Style Of Shortening Research Articles

STRUCTURE AND EVOLUTION OF THE TEREK‐CASPIAN FOLD‐AND‐THRUST BELT: NEW INSIGHTS FROM REGIONAL SEISMIC DATA

The Terek‐Caspian fold‐and‐thrust belt along the northern flank of the Greater Caucasus Mountain Range together with the adjacent foreland basin is one of the oldest oil‐producing regions in Russia. Despite the long history of exploration, recently acquired seismic data has provided new insights about the structural architecture and evolution of this area. Its structural development during the Neogene was constrained by a syn‐extensional tectonic fabric inherited from Jurassic rifting and extension of the Great Caucasus Basin. The structural framework of this basin controlled the distribution of syn‐extensional deposits, and the Cenozoic reactivation of lateral ramps resulted in along‐strike variations in structural style. Thus western, central and SE segments of the Terek‐Caspian foldbelt are recognised and are referred to here as the Terek‐Sunzha fold zone, the Dagestan Promontory, and the Maritime Zone in southern Dagestan.Three principal episodes of Cenozoic compression in the Terek‐Caspian fold‐and‐thrust belt took place. The first episode in the Oligocene resulted in the inversion of pre‐existing normal faults with the coeval development of a foreland basin to the north of the thrust belt. The dominance of sediments of northerly provenance in the foreland basin suggests there was only moderate uplift of the Greater Caucasus at this time. However, significant uplift of the orogenic belt took place during later phases of Sarmatian (Late Miocene) and Akchagylian (Late Pliocene) compression. Erosion of the uplifting Greater Caucasus gave rise to the development of large‐scale, northerly prograding clinoforms which are clearly observed on seismic profiles in the foreland basin. Shortening was largely accommodated by wedge‐shaped thrusting facilitated by the presence of mechanically weak stratigraphic units.Structural development of the Terek‐Sunzha fold zone in the west of the Terek‐Caspian fold‐and‐thrust belt was largely controlled by a Tithonian salt layer which provided an efficient basal detachment surface and which also supplied material to squeezed diapirs in front of the belt. To the east, the plan‐view shape and internal architecture of the Dagestan Promontory were influenced by the areal extent of the Lower‐Middle Jurassic depocentre of the palaeo‐Volga delta which is up to 10 km thick. In the Maritime Zone, the style of shortening was mostly controlled by the presence of a pre‐existing structural high composed of folded Palaeozoic‐Triassic strata in front of the fold‐thrust belt.

Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension, Romero Wash–Tecolote Ranch area, southeastern Arizona

Laramide structures that are exposed along much of the Laramide porphyry copper province of southeastern Arizona have been dismembered and tilted by superimposed Cenozoic normal faults, such that the overall style of shortening (e.g., thin-skinned, low-angle thrusts versus basement-cored, moderate-angle reverse faults) is unclear pending a compelling reconstruction of superimposed extension. This study integrates new geologic mapping in the Romero Wash–Tecolote Ranch area with earlier work and utilizes palinspastic reconstructions of structures related to extension and shortening to determine the geometry of structures in this segment of the Laramide orogenic belt and their relationship to local products of magmatism and hydrothermal alteration. Geologic mapping indicates that multiple generations of Cenozoic normal faults can be identified through crosscutting relationships. Near both ends of the study area, the oldest synextensional strata dip vertically, indicating that the area has been tilted 90° eastward during extension. Structural reconstructions of normal faults reveal two reverse faults within the area, the Romero Wash fault and the Tecolote fault. Once restored, both faults originally dipped in opposite directions at moderate angles and demonstrate clear evidence for associated fault-propagation folds. As a result of 90°E tilting by Cenozoic normal faulting, the Romero Wash fault became an overturned reverse fault, whereas the Tecolote fault became an apparent normal fault. Constraints from the surface geology and forward models indicate that the Tecolote fault has ∼2.8 km of displacement on a fault that had an initial dip of 55°E, whereas the Romero Wash fault has ∼0.75 km of displacement on a fault that had an initial dip of 58°W. Considering their relative displacements and orientations, the Romero Wash fault is interpreted to be a backthrust to the Tecolote fault. These results are consistent with the interpretation that Laramide shortening in southeastern Arizona was dominated by basement-cored uplifts. Based on reconstruction of a cross section, the amount of Laramide shortening ( e = Δ l / l i ) in the study area was 1.8 km or 20%, and the total amount of subsequent Cenozoic extension ( e = Δ l / l i ) was 14.6 km or 200%. Two stocks of granodiorite, with associated porphyry dikes and related hydrothermal alteration, occur within the study area. Sericitic and ­propylitic ­alteration and sparse quartz veins are present without observed potassic altera­tion. A barren porphyry dike (65.9 ± 0.7 Ma, U-Pb zircon) cuts the Romero Wash fault, limiting the upper age of the fault. Similar dikes nearby are sericiti­cally altered, suggesting that alteration postdates shortening, similar to interpretations reached at the nearby porphyry copper systems of Ray, Resolution, and Kelvin-Riverside.

