Reverse fault step at Engel Peaks, Antarctic Peninsula

Abstract

A steep reverse fault at Engel Peaks, Antarctic Peninsula, cuts granophyre and overlying volcanic rocks and incorporates a lower angle zone or ‘fault step’. The reverse fault step differs from the more familiar thrust ramp in that it cuts down-section in the footwall in the direction of tectonic transport. Accommodation of the fault zone to this step and its associated sidewall are discussed with reference to kinematic models for pull-apart zones and for the movement of thrust sheets over non-planar surfaces. During movement on the main fault the step acted as a fault jog and pull-apart zone, preserving cataclasite, breccia and other structures demonstrating a brittle to ductile deformation history. Ultra-fine-grained rock in the step zone is the result of dynamic recrystallization of ultracataclasite. During episodes of brittle slip, the fault step acted as a dilational fault jog between the main steep reverse fault segments. Rotation of mylonite into the reverse fault step and its sidewall involved flexural flow parallel to foliation. Movement of material through the step zone resulted in shear parallel to the base of the step, enhancement of breccia layering, mixing of lithologies and cleavage formation; it also involved extension at a high angle to the main movement direction to accommodate the extra length of the fault zone. Late, steep, reverse fault zones propagating from the main fault folded the breccias in the step and crenulated the earlier cleavage. The fault step was a zone of brittle dilation, hydration, diffusive mass transfer and mineralization.