No abstract

A long-lived hydrothermal system at the Heyuan fault, South China, has led to the development of a giant quartz reef, now partially exhumed along its length for more than 40 km. Systematic analyses and focused microstructural studies have been undertaken to unravel a complex formation history of repeated fracturing, hydrothermal fluid flow and sealing cycles, resulting in a dynamic permeability across the fault zone. The change in morphology and decreasing grain-size with time further indicates the move from slow ductile opening to fast seismic events. Quartz reef formation has been estimated to occur within a range of ~200-350°C, based on evaluation of (i) quartz deformation microstructures; (ii) chlorite and mica geothermometry; and (iii) review of comparable quartz reef studies. Additionally, a set of physico-chemical formation conditions have been identified which compose the 'quartz reef window'. These are: (i) significant volume of fluid; (ii) fluid sources from meteoric, metamorphic and/or from mantle origin; (iii) considerable Time-Integrated Fluid Fluxes; (iv) SiO2 oversaturation due to (a) temperature change, (b) sudden pressure drop, or (c) chemical change e.g. fluid mixing; (v) accommodation space to 'grow' the reef; (vi) channel permeability; and (vii) cap rock/seal to trap the fluid flow. The mechanism of quartz reef growth is here interpreted as the brittleductile analogue of the brittle fault-valve model.

SUMMARYGeothermal energy potential in China is high, and although they currently lead the way in direct heat production, geothermal power generation is still low. Hot spring analysis and surface heat flux data indicate significant potential resources for the major industrial province of Guangdong, South China. This pilot study investigates the Heyuan Fault, Guangdong, as a potential site for a geothermal power plant. Here we line out (i) preferred locations of possible hot spots on fault intersections, (ii) the possible sources of the heat anomalies, (iii) potential pathways for hot fluid circulation in the upper crust, (iv) available hot spring data and (v) the future work plan to investigate the geothermal hot spots.We find that hot springs occur along the NE trending Heyuan Fault, clustering where NNW striking faults crosscut the Heyuan. The increased heat flow can be explained partly by radioactive decay of a large granite pluton beneath the fault, however, additional heat sources may need to be considered to explain the heat flow maxima of above 85 mWm -2 . We postulate that advective (topographically driven) and convective (deep fluids ponding at the brittle-ductile transition) processes may be operating to generate these heat anomalies.Expansive quartz reef systems exposed on the Heyuan Fault, are proposed here, to represent uplifted sections of these deep fluid circulation patterns. A detailed systematic analysis of reef structures will reveal (i) the fluid provenance, (ii) precipitation conditions and (iii) deformation mechanisms, which will ultimately help us understand how fault intersection relations control fluid flow; which is of key significance if it can be utilised for targeting geothermal energy.

How to build a giant quartz reef Tannock et al.

<p>A paleohydrothermal giant quartz reef (at least 75 m wide, 40 km long) and abundant hot springs at the Heyuan fault, South China, provide an excellent opportunity to investigate hydrothermal flows from the Mesozoic through to present-day.</p><p>The giant quartz reef has formed in the extensional regime initiated in the Mesozoic, while a change to  compressional stress on the Heyuan in the Cenozoic led to the development of cross-cutting strike-slip faults and associated vertical fracture network. Here, we present multiscale observations and analyses from the earlier long-term extensional phase.</p><p>Detailed microstructural analyses identified a 'quartz-reef window' of formation occurring between ~200-350˚C, linking in both quasi-static criteria (accommodation space; massive fluid sources; and a cap rock/seal)  and dynamic mechanisms (episodic-dynamic permeability; brittle-ductile cycles; and fluid injection though brittle-ductile equivalent of Sibson's 'fault-valve' behaviour.</p><p>This oscillatory brittle-ductile fault-valve is recorded in the field through its apparent contradiction between idiomorphic 5 cm long quartz crystal growth in mode-I fractures, embedded at large-scale inside far from equilibrium fault zones with mylonitic and cataclastic microstructures. Another characteristic feature is the increasing quartz vein frequency towards the core shown by enrichment of SiO<sub>2</sub>, with depletion of K<sub>2</sub>O and  Na<sub>2</sub>O in tectonites during alteration from the host granite; a reaction partly sourcing the SiO<sub>2</sub> for the quartz reef.<br><br>We present a first theoretical model compatible with the observation of oscillatory macroscale far from equilibrium conditions, followed by long periods of micro-scale local equilibrium. The model can in particular describe mechanisms of abundant SiO<sub>2</sub> dominated fluid release reaching episodically above hydrostatic pressures followed by long periods of SiO<sub>2</sub> precipitation, allowing growth of up to 5 cm long idiomorphic quartz  crystals in subparallel open channels, which presumably were held open by high fluid pressures. In this interpretation, the observations instabilities are seen to stem from the multiscale and multiphysics of the mineral reactions at the brittle-ductile transition, promoted by a slow extensional geodynamic driver at the Heyuan fault.<br><br>The new approach allows interpretation of rock physics properties in terms of recently discovered Thermo-Hydro-Mechanical-Chemical (THMC) multiscale wave-like instabilities. In the model short wavelength chemical dissolution-precipitation reaction waves are bouncing between the phyllonitic cap rock and the mylonitic shear zone below. A resonance phenomenon of constructive interference in a finite width around the future quartz-reef triggers the long-time scale steady-state attractor allowing quartz reef growth over geodynamic time scales. We show that this solitary wave limit forms a standing wave matching the characteristic periodic pattern of mode-I quartz veining around the reef and also explaining the fluid overpressures leading to local hydro-fracturing.</p>