Fabric development in a weathering profile at a basement-cover interface, the sub-Cambrian peneplain, Israel: Implications for decollement tectonics

The present study examines a fossil saprock-saprolite-laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S_1m (magmatic) and S_2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S_3d) with increasing intensity towards the unconformity. Bulk susceptibility (κ_bulk) and anisotropy (P') decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe-Mg-mica lead to increased κ_bulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay-/mica-fabric S_3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces.