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Geological Society, London, Special Publications; 2004; v. 224; p. 141-160;
DOI: 10.1144/GSL.SP.2004.224.01.10
© 2004 Geological Society of London

Contrasting styles of fluid-rock interaction within the West Fissure Zone in northern Chile

C. Janssen, V. Lüders & A. Hoffmann-Rothe

GeoforschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germanyjans{at}gfz-potsdam.de

Geochemical data from three profiles crossing the West Fissure Zone in northern Chile were used to describe the chemical effects of fluids on faulting processes. The results document considerable differences of fluid-rock interactions between the profiles (A–C). Within the area of profile A and C fluid activities have neither led to intensive exchange reactions between fluids and rocks nor to notable changes in whole rock and mineral composition of fault rocks relative to their undeformed host rock. Investigations of stable isotopes ({delta}13C, {delta}18O) and fluid inclusions indicate infiltration of predominantly descending (meteoric) fluids and only subordinate involvement of ascending hydrothermal fluids. In the case of profile B, fluid-enhanced weakening mechanisms are dominant. Dissolution transfer has led to the formation of an alteration zone up to 400 m wide. Along profile B we have no indications for warm fluids derived from a deep source. Rather, a number of lines of evidence (e.g. {delta}18O and {delta}D) show that fault rock alteration took place due to the infiltration of low temperature meteoric water. For this profile, all geochemical data indicate an open fluid system. Moreover, the chemical variations across the West Fissure Zone agree well with the structural variability observed at this traverse. The application of a variety of geochemical analyses of fault rocks shows the heterogeneity of large-scale continental fault zones.