Evolution of a poly-deformed relay zone between fault segments in the eastern Southern Alps, Italy

Abstract In the eastern Southern Alps (NE Italy), Liassic north–south extensional structures are prominent. The southern Trento Platform also experienced extension during the Palaeogene, when reactivation of some pre-existing faults occurred, coupled with nucleation of new faults. During Neogene shortening, these structures were reactivated once again, but with strike-slip kinematics. In this framework, the Gamonda–Tormeno restraining stepover represents the final result of an overlap zone which evolved through time. In the first stage (Lias to Early Cretaceous) a prominent splay developed at the tip of the Gamonda Fault by lateral propagation and breaching of independent segments. At the same time, there was kinematic interaction between the antithetic Gamonda and Tormeno faults, followed by diachronous motion on crossing faults and the development of a narrow graben. During the second stage of extensional tectonics (Palaeocene to Early Oligocene), the reactivation and propagation of the overlapping faults along with the generation of new faults led to deepening of the graben. In the third stage (Miocene to present), the final structure of a strike-slip restraining stepover was accomplished. Due to the mechanical stratigraphy and complex inherited architecture of the relay zone where stratigraphic sequences with different rheological properties are juxtaposed, the style of shortening is different in the western and eastern sides of the stepover. The Gamonda–Tormeno structure represents a unique example of how a relay zone may change through time.

Pure and simple shear plateau uplift, Altiplano-Puna, Argentina and Bolivia

The concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central Andean plateau. Since 10 Ma, discrete segments of the plateau exemplify pure or simple shear modes. Characteristics of the simple shear mode are a flanking thin-skinned thrust belt, and subdued relief and lack of significant Late Miocene to Recent deformation within the plateau. The pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick-skinned foreland deformation involving seismic deformation of the crust to 30 km or more. These modes of crustal thickening correlate with changes in lithosphere thickness, modes of isostatic compensation, broad wavelength topography and back-arc magmatism. The correlation of the boundary between pure and simple shear segments with major changes in pre-existing anisotropies in the foreland of the orogen suggests that events which long pre-date the Andean orogeny indirectly controlled the along-strike variation in the magnitude and style of shortening in the central Andes and may have influenced the development of flat subduction.

The kinematics of inversion in the Danish Central Graben

Summary The Danish Central Graben is a complex mosaic of half-grabens and structural highs that formed during Permo-Triassic and Upper Jurassic rift phases at the confluence of the NW-SE and N-S trending elements of the Central North Sea Rift. The extensional structures forming the margins to the rift were reactivated in a reverse sense in the Late Cretaceous as part of the regional inversion of basins in the southern North Sea. Deposition proceeded throughout the deformation, and thickness variation in the deep marine Chalk succession is directly attributable to compressional deformation of the basin floor. The total shortening across the rift amounts to only a few percent, and the structural expression of the reactivated margins is typically as low amplitude folds with minor associated reverse faulting. Compressional reactivation was restricted to basement faults, and the shallow inclination of the margin faults may have been an important factor in their subsequent reactivation. Zechstein evaporites and overpressured Jurassic shales influenced the development of some of the compressional structures. The orientation of the growth fold axes is consistent with a net convergence in an approximately NE-SW direction. The fold axes are arranged in a segmented pattern which can be related to the underlying basement fabric. This segmentation not only resulted in variations in the amounts and style of shortening along the length of the rift, but seems to have influenced the timing of the deformation. Seismic-stratigraphic analysis of the Upper Cretaceous sequence indicates that the compressional deformation was markedly diachronous. Abrupt changes in the deformational chronology are found to occur across the transverse segment boundaries.