Plate kinematics of the central Atlantic during the Oligocene and early Miocene

A new plate motions model for the northwest Africa–North America Plate pair during the Oligocene and early Miocene is presented. The model is accompanied by a high-resolution isochron map for the central Atlantic region, resulting from a re-examination of 423 ship tracks from the NGDC data base for the area between the 15°20′ FZ and the Azores triple junction. A new digital model of fracture zones for this region and a set of 309 magnetic profiles crossing the Oligocene to recent oceanic crust within the study area allowed to determine accurate finite reconstruction poles for the North America–northwest Africa conjugate plate pair between the early Miocene (Chron 6) and the early Oligocene (Chron 13). For times older than Chron 7 (∼25 Ma), the finite reconstruction poles were calculated using a reliable data set coming exclusively from the region south of the Canary Islands FZ (∼32°N), which allowed to test the rigidity of the northwest African oceanic lithosphere during the Oligocene–early Miocene phase of Atlas orogeny. A comparison of theoretical magnetic isochrons with observed magnetic lineations systematically shows that anomalously high spreading rates occurred in the area north of the Canary Islands FZ before Chron 7, thereby suggesting that the formation of the Atlas mountain, rather than being a localized intracontinental process, was logically linked to the central Atlantic spreading history. Thus, an independent Moroccan Plate could have existed during the Oligocene–early Miocene time interval, which included both the oceanic lithosphere north of the Canary Islands FZ and the northern Maghrebian areas of Morocco, Algeria and Tunisia. In this eventuality, the Atlas mountain belt should be reinterpreted as a giant flower structure associated with dextral transpression

Breakup of Pangaea and plate kinematics of the central Atlantic and Atlas regions

A new central Pangaea fit (type A) is proposed for the late Ladinian (230 Ma), together with a plate motions model for the subsequent phases of rifting, continental breakup and initial spreading in the central Atlantic. This model is based on: (1) a reinterpretation of the process of formation of the East Coast Magnetic Anomaly along the eastern margin of North America and the corresponding magnetic anomalies at the conjugate margins of northwest Africa and the Moroccan Meseta; (2) an analysis of major rifting events in the central Atlantic, Atlas and central Mediterranean and (3) a crustal balancing of the stretched margins of North America, Moroccan Meseta and northwest Africa. The process of fragmentation of central Pangaea can be described by three major phases spanning the time interval from the late Ladinian (230 Ma) to the Tithonian (147.7 Ma). During the first phase, from the late Ladinian (230 Ma) to the latest Rhaetian (200 Ma), rifting proceeded along the eastern margin of North America, the northwest African margin and theHigh, Saharan and Tunisian Atlas, determining the formation of a separate Moroccan microplate at the interface between Gondwana and Laurasia. During the second phase, from the latest Rhaetian (200 Ma) to the late Pliensbachian (185 Ma), oceanic crust started forming between the East Coast and Blake Spur magnetic anomalies, whereas the Morrocan Meseta simply continued to rift away from North America. During this time interval, the Atlas rift reached its maximum extent. Finally, the third phase, encompassing the time interval from the late Pliensbachian (185 Ma) to chron M21 (147.7 Ma), was triggered by the northward jump of the main plate boundary connecting the central Atlantic with the Tethys area. Therefore, as soon as rifting in the Atlas zone ceased, plate motion started along complex fault systems between Morocco and Iberia, whereas a rift/drift transition occurred in the northern segment of the central Atlantic, between Morocco and the conjugate margin of Nova Scotia. The inversion of the Atlas rift and the subsequent formation of the Atlas mountain belt occurred during the Oligocene–early Miocene time interval. In the central Atlantic, this event was associated with higher spreading rates of the ridge segments north of the Atlantis FZ. An estimate of 170 km of dextral offset of Morocco relative to northwest Africa, in the central Atlantic, is required by an analysis of marine magnetic anomalies. Five plate tectonic reconstructions and a computer animation are proposed to illustrate the late Triassic and Jurassic process of breakup of Pangaea in the central Atlantic and Atlas regions

Editorial Style of Scientific Works: Disagreement and Contrast in the Correspondence Govi-Boncompagni

The second half of the nineteenth century in Italy bore witness to a number of scientific projects of great cultural importance that were unique on the European landscape. The correspondence between Govi and Boncompagni testifies to a scientific partnership throughout the twenty years’ life of the «Bullettino di Bibliografia e di Storia delle Scienze Matematiche e Fisiche» (1868-1889). Both these scientists, as scholars of historical studies, bibliophiles and collectors, contributed to the spread of scientific literature and history. Nevertheless they did not agree about the editorial style of the scientific works