科研之友
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摘要
Detailed, high quality, marine total field magnetic data recently acquired over parts of the South Atlantic Ocean off the southwestern margin of South Africa display a pattern of well-defined, NW-SE striking linear magnetic anomalies that can be traced with confidence over distances >150 km. The magnetic anomalies are interpreted to be M-series seafloor spreading anomalies M9 to M11, which are consistent with the initiation of seafloor spreading at approximately 135 Ma (Late Valanginian/Early Hauterivian). Seafloor spreading models indicate a more rapid (44 mm/yr) initial spreading phase between M11 and M4/M5 followed by slower (29 mm/yr) spreading from M4/M5 to MO. This two rate spreading model also matches M-series anomalies previously reported over the conjugate South American margin offshore Argentina where the rates are slightly ( < 10%) slower. The presence of M11 anomalies over both margins suggests an earlier opening of the southern South Atlantic basin than previously recognized.
Breaks in the continuity of the linear anomaly pattern, observed in map view, are oriented approximately NE" SW and are considered sites of possible fracture zones. One such discontinuity, which we have termed the "Cape Lineament" (CL), marks a significant change in crustal character and Cretaceous depositional history, as revealed by gravity data and seismic reflection data, respectively. Crust NW of the CL, in the Orange Basin, is characterized by greater thickness and the presence of seismically-imaged seaward dipping reflectors (SDRs) whereas SE of the CL the crust has a more "normal"oceanic thickness and SDRs are either absent or more limited in areal extent. Magnetic, gravity and seismic data were combined to develop a crustal model of the South African margin north of the CL that includes: (a) a region of rifted/attenuated continental crust landward of magnetic anomaly G, (b) a 60-80 km wide zone of substantially intruded, underplated thin continental crust overlain by SDRs between anomalies G and M11, and (c) a 90 km wide zone of thick oceanic crust associated with a wide intrusion zone and smoother seafloor spreading magnetic anomalies that thins progressively to normal oceanic crustal thickness at the seaward edge of the overlying SDRs. South of the CL, the change in crustal character from attenuated continental crust to normal thickness oceanic crust occurs over a much shorter distance of approximately 35-40 km, possibly indicating the diminished influence of magmatic material. Although linear magnetic anomalies are observed both NW and SE of CL, anomalies to the SE display a better correlation with those predicted by our seafloor spreading model.
We have successfully reconstructed the positions of the African and South American margins south of 32 S at M11 time (i.e., similar to 135 Ma) using a new rotation pole at 38.86 N, 31.46 W. This pole applies only to the southernmost margins of the Austral segment where Mll is observed. Further north at this time the margins are undergoing non -rigid deformation that includes both crustal extension and magmatic underplating.
- 出版日期2018-8
