Tectonic fluid expulsion: U-Pb evidence for punctuated hydrothermal fluid flow and hydraulic fracturing during orogenesis

Abstract

Tectonic fluid expulsion has been linked to fluid flow in permeable foreland basin sequences, but it is unclear how fluids migrate through underlying low-permeability successions. Here we report U-Pb data which indicate that fluids expelled during tectonic compression migrated through older compacted shales inboard of the deforming craton margin by fracture permeability and enhanced diffusion. From a ∼350 m drill-core interval of Neoarchean shale (drill-hole ABDP9), Hamersley region, Australia, we document abundant bedding-parallel veins indicating pervasive hydraulic fracturing. Thermometry of vein-filling chlorite indicates temperatures between 329-374°C, which is higher than estimates (175-280°C) from metamorphic assemblages in underlying basalts, implying the passage of hydrothermal fluids. New in situ secondary ion mass spectrometry (SIMS) U-Pb geochronology of monazite and xenotime in bedding-parallel veins verifies and expands earlier results, showing that microfracturing and hydrothermal fluid flow spanned ∼650 million years and was restricted to five short intervals: 2.30 Ga, 2.20 Ga, 2.10 Ga, 2.05 Ga and 1.65 Ga. Three of the five age populations coincide with known fabric-forming orogenic events (i.e. 2.30 Ga Sylvania orogeny; 2.22-2.15 Ga Ophthalmia orogeny; 1.68-1.65 Ga Mangaroon orogeny), which are focused along the southern Pilbara Craton margin. Our results show that the timing of veining was not random, but coeval with major orogenic events, linking fluid overpressure and hydraulic fracturing to the expulsion of heated orogenic fluids 100s of km inboard of the craton margin. The crack-seal veins indicate repeated cycles of fluid overpressure resulting in hydrofracturing and fluid drainage, followed by cavity collapse. The tectonic expulsion of pressurized hydrothermal fluids produced fracture and matrix permeability in Earth's upper crust, leading to episodic open-system behaviour with enhanced fluid-rock reactions and the large-scale transfer of heat and mass.