Ground Penetrating Radar Reveals Differences in Fracture Spacing Across Two Geochemically Distinct Granitic Lithologies

Abstract:

Weathering at Earth’s surface breaks rock down, liberating mineral-bound nutrients and producing a blanket of loose regolith that serves as a porous, life-sustaining substrate for overlying ecosystems. Lithology is a long-recognized regulator of weathering due to variations in bedrock composition and fracture spacing. However, while the role of bedrock composition has been widely studied, the role of fracture spacing has received comparatively little attention. This is because fracture spacing is difficult to quantify, particularly in regolith-mantled landscapes, where fractures are shielded from direct observation by overlying soil. We overcame this limitation by quantifying fracture density, orientation, and length using ground penetrating radar (GPR) at two forested, non-glaciated, granitic sites in the Sierra Nevada, California. The sites differ in bedrock composition, but are just ~7 km apart at roughly the same elevation. Hence, they have roughly the same climate and a similar history of cooling and exhumation through the crust. However, the sites differ markedly in regolith thickness and forest cover, suggesting that the differences in lithology drive differences in weathering and ecosystem productivity. To explore this possibility, we collected multiple common offset GPR profiles with a 100 MHz antenna at each site. Reflections are visible beyond 400 ns, at depths greater than 24 m (for velocity = 0.12 m/ns). We observe long (>40 m) continuous reflections surrounded by distinct regions with no reflections at the site with less forest cover, less biotite, and thinner regolith. In contrast, at the site with greater forest cover, more biotite, and thicker regolith, the GPR profiles have more reflections, and these reflections are all short and sporadic, suggestive of greater fracture density. Our results are consistent with a lithologically mediated feedback in which greater fracture spacing enhances mineral dissolution and tree growth by promoting greater storage and throughflow of water, which are further increased by the opening of pore space caused by dissolution and root wedging. However, more work is needed to resolve the relative importance of fracture spacing and other lithologic controls on weathering, which include nutrient content and mafic mineral concentrations at these sites.