Paleomagnetism of the Duke Island, Alaska, ultramafic complex revisited

The Duke Island ultramafic intrusion was emplaced into the Alexander terran e immediately preceding development of a regional mid-Cretaceous thrust bel t. Paleomagnetic samples were collected from exposures of ultramafic rock w ith cumulate layering northwest of Judd Harbor and northwest of Hall Cove. Thermal demagnetization results were analyzed using principal component ana lysis to isolate the characteristic remanent magnetization. Site-mean chara cteristic directions determined from 16 sites fail the fold test at 95% con fidence, indicating that cumulate layering attitudes were highly contorted at the time of magnetization, at least on a scale of tens of meters. Variat ions in cumulate layering attitudes probably resulted from the combined eff ects of thermal convection phenomena during crystallization and deformation following crystallization but prior to magnetization. Analysis of cumulate layering over larger structural domains indicates that kilometer-scale def ormation produced southwest plunging folds within the Hall Cove and Judd Ha rbor bodies. Bogue et al. [1995] proposed that a compound structural correc tion involving unplunging of fold axes followed by unfolding of average cum ulate layering could restore cumulate layering to horizontal. However, usin g the full set of 21 site-mean paleomagnetic directions from Duke Island (1 6 from the current study and 5 from Bogue et al. [1995]), the compound stru ctural correction yields mean paleomagnetic directions from the Judd Harbor and Hall Cove areas that are statistically distinguishable at 99% confiden ce. This result indicates that even on the kilometer-scale, cumulate layeri ng within the Duke Island ultramafic intrusion was neither coplanar nor hor izontal at the time of magnetization. Observations of cumulate layering in other ultramafic intrusive rocks indicate that this layering can significan tly depart from horizontal by 10 degrees -20 degrees even on the kilometer scale. Therefore use of cumulate layering of ultramafic rocks as a proxy fo r paleohorizontal is not justified, and paleomagnetic directions from the D uke Island ultramafic intrusion cannot be used to infer the Cretaceous pale olatitude of the Insular superterrane.