Oblique-to-the-orogen fault systems and it causal relationship withvolcanism and geothermal activity in Central Southern Chile: Insights on ENE and NW regional lineaments”

Abstract

Major crustal faults systems at convergent margins are commonly organizedinto margin-parallel, high-strain domains that appear to be continuous over hundreds of kilometers.This major architecture shapes the structural grain of a given mountain belt and is thought to representthe long-term record of its deformation history. However, second-order transverse structures, crosscuttingthe orogen main structural grain, also are common. In the Andes they play a key role in thetectonic, magmatic and hydrothermal history. Although the relevance of these oblique-to-the-orogenstructures is widely recognized, little is known regarding their nature, kinematics and timing and alsotheir long and short-term tectono-magmatic role.The present work address the tectono-magmatic significance of WNW and ENE-striking basementstructures, through a combination of field geology and geophysics. Our working hypothesis is thatWNW and ENE-striking structures correspond to long-lived fault zones that play a fundamentaltectono-magmatic role in Andean evolution. They have accommodated part of the upper platedeformation arising from the ~ ENE-WNW-trending shortening and –at the same time- they haveprovided episodic pathways for magma and hydrothermal fluid transport in the lithosphere.Furthermore, we speculate that because WNW-striking fault zones are severely misoriented withrespect to the prevailing stress field, they reactivate under supra-lithostatic fluid pressures. ENEstrikingfaults, in turn, are favorably oriented and do not require supra-lithostatic fluid pressures toreactivate.The problem is being tackled by selecting two outstanding case studies in the Andes of Central Chile:the ENE-oriented Tatara-San Pedro-Pellado volcanic complex – Laguna del Maule volcanic fieldalignment (TPMA) and the WNW-oriented Cortaderas-Chillán lineament (CChL). Observations onsatellite images combined with preliminary field studies suggest that WNW-striking faults and ENEstriking faults show sinistral-reverse and dextral-normal displacement respectively. Both systemscrosscut each other and their activity is younger than Late Pleistocene. Furthermore, Late Pleistocenemafic dikes, vein systems and fault-controlled fumaroles appear to be synkinematic with bothtransverse crustal faults.