ANISOTROPY OF MAGNETIC SUSCEPTIBILITY OF THE TSÉ BIT' A'Í (SHIP ROCK) DIATREME AND RADIAL DIKE SWARM, DINE′ NATION, NEW MEXICO

Tsé Bit' A'í, referred to as Ship Rock in the geological literature, is an ~27 Ma alkalic maar-diatreme well-known for its associated spectacular radial dike swarm of principally minette mineralogy (phlogopite/biotite, K-feldspar, pyroxene, magnetite). Structural study of a dike northeast of Ship Rock by Delaney and Pollard (1981) suggests primarily vertical magma flow due to segmentation of the dike along its length. In this study we expand upon these earlier results and define magmatic flow directions with the major dikes of the complex through a combination of field criteria of flow on dike margins as well as through determination of anisotropy of magnetic susceptibility (AMS) from oriented samples. Our study is part of a much larger geophysical survey of the Ship Rock region by the Keck Geology Consortium. Due to sampling restrictions, we collected 24 oriented blocks (rather than cores) from each of the major dikes surrounding Ship Rock. These include the South Dike, West Dike, and several dikes in the area northeast of Ship Rock as well as a single sample from the eastern margin of the diatreme. Each dike possessed well-defined macroscopic indicators of magmatic flow direction on their margin surfaces. Indicators include hot slickenlines, stretched vesicles, and oriented inclusions of Mesozoic wall rock. While variable locally, the average flow direction indicated by these field indicators is shallowly inclined toward the central diatreme. Sixteen of the 25 blocks gave interpretable (AMS) results which are generally parallel to field indicators of flow. AMS is low (ave. = <2%) and representation ellipsoids are oblate probably reflecting control on late magnetite by the flow-oriented phlogopite/biotite structure. Because of the inherent problems associated with working on oriented blocks, we are not confident in interpreting angular offsets between AMS K1 directions and dike margins in terms of flow-determined imbrication. AMS is, however, reasonably interpreted as indicating primarily radial, near-horizontal flow. A single oriented sample of the steep, eastern diatreme (tuff breccia) margin shows a strongly down-dip K1 direction, probably indicating laminar flow during maar ejection rather than late chaotic slumping. Remanence directions are highly scattered due to control by large, multidomain magnetite.