Paleomagnetism of Jurassic plutons in the central Klamath Mountains, southern Oregon and northern California Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1831cn082

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  • An understanding of the tectonic history of the Klamath Mountains is crucial for a valid paleogeographic reconstruction of the Pacific Northwest. However, prior to this study there were very few paleomagnetic (PM) data from the Klamath Mountains (KN), which resulted in conflicting interpretations about the role of the KM province in the tectonic evolution of western North America. Twenty-eight sites from five unmetamorphosed Middle Jurassic KM plutons with K-Ar ages ranging from 161 to 139 m.y.B.P. yielded stable PM results showing (1) a direction for the 160 in.y.B.P. Ashland pluton (D=324°, I=163°, α₉₅=8°, n=6) nearly concordant with the coeval expected direction (D=337°, 1=54°) and (2) clockwise rotated directions for the plutons of Grants Pass (D=045°, I=67°, α₉₅=12°, n=4), Greyback (D=083°,I=63°, α₉₅=9, n=9), and the Wooley Creek batholith and Slinkard pluton combined (D=037°, 160°, α₉₅=ll°, n=9). Tectonic interpretations of these PM data are difficult; two interpretations are offered to explain the observed directions. In the first, the mean PM direction of the four plutons with discordant directions (D=057°, 1=65°, α₉₅=7°, n=22) is restored to the expected 150 m.y.B.P. (the average K-Ar age for these four plutons) direction by rotation of a rigid block ~87° in a counterclockwise sense about a vertical axis (the possibility of tilt of these four plutons is disregarded in this interpretation). The Ashland pluton which shows no rotation is problematic. Either there was (is) a tectonic boundary west of the Ashland pluton, separating it from the rotation of the others, or the Ashland pluton was influenced both by clockwise rotation and tilt, the combined effect producing an essentially concordant PM direction. In the second interpretation we distinguish between the northern KN, intruded by the Grants Pas and Greyback Mountain plutons, and the southern region intruded by the Wooley Creek batholith and the Ashland and Slinkard plutons. The bases for this distinction are recent geologic and gravity studies which suggest that post-Middle Jurassic uplift of the domal Condrey Mountain Schist may have caused radially outward tilt of its adjacent terranes and plutons intruded therein, causing some of the observed discordances in their PM directions. Thus, in the second interpretation it is envisioned that (a) the northerly portion of the KM, intruded by the Grants Pass and Greyback plutons, was affected primarily by clockwise rotation about a vertical axis, and (b) discordant directions for the remaining plutons intruded farther south are due primarily to tilt in response to Condrey Mountain uplift. Based on the observed inclinations, there is no evidence of transport of the Klamath Mountain province along lines of longitude since Middle Jurassic time. Tectonic interpretations of the PM results of this study are consistent with significant post-Middle Jurassic clockwise rotation of the Klamath Mountains. The first interpretation above yields ~87° of clockwise rotation of the terrane examined. According to the second interpretation, a clockwise rotation of ~l00° is inferred from the average of the PM results of the northern Grants Pass and Greyback plutons. Therefore, 10° to 25° of clockwise rotation of the KM may have occurred prior to the formation of the Oregon Coast Range (~55 m.y.B.P.) and the two provinces may have rotated together since post-Lower Eocene time.
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  • File scanned at 300 ppi (Monochrome, 8-bit Grayscale) using ScandAll PRO 1.8.1 on a Fi-6670 in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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