Cretaceous palaeopositions of the Falkland Plateau relative to southern Africa using Mesozoic seafloor spreading anomalies

Abstract

Mesozoic seafloor spreading anomalies M0—M10 have been identified in the Natal Valley between the south-eastern African margin and the Mozambique Ridge. These identifications confirm that seafloor spreading north and south of the Falkland Agulhas Fracture Zone (FAFZ) began simultaneously. The Natal Valley anomalies are offset ∼ 1300 km by the FAFZ from their equivalents in the southern Cape Basin. Their positions confirm that the Tugela Ridge marks the continent/ocean boundary (COB) in the Natal Valley. We have computed successive Falkland Plateau palaeo-positions using Natal Valley and Georgia Basin anomalies. These show that the offset in spreading ridges at the FAFZ remained ∼ 1300 km long from M10 to M0 time. By anomaly 34 time, the offset was ∼ 1270 km. Therefore no major ridge jumps had occurred by then. Dating M0 as 108 Myr bp and anomaly 34 as 80 Myr bp, the average half-spreading rate immediately south of the FAFZ for the Cretaceous Normal Polarity Epoch is 4.2 cm yr-1. Using this, we date: (a) the change in early pole of rotation at 105 Myr; (b) a reconstruction which juxtaposes salt boundaries in the Brazil and Angolan basins at 103.7 Myr; (c) final separation of the Falkland Plateau from southern Africa at 98.3 Myr; (d) the formation of the oceanic northern part of the Agulhas Plateau at 97.3–90.7 Myr. Comparison of magnetic data with implied COB positions in the southernmost Cape and Argentine Basins, and the Georgia Basin suggests continental separation began 122–127 Myr bp with undeformed magnetic anomalies dating from 122 Myr (M10). These dates are consistent with micropalaeontological and sedimentological data around southern Africa.