Early tectonic dewatering and brecciation on the overturned sequence at Marble Bar, Pilbara Craton, Western Australia: dome-related or not?

Abstract

Cataclastic breccias and hydrothermal fault arrays of likely c. 3400 Ma timing are well developed and exceptionally well exposed in the Marble Bar Chert Member of the Pilbara Craton. Brecciation involved centimetre- to metre-scale clast transport distances, in breccia zones up to 5 m wide, cutting the c. 60 m thick chert in a series of right-lateral fault zones. Our observations of downward facing pillow basalts, the geometry of the breccias, and oxygen isotope data for rocks and the breccia matrix suggest the rocks were at least steeply overturned on this flank of the Mt Edgar Dome prior to brecciation. The breccias are inferred to represent steep conjugate fault zones developed by local transtension. The history of overturning and brecciation predates the formation of dome-related regional foliation and metamorphism, and therefore occurred between 3460 and 3320 Ma, the established ages for deposition of the underlying Duffer Formation and intrusion of the Mt Edgar Batholith respectively. Local overturning of the Marble Bar sequence prior to both brecciation. and the main phase of dome formation suggests a protracted deformation history for this segment of the Pilbara Craton. The transtensional movement along the breccias may be representative of strain accommodation accompanying an early doming phase, or could be a deformation event that developed independently of doming. Fluids involved in brecciation were most likely formation waters expelled from the cherts and basalts in response to overpressuring induced by the overturning and progressive burial.