COMBINED PALEOMAGNETIC AND MAGNETIC ANISOTROPY ANALYSES OF PLUTONS: TOWARDS PLACING FABRIC DATA WITHIN A TECTONIC FRAMEWORK

The use of magnetic anisotropy has led to the recognition that nearly all plutons emplaced within active orogenic belts preserve mineral fabrics formed during pluton crystallization. Such fabrics can yield information on the orientation of tectonic strains, which in turn can be used to evaluate the kinematics within that orogenic belt. Relating rock fabrics to plate tectonic processes on an orogenic scale requires the (often-unstated) assumption that the orientation of fabrics measured in-situ faithfully reflect the orientation of the "paleo-strain" shortly after the pluton was intruded. If significant tilt or vertical axis rotation of the pluton occurred, the present orientation of the structures and fabrics contained within the pluton will no longer be in their proper tectonic reference frame. Using such rotated fabrics in their in-situ orientations can lead to incorrect interpretations of the kinematics of orogenic belts.

The temporal relationship between the acquisition of mineral fabrics and recording of remanent magnetism in plutonic rocks can be exploited to evaluate, and if need be correct for, rotation and tilt of plutons and their fabrics. Pluton fabrics, be they magmatic or solid-state, will form at temperatures exceeding the Curie temperature of common magnetic minerals, with the result that such fabrics predate the acquisition of the paleomagnetic signal recorded by a pluton. If the pluton records a stable paleomagnetic direction, this can be compared to an expected direction for its tectonic plate or terrane. Any difference between this expected direction and the observed direction can be used to restore the mineral fabrics to a common tectonic framework.

We will present case studies from the North Cascades-Coast Plutonic Complex illustrating how paleomagnetic and magnetic anisotropy data can be combined to place fabrics within a plate tectonic framework. The Eocene Cooper Mt. and Golden Horn plutons have, after rotation-correction, sub-horizontal and N to NNW trending AMS lineations that are consistent with orogen-parallel extension at this time. Results from older (late Cretaceous) plutons (Mt. Stuart, Ecstall, and Spetch Creek) provide examples of the complexities encountered in fabric interpretation when polyphase deformation and possible terrane translation need to be considered.