Detrital zircon U-Pb ages of a late-Variscan Carboniferous succession associated with the Porto-Tomar shear zone (West Portugal): provenance implications

Laser ablation ICP-MS U-Pb zircon ages are used to investigate the sediment provenance of a late-Variscan Carboniferous succession from Buçaco Basin. The Buçaco Basin is associated with the Porto-Tomar Shear Zone (PTSZ), which is a major transcurrent Variscan structure and tectonic boundary within Iberia. The clastic succession is interpreted to be deposited within a pull-apart basin, where alluvial fans radiating from surrounding range fronts grade into an axial valley. Coarse-grained sediments attributed to short streams that drained the eastern basin margin exhibit mostly Pan-African ages. In contrast, the sandy sediments associated with longer fluvial systems tend to have higher proportions of Grenvillian and Paleozoic ages. Limited amounts of Variscan zircons were found in upper beds. These results suggest that the intermontaine Buçaco Basin had a catchment area that increased progressively eventually exceeding the range front. It is proposed that rivers striking parallel to the PTSZ played a major role in supplying Grenvillian and Paleozoic ages while lateral supply sourced mainly Pan-African ages. Given the prevalence of short sediment supply, the proportion of Variscan grains is always limited. The higher proportion of Mesoproterozoic ages in some Carboniferous beds than has been presented for central west Iberia indicates that nearby basement units enriched in Grenviilian zircons were exhumed during the Variscan orogeny.