Age and Origin of the Messejana Dolerite Fault-Dike System (Portugal and Spain) in the Light of the Opening of the North Atlantic Ocean

Abstract

The western Iberian Peninsula is transected by a northeast-striking dike system into which the Messejana Dolerite, at least 530 km long, was emplaced along the Messejana Fault. This left-lateral wrench fault drags, cuts and offsets (up to 4 km) the Hercynian structures and granites. The Messejana Dolerite magma intruded when dilatational fractures formed both along the fault and locally on either side of it. Deep-reaching fissures permitted the rise of tholeiitic magma. The dolerite dikes are undeformed and have contact-metamorphosed the folded, cleaved and regionally metamorphosed wall rocks. Locally, the fault zone was rejuvenated once or several times from the middle Tertiary onward, with normal faulting. The original wrench fault determined the course of later dislocations under different stress patterns. K-Ar whole-rock dating gives ages of Ma (Middle Jurassic) for samples from South Portugal and Ma (Early Jurassic) for samples from Spain. Other published K-Ar dates, both from the Messejana Dolerite and other dike suites along the Atlantic Ocean, fall in the same age range. The Messejana Fault originated probably during the initial rifting leading to continental breakup, in Early to Middle Triassic times. The Messejana Dolerite resulted from multiple intrusion during the Early and Middle Jurassic, possibly starting already in the latest Triassic, and spanning the initial rifting and subsequent spreading stages.