Exploring Elongation-Inclination Relationships in Datasets from Plutons and Remagnetized Sediments: Examples from the North Cascades and the Blue Mountains

Tectonic applications of paleomagnetism rely upon establishment of paleohorizontal at the time of magnetization. Paleohorizontal can be established in sedimentary rocks and volcanics, but is poorly constrained in plutonic rocks and areas that have experienced regional remagnetizations. This study will explore another latitudinal-dependent property of the geomagnetic field- elongation of elliptical distributions of directional data- to evaluate whether the combination of elongation and inclination can be used to constrain effects of tilt or other paleohorizontal uncertainties in paleomagnetic datasets. This work is inspired by the application of the E-I relationship proposed by Tauxe and Kent (2004) to evaluate effects of inclination error in sedimentary rocks.

The first example is from the Blue Mountains of eastern OR. Remagnetized Permian-Jurassic sedimentary rocks (Hillhouse et al, 1982, Harbert et al, 1995, Housen, 2007, Kalk, 2008) have magnetizations that match those of Late Jurassic-Early Cretaceous plutons (Wilson and Cox, 1980, Housen, 2007). Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k= 26, α₉₅ = 3.7°. The E-I method can be used to determine the effects of calculated paleohorizontal errors by finding an optimal paleohorizontal error that results in the best agreement between E and I for a set of data. For the Blue Mountains rocks, the optimal E-I relationship yields a corrected inclination of I = 65° (+7°/-4°), and estimated paleolatitude of 47°N (42° to 57°).

The second example is from the Cretaceous Mt Stuart batholith in the North Cascades of central WA- these 95-88 Ma plutonic rocks have well defined magnetizations (Housen et al, 2003). Directions from 89 samples have a mean of D = 350°, I=44°, k=50, α₉₅ = 2.1°. The E-I relationship suggests a corrected mean inclination of I=46° (+12°/-3°), and estimated paleolatitude of 27°N (25° to 39°).

For the Blue Mountains, this comparison indicates that the plutons and remagnetized rocks have a mean I and E that agree well and suggest little paleohorizontal error. For the Mt Stuart batholith- the E/I corrected inclination suggests only moderate tilt is likely- and agrees well with the inclination of these rocks corrected independently using barometry to estimate paleohorizontal (I= 51°).