Uniform lithospheric extension predicts basic properties of non-volcanic rifted margins but fails to explain other important characteristics. Significant discrepancies are observed at 'type I' margins (such as the Iberia-Newfoundland conjugates), where large tracts of continental mantle lithosphere are exposed at the sea floor, and 'type II' margins (such as some ultrawide central South Atlantic margins), where thin continental crust spans wide regions below which continental lower crust and mantle lithosphere have apparently been removed. Neither corresponds to uniform extension. Instead, either crust or mantle lithosphere has been preferentially removed. Using dynamical models, we demonstrate that these margins are opposite end members: in type I, depth-dependent extension results in crustal-necking breakup before mantle-lithosphere breakup and in type II, the converse is true. These two-layer, two-stage breakup behaviours explain the discrepancies and have implications for the styles of the associated sedimentary basins. Laterally flowing lower-mantle cratonic lithosphere may underplate some type II margins, thereby contributing to their anomalous characteristics.

[1] Strain-dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones. We investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments. Parametric softening is specified as a linear decrease of the effective internal angle of friction, the effective viscosity, or both in the model rheologies. The sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere. Results are classified according to the symmetry (S) or asymmetry (A) of the deformation of the upper and lower lithosphere during rifting. Strain softening is required for rifting asymmetry but is not always sufficient. A range of model tectonic styles occurs including pure and simple shear modes with focused shear zones. Mode selection is mostly determined by the feedback between two primary controls, the ''dominant'' rheology and the parametric strain-softening mechanisms listed above. Softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology. Model results are consistent with the proposed primary controls and the factors that contribute to these controls. In particular, decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology. Asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities.

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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.

Many areas of the Earth’s crust deform by distributed extensional faulting and complex fault interactions are often observed. Geodetic data generally indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where crustal extension and devastating earthquakes occur in response to regional surface uplift. We constrain fault slip-rates since ~18 ka using variations in cosmogenic 36Cl measured on bedrock scarps, mapped using LiDAR and ground penetrating radar, and compare these rates to those inferred from geodesy. The 36Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar length time intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (104 yr; 102 km) but over shorter timescales most of the deformation may be accommodated by <30% of the across-strike fault array. We attribute the shifts in activity to temporal variations in the mechanical work of faulting.