Out-of-sequence shortening in an active accretionary prism by km-scale buckle folding revealed by structural and thermochronometric data from the Eastern Greater Caucasus

During subduction, accretionary prisms deform internally to maintain a characteristic shape as their volume increases by addition of lower plate sediment. Subduction zone kinematics and hazards are influenced by when and how accretionary prisms deform internally. We present new structural and thermochronometric data that constrain the timing and style of internal deformation in an active, sub-aerially exposed accretionary prism, the Eastern Greater Caucasus (EGC) orogen in western Asia.

New field mapping reveals pervasive folding of the turbiditic strata that constitute the bulk of the EGC orogen. Folds vary from open to isoclinal and wavelengths vary from 0.5 - 5 km according to the thickness and continuity of over- and underlying sandstone layers, suggesting formation by buckling during horizontal shortening.

Our thermochronometry data include apatite (U-Th)/He (AHe) and apatite fission track (AFT) data from two transects (~ 1 km vertical relief each) collected from sub-vertically dipping fold limbs in the core of the EGC, as well as individual samples from across the range. The toe of the EGC prism consists of a deformed volcaniclastic sequence of lower plate affinity that is juxtaposed against upper plate turbidties by a major fault and is characterized by unreset AFT ages and 1-3 Ma AHe ages. Thermal modeling of these ages indicates slow burial followed by rapid exhumation from 5 Ma to present, likely reflecting accretion of the volcaniclastic sequence at 5 Ma. Samples from the sub-vertical fold limbs in the turbiditic core of the range contain 6-15 Ma AFT ages and 3-5 Ma AHe ages. Notably, the youngest cooling ages are on the structurally highest part of the fold limbs and the oldest cooling ages are on the structurally lowest part of the fold limbs, opposite the age-elevation relationship expected for an exhumed rigid block. The ages are consistent with a model in which exhumation occurred as the strata moved up a thrust ramp, followed by buckle folding after cooling. The inferred buckle folding must have occurred after the youngest cooling ages of these rocks (3 Ma) and therefore followed the accretion of new material to the toe of the prism at 5 Ma. Our data suggest that out-of-sequence, km-scale buckle folding may be an important mechanism of internal thickening to maintain critical taper during accretionary prism growth.