Paleomagnetism of intrusive rocks in the Coast Range of Oregon: Microplate rotations in middle Tertiary time

Beck, Myrl E.; Plumley, Peter W.

doi:10.1130/0091-7613(1980)8<573:poirit>2.0.co;2

Cited by 36 publications

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“…since at least 50 Ma (Simpson and Cox, 1977;Beck and Plumley, 1980;Magill et al, 1981;Grommé et al, 1986;Wells et al, 1998;McCaffrey et al, 2007). The SRV is rotated more than 70°, and successively younger strata are rotated lesser amounts.…”

Section: Html) Folds Are Parallel To Precollision Margin B-b′ Is a mentioning

confidence: 99%

Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot

et al. 2014

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Siletzia is a basaltic Paleocene and Eocene large igneous province in coastal Oregon, Washington, and southern Vancouver Island that was accreted to North America in the early Eocene. New U-Pb magmatic, detrital zircon, and 40 Ar/ 39 Ar ages constrained by detailed fi eld mapping, global nannoplankton zones, and magnetic polarities allow correlation of the volcanics with the 2012 geologic time scale. The data show that Siletzia was rapidly erupted 56-49 Ma, during the Chron 25-22 plate reorganization in the northeast Pacifi c basin. Accretion was completed between 51 and 49 Ma in Oregon, based on CP11 (CP-Coccolith Paleogene zone) coccoliths in strata overlying onlapping continental sediments. Magmatism continued in the northern Oregon Coast Range until ca. 46 Ma with the emplacement of a regional sill complex during or shortly after accretion. Isotopic signatures similar to early Columbia River basalts, the great crustal thickness of Siletzia in Oregon, rapid eruption, and timing of accretion are consistent with offshore formation as an oceanic plateau. Approximately 8 m.y. after accretion, margin parallel extension of the forearc, emplacement of regional dike swarms, and renewed magmatism of the Tillamook episode peaked at 41.6 Ma (CP zone 14a; Chron 19r). We examine the origin of Siletzia and consider the possible role of a long-lived Yellowstone hotspot using the reconstruction in GPlates, an open source plate model. In most hotspot reference frames, the Yellowstone hotspot (YHS) is on or near an inferred northeast-striking KulaFarallon and/or Resurrection-Farallon ridge between 60 and 50 Ma. In this confi guration, the YHS could have provided a 56-49 Ma source on the Farallon plate for Siletzia, which accreted to North America by 50 Ma. A sister plateau, the Eocene basalt basement of the Yakutat terrane, now in Alaska, formed contemporaneously on the adjacent Kula (or Resurrection) plate and accreted to coastal British Columbia at about the same time. Following accretion of Siletzia, the leading edge of North America overrode the YHS ca. 42 Ma. The voluminous high-Ti basaltic to alkalic magmatism of the 42-35 Ma Tillamook episode and extension in the forearc may be related to the encounter with an active YHS. Clockwise rotation of western Oregon about a pole in the backarc has since moved the Tillamook center and underlying Siletzia northward ~250 km from the probable hotspot track on North America. In the reference frames we examined, the YHS arrives in the backarc ~5 m.y. too early to match the 17 Ma magmatic fl are-up commonly attributed to the YHS. We suggest that interaction with the subducting slab may have delayed arrival of the plume beneath the backarc.

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Section: Html) Folds Are Parallel To Precollision Margin B-b′ Is a mentioning

confidence: 99%

Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot

et al. 2014

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“…Nearly all of these rocks had declinations of this the "magnetic terrain effect." They further suggest that as a remanent magnetization that indicated clockwise rotation (discussed consequence of the magnetic terrain effect, directions of remanent elsewhere by Wells and Heller [1988], Beck and Plumley [1980], magnetization of lava flows in general are suspect as indicators of and Magill and Cox [1981], among others). Beck's [1984] focus past directions of the geomagnetic field.…”

Section: Introductionmentioning

confidence: 99%

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Baag¹,

Helsley²,

Xu³

et al. 1995

J. Geophys. Res.

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In the summers of 1989 and 1991 we made 344 near‐ground level measurements of the ambient geomagnetic field above recent basalts on the island of Hawaii using a three‐component fluxgate magnetometer. We studied 12 surface features, including a lava pond, lava channels, long tilted blocks, smooth sloping surfaces, two fissures, and a deep U‐shaped road cut. We observed substantial differences (up to 20°) between the observed and expected (International Geomagnetic Reference Field, IGRF) magnetic field directions over these features except those composed of shelly pahoehoe and a flat (horizontally) thin lava pond. We also observed inclinations that were systematically shallower than the IGRF field by up to 5°. We show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island. We found that all of the observed inclination deflections can be explained by simple two‐dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain. Our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled. Since this information is rarely available, it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation (PSV). We therefore conclude that PSV dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows. We also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect. It is unnecessary to invoke ad hoc mechanisms such as clastic, block, or crustal rotations, distortion of the top crust, or flow deformation to explain the large between‐site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks. Our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks, as a systematic inclination deflection due to local and regional slopes and irregular terrain, such as those we observed, would lead to a corresponding error in. the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.

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“…This fact ordinarily is interpreted as evidence that large tracts of land, including the Oregon and Washington Coast Range (CR) and the Oregon and Washington Cascades, have rotated clockwise about a nearby vertical axis during Tertiary time. Evidence from the Oregon CR, in fact, shows quite clearly that rotation took place and, moreover, took place while CR rocks were in the process of formation [Simpson and Cox, 1977;Beck and Plumley, 1980;Magill et al, 1981]. Thirteen separate studies involving at least 14 distinct rock units now support clockwise rotation (summaries in the work of and Magill et al [1981]).…”

Section: Introductionmentioning

confidence: 99%

Paleomagnetism of small basalt exposures in the West Puget Sound Area, Washington, and Speculations on the accretionary origin of the Olympic Mountains

Beck¹,

Engebretson²

1982

J. Geophys. Res.

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Twenty‐two sites from two small exposures of basalt of probable Eocene age immediately east of the Olympic Mountains have the following mean paleomagnetic direction: declination, 176.5°; inclination, −66.5°; circle of 95% confidence, 8.6°. This is the only paleomagnetic study of pre‐Miocene rocks anywhere in western Washington or Oregon that matches the polar wandering curve for North America. However, a marked fanning of paleomagnetic declinations around the Olympic core suggests that concordance in this case is fortuitous and results from oroclinal bending during or immediately after Olympic orogenesis. Creation of the Olympic Mountains involved accretion of thickened basaltic crust followed by underthrusting of large quantities of oceanic sedimentary debris, as the Farallon plate pressed northeastward into a recess formed by the thick crystalline blocks of Vancouver Island and the North Cascades.

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Paleomagnetism of intrusive rocks in the Coast Range of Oregon: Microplate rotations in middle Tertiary time

Cited by 36 publications

References 0 publications

Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot

Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot

Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Paleomagnetism of small basalt exposures in the West Puget Sound Area, Washington, and Speculations on the accretionary origin of the Olympic Mountains

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