DEFORMATIONAL HISTORY OF THE GRANJENO SCHIST, CIUDAD VICTORIA, MEXICO

The Paleozoic Granjeno Schist of NE Mexico forms part of the Huizachal-Peregrina Anticlinorium in the front ranges of the Sierra Madre Oriental. Comprising polydeformed metasedimentary and metavolcaniclastic sequences and serpentinized mafic-ultramafic units of probable ophiolitic origin, the schist is juxtaposed against Grenville-aged basement (Novillo Gneiss) that is overlain by Lower Paleozoic fossiliferous strata of Gondwanan affinity. The Granjeno Schist has been deformed by at least three sets of structures: (1) Steeply inclined NW- or SE- plunging tight to isoclinal folds (F1) of presently unknown kinematics that deform bedding and have a strong axial planar slaty cleavage (S1). (2) Small-scale N- or S-plunging folds (F2) that deform S1 and S1-parallel quartz veins and possess a moderately developed axial planar crenulation cleavage (S2) most evident in the pelitic lithologies. (3) Rare, steeply inclined NNW- or SSE-plunging close folds (F3) that deform S2 but do not possess a cleavage. Juxtapositioning of the Granjeno Schist against the Novillo Gneiss occurred as a result of NNE-SSW dextral strike slip. This deformational sequence invites comparison with that recorded in the Acatlán Complex in southern Mexico, which exposes Paleozoic schists of similar lithologies that are affected by three comparable phases of folding and have been juxtaposed against Grenville-aged basement (Oaxacan Complex) by dextral shear. Tectonothermal events within the Acatlán Complex of Late Ordovician-Early Silurian, Late Devonian and Early Permian age suggest linkages with the Appalachian orogen. Correlation with the Granjeno Schist would extend this belt of Paleozoic schists and Grenville-aged gneisses the length of Mexico, lending support to the existence of Oaxaquia, a microcontinental block of Grenville age believed to underlie much of the country.