RALSER, Steven. Dept oF Earth & Environmental Sciences, New Mexico Tech, Socorro, NM, 87801.

Granitic gneisses in the Joyita Hills (ca. 35 km NE of Socorro, NM) record a transition from L-tectonites to L-S tectonites. Two granitic gniesses are recognized; a massive coarse grained gneiss, consisting predominantly of megacrysts of K-feldspar (up to 40 mm in size and comprising greater than 50% of the rock) and a megacrystic gneiss consisting of megacrysts of K feldspar (<20 mm in size) in a finer grained matrix of quartz, feldspar and biotite. The megacrystic gneiss is associated with deformed aplite dykes and both show an intrusive relation into the massive gneiss. The dominant structural feature within the Joyita Hills is a strong stretching lineation, trending 50¡ to 150¡. Foliations show a wide range in orientation from northeast to southeast trending, and generally east dipping. However poles to all foliations lie on a single great circle, normal to the stretching lineation. 2 structural domains are defined on the basis of these foliation orientations. In the southern domain foliations show the whole range of orientations, but are best developed where there is a strong rheological contrast between the granitic gneiss and some planar structure (e.g. veins and aplite dikes). The dominant structural feature in this domain is the strong stretching lineation The northern domain is characterized by an east dipping, N-NE trending foliation which is locally developed in the massive gneiss. The stretching lineation is not as pervasively developed in this domain. The boundary between these 2 domains is gradational. Asymmetric tails around K-feldspar porphyroclasts in the megacrystic gneiss indicate a SE over NW movement sense (in their present orientation). If the effects of Phanerozoic tilting are removed, foliations become near vertical, with a near vertical stretching lineation, with the S side up relative to the N side. Both the nature and distribution of the structures suggests the presence of a wide shear zone, with a minimum thickness of 2-3 km. The zone of L-tectonites, which shows evidence of the most intense deformation, has a thickness of at least 1.5 km. Rocks showing such constrictional strains occur elsewhere in the Proterozoic of New Mexico; however, the significance of such zones is currently unknown.