Thermochronology of high heat‐producing crust at Mount Painter, South Australia: Implications for tectonic reactivation of continental interiors

Abstract

The Mount Painter Region in the Northern Flinders Ranges, South Australia, contains a Mesoproterozoic gneissic complex characterized by extraordinary heat production (∼16 μW m−3), resulting in the development of elevated middle‐upper crustal thermal gradients through much of the Paleozoic. Early Paleozoic deformation and metamorphism attained amphibolite facies (>500°C) in the deepest parts of the metamorphic pile (∼10–12 km) during the ∼500 Ma Cambro‐Ordovician Delamerian orogeny. The subsequent thermal history of these rocks is assessed through new K/Ar and 40Ar/39Ar age measurements on amphiboles and micas and multiple‐diffusion‐domain thermal modeling of K‐feldspar 40Ar/39Ar data. The preferred interpretation of these data is that the deepest rocks were at ∼500°C until around 430 Ma, requiring average upper crustal thermal regimes of the order of 40°C km−1 for at least 70 million years. At around 430 Ma, and again at 400 Ma, the terrane underwent periods of moderately fast cooling, possibly separated by a period of isothermal residence. Following cooling at 400 Ma, the terrane entered a second period of relative tectonic quiescence remaining essentially isothermal until ∼330 Ma. This long residence near the closure temperature of biotite resulted in variable argon loss from biotites in the 400 Ma to 330 Ma interval. Tectonic and thermal stability was terminated by a further period of moderately fast cooling (∼4°–8°C Ma−1) in the interval between 330 and 320 Ma. The three cooling episodes at around 430 Ma, 400 Ma, and 330 Ma are interpreted to be the result of exhumation resulting in a combined minimum of 6–7 km of denudation. We attribute this exhumation to the Alice Springs orogeny, a major intraplate tectonothermal event known throughout central Australia but not previously recognized as a significant tectonic event in the Adelaide Fold Belt. These new data provide compelling evidence that thermally modulated variations in lithospheric strength control the distribution of intraplate deformation at the continental scale.

Modes of active intraplate deformation, Flinders Ranges, Australia

“…This sedimentary package and its underlying crystalline basement were deformed in an orogenic setting in the Late Cambrian -Early Ordovician ($500 -490 Ma) Delamerian Orogeny, to form a distinctive oroclinal system now expressed in the strike ridge dominated topography of the Flinders Ranges. Subsequent reactivation during the Devonian and Carboniferous ''Alice Springs'' events is indicated by a number of low-temperature thermochronometric studies [Gibson and Stuwe, 2000;McLaren et al, 2002].…”

“…[17] Following the Delamerian Orogeny a period of tectonic quiescence ensued with mild thermal perturbations attributed to the late Paleozoic Alice Springs Orogeny [Gibson and Stuwe, 2000;McLaren et al, 2002]. During the Mesozoic the region was reduced to a peneplain, after which episodes of fluvial to lacustrine deposition occurred in the Cretaceous, Eocene and Miocene.…”

Genesis and Preservation of a Uranium-Rich Paleozoic Epithermal System with a Surface Expression (Northern Flinders Ranges, South Australia): Radiogenic Heat Driving Regional Hydrothermal Circulation over Geological Timescales

“…In the absence of proximal volcanism, the establishment of the heat flow regime that drove hydrothermal circulation in the late Paleozoic most probably resulted from a combination of high geothermal gradients caused by the radiogenic basement (Neumann et al, 2000) coupled with rapid uplift (McLaren et al, 2002). Sandiford et al (1998) estimated the mean heat production of the MPI to be 9.9 · 10 -3 W m -3 , which is 4 times the heat production for average granites (2.5 · 10 -3 W m -3 ).…”

“…Painter area during the Paleozoic, and smaller-scale activity occurred throughout the Mesozoic and Cenozoic. To the present day, small-scale (limited by low rainfall) hydrothermal activity has occurred along the Paralana Fault (Foster et al, 1994;McLaren et al, 2002;Mitchell et al, 2002;Brugger et al, 2005).…”

Reservoir Definition

“…High heat producing (HHP) granites (e.g., in Australia, McLaren et al, 2002) can be considered as permanent crustal heat sources, inducing heating of the surrounding rocks over a long time. Consequently, thermal regime around HHP granites exhibits higher temperatures than elsewhere, yielding promising areas for geothermal reservoirs.…”