Impact of mechanical stratification on the structural style of the Lublin Basin, SE Poland: results of seismic interpretation and implications for quantification of deformation within the frontal parts of thin-skinned fold-and-thrust belts
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Abstract We demonstrate how lithological and mechanical stratification of Ediacaran–Carboniferous sedimentary package governs strain partitioning in the Lublin Basin (LB) which was incorporated in the marginal portion of the Variscan fold-and-thrust belt. Based on the geometry of seismic reflectors, the pre-Permian–Mesozoic sedimentary sequence was subdivided into two structural complexes differing in structural style. The lower one reveals forelandward-vergent imbrication, while the upper one comprises fold train, second-order deformations, and multiple local detachments. Lithological composition of the upper structural complex controlled geometry, kinematics, and position of compressional deformations in stratigraphic profile. System of foreland-vergent thrusts which links lower and upper detachment developed due to efficiency of simple shear operating in heterogeneous clastic-carbonate-evaporitic strata of the Lower–Upper Devonian age. Internal homogeneity promoted the formation of conjugate sets of thrusts in Silurian shales and Upper Devonian limestones. Structural seismic interpretation combined with sequential restoration revealed localised thickening of Devonian strata and up to 5% difference in length of Devonian horizons. This mismatch is interpreted as a manifestation of distributed shortening, including layer-parallel shortening (LPS), which operated before or synchronously to the initiation of folding. The amount of distributed strain is comparable with numbers obtained in external parts of other fold-and-thrust belts. The outcomes derived from this study may act as a benchmark for studying variability in a structural style of multilayered sequences which were incorporated in the external portion of other fold-and-thrust belts.Keywords:
Devonian
Imbrication
Stylolite
Strain partitioning
Abstract We demonstrate how lithological and mechanical stratification of Ediacaran–Carboniferous sedimentary package governs strain partitioning in the Lublin Basin (LB) which was incorporated in the marginal portion of the Variscan fold-and-thrust belt. Based on the geometry of seismic reflectors, the pre-Permian–Mesozoic sedimentary sequence was subdivided into two structural complexes differing in structural style. The lower one reveals forelandward-vergent imbrication, while the upper one comprises fold train, second-order deformations, and multiple local detachments. Lithological composition of the upper structural complex controlled geometry, kinematics, and position of compressional deformations in stratigraphic profile. System of foreland-vergent thrusts which links lower and upper detachment developed due to efficiency of simple shear operating in heterogeneous clastic-carbonate-evaporitic strata of the Lower–Upper Devonian age. Internal homogeneity promoted the formation of conjugate sets of thrusts in Silurian shales and Upper Devonian limestones. Structural seismic interpretation combined with sequential restoration revealed localised thickening of Devonian strata and up to 5% difference in length of Devonian horizons. This mismatch is interpreted as a manifestation of distributed shortening, including layer-parallel shortening (LPS), which operated before or synchronously to the initiation of folding. The amount of distributed strain is comparable with numbers obtained in external parts of other fold-and-thrust belts. The outcomes derived from this study may act as a benchmark for studying variability in a structural style of multilayered sequences which were incorporated in the external portion of other fold-and-thrust belts.
Devonian
Imbrication
Stylolite
Strain partitioning
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Stylolite
Pressure solution
Strain partitioning
Cataclastic rock
Overburden pressure
Brittleness
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Anticline
Imbrication
Syncline
Outcrop
Thrust fault
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Bedding
Stylolite
Imbrication
Décollement
Bed
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Imbrication
Molasse
Monocline
Wedge (geometry)
Accretionary wedge
Allochthon
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In the western Sicilian fold and thrust belt, large clockwise rotations of allochthons occurred during late Cenozoic contraction of part of the southern Tethyan margin. The magnitude of rotation decreases stepwise from over 120° in the upper sheets, lying on the north coast of Sicily, to no appreciable rotation in the frontal portion of the belt flanking the southern coast of the island. The allochthons are composed of imbricate thrust sheets derived primarily from individual basin and platform assemblages of the old Tethyan margin. Paleomagnetic and structural data indicate that the rotation of the allochthons was accommodated by coherent torsional displacements on relatively low‐angle detachment surfaces. Timing relations for the imbrication history of the Sicilian fold and thrust belt are derived from stratigraphic overlap and local involvement of sediments deposited in a series of foreland and piggyback basins. The locus of deposition within successive foreland basins first migrated easterly then southerly during progressive deformation in the orogen. Imbrication began in the early Miocene (Burdigalian‐Langhian) and continued at least through the early Pleistocene and appears to be continuing today. Rotation is related to thrusting and accompanies a 70° change in the tectonic transport direction from easterly to southerly. Easterly striking, right‐oblique transpressional faults and associated northeasterly trending folds postdate thrust sheet rotation in the interior of the thrust belt and were active contemporaneously with south‐directed thrusting in the foreland region. Pleistocene and possibly older (late Pliocene?) extension strongly modified the older thrust morphology along the Tyrrhenian coast of northwestern Sicily, with the development of down‐to‐the‐north listric normal faults. The extensional structures apparently are related to the opening and subsequent deformation of the Tyrrhenian Sea to the north.
Imbrication
Sicilian
Clockwise
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Foothills
Imbrication
Thrust fault
Mountain formation
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Stylolite
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Emplacement‐driving mechanisms for thrust systems have been briefly reviewed and compared with distinctive structural styles in thrust belts. A B‐type mode, which implies a push from the rear and an elastic‐brittle behavior of the rock mass, appears to be suitable as a general model for the emplacement of thrust sheets in thin‐skinned fold and thrust belts where stratal shortening of sedimentary covers and imbrication of upper level crustal rocks occur. In such an environment that the mechanical behavior of the crustal section involved the mode, and timing of deformation, and the geometry of thrust systems are all strongly dependent upon the coupling between induced tectonic strain and transient, close to lithostatic pore fluid pressure buildup over a critical areal extent. The model presented here does not require a weak basal horizon for the detachment of thrust sheets, and pore pressure P p does not need to be high at once under the entire length of the thrust wedge. Rather, the transient buildup of P p and hence the spatial and temporal progressive weakening of the rock mass are directly coupled with deformation.
Imbrication
Wedge (geometry)
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