Parasitic folds with wrong vergence: How pre-existing geometrical asymmetries can be inherited during multilayer buckle folding

Parasitic folds are typical structures in geological multilayer folds; they are characterized by a small wavelength and are situated within folds with larger wavelength. Parasitic folds exhibit a characteristic asymmetry (or vergence) reflecting their structural relationship to the larger-scale fold. Here we investigate if a pre-existing geometrical asymmetry (e.g., from sedimentary structures or folds from a previous tectonic event) can be inherited during buckle folding to form parasitic folds with wrong vergence. We conduct 2D finite-element simulations of multilayer folding using Newtonian materials. The applied model setup comprises a thin layer exhibiting the pre-existing geometrical asymmetry sandwiched between two thicker layers, all intercalated with a lower-viscosity matrix and subjected to layer-parallel shortening. When the two outer thick layers buckle and amplify, two processes work against the asymmetry: layer-perpendicular flattening between the two thick layers and the rotational component of flexural flow folding. Both processes promote de-amplification and unfolding of the pre-existing asymmetry. We discuss how the efficiency of de-amplification is controlled by the larger-scale fold amplification and conclude that pre-existing asymmetries that are open and/or exhibit low amplitude are prone to de-amplification and may disappear during buckling of the multilayer system. Large-amplitude and/or tight to isoclinal folds may be inherited and develop type 3 fold interference patterns.     

An alternative interpretation for the formation of doubly plunging folds in sandstone terrains

Folds are marvellous features of mountain terrains, but despite extensive research, many fundamental problems have still not been solved. In terrains of sandstone, fold hinges are rarely straight lines but curved, forming a pattern characterized by doubly plunging, elliptical dome‐and‐basin structures. Such structures are an obvious manifestation of coeval or successive shortening deformation in two orthogonal principal directions in the horizontal plane. Based on an anatomic investigation of the fold pattern of sandstone beds at the rocky beaches of Saint‐Jean‐Port‐Joli (Quebec, Canada), we propose that the doubly plunging folds may result from a transition from a plane deformation to a constrictional deformation due to auxetic effects of quartz‐rich rocks. The sandstone beds possessed potentially such negative values of Poisson's ratio that, when placed under compression in one direction, they become contracted in the transverse direction, producing a series of doubly plunging folds. Further work is needed to approve or disapprove the interpretation.     

The glaciotectonic deformation of Quaternary sediments by fault-propagation folding

Fault-propagation folding is an important yet seldom recognised structural style within sediments affected by glacier-induced deformation. Fault-propagation folds develop in the hanging wall of low angle thrust faults and compensate part of the slip along the fault. Field examples are recognised across northern Europe, in glaciotectonic complexes of north Germany, Wales and the Isle of Man. The recognition of the fault-propagation fold mechanism in glaciotectonic deformation is important because resultant structures are related to exactly the same phase of deformation (i.e. the same phase of ice advance), and thus play a critical role in analyses of the temporal and spatial evolution of glacier-induced deformation. Some field examples show monoclinal geometries that are in good agreement with predictions of trishear kinematic theory. The trishear approach is appropriate to model these structures because the structures analysed in the field and simulated below show characteristics that are compatible with fault-propagation folds that were produced by trishear kinematics. The curved forelimb and the monocline geometry of the fault-propagation folds fit to the trishear model. The occurrence of footwall synclines is also in good agreement with trishear kinematics. These synclines show the typical thickening of the strata in the hinge. With respect to the modelling output, most important factors for the structural evolution of the fault-propagation folds is the ramp angle of the thrust, the position of the tip line and the propagation-to-slip ratio along the fault. This fits to observations made by previous studies at large scale fault-propagation folds in fold-and-thrust belts.     

Regional significance of kilometric-scale north-east vergent recumbent folds associated with east to south-east directed shear on the southern border of the Central Iberian Zone (Hornachos-Oliva region,Variscan belt,Iberian Peninsula)

On the southern border of the Central Iberian Zone there are two sectors with different styles of deformation. To the south-west, in the Hornachos sector, large-scale recumbent folds associated with ductile shearing can be seen. This shearing is characterized by a direction of movement parallel to the fold axes and can be correlated for 150 km along strike. The K-values of the strain ellipsoid range from 0.8 to 2.0. Stretching in the X direction, parallel to the recumbent fold axes, is more than 100%. To the north-east, in the Oliva sector, first-phase folds are upright and the strain intensity is lower than in the Hornachos sector. Metamorphic, geometric and kinematic considerations lead us to conclude that the shearing in the Hornachos sector is better explained as conjugate to a main shear zone along which the southern border of the Central Iberian Zone is moved onto the Ossa-Morena Zone. This main thrust is at present obliterated by a left-lateral extensional shear zone that affects a high pressure exotic unit located between the Central Iberian and the Ossa-Morena Zones. This high pressure unit constitutes a suture of the Variscan belt in the Iberian Peninsula.     

Recent Tectonic Structures in a Transect of the Central Betic Cordillera

—The Betic Cordillera has undergone recent Alpine deformations related to the Eurasian-African plate interaction boundary. Most of the present-day relief has been built up since Tortonian times, and is related to the development of folds and faults that are overprinted on older deformations, and some of the faults may be considered as out-of-sequence. The combination of geophysical and geological data makes it possible to determine the main features of the recent tectonic structures, or those recently active, in its central transect. The main fault is a crustal detachment that separates a footwall constituted by the Iberian Massif and a hanging wall formed by the rocks of the Betic Cordillera. While the footwall is practically undeformed, the hanging wall has been folded and faulted. The folds are mainly E-W to NE-SW and have larger sizes and higher related relieves towards the South. The reverse faults are mainly concentrated in the northern mountain front. However, normal faults affect the southern part of the Cordillera and are associated with the development of large asymmetrical basins such as the Granada Depression. In this setting, the slip along the crustal detachment is variable and should increase southwards. The model of the recent tectonics in the central transect of the Cordillera is compatible with the presence of an active subduction in the Alboran Sea, and contrasts notably with the setting of the eastern Betic Cordillera, mainly deformed by transcurrent faults.     

