ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) OF THE NORTHEAST MARGIN OF THE PALEOARCHEAN SHAW DOME, EAST PILBARA TERRANE, WESTERN AUSTRALIA

Earth’s earliest continents commonly preserve large, multi-component gneiss domes. The way in which these gneiss domes have been deformed at their boundaries can reveal important aspects of their growth histories. This study examines structures within a portion of the Paleoarchean (3.6-3.2 Ga) Shaw dome in the East Pilbara Terrane, Western Australia, in order to increase understanding about the deformational history and structural relationships between different domes in the Pilbara. We present field measurements, anisotropy of magnetic susceptibility (AMS), and microstructural observations with the goal of understanding the orientation, geometry, and kinematics of the sheared northeast boundary of the Shaw dome. All data were collected at 11 stations along margin-parallel transects through the Cooliya Creek Granodiorite. Samples in the first transect (AME23-039 through AME23-045) on the northeast margin of the Shaw Dome have unidirectional lineations that trend E-NE, not quite perpendicular to the dome margin, with variable dips and foliations striking WNW-ESE that match the Cooliya Creek Granodiorite’s border with the greenstones and Fortescue Basalt. These samples all display oblate strain geometry indicating they are foliation dominated. The trend of lineations are sub-parallel to trends of lineations at the nearest margin of the Mt Edgar dome, the granitic dome to the northeast of the Shaw. The samples in the second transect have variable orientations and display no clear trend. These samples also display strong oblate geometry, again with the exception of AME23-050, which displays prolate geometry. The high magnetic susceptibility (bulk susceptibility of ~103) of most samples from both transects suggests that they are magnetite-bearing. We present several dome formation histories that can explain the orientation, geometry, and kinematics of fabrics on the northeast margin of the Shaw dome.