Comment and Reply on "Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement"

Umhoefer, Paul J.; Magloughlin, Jerry F.; Butler, Robert F.; Gehrels, George E.; McClelland, W.; May, Steven R.; Klepacki, D.

doi:10.1130/0091-7613(1990)018<0800:carodp>2.3.co;2

Cited by 6 publications

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“…The southwestern margin of the batholith is in contact with southwesterly dipping clastic strata of the lower Eocene Swauk Formation. The nature of this contact-whether stratigraphic or structural-remains unresolved despite considerable debate (see Butler AGUE AND BRANDON et al, 1989;Miller et al, 1990;Umhoefer and Magloughlin, 1990;Tabor et al, 1993; Fig. 2A).…”

Section: Regional Tilt Mount Stuart Batholith Washingtonmentioning

confidence: 99%

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

Ague

Brandon

1996

Geological Society of America Bulletin

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We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93-96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressure estimation. High-contrast back-scattered electron imaging was used to ensure that the analyzed hornblendes were not significantly affected by subsolidus alteration. The success of the AH barometry is indicated by two observations. First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (Ϸ650 ؇C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from Ϸ0.3 GPa in the northwest to Ϸ0.15 GPa in the southeast, consistent with independent barometry for the contact aureole of the batholith and regional structural and stratigraphic relations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43؇ ؎ 30.4؇ and dip of 7؇ ؎ 2.0؇ southeast (؎95% confidence). Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence strongly suggest that the published directions were acquired shortly after emplacement of the batholith. (1) The ''stable'' sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42؇ ؎ 11؇ clockwise rotation and 3100 ؎ 600 km of northward offset (؎95% confidence). This result verifies Beck et al.'s original interpretation that the Mount Stuart batholith originated at the paleolatitude of northern Mexico.

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Section: Regional Tilt Mount Stuart Batholith Washingtonmentioning

confidence: 99%

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

Ague

Brandon

1996

Geological Society of America Bulletin

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“…However, it is important to note that available geologic data provide no hard evidence concerning the tilt geometry of the batholith. Many authors have attempted to deduce tilt of the batholith using the patterns of metamorphism and cooling ages within the Mount Stuart block Umhoefer and Magloughlin, 1990;Miller et al, 1990;Beck, 1991). We believe that these approaches are problematic because the Mount Stuart block has a complex metamorphic and thermal history.…”

Section: Regional Relationshipsmentioning

confidence: 99%

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and reply

Paterson¹,

Miller²,

Ague³

1998

Geological Society of America Bulletin

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Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex

Bogue¹,

Grommé²,

Hillhouse³

1995

Tectonics

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We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540øC) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the sitemean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56 ø ñ 10 ø) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (ñ 1300 kin) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes. Introduction The Canadian Cordillera, including southeastern Alaska, is divided into a series of physiographic/geologic provinces that are roughly parallel to the continental margin. Paper number 95TC01579. 0278-7407/95/95TC-01579510.00 Mountain Belt. Of these, both the Insular Belt (Terrane II) and the Intermontane Belt (Terrane I) are superterranes composed of smaller tectonostratigraphic terranes that are termed "suspect" because they are evidently allochthonous with respect to the North American craton [e.g., Monger et al., 1982]. The Coast Plutonic Complex and the Omineca Crystalline Belt are the broad loci of sutures joining these two allochthonous superterranes to each other and to the continent. The Intermontane superterrane accreted to North America in Middle Jurassic time, and the outboard Insular superterrane accreted to the Intermontane superterrane sometime between the Middle Jurassic and Late Cretaceous [Monger et al., 1982; van der Heyden, 1992; Rubin et al., 1990]. A regionall...

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Comment and Reply on "Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement"

Cited by 6 publications

References 0 publications

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and reply

Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex

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