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Geological Society, London, Special Publications; 2004; v. 238; p. 251-283;
DOI: 10.1144/GSL.SP.2004.238.01.16
© 2004 Geological Society of London

Igneous Fabrics

Magmatic flow paths and palaeomagnetism of the Miocene Stoddard Mountain laccolith, Iron Axis region, Southwestern Utah, USA

M. S. Petronis1, D. B. Hacker2, D. K. Holm2, J. W. Geissman1 & S. S. Harlan3

1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, 87131, USA mspetro{at}unm.edu
2 Department of Geology, Kent State University, Kent, Ohio 44242, USA
3 Environmental Sciences and Policy, George Mason University, Fairfax, VA 22030, USA

The Stoddard Mountain laccolith is part of a complex of Early Miocene laccoliths intruded along the western edge of the Colorado Plateau in the Iron Axis region of Southwestern Utah. Most Colorado Plateau laccoliths (e.g. Henry and La Sal Mountains) are considered to be fed by a central axial feeder system. However, detailed mapping in the Iron Axis region suggests that the Stoddard Mountain laccolith was fed laterally from the west. Structural and field data suggest the quartz monzonitic magma initially migrated laterally eastward at ~1 km depth as a sill before spreading laterally north-south where it inflated to ~1–1.5 km thickness. To test the model of a lateral feeder system, data were collected from 32 palaeomagnetic sites and 76 AMS stations (763 accepted specimens) sampled over the ~54 km2 exposed part of the N-S oval-shaped laccolith. The in situ AMS fabrics, inferred to correlate with magmatic fabrics, typically show NE trending lineations in the north and S-SE trending lineations in the south part of the intrusion. The palaeomagnetic data are interpreted to indicate a very minimal amount of post-emplacement deformation of the intrusion. The overall lack of westerly-directed and steep magnetic lineations argues against emplacement via a central axial feeder system.