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. / Graduation date: 1983
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29012 |
| Date | 11 February 1983 |
| Creators | Schultz, Karin L. |
| Contributors | Levi, Shaul |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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