Internal structure of the Paleoarchean Mt Edgar dome, Pilbara Craton, Western Australia

Abstract

The Paleoarchean East Pilbara Terrane of Western Australia is a dome-and-keel terrane that is often highlighted as recording a vertically convective tectonic regime in the early Earth. In this model, termed 'partial convective overturn', granitic domes diapirically rose through a dense, foundering mafic supracrustal sequence. The applicability of partial convective overturn to the East Pilbara Terrane and to other Archean dome-and-keel terranes is widely debated and has significant implications for early Earth geodynamics. A critical data gap in the East Pilbara Terrane is the internal structure of the granitic domes. We present field-based, microstructural, and anisotropy of magnetic susceptibility (AMS) data collected within the Mt Edgar dome to understand its internal structure and assess its compatibility with existing dome formation models. Field and microstructural observations suggest that most fabric development occurred under submagmatic and high-temperature solid-state conditions. The AMS results reveal a coherent, dome-wide structural pattern: 1) Sub-vertical lineations plunge radially inward towards the center of the dome and foliations across much of the dome consistently strike northwest; 2) Shallowly plunging lineations define an arch that extends from the center of the dome to the southwest margin; and 3) Migmatitic gneisses, which represent the oldest granitic component of the dome, are folded and flattened against the margin of the dome in two distinct lobes. The structural relationships between rocks of different ages indicate that units of different crystallization ages deformed synchronously during the last major pulse of granitic magmatism. These data are broadly consistent with a vertical tectonics model, and we synthesize our structural results to propose a three-stage diapiric evolution of the Mt Edgar dome. The critical stage of dome development was between 3.3 and 3.2 Ga, when widespread, melt-assisted flow of the deep crust led to the formation of a steep-walled, composite dome. These data suggest that diapiric processes were important for the formation of dome-and-keel terranes in the Paleoarchean.