Basin inversion

The Late Eocene to Early Miocene Alpine-Carpathian fold-and-thrust belt (FTB) lies in the transition between the Eastern Alps and the Western Carpathians, southeast of the Bohemian crystalline massif. Our study shows the involvement of crystalline basement from the former European Jurassic continental margin in two distinct events: a first extensional event coeval with Eggerian to Karpatian (ca. 28-16 Ma) thin-skinned thrusting reactivated the rift basement fault array and resulted from the large degree of lower plate bending promoted by high lateral gradients of lithospheric strength and slab pull forces. Slab-break off during the final stages of collision around Karpatian times (ca. 17-16 Ma) promoted large wavelength uplift and an excessive topographic load. This load was reduced by broadening the orogenic wedge through the reactivation of the lower plate deep detachment beneath and ahead of the thin-skinned thrust front (with the accompanying positive inversion of the basement fault array) and ultimately, by the collapse of the hinterland summits enhanced by transtensional faulting. Although this work specifically deals with the involvement of the basement in the Alpine-Carpathian Junction, the main conclusions are of general interest to the understanding of orogenic systems.

This work presents numerical experiments of inversion of rift basins and consequent sub-thrust imbrication in tectonic wedges. Half-graben basins initially develop and then are covered with a post-rift sequence bearing a décollement-prone horizon (i.e., the upper décollement). A total of twelve models of tectonic inversion have been conducted varying (i) the strength of inherited extensional fault arrays and (ii) applying different fluid pressure ratios (i.e., strength) within syn-rift strata. Combinations of those were simulated using different internal angles of friction for the inherited faults, different strengths for the syn-rift infill and for the upper décollement. Results show that changes in relative strength between inherited faults, syn-rift deposits and the upper crustal décollement leads to important variations in structural styles. Weak faults systematically favour the compressional reactivation of inherited extensional faults. Weak syn-rift sediments favour hanging wall by-pass structures instead of fault reactivation and less internal deformation of the syn-rift deposits. Weak upper décollements supports the accretion of basement in a hinterland antiformal stack, decoupling of basement and cover, and forward tectonic transport of rift basins. Strong upper crustal décollements favours basement and cover coupling, can lead to fault reactivation in the absence of weak faults and syn-rift sediments, however combinations of weak faults and strong upper décollement shows fault reactivation, weak syn-rift sediments and strong upper décollement form hanging wall by-pass structures. Modelling results are compared to natural case studies.

In the Spanish Pyrenees, the Sant Corneli-Bóixols
thrust-related anticline displays an outstandingly preserved
growth strata sequence. These strata lie on top of a major
unconformity exposed at the anticline’s forelimb that divides
and decouples a lower pre-folding unit from an upper
syn-folding one. The former consists of steeply dipping to
overturned strata with widespread bedding-parallel slip indicative
of folding by flexural slip, whereas the syn-folding
strata above define a 200m amplitude fold. In the inner and
outer sectors of the forelimb, both pre- and syn-folding strata
are near vertical to overturned and the unconformity angle
ranges from 10 to 30. In the central portion of the forelimb,
syn-folding layers are gently dipping, whereas the angular
unconformity is about 90 and the unconformity surface displays
strong S–C shear structures, which provide a top-toforeland
slip sense. This sheared unconformity is offset by
steeply dipping faults, which are at low angles to the underlying
layers of the pre-folding unit. Strong shearing along
the unconformity surface also occurred in the inner sector of
the forelimb, with S–C structures providing an opposite, topto-
hinterland slip sense. Cross-cutting relationships and slip
senses along the pre-folding bedding surfaces and the unconformity
indicate that regardless of its orientation, layering
in the pre- and syn-folding sequences of the Sant Corneli-
Bóixols anticline were continuously slipped. This slipping
promoted an intense stress deflection, with the maximum
component of the stress tensor remaining at low angles to
bedding during most of the folding process.

