Abstract
The Sierras Australes of eastern Argentina record the progressive suturing of the Patagonian terrane with the South American craton during Permo-Triassic time. On the South American side of the suture, fold axes and axial plane cleavage show a systematic variation in orientation across the region, rotating counterclockwise from the southern and central segments to the northwestern segments. These data, in combination with finite strain measurements indicative of extension parallel to fold axes, suggest progressive, horizontal, simple shear with a left-lateral sense. Thus, the suturing probably had a strong, transpressive component. This conclusion explains the characteristic sigmoidal shape of the sierras and is supported by comparison with scale-model experiments conducted by other investigators. Furthermore, this interpretation is in agreement with paleomagnetic data.
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