磷灰石裂变径迹数据的热史反演及其局限性

应用一种局部优化方法——模拟退火法对磷灰石裂变径迹长度分布及径迹年龄进行热史的反演。反演中采取了Laslet等(1987)的扇型径迹退火模型。通过对理论热史模型及实测资料的反演,结果表明应用磷灰石裂变径迹数据提取热史信息是有效的。但是,由于径迹的退火主要受温度的控制,因此以前记录在径迹中的热史信息有可能被后期高温改造而难以反映。指出磷灰石径迹资料在冷却盆地的应用可以获取较精确的热史信息,而在有复杂演化史的盆地中精度较差。

INVERSE OF APATITE FISSION TRACK DATA FOR THERMAL HISTORY INFORMATION AND ITS LIMITATION

Apatite fisson track data(apatite track length distribution and fission track age)are used to restore thermal history information.In the inverse method,a local optimization technique(simulated annealing)which is able to climb out of local minima in search of a global minimum state with a certain probability is used to determine the range of thermal histories compatible with apatite fission track data,and the forward model is based on the fanning annealing model given by Laslett et al.(1987),and thermal history is simulated by Chebyshev polynomials. With the inverse method,two theoretical thermal histories and two observed data sets are used.At the first time,with the forward model,thoretical thermal histories are used to calculated track length distributions and track ages,then the results are inversed.The first model delegates an uplifting,cooling geological process,the cooling rate is 0.6℃/Ma,the inverse result fits the model very well,which indicates that the method is efficient for this kind of cooling process.The second model is more complex,which delegates that at the time of 50Ma the temperature was up to the maximum of 90℃,then cooling with uplifting process and temperature decrease,later followed by a burying process with an increasing temperature and finally it is 70℃ now.The inverse result reflects the trend of thermal history,and the maximum temperature of 90℃ at time of 50Ma is reflected too,but some local details have not been reflected accurately.Two observed samples come from Dongying Depression.The first observed data set is from fine sand in the Guangtou group of some well.The result indicates that at time of 50Ma,the temperature the apatite grains had experienced in source area was about 100℃ at that time,the Dongying depression was extented on large scale and the source area was hot,so the result is compatible with the actual process,but,again,some local details have not been reflected accurately.The result of the second observed data set is little like the first one. The discussion indicates that apatite fission track data are efficient in applying to restore thermal information.However,since the annealing of tracks is mostly controlled by temperature,the older thermal information accorded in tracks may be remade by later higher temperature and can not be reflected.This paper shows that objective thermal history information can be given by using apatite fission track data in cooling basins,while in those basins with complex thermal history,the results are not so ideal.