Because the recognition of sea-floor anomalies that are spreading dates the postrift development of ocean crust. Usually the very first clear marine magnetic anomalies are found instead far seaward from the margin, due either towards the existence of instead weak anomalies of uncertain beginning nearer to the margin (southern Newfoundland and Labrador margins) or even to the possible lack of magnetic reversals (Scotian and north Newfoundland margins) throughout the Jurassic and Cretaceous Normal Polarities (
210-160 Ma and 118-83 Ma, respectively). More particular times for rifting would originate from exposures on land and/or drilling of syn-rift sequences that are sedimentary. Other quotes could be created by extrapolating the prices of sea-floor spreading to your margin or by dating of sedimentary sequences or stones on land.
Such times claim that rifting of this older margins could have happened over a extensive duration before the forming of ocean crust and can even have affected adjacent margin sections. Initial rifting began as soon as the belated Triassic to Early Jurassic, as evidenced by a wide-spread volcanic pulse understood while the CAMP occasion at 200 Ma (Marzoli, 1999) while the existence of rift successions experienced in marginal basins ( ag e.g. (more…)