Kinematics of large scale asymmetric folds and associated smaller scale brittle — Ductile structures in the proterozoic somnur formation, pranhita — Godavari Valley, South India

The development of structural elements and finite strain data are analysed to constrain kinematics of folds and faults at various scales within a Proterozoic fold-and-thrust belt in Pranhita-Godavari basin, south India. The first order structures in this belt are interpreted as large scale buckle folds above a subsurface decollement emphasizing the importance of detachment folding in thin skinned deformation of a sedimentary prism lying above a gneissic basement. That the folds have developed through fixed-hinge buckling is constrained by the nature of variation of mesoscopic fabric over large folds and finite strain data. Relatively low, irrotational flattening strain (X:Z-3.1-4.8, k<1) are associated with zones of near upright early mesoscopic folds and cleavage, whereas large flattening strain (X:Z-3.9-7.3, k<1) involving noncoaxiality are linked to domains of asymmetric, later inclined folds, faults and intense cleavage on the hanging wall of thrusts on the flanks of large folds. In the latter case, the bulk strain can be factorized to components of pure shear and simple shear with a maximum shearing strain of 3. The present work reiterates the importance of analysis of minor structures in conjunction with strain data to unravel the kinematic history of fold-and-thrust belts developed at shallow crustal level.     

南天山库车秋里塔格褶皱带三维构造分析

笔者利用库车秋里塔格地区3000km的二维地震反射资料，结合地表构造测量成果，分段叙述秋里塔格褶皱带的构造几何学和运动学性质，说明构造交汇部位断层和褶皱的叠加过渡关系，并通过二维构造剖面的组合，建立秋里塔格褶皱带的三维构造几何模型。研究发现秋里塔格褶皱带为浅部断层传播褶皱与深部断层转折褶皱叠加形成的复合型背斜带，深部台阶状逆断层的叠加作用、叠加断层位移量的转换、断层断坡高度的变化造成地表背斜沿走向发生变化，笔者通过测量断层叠加方式、断层位移量转换、断层断坡高度，说明秋里塔格褶皱带背斜叠加、扭曲、分叉现象的构造机理，并且给出了秋里塔格褶皱带断层的滑移量。     

金沙江溪洛渡地区叠加褶皱及工程地质意义

金沙江溪洛渡地区古—中生代沉积地层中发育复杂的褶皱构造，它们是喜马拉雅运动期间经3期褶皱叠加而成，其叠加方式主要有跨褶、移褶、限褶、重褶等。大比例尺填图及构造解析表明，区内第3期褶皱——箐口复式背斜形成地下分水岭，构成了金沙江溪洛渡水电站库首至下游可能渗漏路径上的挡水墙，岩溶管道式渗漏不会发生。区内第3期褶皱的首次厘定及研究，对溪洛渡巨型水电站库首渗漏研究具有极为重要的意义。     

Les plis hercyniens kilométriques couchés vers l'ouest-sud-ouest dans la région du pic du Midi d'Ossau–col du Somport (zone axiale des Pyrénées occidentales)Kilometric westward Hercynian recumbent folds in the western Pyrenees (Pic du Midi d'Ossau area)

The Palaeozoic of the western Pyrenees shows two superposed tectonics easily defined by their different geometry and the major unconformity of the Permian sediments and volcanics on the Devono-Carboniferous series: an Hercynian tectonic found only in the pre-Permian series, characterised by kilometric westward recumbent folds with a weak cleavage; a Pyrenean tectonic, characterised by tight east–west folds, upright to overturned to the south with slaty cleavage, which is the only deformation found in the Permian and Mesozoic series and the second deformation in the pre-Permian Palaeozoic. The Hercynian folding, roughly perpendicular to the trend of the Pyrenees characterises the northern branch of the Ibero-Armorican virgation. To cite this article: P. Matte, C. R. Geoscience 334 (2002) 773–779.     

Mountain Front Development by Folding and Crustal Thickening in the Internal Zone of the Betic Cordillera-Alboran Sea Boundary

The boundary between the Alboran Sea and Betic Cordillera is a good example of a fold related mountain front in the Internal Zone of an alpine mountain range. Since the late Miocene, NNW-SSE convergence between the Eurasian and African plates has produced shortening and related orthogonal extension. To improve the characterisation of the geometry of the deep structure in the region and to establish the recent tectonic evolution of the mountain front, well logs and newly acquired geophysical data (multichannel reflection seismic and gravimetric surveys) have been interpreted and integrated with available surface data. The most marked tectonic structure corresponds to large antiforms and synforms of ENE-WSW trend which are related to mountain ranges and basins, respectively. The fold belt continues toward the northern continental shelf of the Alboran Sea. The fold vergence is generally northwards and its amplitude decreases progressively towards SSE, until disappearring in a sharp boundary where the reflectors are undeformed. The deep geometry suggests that fold growth started during upper Tortonian times and continued its activity up to Pliocene or even Quaternary times. The NNW-SSE compression produces crustal thickening and a regional and progressive southwards emersion. The location of main present-day deformation fronts in the Internal Zones contrasts with classical models where the deformation progresses towards the frontal part of External Zones of cordilleras. In addition, this fold-related deformation mountain front has features different from fault related fronts, as it does not show a sharp boundary, and folds that determine rectilinear mountain boundaries decrease progressively in amplitude or in wavelength up to undeformed areas.