DISCUSSION

The length of fission tracks in apatite provide the means of interpreting age: each length represents a different portion of the time-temperature history (tT) story, and length distributions record the integrated thermal history. In the data from Site 976 there is a high degree of similarity of track-length data with mean track lengths (MTL) that range from 14.78 0.21 m to 13.83 0.24 m and standard deviations from 0.83 to 1.29 (Table 1). Distributions shown in Figure 1 also indicate an absence of short tracks <10 m (except Sample 161-97R-2 (Piece 1), the sample with the shortest MTL). Long track lengths (>14.5 m) and tight distributions are characteristic of rapid cooling, such as found in thermally undisturbed volcanic rocks (Gleadow et al., 1986). The track-length data, together with the homogeneous, low-dispersion ages, provide strong evidence for rapid cooling of the Site 976 samples from temperatures above total track annealing to temperatures <60C at the time given by measured age: ~18-19 Ma.

Numerical modeling of the data has used a form of Monte Carlo simulation to select four time-temperature points from within the bounds 30 30 Ma and 60 60C. This thermal history, together with a present-day temperature of 30 10C, was used to predict FT parameters. Those predictions were then quantitatively compared to the observed values. Time-temperature points were selected randomly to start with, after which a genetic algorithm was used (Gallagher, 1995). A total of 5000 thermal histories were tested. Figure 3A shows the ten modeled runs whose predicted track ages and length parameters fit best with the measured values. In each case the passage through the partial annealing zone (PAZ) is similar, with cooling from 120 to 60C in about 2 Ma. Figure 3B shows the overall best-fit modeled run, which shows a very close agreement between measured and predicted length distributions and ages. (Note that the subsequent variation at temperatures <60C is irrelevant because no significant track annealing occurs at these temperatures.)

Details of thermal history are often revealed where individual samples yield differences in measured age and track length. Each sample at a different crustal level in effect preserves an individual record of part of the overall regional thermal history (e.g., see Lewis et al., 1992). Similar plots of the age and length data measured for the Site 976 samples (Fig. 4) show no such trends, which argues against complexity and favors a relatively simple thermal history. Although the samples come from within a faulted and fault-bounded horst block of Alboran Sea basement, the FT data suggest a similar thermal history for all the samples since early Miocene time, with rapid cooling at ~18-19 Ma succeeded by residence at <~60C. The basement in Hole 976B is directly overlain by Serravallian (11-12 Ma) sediment. Preservation of the FT record indicates negligible (probably zero) track annealing in the basement subsequent to the early Miocene cooling. Such a restriction to a maximum temperature of ~60C imposes limits on any additional sediment burial that might subsequently have been eroded.

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