干旱区生态环境知识资源中心

Apatite fission track ages and confined track length distributions from the Old Woman Mountains area, southeastern California, record evidence for periods of rapid cooling during the Late Cretaceous following Mesozoic orogenesis and during Miocene crustal extension. Apatites from the Clipper, Marble, and Turtle Mountains give fission track ages between approximately 62 and approximately 69 Ma as a result of cooling of these upper crustal rocks after regional Late Cretaceous plutonism and metamorphism. In the Piute Mountains, apatite fission track ages decrease from > 70 Ma with mean track lengths of approximately 13.5-mu-m to < 20 Ma with mean track lengths of > 14-mu-m from west to east. Intermediate aged samples from the Piute Mountains have mean track lengths between 11 and 13-mu-m due to their extended residence in the apatite annealing zone during the early Tertiary. This pattern is the result of 20-degrees-30-degrees of westward tilting of the Piute Mountains block as indicated by the dip of Tertiary strata. The apatite data indicated that tilting occurred after approximately 18 Ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range. Knowing the amount of tilting and the apparent paleotemperatures across the Piute Mountains allows us to estimate that the synextensional geothermal gradient was 50-degrees +/- 20-degrees-C/km. Apatites from the northeastern Old Woman Mountains yield fission track ages of approximately 68 Ma with mean track lengths of 13.8-mu-m. These samples record cooling after batholith emplacement at 73 Ma. In the northwestern Old Woman Mountains, apatite fission track ages decrease to < 20 Ma and have mean track lengths approximately 15-mu-m indicating very rapid cooling. We interpret this age pattern to be the result of minor eastward tilting of the Old Woman Mountains block after 18-19 Ma or flexural unroofing of the northwestern Old Woman Mountains due to movement on the Ship Mountains fault. Extension in the Old Woman Mountains area occurred between 19 and 16 Ma, which is coincident with extension in the Colorado River Extensional Corridor and later than major extension in the Mojave Extensional Belt.