The Asturian Basin is located on the coastline of the North Iberian Margin. This basin is dissected by long-lived E-, NE- and NW-striking faults that delineate a series of extensional fault blocks that became shortened during the Upper Cretaceous to Cenozoic Alpine convergence. In the Conejera cove, the NE-striking and SE-dipping Conejera Fault displays a remarkable example of contractional deformation, promoted by the mechanical contrast within the Lower to Middle Jurassic stratigraphic series. Field observations and structural analysis carried out in this study reveal: i) a first system of orthogonal cross-joints oblique to the Conejera Fault and other major onshore boundary faults, ii) a second system of meso-extensional faults parallel to the Conejera Fault, and developed by the reactivation and linkage of the orthogonal cross-joints and iii) a series of contractional folds, thrusts and
pressure solution with a predominant NE to ENE trend. Observed relationships and structural analysis suggest an obliquity between the here inferred direction of the Late Jurassic-Early Cretaceous stretching (i.e. about N015E) and the onshore boundary faults, whereas the contractional structures are broadly parallel to the NE-striking Conejera Fault and suggest a roughly SSE- to SE-oriented Alpine convergence.

The Maestrat basin was one of the most subsident basins of the Mesozoic Iberian Rift
system, developed by a normal fault system which divided it into sub-basins. Its Cenozoic inversion
generated the N-verging Portalrubio–Vandellòs fold-and-thrust belt in its northern margin, detached in
the Triassic evaporites. In the hinterland, a 40 km wide uplifted area, in the N–S direction, developed,
bounded to the N by the E–W-trending, N-verging Calders monocline. This monocline is interpreted
as a fault-bend fold over the ramp to flat transition of the E–W-trending, N-verging Maestrat Basement
Thrust, and also indicates the transition from a thick-skinned (S) to a thin-skinned (N) style of
deformation. This paper presents a kinematic evolutionary model for the northern margin of the basin
and a reconstruction of the Maestrat Basement Thrust geometry, generated by the inversion of the
Mesozoic normal fault system. It contains a low-dip ramp (9°) extended southwards more than 40 km,
attaining a depth of 7.5 km. As this thrust reached the Mesozoic cover to the foreland, it propagated
across the Middle Muschelkalk evaporitic detachment, generating a nearly horizontal thrust which
transported northwards the supra-salt cover, and the normal fault segments within it, for c. 11–13 km.
The displacement of the basement in the hanging-wall of the low-dip basement ramp generated the
40 km wide uplifted area, while the superficial shortening was accumulated in the northern margin
of the basin – which contains the thinnest Mesozoic cover – developing the Portalrubio–Vandellòs
fold-and-thrust belt.

The Maestrat Basin experienced two main rifting events: Late Permian-Late Triassic and Late Jurassic-Early Cretaceous, and was inverted during the Cenozoic Alpine orogeny. During the inversion, an E-W-trending, N-verging fold-and-thrust belt developed along its northern margin, detached in the Triassic evaporites, while southwards it also involved the Variscan basement. A structural study of the transition between these two areas
is presented, using 2D seismic profiles, exploration wells and field data, to characterize its evolution during the Mesozoic extension and the Cenozoic contraction.
The S-dipping Maestrat basement thrust traverses the Maestrat Basin from E to W; it is the result of the Cenozoic inversion of the lower segment–within the acoustic basement–of the Mesozoic extensional fault system that generated the Salzedella sub-basin. The syn-rift Lower Cretaceous rocks filling the Salzedella sub-basin thicken progressively northwards, from 350m to 1100m. During the inversion, a wide uplifted area–40km wide in the N-S direction–developed in the hanging wall of the Maestrat basement thrust. This uplifted area is limited to the North by the E-W-trending Calders monocline, whose limb is about 13km wide in its central part, dips about 5ºN, and generates a vertical tectonic step of 800-1000m. We interpreted the Calders monocline as a fault-bend fold; therefore, a flat-ramp-flat geometry is assumed in depth for the Maestrat basement thrust. The northern synformal hinge of the Calders monocline coincides with the transition from thick-skinned to thin-skinned areas. The vast uplifted area and the low-dip of the monocline suggest a very low-dip for the basement ramp, rooted in the upper crust. The Calders monocline narrows and disappears laterally, in coincidence with the outcrop of the Maestrat basement thrust.
The evaporitic Middle Muschelkalk detachment conditioned the structural style. Salt structures are also related to it; they developed during the Late Triassic extension, as deduced from the Keuper seismic reflectors that onlap the folded Upper Muschelkalk and form growth strata above some basement normal faults.