STRAIN AND RHEOLOGY WITHIN A TRACHYTIC FLOW: A meso-scale example from the Terceira Island (Azores Archipelago)

Abstract

Fault kinematic indicators are widely used to constrain local and regional stress fields at the origin of a fault population. We present clear tectonic like structures, which were developed and are confined within a trachytic flow at the Terceira Island. Its tectonic pattern when considered as a whole is consistent with the dynamic history of the cooling flow rather than with the regional tectonic stress field. The studied trachytic flow (Ponta da Serreta) is several tens of meters high and is capped by a lava clinker. Observed structures are brittle to ductile shear zones developing low angle frontal (synthetic and antithetic), lateral ramps and conjugate sets of vertical tear faults accommodating differential movements within the flow. Identified fault kinematic criteria are fault grooves and striations sometimes presenting cross-cutting relations, and ranging from dip slip to oblique slip. The stereographic plot of existing fault planes and fault kinematic criteria is consistent with a local stress field generated by the gravity flow of the freezing lava. Observed structures, at the meso-scale, express distinct strain mechanisms imposed by significant rheological variations, resulting from drastic centrifugal thermal gradients. Flow core, corresponding to the front of the outcropping lava lobe, presents dominantly ductile deformation. It displays a fan like pattern of stretching lineations materialized by alignment of feldspar crystals growing upon complex magmatic foliations. Ductile to brittle transitions within volcanic flow are observed from either co-planar or oblique discrete fault surfaces developing in lava lobe outer shells and they are interpreted as incremental colder pulses postdating quasi-coeval magmatic foliations and sharing a common kinematic history for the entire flow unit. The physical similarity between such cooling induced structures and those produced by a regional tectonic stress field imply that interpretation of volcanic lavas outcrops from past terrains have to be evaluated in a careful manner as they might produce erroneous tectonic interpretations otherwise.