Because the recognition of sea-floor anomalies that are spreading dates the postrift development of ocean crust. Usually the very very first marine that is clear anomalies are found instead far seaward from the margin, due either towards the presence of instead poor 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) through the Jurassic and Cretaceous Normal Polarities (
210-160 Ma and 118-83 Ma, correspondingly). More certain times for rifting would originate from exposures on land and/or drilling of syn-rift sedimentary sequences. Other quotes could be produced by extrapolating the rates of sea-floor spreading to your margin or by dating of sedimentary sequences or stones on land.
Such times declare that rifting associated with older margins might have taken place over a extensive duration before the synthesis of ocean crust and might have impacted adjacent margin portions. Initial rifting began as soon as the belated Triassic to Early Jurassic, as evidenced by a wide-spread volcanic pulse known while the CAMP occasion at 200 Ma (Marzoli, 1999) together with existence of rift successions experienced in marginal basins ( ag e.g. Continue reading