THE RECORD OF STRESS CYCLES IN FRONT OF A PROPAGATING THRUST FAULT

Juvenile faults grow by propagation. If a fault grows during repeated earthquakes, the crust facing the fault front is subject to consecutive stress cycles as the fault front approaches the site of observation. When the front of a thrust fault has passed the site of observation, the hanging wall becomes decoupled from its former substratum, and can be transported as a nappe. If so, the pre-decoupling stress history may be preserved in the rocks at the nappe base.

The base of the Preveli nappe, Crete, is formed by a 30 m thick cataclasite. Above the cataclasite, the metamorphic rocks in the interior of the nappe show structures, indicating (1) deformation of quartz by dislocation creep at high differential stress, (2) brittle failure and vein formation controlled by ductile creep of the host rock, and (3) deformation of the vein quartz in the regime of low-temperature plasticity. Overprinting relations show that these features formed in a systematic sequence. All microstructures are also observed inside the rotated clasts in the cataclasite. The cataclasite is crosscut by undeformed pseudotachylite veins.

For our site of observation we propose: (1) The stress cycles recorded in the quartz microfabrics are related to a sequence of earthquakes, during which the fault front propagated closer to the site of observation. (2) The cataclasite formed immediately after passage of the fault front from an intensively fractured wake left behind by the process zone. (3) Formation of pseudotachylite veins indicates injection of frictional melts after cataclasite formation. (4) The undeformed preservation of these pseudotachylite veins indicates localization of any subsequent deformation into the underlying weak thrust plane. Notably, nappe transport after pseudotachylite injection did not leave a discernible imprint at the site of observation. (5) All this happened in the middle crust, as indicated by the composition of phengites grown in the pseudotachylite matrix during devitrification. We propose that cataclasite and pseudotachylite formation mark the end of a history of coseismic loading and postseimic stress relaxation at the site of observation. Such type of record may be systematically found at the base of thrust sheets decoupled in the middle crust.