In natural doubly vergent orogens, the relationship between the pro‐ and retro‐wedges is, as yet, poorly constrained. We present a detailed tectonostratigraphic study of the retro‐wedge of the Eastern Pyrenees (Europe) and link its evolution to that of the pro‐wedge (Iberia) in order to derive insight into the crustal‐scale dynamics of doubly vergent orogens. Based on cross‐section restoration and subsidence analyses, we divide the East Pyrenean evolution into four phases. The first phase (Late Cretaceous) is characterized by closure of an exhumed mantle domain between the Iberian and European plates and inversion of a salt‐rich, thermally unequilibrated rift system. Overall shortening (~1 mm/yr) was distributed roughly equally between both margins over some 20 Myr. A quiescent phase (Paleocene) was apparently restricted to the retro‐wedge with slow, continuous deformation in the pro‐wedge (~0.4 mm/yr). This phase occurred between closure of the exhumed mantle domain and onset of main collision. The main collision phase (Eocene) records the highest shortening rate (~3.1 mm/yr), which was predominantly accommodated in the pro‐wedge. During the final phase (Oligocene), the retro‐wedge was apparently inactive, and shortening of the pro‐wedge slowed (~2.2 mm/yr). Minimum total shortening of the Eastern Pyrenees is ~111 km, excluding closure of the exhumed mantle domain. The retro‐wedge accommodated ~20 km of shortening. The shortening distribution between the pro‐ and retro‐wedges evolved from roughly equal during rift inversion to pro‐dominant during main collision. This change in shortening distribution may be intrinsic to all inverted rift systems.

Seismological data from recent subduction earthquakes suggest that megathrust earthquakes induce transient stress changes in the upper plate that shift accretionary wedges into an unstable state. These stress changes have, however, never been linked to geological structures preserved in fossil accretionary complexes. The importance of coseismically induced wedge failure has therefore remained largely elusive. Here we show that brittle faulting and vein formation in the palaeo-accretionary complex of the European Alps record stress changes generated by subduction-related earthquakes. Early veins formed at shallow levels by bedding-parallel shear during coseismic compression of the outer wedge. In contrast, subsequent vein formation occurred by normal faulting and extensional fracturing at deeper levels in response to coseismic extension of the inner wedge. Our study demonstrates how mineral veins can be used to reveal the dynamics of outer and inner wedges, which respond in opposite ways to megathrust earthquakes by compressional and extensional faulting, respectively.

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~320°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.,~150-350°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.

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