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

Abstract

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...

Concordant paleolatitudes for Neoproterozoic ophiolitic rocks of the Trinity Complex, Klamath Mountains, California

“…Their paleomagnetic/rock magnetic study convincingly shows that the remanence in those rocks passes the fold test when the layering is restored to horizontal. Bogue et al 's [1995] interpretation gains additional support because the restored paleomagnetic inclination yields a paleolatitude that agrees with other results from the Insular superterrane and Coast Plutonic Complex of British Columbia. (However, see Butler et al [2001] for a differing opinion.…”

“…Although the Bogue et al [1995] study seems compelling to us, their results may not apply to all similar intrusions or to others that are less mafic. One of the earliest successful applications of the use of cumulate layering for structural correction [ Gough and Van Niekerk , 1959] was in a study of the gabbroic sequence of the Proterozoic Bushveld Complex, South Africa (see papers by Hattingh [1986a, 1986b] which supercede the earlier results).…”

“…Unfortunately, few paleomagnetic studies addressing this problem are available. Probably the most thorough, recent study is that by Bogue et al [1995] of the 110‐Ma, zoned ultramafic complex of Duke Island, Alaska. Their paleomagnetic/rock magnetic study convincingly shows that the remanence in those rocks passes the fold test when the layering is restored to horizontal.…”

A compaction correction for the paleomagnetism of the Nanaimo Group sedimentary rocks: Implications for the Baja British Columbia hypothesis

“…The 110 m.y. old Duke Island ultramafic complex also yielded a shallow inclination, after a tilt correction based on the regional trend of cumulate layers, indicating 2970 ± 1360 km of translation for the Insular superterrane [ Bogue et al , 1995]. However, the paleomagnetic directions after a tilt correction using cumulate layers at the site level are reported to yield a scattered result [ Butler et al , 2001b].…”

Deciphering shallow paleomagnetic inclinations: 2. Implications from Late Cretaceous strata overlapping the Insular/Intermontane Superterrane boundary in the southern Canadian Cordillera

“…Paleomagnetic studies from the Cretaceous Spences Bridge Group (∼105–100 Ma) of the Intermontane Superterrane suggest that these rocks have been translated ∼1100 km northward along a dextral fault that lay within the eastern third of Quesnellia, west of the Omineca Belt (Figures 1 and 2) [ Thorkelson and Rouse , 1989; Irving and Thorkelson , 1990; Irving et al , 1995; Haskin et al , 2003]. Data from the Insular Superterrane, west of the Intermontane Superterrane, indicate substantially greater displacement [ Ague and Brandon , 1996; Wynne et al , 1995; Bogue et al , 1995]. Paleomagnetic data from the Upper Cretaceous Silverquick conglomerate and overlying Powell Creek formation [ Garver , 1992; Wynne et al , 1995] of the Methow Terrane require ∼3000 km of dextral displacement along a fault between the westernmost Intermontane strata and easternmost Insular Superterrane (i.e., Methow Terrane) strata.…”