The Alto allochthon is a postmetamorphic crystalline thrust sheet within the metamorphic core of the southern Appalachian orogen. The Toccoa Falls-Shorts Mill thrust fault separates structurally overlying migmatitic sillimanite- bearing Tallulah Falls Formation (?) rocks of the Alto allochthon from upper-greenschist/lower-amphibolite-facies rocks of the Chauga belt. Early D1-D2 structures are truncated by the Toccoa Falls-Shorts Mill fault, suggesting emplacement of the allochthon subsequent to their formation. Later D4-D5 structures appear to postdate thrusting, and the allochthon is preserved in an open F4 synform. Emplacement of the allochthon was probably coincident with the formation of D3 structures. Two distinct episodes of mylonitization have affected the rocks of the Alto allochthon. Early middle- to upper-greenschist-facies mylonites are locally developed along the Toccoa Falls-Shorts Mill fault. Along the northwestern margin,these early mylonites are overprinted by a more pervasive episode of lower temperature (lower-greenschist-facies) retrogressive mylonitization associated with deformation within the Brevard fault zone. Lithologic sequences within the Alto allochthon may be correlated with successions in both the eastern Blue Ridge (Tallulah Falls Formation) and the Inner Piedmont. Structural and stratigraphic data suggest that the allochthon is rooted to the southeast in the Inner Piedmont.

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