Gilda Lopes

Superimposed thermal histories in the southern limit of the Ossa Morena Zone – Portugal

The Mississippian volcano-sedimentary complex in the Toca da Moura – Cabrela areas represents remnants of intra-volcanic marine sedimentary basins, formed during the collision between the Ossa Morena Zone with the South Portuguese Zone. These rock units are unconformably overlain by the Pennsylvanian intramontane coal-bearing Santa Susana Basin. Vitrinite reflectance determinations from rocks of these two basins indicate two episodes of thermal maturation. During the first episode, the Toca da Moura – Cabrela volcano-sedimentary complexes attained high maturation levels, equivalent to anthracite coal rank (3.0–3.5% R oran ), which pre-dates the middle Moscovian Santa Susana Basin. The Santa Susana Basin attained moderate maturation levels equivalent to bituminous coal rank (1.35–1.5% R oran ) recording a second episode of thermal maturation. Here, peak thermal conditions did not overprint the first maturation episode. The observed effects of magmatic intrusion on the thermal maturity and the lack of any increase in vitrinite reflectance with depth through c. 400 m of section in borehole SDJ-1 indicate high geothermal gradients during the first maturation episode. A contemporaneous magmatic event associated with the c. 335–320 Ma Cuba-Alvito Gabbros/Diorites of the Beja Massif was the possible cause for the high geothermal gradients postulated for the first maturation episode. Burial under a post-upper Moscovian sedimentary cover was the most likely process to account for the maturation levels determined for the Santa Susana Basin and for the second episode of thermal maturation.