The Mesoarchean Mulgandinnah shear zone, Pilbara Craton: the world’s oldest arc-slicing transform fault

The Paleo-Neoarchean Pilbara craton of western Australia is one of the best-exposed, most studied early Archean cratons on Earth, yet ideas about its origin are split between plate tectonic and non-plate tectonic models (Kusky et al., 2021). Some suggest that the domes-and-basins of the eastern Pilbara represent non-plate tectonic drips where thickened dense volcanic outpourings sagducted into a warm pre-existing sial. Other models propose an accretionary orogen model where oceanic affinity units were off-scraped in a subduction setting, and progressively intruded and re-deformed by arc-related plutons, forming the domes-and-basins. Based on our field, drone- and satellite-assisted structural mapping and analysis, along with processing of geophysical data, we suggest the latter model is more consistent with the geologic and geophysical data.  Here, we report results from our mapping and data synthesis of the craton-cutting Mulgandinnah shear zone.

Arc-slicing transform faults represent an integral component of convergent margin tectonics. They are developed above oblique subduction systems, cutting through and displacing the entire magmatic section of arcs, leading to tectonic repetition of segments of the overriding plate in the ensuing orogenic collage. Extant examples clearly show this process in Sumatra, New Zealand, and the Philippines, while ancient examples are reported from the Paleozoic Altaids, and Neoarchean Superior and Yilgarn cratons. Here, we report data that documents that the Paleo-Mesoarchean Eastern Pilbara craton, recently interpreted to be a preserved mid-upper crustal level of a magmatic arc, is cut and repeated by a major 3.0-2.93 Ga arc-slicing fault, the sinistral Mulgandinnah shear zone (Aldoud et al., 2024), which sliced a previously 600 x 100 km segment of a Mesoarchean arc system, laterally moving different segments to their presently juxtaposed 200 x 200 km preserved fragment. We document the offsets using offset strata and plutons (i.e. domes), both from surface mapping and high-resolution Reduced to Pole (RTP) magnetic datasets for buried plutons. We document the sense of shear using classical kinematic indicators, and also show that that shearing occurred during partial melting at ~3.0 Ga. Geophysical data (reflection profiles) reveal that the domes-and-basins are confined to the upper crust, with a flat reflector (detachment shear zone?), into which listric thrust fault merge, at 10 sec (~30 km) depth, and no trace of any drips sinking deeper. This evidence demonstrates lateral plate motions by 3.0 Ga, and shows oblique subduction, arc plutonism, arc-slicing and repetition, reflecting that crustal growth in some style of convergent margins was in operation by the Mesoarchean. To our knowledge, this is currently the oldest well-documented arc-slicing fault system on Earth, although this by no means that others may not have operated previously, and have been eroded, or remain to be documented.

 

Aldoud, A., Kusky, T.M.*, Wang, Lu, 2024, Is the Mulgandinnah shear zone, Pilbara craton, the world’s oldest arc-slicing transform fault? Geology 52, 801-806. https://doi.org/10.1130/G52360.1

Kusky, T.M.*, Windley, B.F., Polat, A., Wang, L., Ning, W.B., and Zhong, Y.T., 2021, Archean dome-and-basin style structures form during growth and death of intraoceanic and continental margin arcs in accretionary orogens, Earth-Science Reviews 220, 103725.  https://doi.org/10.1016/j.earscirev.2021.103725