THE SALADO RIVER FAULT: A MAJOR TRANSFER FAULT ASSOCIATED WITH LARAMIDE DEFORMATION IN SOUTHERN MEXICO

At 25 km long, the Salado River fault (SRF) is a prominent geological feature exposed in northwestern Oaxaca State in southern Mexico that reveals a complex history of reactivation. It is a subvertical, E-W to ESE-WNW trending lineament that roughly constitutes the boundary between a structural high to the north represented by extensive outcrops of the Paleozoic Acatlán Complex and a ~2000 m sequence of Mesozoic terrestrial and marine beds to the south. Late Cretaceous-Paleocene orogenic deformation, generally associated with Laramide deformation, displays notable contrasts south and north of the SRF. E-W shortening was less intense in the more rigid block to the north, but to the south produced NNW-SSE trending, open to moderately closed, and asymmetric folds in what is now known as the Diquiyú anticlinorium. The northern terminations of the fold axes of the anticlinorium show a marked flexure to the west and become parallel to SRF, consistent with sinistral displacement along the SRF.

Sheared Paleozoic rocks show evidence of crystal-plastic deformation. Observations in thin sections reveal fabrics typical of low to medium grade (>400º C) conditions (i.e. chessboard pattern in quartz). Kinematic indicators at outcrop and microscopic scales (S-C fabric, mica fish) in oriented samples confirm sinistral displacement, and brittle fabrics overprint crystal-plastic fabrics, attesting to a later brittle phase. In contrast, in Mesozoic rocks fault surfaces displaying kinematic indicators of left lateral movement formed in a brittle regime.

At one site, 40Ar-39Ar ages obtained in muscovite from the fault gouge in Paleozoic rock vary from approximately 300 to 400 Ma, and most likely correspond to several Paleozoic tectonothermal events related to regional metamorphism, as reported by other authors. At another site, muscovite was completely reset and 40Ar-39Ar dating yielded Early Jurassic ages, probably associated with left lateral dip-slip motion during the development of the Mesozoic basin. In the last major stage of activity, in which temperatures were not high enough to reset muscovite in the shear zone, brittle deformation overprinted earlier crystal plastic deformation and sinistral strike slip movement associated with Laramide deformation dragged the anticlinal fold axes to the west.