Early establishment of seafloor hydrothermal systems during structural extension: paleomagnetic evidence from the Troodos ophiolite, Cyprus

Paleomagnetic data from the Troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large-tonnage sulfide deposits at oceanic ridges. We have sampled dikes from the Troodos sheeted complex that have been subjected to variable hydrothermal alteration, from greenschist alteration typical of the low water/rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks (epidosites) altered within high water/rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits. These dikes are moderately to highly tilted from their initial near-vertical orientations due to rotations in the hangingwalls of approximately dike-parallel, oceanic normal faults. Comparison of characteristic remanence directions from these dikes with the Late Cretaceous Troodos reference direction, therefore, allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting. Remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the Late Cretaceous Troodos reference direction upon restoration of dikes to near-vertical positions about a NNW-trending, horizontal axis. These data, along with field observations of focused alteration along normal faults, suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation. This sequence of events is similar to that observed for creation of large-tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension. We suggest that the dike-parallel normal faults were initiated as extensional fractures during this early stage of crustal extension, thus providing the necessary permeability for focused fluid flow, and that later slip along these structures during rotational-planar normal faulting caused reduction in permeability due to gouge formation.