40AR/39AR GEOCHRONOLOGY AND KINEMATIC ANALYSIS OF SIERRA DE MAZ: IMPLICATIONS FOR TIMING AND ORIENTATION OF DEFORMATION ON THE PALEOZOIC MARGIN OF WESTERN GONDWANA

The Valle Fértil fault zone is an orogen scale (>300 km long) Paleozoic shear zone that separates the Cambro-Ordovician Famatina arc from accreted terranes. We identified a major shear zone along strike of the Valle Fértil fault zone to the north within the Sierra de Maz which separates lower amphibolite facies metapelites in the west from upper amphibolite to granulite facies rocks in the east that are dominantly meta-igneous. We present preliminary ⁴⁰Ar/³⁹Ar geochronology and structural analyses from the Maz shear zone and surrounding regions, in combination with other geochronology techniques (see abstracts Tholt et al., 2017 and Webber et al., 2017), to constrain the ductile deformation history of this region.

Structural analysis shows that the Maz shear zone is a 1 km-wide mylonitic high-strain zone. Macro- and microscopic fabrics record sinistral strike-slip kinematics at amphibolite facies conditions (vertical foliation oriented NNW, horizontal stretching and mineral lineations). Smaller shear zones to the east are subparallel to the Maz shear zone but have oblique lineations with a range of rakes trending north and south. These shear zones record lower temperature deformation than the Maz shear zone and do not represent significant terrane boundaries, although a folded dike dated to 431 ± 3 Ma (U/Pb zircon) indicates deformation in the east was occurring at the same time as the Maz shear zone.

Preliminary geochronology results show multiple generations of metamorphism recorded in the unit east of the shear zone, with the youngest at 440-430 Ma. The highest strain part of the shear zone contains deformation initiating at amphibolite facies and continuing through greenschist facies conditions. The deformation is bracketed by 433-423 Ma titanite and garnet ages and a 414 ± 3 Ma ⁴⁰Ar/³⁹Ar hornblende cooling age. The unit west of the Maz shear zone records a metamorphic event at lower amphibolite facies (419 ± 4 Ma, ⁴⁰Ar/³⁹Ar hornblende and 423 ± 10 Ma, Lu/Hf garnet). The similarity of this event to the timing and metamorphic grade of the Maz shear zone suggests some component of convergence at ~420 Ma. The kinematics of the smaller shear zones support this, suggesting that the Sierra de Maz range records prolonged sinistral oblique convergence, with deformation distributed along multiple shear zones.