Paleoarchean Dome-and-Keel Structures of the East Pilbara Craton: A Structural Geology and Potential

Описание: Paleoarchean Dome-and-Keel Structures of the East Pilbara Craton: A Structural Geology and Potential Field Perspective

Basil Tikoff

University of Wisconsin

The Paloearchean East Pilbara craton contains a dome-and-keel map pattern, similar to those observed in several of Earth’s earliest continents. Large (~50 km diameter) granitic-gneiss domes intrude variably deformed supracrustal sequences (greenstones). Various models exist for the formation of these structures: we took an empirical approach and collected data that would allow us to distinguish between different models. The domes contain complex yet interpretable internal fabrics, formed over significantly long time period (unlike Phanerozoic plutons). We have also investigated the belt of deformed supracrustal (greenstone) rocks between the Mt. Edgar and Corunna Downs Granitic Complexes known as the Warrawoona Syncline. Geochronology data suggests that garnet growth in this zone occurred 30 m.y. after deposition of the host units. Moreover, part of the Warrawoona Syncline has vertical linear fabrics and was proposed to reflect a “zone of sinking”, functioning as a downwelling zone associated with sagduction. The microstructural data in this region supports constrictional fabrics consistent with the sagduction interpretation. We conducted a detailed gravity study of this region to test the “zone of sinking” hypothesis; gravity is a particularly good tool because the proposed model is buoyancy (density) driven. Our results reveal no anomaly at depth below the zone of sinking, and that the greenstone do not continue to great depth ( less than 5 km). Rather, the zone of sinking occurs along a gradient of increasing positive gravity anomaly to the west. Thus, either sagduction did not occur at this location or the original evidence has been effectively erased. If the former is true, which seems the most likely alternative, the origin of the constrictional fabrics remains unresolved. Future work is needed to further untangle the formation and evolution of the dome-and-keel structures.