TECTONIC EVOLUTION OF THE CENOZOIC OLYMPIC SUBDUCTION COMPLEX, WASHINGTON-STATE, AS DEDUCED FROM FISSION-TRACK AGES FOR DETRITAL ZIRCONS

被引:225
作者
BRANDON, MT [1 ]
VANCE, JA [1 ]
机构
[1] UNIV WASHINGTON,DEPT GEOL SCI,SEATTLE,WA 98195
关键词
D O I
10.2475/ajs.292.8.565
中图分类号
P [天文学、地球科学];
学科分类号
07 ;
摘要
The Olympic subduction complex (OSC), exposed in the Olympic Mountains of northwest Washington State, consists of an imbricated assemblage of Cenozoic sandstone, mudstone, and minor pillow basalt. The tectonic evolution of the OSC has been difficult to resolve because of poor age control, especially for the more easterly parts of the complex. The fission track (FT) method is used here to date detrital zircons from sandstones of the OSC in order to define better the timing of deposition, subduction accretion, and metamorphism. The external detector method was used to determine grain ages for individual detrital zircons. For unreset samples, the detrital zircons retain pre-depositional FT ages, which are related to the thermal history of the source region from which the zircons were derived. The samples are inferred never to have seen temperatures in excess of about 175-degrees to 185-degrees-C. For these samples, determination of the FT age of the youngest population of grain ages is especially useful because it represents the maximum age for deposition of the sediment. For reset samples, maximum temperatures are inferred to have exceeded 240-degrees 245-degrees-C, so that the zircons were annealed after deposition. Zircon grain ages from these samples show a restricted range of ages which can be used to define a cooling age for the rock following metamorphism. We report FT results for 19 sandstone samples with a total of 928 grain ages. Of the 15 samples from the OSC, 11 are unreset, and 4 are reset. The remaining 4 samples are from unmetamorphosed Eocene basin sequences to the east and southeast of the OSC and are included here for purposes of comparison. The statistical methods outlined in Brandon (this issue) are used to interpret each of the unreset grain-age distributions. The chi2 age method estimates the FT age of the youngest population of "plausibly related" grains, defined as the chi2 'age. The peak-fitting method is used to decompose an entire grain-age distribution into a set of component grain-age populations, each of which is distinguished by a peak age, defined as the average FT age of the component population. The unreset Eocene basin samples have chi2 ages of 51 to 39 Ma. Comparisons between chi2 ages and independently determined depositional ages indicate a lag time of less than approximately 5 my between the cooling age of the youngest zircons and deposition. Thus, the chi2 age is assumed to be a reasonable proxy for the depositional age of the sandstone. The chi2 age usually conincides with the youngest peak age as determined by the peak-fitting method. Thus, these ages correspond to a specific population of young zircon grains. We refer to this young population as a moving peak because the age of the population appears to shift with but lag slightly behind the depositional age of the sandstone. A likely source for these zircons is the Cascade volcanic arc, which has been active continuously from approximately 36 Ma to present. All the unreset samples from the OSC and the Eocene basin localities contain a common set of older peaks, which we call static peaks because they remain relatively constant in age regardless of the depositional age of the sample. The peak-fitting method indicates three static peaks with ages of 43, 57, and 74 Ma. Possible source areas for the static peaks include unreset Late Cretaceous plutons and Eocene reset terrains located on the eastern side of the Omineca belt and in the North Cascade Mountains. The relatively constant age of the static peaks from sample to sample provides further evidence that samples designated as unreset have not been affected to any significant degree by partially resetting after deposition. The FT results for the OSC samples define a concentric pattern with reset samples (cooling age = 14 Ma) restricted to the central part of the OSC, unreset Upper Oligocene and Lower Miocene clastic rocks (chi2 = 27 to 19 Ma) lying around the perimeter of the reset area, and unreset Middle Eocene to Lower Oligocene clastic rocks (chi2 = 48 to 32 Ma) at a still greater distance. This pattern is interpreted as part of a domal structure that formed during emergence an uplift of the modern Olympic Mountains, starting at approximately 12 Ma. The reset region in the central OSC represents the youngest and most deeply exhumed part of the OSC. It lies in the center of the Olympic Mountains and coincides with the area of highest topographic relief. Rocks in this area are interpreted to have been accreted to the base of the accretionary wedge at about 17 Ma and to have been metamorphosed at temperatures greater than about 240-degrees to 245-degrees-C and al a dept of ab ut 12 km. At about 14 Ma, these rocks cooled below the closure temperature approximately 239-degrees-C at a rate of about 16.5-degrees-C/my. Erosion of the surface of the wedge began at about 12 Ma when the wedge first became subaerially exposed. Erosional unroofing of the wedge was apparently driven by continued underplating and ductile thickening within the wedge. About 12 km of rocks were removed in approximately 12 my, indicating an average erosion rate of approximately 1 km/my. At present, the central reset area is underlain by another 30 km of accreted sedimentary materials, which indicates an average rate of wedge thickening of 1.75 km/my over the last 17 my.
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页码:565 / 636
页数:72
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