Paleomagnetism and tectonics of the Cretaceous Mt. Stuart Batholith of Washington: translation or tilt?

Abstract

The Mt. Stuart Batholith is a composite pluton of Late Cretaceous age that intrudes the crystalline North Cascades terrane of northwestern Washington. Its paleomagnetic direction ( D=10.0°, I=45.5°; α 95=4.9° ) is markedly different from the direction expected for Cretaceous rocks from northwestern Washington ( D=330.5°, I=73.0°), which means that the Mt. Stuart Batholith either has moved relative to the North American interior, or has been tilted through a substantial angle, or both, since it acquired its magnetism. Either tilt or translation are possible, geometrically, but translation is more likely, because: (1) local geology apparently does not support tilt in the required direction; (2) it probably is not possible for a panel of rock the size of the Mt. Stuart Batholith to tilt through the necessary angle (ca. 35°) after its deeper parts have cooled sufficiently to retain remanent magnetization; (3) the sense of paleomagnetic discordance found in the Mt. Stuart Batholith (clockwise-rotation of declination, flattened inclination) is identical to that found in nearly every other allochthonous terrane in the western North American Cordillera, including every other Cretaceous batholith studied except the Sierra Nevada.