Research Projects

Characterization of the Mn-horizon in the Tusas Mts., Northern New Mexico

Above Left: Map of the SW US showing quartzite and rhyolite localities and their relation to province boundaries (from Kopera, 2003).
Above Right: Map showing the location of the Tusas Mts. in relation to other basement-cored uplifts of Paleoproterozoic age (from Williams, 1987).

The cratonic core of the North American continent was assembled in the Proterozoic from smaller Archean microcontinents (Hoffman, 1988). This assembly was followed by the genesis of a long-lived, active margin at the southern edge of the craton, at which significant amounts of crust were added through accretionary orogenic events of the Proterozoic (Karlstrom and Bowring, 1987). During this time, the North American continent grew considerably southward. Rocks of the accreted Proterozoic terrane are well exposed in the southwestern US (see maps above) and represent the ideal location for the investigation of Proterozoic plate tectonic processes. A distinctive feature of this terrane is the occurrence of km-thick sequences of mature, syn/intra-orogenic, metamorphosed quartzarenite. In the Tusas Mts and across the southwestern US, an anomalous Mn-horizon can be found spanning across the basal contact of the quartzites and characterized by the occurrence Mn-rich phases (esp. Mn-andalusite) in areas of higher metamorphic grade (Williams, 1987). The source of the Mn-horizon is still unknown, although presence of Mn-rich phases on cross-beds in the quartzite points to a sedimentary origin.

Studies of structural fabrics and metamorphic assemblages linked with geochronology have constrained three periods of Proterozoic deformation (D1, D2, D3) (Williams, 1991). Interpretation of the character, intensity, and timing of the different deformations has become an issue of debate. Careful interpretation of the tectonic and deformational history of the southwestern US still remains key to understanding how the southern margin of the North American continent evolved in the Proterozoic.

In this project, detailed study of the Mn-rich phases and related assemblages will be used:
(1) as a tool to understand the origin of the Mn-horizon (particularly if linked to quartzite deposition),
(2) as a marker unit to correlate quartzite localities across the southwest, and
(3) as a tool to compare the timing of the growth and deformation of Mn-phases

A detailed study of the Mn-horizon could serve as the critical link when comparing isolated Proterozoic outcrops across the southwestern US. It may be the marker unit that links southwestern US quartzite localities together both in origin and in character of depositional environment (a character that could represent conditions unique to this time in the Proterozoic). The Mn-horizon may also be the link that finally provides the tools necessary to compare the effects of deformational events on a regional scale and shed light on the extent to which these events may have played a role in the accretion and evolution of crust at the southern margin of the North American continent in the Proterozoic.

Hopewell Lake-Jawbone Mt. Area
Above: Photo of the a small ridge of quartzite of the Ortega Fm near the base of the unit where the Mn-horizon can be found.
Right: A photomicrograph of a Mn-andalusite crystal from the locality featured above (x-pol). Variation in birefringence directly cooresponds to Mn abundence. Oxide inclusion trails trace the dominant fabric.

Kiawa Moutain
Above Left: Photo from the peak of Kiawa Mt. where the occurance of Mn-andalusite marks the location of the Mn-horizon.
Above Right: A photomicrograph of a rock from Kiawa Mt. showing relationships between kyanite, Mn-andalusite, and accessory phases.


Hoffman, P.F. 1988. United Plates of America, the birth of a craton. Annual Reviews of Earth and Planetary Sciences, 16, 543-603.

Karlstrom, K.E. and Bowring, S.A., 1987. Early Proterozoic assembly of tectonostratigraphic terranes in southwestern North America. Journal of Geology, 96, 561-576.

Kopera, J., 2003. Monazite geochronology of the Ortega Quartzite: Documenting the extent of 1.4 Ga tectonism in northern New Mexico and across the orogen. Unpub. M.S. Thesis, University of Massachusetts, Amherst.

Williams, M.L., 1987. Stratigraphic, structural, and metamorphic relationships in Proterozoic rocks from northern New Mexico. Unpub. PhD. Thesis, University of New Mexico, Albuquerque.

Williams, M.L., 1991. Heterogeneous deformation in a ductile fold-thrust belt: The Proterozoic structural history of the Tusas Mountains, New Mexico. GSA Bulletin, 103, 171-188.

Research Projects
Structure and Tectonics Research Group

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