Paleomagnetism of Cretaceous and Paleocene Sedimentary Rocks Across the Castle Mountain Fault, South Central Alaska
- John W. Hillhouse
Published Online: 18 MAR 2013
Copyright 1989 by the American Geophysical Union
Deep Structure and Past Kinematics of Accreted Terranes
How to Cite
Stamatakos, J. A., Kodama, K. P., Vittorio, L. F. and Pavlis, T. L. (1989) Paleomagnetism of Cretaceous and Paleocene Sedimentary Rocks Across the Castle Mountain Fault, South Central Alaska, in Deep Structure and Past Kinematics of Accreted Terranes (ed J. W. Hillhouse), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM050p0151
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875904542
Online ISBN: 9781118666609
- Geology, Structural—Congresses;
Paleomagnetic and rock magnetic analyses on 217 samples from 13 sites in the Paleocene Chickaloon Formation south of the Castle Mountain Fault and 111 samples from 9 sites in the coeval, but lithologically distinct, Arkose Ridge Formation north of this fault indicate that these rocks contain a pre-folding magnetization carried by fine grained (<1.0μm) single domain magnetite. Secondary magnetizations are common, possibly as the result of the presence of authigenic or hydrothermal pyrrhotite. Although characteristic magnetizations were isolated for the Chickaloon and Arkose Ridge rocks, the best results were obtained from demagnetization plane analysis which estimates the location of the paleomagnetic pole for the Chickaloon Formation at 50.5°N, 277.2°E, δm = 12.2°, δP = 7.77deg;, and a paleomagnetic pole for the Arkose Ridge Formation at 60.4°N, 138.6°E, δm = 11.6°, δp = 6.4°. These results suggest that there is no paleomagnetically discernible latitudinal offset across the Castle Mountain Fault since Paleocene time, but that both the Chickaloon and Arkose Ridge rocks, as part of the Peninsular terrane, originated approximately 1600±1200 km south of their present position with respect to North America. One possible explanation of these data is that the Peninsular terrane was accreted to North America at mid-latitudes in the Cretaceous and was subsequently translated northward by right-lateral strike-slip faulting parallel to the North American margin. Hence, the Arkose Ridge and Chickaloon results may be indicative of the cumulative right-hand displacement occurring on these faults since Paleocene time. However, a calculation using the pole to the small circle fit of the present-day curvature of the Tintina-Northern Rocky Mountain Trench and Denali fault systems, and the maximum amount of structurally estimated offset across these fault systems, indicates that motion on these faults can account for no more than half of the paleomagnetically observed latitudinal displacement. Thus, unless these faults had significantly different orientations in Paleocene time, or the structural analyses have underestimated the amount of slip on these strike-slip faults, the actual latitudinal displacement predicted for the Arkose Ridge and Chickaloon rocks, based on the 95% confidence limits, may be closer to the lower limit of 400 km.
Paleomagnetic and rock magnetic results from 174 samples from 10 sites in the Matanuska Formation indicate that the magnetic signal is also carried by single domain magnetite. The 4 Matanuska sites north of Castle Mountain Fault yielded a paleomagnetic pole at 50°N, 131° E, K = 9.8, suggesting that these rocks were deposited 2880±2000 km south of their present latitudinal position with respect to North America. Based on tectonic arguments similar to the one outlined above, these results suggest that either there was rapid motion of the Peninsular terrane in the Late Cretaceous or that like the Chickaloon and Arkose Ridge rocks, the actual amount of latitudinal displacement of the Matanuska Formation is closer to its lower 95% confidence limit of 800 km. South of Castle Mountain Fault, some Matanuska rocks carry only a viscous present-day field direction and others an Eocene PTRM caused by Eocene-age sills intruding into the the Cretaceous and Tertiary strata in the western part of the Matanuska Valley.