New findings of pseudotachylytes and their paleoseismic significance, Sierras Pampeanas of San Juan, Argentina

Abstract

In this contribution, the first occurrence of pseudotachylyte veins in the crystalline basement of the Sierras Pampeanas of San Juan province, Argentina, is presented. The Pan de Azúcar and El Retamo pseudotachylyte veins were found closely related to the NNW-SSE trending Valle Fértil and Portezuelos fault zones, two major ductile-brittle shear zones that were tectonically active since the early Paleozoic. Macro- and microstructures observed in both pseudotachylytes suggest that they are the product of rapid cooling of silicate melts along paleoseismic zones during faulting. This inference is supported by the presence of distinctive features like intrusive margins, flow banding and melt-corroded rims in clasts. Acicular and globular microlites, as much as lesser quantities of spherulites, were identified in the matrix of these rocks are considered as a product of direct crystallization from the melt. The presence of vesicles and amygdules indicates a possible formation depth of less than 7 km. The chemical composition of pseudotachylyte veins closely resembles that of their host rocks, greenschist facies mylonitic rocks and mylonitized granitoid rocks, indicating that they were formed by frictional melting. The pseudotachylytes described from the Pan de Azúcar and El Retamo fault zones were found occurring in veins, which is interpreted as an “in situ” origin. These pseudotachylytes were formed long after host rocks mylonitization. Well preserved microstructures and absence of subsequent overprinting suggest that they were formed in the late stages of faulting.