Bimodal volcanism in a tectonic transfer zone: Evidence for tectonically controlled magmatism in the southern Central Andes, NW Argentina

This field-based and analytical laboratory study focuses on the genetic relationship between bimodal volcanic centres and fault types of an important tectonic transfer zone in the southern Central Andes, the NW–SE striking Calama–Olacapato–Toro (COT) volcanic belt. More specifically, tectono–magmatic relationships are examined for the 0.55 Ma Tocomar, the 0.78 Ma San Jerónimo and the 0.45 Ma Negro de Chorrillos volcanic centres in the Tocomar area (66°30 W–24°15 S). Structures of the COT volcanic belt, notably NW–SE striking strike-slip faults and NE–SW trending normal faults, accommodated differential shortening between major N–S striking thrust faults on the Puna Plateau. We present evidence that bimodal volcanism was contemporaneous with activity of these fault types in the COT volcanic belt, whereby eruption and composition of the volcanic rocks in the Tocomar and San Jerónimo–Negro de Chorrillos areas appear to have been controlled by the kinematics of individual faults. More specifically, rhyolitic centres such as the Tocomar are associated with normal faults, whereas shoshonitic–andesitic monogenetic volcanoes, e.g., the San Jerónimo and Negro de Chorrillos centres, formed at strike-slip dominated faults. Thus, the eruption of higher viscous rhyolite magmas appears to have been facilitated in tectonic settings characterized by horizontal dilation whereas ascent and effusive volcanic activity of less viscous and hot basaltic andesites to shoshonites were controlled by subvertical strike-slip faults. While the Tocomar rhyolites are interpreted to be derived from an anatectic crustal source, geochemical characteristics of the San Jerónimo and Negro de Chorrillos shoshonitic andesites are in agreement with a deeper source. This suggests that the composition of erupted volcanic rocks as well as their spatial distribution in the Tocomar area is controlled by the activity of specific fault types. Such volcano–tectonic relationships are also evident from older volcanic centres of the COT volcanic belt. This points to a close genetic relationship between bimodal volcanism and the upper-crustal deformation regime, whereby episodes of increased volcanic activity correlate with deformation episodes in the Central Andes.