Long-lived crustal-scale fluid flow: the hydrothermal mega-breccia of Hidden Valley, Mt. Painter Inlier, South Australia

The Palaeozoic Hidden Valley breccia in the Northern Flinders Ranges of South Australia is exceptional for (1) its size of about 10 km², (2) the large continuous range in clast sizes from tens of microns to hundreds of metres and (3) thorough mixing of lithologies of different provenance, some originally kilometres apart stratigraphically. The size distribution follows a single fractal dimension of about unity over at least 6 orders of magnitude, implying that a single process was responsible for diminution from the 100 m scale, down to < mm. The breccia formed during >12 km exhumation which lasted about 200 Myrs, starting during the ~500 Ma Delamerian Orogeny and continuing during the Alice Springs Orogeny. Fluids released during exhumation were structurally focussed towards Hidden Valley, where an estimated 20 (5–30) km³ total fluid volume caused the extensive brecciation. Brecciation initiated in Neoproterozoic cover metasedimentary rocks, at a level that is now fully exhumed. As hydrothermal fluid ascent continued with ongoing exhumation, the level of brecciation shifted down into the underlying Mesoproterozoic basement rocks, taking with it clasts of cover rocks. In this model, rocks pass through the zone of brecciation, which can thus incorporate a variety of lithologies that were originally kilometres apart.