4. APATITE FISSION TRACK ANALYSIS
OF SITES 959 AND 960
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Four samples of sandstone from deformed Aptian–Albian sediments and their less deformed Turonian cover at Sites 959 and 960 on the Côte d’Ivoire-Ghana Marginal Ridge yielded sufficient apatite for fission track analysis. Reduced mean track length values of 12.42 ± 0.16 µm to 13.67 ± 0.11 µm indicate that measured central ages, which range from 88 ± 4 Ma to 113 ± 4 Ma, have undergone partial resetting and are therefore apparent ages. Stochastic modeling of the fission track age and length parameters indicate that the samples have a strong predepositional signal, slightly modified by minor levels of prolonged postdepositional annealing (maximum temperatures <80°C). The preserved provenance-related signal records cooling through the apatite partial annealing zone (60°-110°C) during the Late Jurassic and Early Cretaceous, prior to 100 Ma (Albian). The absence of substantial postdepositional annealing suggests that the Marginal Ridge was not significantly heated by the passage of an oceanic spreading center along the transform margin during the Turonian (approximately equal to 90 Ma). Instead the fission track data record cooling in response to denudation of the sediment source terrain initiated during the intracontinental wrench phase of tectonism. The 110°C of cooling corresponds to approximately 4 km of denudation, assuming normal geothermal gradients, a figure that is comparable to measured erosion along the flanks of the northern San Andreas fault or inferred from the Dead Sea. |
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