New SHRIMP zircon and monazite 206 Pb/ 238 U and 208 Pb/ 232 Th ages on structurally controlled units and 40 Ar- 39 Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

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Abstract. Mountain belts developed in an intracratonic setting during the Neoproterozoic in the center of Australia. Mesoproterozoic granulite and amphibolite facies gneisses of the Musgrave Block were overprinted during the Petermann Orogeny (-550 Ma)with zones of high strain concentrated along broadly east-west trending shear zones. These zones formed under eclogite facies (T-650øC and P-12 kbars) to greenschist facies conditions and may form part of a strikeslip-related, crustal-scale flower-type structure. Granulite facies gneisses preserve ages older than 700 Ma, suggesting that these rocks did not experience temperatures greater than

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