Crustal shortening, exhumation, and strain localization in a collisional orogen: The Bajo Pequeño Shear Zone, Sierra de Pie de Palo, Argentina

JOSHUA M. GARBER; SARAH M. ROESKE; JESSICA WARREN; SEAN R. MULCAHY; WILLIAM C. MCCLELLAND; LAUREN J. AUSTIN; PAUL R. RENNE; GRACIELA I. VUJOVICH

TECTONICS

AMER GEOPHYSICAL UNION

Año: 2014 vol. 33 p. 1277 - 1303

0278-7407

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening andexhumation associated with the establishment of a new plate boundary, and its placement in a regionalstructural context suggests that local- to regional-scale strain localization occurred with progressive deformation.A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocksindicates an early Devonian (405?400 Ma) phase of deformation on the western margin of Gondwanancontinental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data,involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnetgrowth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZactivity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized bystrain localization to the center of the shear zone coincident with cooling and exhumation. These andother data suggest that significant regional tectonism persisted in the Famatinian orogenic system for60?70 million years after one microplate collision (the Precordillera) but ceased 5?10 million years prior toanother (Chilenia). A survey of other synchronous structures shows that strain was accommodated onprogressively narrower structures with time, indicating a regional pattern of strain localization and broadthermal relaxation as the Precordillera collision evolved.1.