Structural control of weathering processes within exhumed granitoids: Compartmentalisation of geophysical properties by faults and fractures

Abstract

In the latter stages of exhumation processes, rocks undergo weathering. Weathering halos have been described in the vicinity of structures such as faults, veins or dykes, with a lateral size gradually narrowing with depth, symmetrically around the structures. In this paper, we describe the geophysical characterisation of such alteration patterns on two granitoid outcrops of the Catalan Coastal Ranges (Spain), each of which is affected by one major fault, as well as minor faults and fractures. Seismic, electric and ground penetrating radar surveys were carried out to map the spatial distribution of P-wave velocity, electrical resistivity and to identify reflectors of electromagnetic waves. The analysis of this multi-method and complementary dataset revealed that, at shallow depth, geophysical properties of the materials are compartmentalised and asymmetric with respect to major and subsidiary faults affecting the rock mass. This compartmentalisation and asymmetry both tend to attenuate with depth, whereas the effect of weathering is more symmetric with respect to the major structure of the outcrops. We interpret such compartmentalisation as resulting from the role of hydraulic and mechanical boundaries played by subsidiary faults, which tend to govern both the chemical and physical alterations involved in weathering. Thus, the smoothly narrowing halo model is not always accurate, as weathering halos can be strongly asymmetrical and present highly irregular contours delimiting sharp contrasts of geophysical properties. These results should be considered when investigating and modelling fluid storage and transfer in top crystalline rock settings for groundwater applications, hydrocarbon or geothermal reservoirs, as well as mineral deposits. .

Review: Hydrogeology of weathered crystalline/hard-rock aquifers—guidelines for the operational survey and management of their groundwater resources

“…Authors such as Chilton and Smith-Carington (1984); Acworth (1987); Sander (1997); Owen et al (2007); Dewandel et al (2011) and Place et al (2015), describe preexisting heterogeneities within the HR-such as quartz, pegmatite and aplite veins; dolerite dykes, ancient faults and joints; and contacts between lithological units (Neves and Morales 2007;Le Borgne et al 2004Durand et al 2006;Roques et al 2016), that act as sites for preferential weathering. At such sites, local deepenings of the weathering profile are reported to reach several hundreds of metres below ground surface (Roques et al 2014b).…”

Comment on se-2021-118

“…The effective fracture aperture largely governs the hydraulic conductivity of fractures. Due to exhumation and weathering processes, measured aperture values at near-surface outcrops are usually not reliable (Place et al, 2016). Instead, an exponential distribution of the apertures was assumed, and three possible mean values (10 μm, 50 μm and 100 μm) were tested (Sausse and Genter, 2005b).…”

Structural Architecture and Permeability Patterns of Crystalline Reservoir Rocks in the Northern Upper Rhine Graben: Insights from Surface Analogues of the Odenwald

“…Basin-scale studies show the importance of fault zones, in the compartmentalisation of fluid flow and control, on the architecture of a geothermal system in sedimentary and crystalline rocks [8][9][10][11]. However, datasets of quantified structural network properties and their local influence on flow properties within crystalline units are only sparsely available in the literature [12][13][14][15][16]. The petrophysical and geo-mechanical behaviour of crystalline rocks has been investigated in numerous studies at the sample scale in laboratory conditions [17,18].…”

“…Significant structures and lithological boundaries within the basement have Energies 2022, 15, 1310 2 of 30 been determined and modelled by geophysical methods [3,19,20]. Possible architectures of the fracture network and some conceptual models also exist [14,[21][22][23]. In the area of interest, i.e., the Northern Upper Rhine Graben and its crystalline shoulders, some local studies characterise the fracture network at the outcrop scale in the granodioritic [15] and granitic basement [24].…”