indicate inward dipping lineations toward a “zone of sinking”, above which occur vertical lineation and constrictional strain. These patterns are interpreted to record downward flow of the Warrawoona greenstone belt between the Mt Edgar and Coruna Downs batholiths, driven by density inversion.

This model was tested with a gravity study over the Warrawoona “zone of sinking” and adjacent Mt Edgar and Coruna Downs batholiths. There were 535 new ground gravity station collected at a station spacing of 0.5-1 km. This data is supplemented by the Geological Survey of Western Australia gravity data from this region, which is primarily airborne data with a 2.5 km spacing. A maximum of a 20 mGal positive Bouguer anomaly is observed over the Warrawoona syncline on a ~NS-oriented transect from the Mt Edgar batholith to the Coruna Downs batholith, which crosses the proposed zone of sinking. On a transect along the axis of Warrawoona syncline, however, there is a gradual increase (~30 mGal) in gravity values toward the WNW. No closed contour Bouguer gravity anomaly is observed above the proposed zone of sinking. Consequently, our data do not support the “zone of sinking” model. Further, because convective overturn is a density-driven process, the lack of a gravity signal is a compelling test of this model. Inverse and forward models of the gravity data will constrain the depth extent of the greenstones associated with the Warrawoona syncline.

If convective overturn occurs in the Warrawoona region, it requires alternative locations for the sinking zones in the greenstone wall rocks. One possibility is an intermediate position between the Shaw, Mt Edgar, and Coruna Downs batholiths, significantly (>10 km) west of the previously proposed “zone of sinking” defined by constrictional strain and near vertical lineations.