ALBERT

Description: Surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts. The first-order effects of these parameters have been studied extensively throughout the last decades, but their relative importance remains notoriously difficult to assess and document. We use lithospheric scale plane-strain thermo-mechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins. Extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion. Surface processes are modeled through the combination of a simple sedimentation algorithm, where all negative topography is filled up to a prescribed reference level, and an elevation-dependent erosion model. Our results show that (1) extensional inheritance facilitates the propagation of basement deformation in the retro-wedge and (2) increases the width of the orogen; (3) sedimentation increases the length-scale of both thin-skinned and thick-skinned thrust sheets and (4) results in a wider orogen; (5) erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro-wedge. A comparison of the modeled behaviors to the High Atlas, the Pyrenees and the Central Alps, three extensively studied natural examples characterized by different degrees of inversion, is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure.

Description: We present a new method that can be used to quantitatively evaluate the consistency between balanced section restorations and thermochronological datasets from orogenic belts. We have applied our method to a crustal-scale area-balanced cross-section restoration along a profile in the Central Pyrenees. This restoration is well constrained and supported by a wide variety of geological and geophysical data. Moreover, an extensive thermochronological dataset has been collected independently in the area. We use the structural-kinematic software 2D-Move™ to constrain a set of velocity fields that describes the kinematics of the Central Pyrenees. Using these velocity fields as input for the thermo-kinematic code PECUBE, we derive predictions of the thermal history and a range of thermochronometric ages for the modeled area. We find that the kinematic history of the belt as inferred from section balancing is in good agreement with the published thermochronological data. High-temperature (zircon fission-track and K-feldspar Ar-Ar) data constrain the thermal structure of the belt as well as the timing of underplating. Low-temperature (apatite fission-track and (U-Th)/He) data require late syn-orogenic sedimentary burial of the southern flank of the Pyrenees between Late-Eocene (40 Ma) to Late-Miocene (9 Ma) times, consistent with previous studies, and imply that no such burial occurred on the northern flank.

Description: The origin of large subsidence in intracratonic basins is still under debate. We propose a new and self-consistent model for the formation of those basins, where lithospheric shortening/buckling triggers metamorphism and densification of crustal mafic heterogeneities. We use a forward thermo-mechanical finite element technique to evaluate this mechanism for the typical example of the East Barents Sea basin (EBB) where a very large and compensated subsidence, accommodating an up to 20 km thick sediment succession, is observed. The lower crust in the dynamic model is modelled with petrologic consistent densities for a wet mafic gabbroic composition that depend on pressure and temperature taking into account de-hydration at high PT conditions. The model successfully explains the main characteristics of the EBB, notably the large anomalous and fast subsidence during the late Permian-Early Triassic, its present-day geometry and the absence of a significant gravity anomaly. This article is protected by copyright. All rights reserved.

Description: Many mountain belts exhibit significant along strike variation in structural style with changes in the width of the orogen, the geometry and kinematics of the crustal-scale thrust system, and the degree of partitioning between pro- and retro-wedge deformation. While the main factors controlling first order structural style are understood, the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenees, characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part. Similarly, the prior Mesozoic rifting event was characterized by significant lateral variation in structure. Here we integrate available geological and geophysical data with forward lithospheric scale numerical models. We show that lateral variation in crustal strength attributed to inherited Variscan crustal composition accentuated during Mesozoic rifting explains the variation in structural style observed during Pyrenean mountain building. This article is protected by copyright. All rights reserved.

Description: Abstract We investigate the factors that control the shortening distribution and its evolution through time in orogenic belts using numerical models. We present self‐consistent high‐resolution numerical models that simulate the inversion of a rift to generate an upper crustal antiformal stack, a wide outer pro‐wedge fold‐and‐thrust belt, characterised by a two‐phase evolution with early symmetric inversion followed by formation of an asymmetric doubly‐vergent orogen. We show that a weak viscous salt décollement promotes gravitational collapse of the cover. When combined with efficient erosion of the orogenic core and sedimentation in adjacent forelands, it ensures the thick‐skinned pro‐wedge taper remains subcritical, promoting formation of an upper crustal antiformal stack. Rift inheritance promotes a two‐phase shortening distribution evolution regardless of the shallow structure and other factors. Comparison to the Pyrenees strongly suggests that this combination of factors led to a very similar evolution and structural style.

Description: 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 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 characterised 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/y) was distributed roughly equally between both margins over some 20 My. A quiescent phase (Paleocene) was apparently restricted to the retro-wedge with slow, continuous deformation in the pro-wedge (~0.4 mm/y). 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/y), 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/y). 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.