Transtension driving volcano-edifice anatomy: Insights from Andean transverse-to-the-orogen tectonic domains

摘要

Dykes and minor eruptive vents are not randomly distributed within tectonically controlled volcanic arcs, which is reflected in the resulting morphology of stratovolcanoes. As a first-order approach, regional stress field controls the spatial distribution and geometry of dykes and minor eruptive vents. However, favourably oriented pre-existing weakness zones, such as inherited fault systems parallel or close-to-parallel to the orogen-scale shortening axis may reactivate and be the loci for magma migration within the upper crust in intra-arc regions. Field mapping and satellite image analyses were used to establish the Callaqui Stratovolcano (CSV) structural setting in the Andean Southern Volcanic Zone (Central-Southern Chile). Geometrical and morphometric analyses of intravolcanic dykes and minor eruptive vents, respectively, yields a consistent model of dykes propagating and erupting into either: (1) N similar to 60 degrees E +/- 5 degrees oriented inherited pre-existing fractures in central portions of the volcanic edifice and/or (2) regional stress controlled N similar to 70 degrees E +/- 10 degrees self-propagating fractures at distal zones. A wing-crack like structural array of the intra-volcanic dyke swarm documents a local right-lateral transtensional setting driving the late-Pleistocene to Holocene volcano building-up process. The latter plays a fundamental role in the progressive shaping of the CSV elongated anatomy in space and time. Parallel dyking and fissural eruptions contribute to successive volcanic output in a predominant ENE orientation. On the other hand, a prominent elongated relief enhances normal-to-the-volcano-elongation flank instability, developing steeper flanks that could trigger sector-collapses. This may promote an increase in the extensional component reinforcing further parallel dyking and subsequent eruptions. These results improve the understanding of the tectonic role in magma migration processes within the upper crust, which is crucial to understand geothermal systems and to minimize hazards related to volcano instability. (C) 2016 Elsevier Ltd and INQUA. All rights reserved.