The accretion of foreland basin sediments during early stages of continental collision in the European Alps and similarities to accretionary wedge tectonics

We present structural observations from foreland basin sediments that were incorporated into the orogenic wedge of the central European Alps during early stages of continental collision. Our analysis focuses on the prograde evolution and considers the full history of the sediments ranging from their deposition in the basin to deep burial and metamorphism at temperatures of similar to 320 degrees C. The tectonic evolution is matched with constraints on the diagenetic alteration of the sediments. For this purpose, we calculate the temperatures and depths of sediment compaction and illitization as well as the associated fluid liberation. The data set highlights that the tectonic incorporation of the sediments into the orogenic wedge was strongly controlled by their diagenetic state. Earliest deformation took place during imbrication and frontal accretion of unconsolidated and fluid-saturated sediments. Ductile folding of the sediments occurred already at this stage and was assisted by particulate flow. With the progressive consolidation of the sediments the elastic strength increased, which resulted in an overall embrittlement. This rheological change is recorded by the onset of out-of-sequence thrusting, brittle faulting, and the formation of massive quartz-calcite veins, which took place in the approximate temperature range of the seismogenic zone (i.e., similar to 150-350 degrees C). Moreover, widespread pressure solution resulted in the formation of a penetrative cleavage and records slow but long-lasting deformation at low background strain rates. In summary, the prograde tectonic evolution of the frontal Alpine wedge exhibits many similarities with the structural and mechanical evolution of accretionary wedges at active subduction zones.