Fossil Palaeoweathering Profiles and their Relation to Deformation at Basement-Cover-Interfaces : Case studies from Israel, Sweden and Spain

Abstract

Detachment horizons are associated with basement-cover-interfaces in many regions of compressive or extensional regimes. The fabric evolution and alteration of ancient fossil palaeoweathering profiles and their relation to deformation at basement-cover-interfaces are the object of this thesis. Three cases of palaeoweathered granite and one case of folded sedimentary basement rocks are investigated by means of structural, mineralogical, whole-rock geochemical and magnetic susceptibility analyses. In the case study Negev, South Israel, the deformed Roded Granite (Pan-African basement) weathered under warm and humid to (semi-) arid conditions to a saprock-saprolite-laterite sequence, buried beneath Cambrian red beds. The laterite was compacted beneath 2.8 ± 0.1 km overburden to 73% of its original thickness (plane strain). In the case studies Langviken and Hara, Central Swedish Caledonian margin, the deformed Revsund Granite (Fennoscandian basement) weathered in a temperate to cold climate to weathering-breccias, which were buried beneath sequences of Vendian conglomerates and Cambrian black shales, respectively. Caledonian detachment tectonics overprinted and duplicated the autochthonous weathering-breccia in basement slices under anchizonal cataclasis. In the case study Narcea Antiform, Cantabrian Mountains in North Spain, sedimentary basement rocks, folded during the Cadomian orogeny, weathered moderately without significant textural changes in an arid climate and were buried beneath Cambrian siliciclastics. The Variscan Orogeny overprinted the rocks under regional anchimetamorphic conditions. It reactivated the angular unconformity, locally under higher metamorphic conditions causing ductile quartz deformation. The intensity of weathering features, which are preserved in all cases, is a function of proximity to the unconformity. Hydrolysis of Na-feldspar and chlorite is the predominant chemical weathering process, leading to clay precipitation, whole-rock Al-enrichment and Na-, Ca-leaching. SiO2-dissolution and partly reprecipitation is locally observed, as well as magnetite dissolution (Langviken, Negev), martitization of magnetite (Negev) and microcrystalline haematite precipitation (Negev and Narcea Antiform). The bulk susceptibility (kappa_bulk) can be used as a tracer of weathering intensity. However, the alteration of magnetic carriers has to be known. The particular weathering of ferrimagnetic lithologies led to a decrease of kappa_bulk, whereas weathering of paramagnetic lithologies caused a kappa_bulk-increase. kappa_bulk was overprinted in particular horizons, e.g. by the passive enrichment of Fe-phases during compaction of the Negev laterite and by ferrimagnetic pyrrhotite precipitation in Langviken cataclasite. The magnetic field dependence of pyrrhotite-bearing rocks is dependent on pyrrhotite grain size, but also on its modal fraction. A corrected field dependence parameter is calculated, in order to eliminate the influence of the matrix susceptibility. The obliteration of primary petrofabrics by palaeoweathering mainly depends on the intensity of physical brecciation and not necessarily on the intensity of chemical alteration. Fabrics have been lost in the Swedish cases due to weathering-brecciation and associated grain size reduction. Granitic foliations are preserved in the absence of grain size reduction (Narcea Antiform) and even in a laterite, where extreme hydrolysis and dissolution processes took place (Negev). Unconformity-parallel compaction fabrics developed by pressure solution and rotation of phyllosilicates in clay-rich weathering-zones during the diagenetic stage. Intensity of compaction depends on the burial depth, the weathering mineralogy (clay-richness) and the porosity. A general positive covariance between the intensities of palaeoweathering and horizontal fabric is suggested, which is reflected by a stronger horizontal fabric imprint with proximity to the unconformity. AMS fabrics are results of the superposition of the compaction fabric with primary (older than compaction) or secondary (younger than compaction) fabrics. The gradual change of compaction fabrics is quantified in the AMS by a systematic decrease of the magnetic anisotropy P’. Flat-lying magnetic lineations, associated with unconformity-parallel fabrics, appear to be a common feature in fossil palaeoweathering profiles, which were subject to at least moderate textural rearrangement. Primary and secondary contrasts of lithologies above and beneath the unconformity developed during the evolution of the basement-cover-interfaces. Primary refers to the lithological differences (palaeoweathering, sedimentary lithology) and the unconformity topography, whereas secondary refers to texture-modification and mineral alterations related to diagenesis and later processes (e.g. by compaction, fluid flow). Collectively, most of the contrasts lowered the shear strength of the palaeoweathering zones and created a permeability-anisotropy along the unconformity. Overpressured fluids beneath the “sealed” unconformities and increased horizontal shear stresses enabled (approximately unconformity-parallel) rock failure in the palaeoweathering zone (Sweden) or frictional sliding along the unconformity (Narcea A.). Showing common characteristics of textural evolution, fluid-rock-interaction and associated shear-strength weakening, this thesis demonstrates the importance of fossil palaeoweathering zones for upper crustal detachment tectonics, as well as for the characteristics of fluid-flow in the basement and related element mobilization and potential ore formation.