Rigid Basement Research Articles

Some characteristics of the orogenic belts in Qinghai-Tibet plateau

The Qinghai-Tibet plateau can be divided into a northern and a southern part, with the Kunlun central thrust forming the boundary. The nothern orogenic belts developed on the rigid basement, starting with rifting and geosynclines in the preorogenic stage, followed by collision to form mountains, and postorogenic deformation, mainly producing overthrusting and overlapping structures. The southern counterparts developed on a flexible basement, characterized by strong activity, and polycyclic opening and closing, with well defined oceanic basins and widespread terrigenous deposits on both flanks in the preorogenic stage, and complicated deformation in the postorogenic stage with a large number of strike-slip faults besides overthrusting and overlapping structures. The overall tectonic framework is a series of unique orogenic belts in the Qinghai-Tibet plateau. There are numerous orogenic belts developed in the Qinghai-Tibet plateau, such as the Himalayas, Gangdise, Tanggula, Hoh Xil, Kunlun and Qilian mountains. These belts provide valuable information on the evolution of the lithosphere. Therefore, it is important to summarize the main characteristics of the orogenic belts.

Control on thrust tectonics in the Himalayan foothills: a view from a numerical model

The evolution of an intracontinental accretionary wedge is controlled by: (a) the displacement of several thrust sheets along faults and an underlying décollement; (b) the vertical motion of the rigid basement beneath the accretionary wedge; and (c) superficial processes (erosion/sedimentation). A numerical model is proposed based on the Coulomb wedge theory, a 2.5-D forward kinematic system, progressive tilting of the underthrust basement, and an erosion/sedimentation model in order to characterise the sequence of fault activation in a thrust system and the syn-orogenic sedimentation in such a system. The Siwalik belt, the frontal thrust system of the Himalayas, represents a structure on which the model can be applied. It presents mainly simple structures made of one fault system branched on an underlying décollement. Conglomerate deposits of the Plio-Pleistocene (Upper Siwalik) are overthrust by Miocene formations (Lower Siwalik) at the front of the wedge and along inner faults. Structural maps show that thrust sheets are laterally relayed and simultaneously emplaced. The comparison between the deformation pattern in the Himalayan foothills and the modelling results supports the concept that the Himalayan thrust wedge has a steady-state-type evolution controlled by horizontal convergence and superficial mass transport, and characterised by an irregular spatial and temporal distribution of shortening within the whole wedge. During this evolution, piggy-back basins or strongly displaced internal sheets hide some thrust sheets. The exhumation path of these sheets is then discontinuous, though the shortening of the wedge is regular.

Propagation of normal modes due to impulsive loading to 3‐D medium on a rigid basement

AbstractGenerally, waves in layered media include several surface wave modes showing dispersive properties. These properties often complicate the understanding of the propagation characteristics of elastic waves. In this paper, not only a method for decomposing transient elastic waves into normal modes is presented but also investigations are applied to decomposed normal modes. This method is developed by means of DWFE and the modal superposition method. In the formulation process, transfer functions describing the relationship between the amplitude of normal modes and that of force density are defined. It was found that the propagation properties of the normal modes were well explained by the dispersion curves and transfer functions. From investigations of the normal mode propagation, properties of each normal mode can be discussed.

A discrete wavenumber and normal-mode superposition method for synthetic seismograms

Abstract A discrete wavenumber and normal-mode superposition method for synthetic seismograms is presented. This method is based on representing the solution of an elastic layered medium in the form of a discrete wavenumber series for the normal modes obtained by the finite-element technique. A rigid basement is added to the bottom of the layered structure to obtain normal modes. Causality arguments are used to define time windows for which the responses are not affected by the rigid basement. A comparison is made between the solutions of the present method and those of the Cagniard de Hoop method, DWFE, by Olson et al. (1984) and the locked mode approximation by Harvey (1981).

Coupled modes in a waveguide with a range-dependent rigid basement

Previous coupled mode approaches to acoustic propagation in ocean waveguides either (1) apply strictly to the case where interfaces other than the (free) ocean surface are range-independent, or else (2) they satisfy the boundary conditions at such interfaces approximately. In this paper coupled mode theory is extended to give a solution which exactly satisfies the boundary condition on a range-varying rigid basement. This is achieved by transforming the coordinate system so that at each range the basement coincides with a coordinate surface, and the partial separation of variables is construed with respect to the coordinate that is normal to that surface. Related work by other researchers is discussed, and the numerical implementation of the theory is described. Results for basements exhibiting both upsloping and downsloping regions are presented and discussed.

剛基盤上の弾性層を伝わるレーリー波について

Rayleigh waves in an elastic medium lying over a rigid basement were analyzed in the frequency-wavenumber domain. First, the Rayleigh function and representation of displacements were derived. Next, the function was closely investigated with respect to causality and locations of its zeros in the complex wavenumber plane. This is because, in case of the elastic and rigid basement model, all the singularities of the representation of the displacements are due to the zeros of the Rayleigh function. The Rayleigh function for the rigid basement model is known to have the Rayleigh poles with negative group velocity. In this study, it was shown that these poles violate causality. Finally, surface displacements were calculated by means of both the mode superposition and the discrete wavenumber method for several epicentral distances, and compared with each other in the frequency and time domain. The comparison showed that the mode superposition method tends to give considerable errors due to neglected complex poles especially at a short epicentral distance and in a low frequency range.

Relationships between strike-slip movement and fold trends in thin-skinned tectonics: Analogue models

In thin-skinned tectonics, a sediment cover has been sheared off from an underlying basement. In many cases, such detachments are located in salt horizons. During deformation, faults and narrow anticlinal folds can develop in the cover. Analogue models show that fold axis patterns are related to the compression direction and to the position and shape of the faults. In each model, a multilayer made of paraffin wax and grease represents the cover sliding over a rigid basement made of plexiglass. In each of the three experiments a fault is cut before deformation. This fault may be located: (1) in the basement; (2) in the cover, with a limited length; or (3) in the cover, with an unlimited length. The fold axis patterns are very heterogeneous around a limited-length fault in the multilayer where the fold axes deviation can reach 90° around the fault tips. They are quite homogeneous around an unlimited-length fault, irrespective of its location. An unlimited-length fault mainly induces a slight deviation of the fold axes due to strike-slip movement. The detachment horizon reduces the transmission of stresses from the basement and in such cases, the final deformation is very sensitive to local heterogeneities inside the overriding multilayer.

Évolutions sédimentaires et tectoniques des plates-formes du sud-est de l'Espagne au cours du Cénomanien supérieur-Coniacien inférieur

A late Cenomanian to early Coniacian unconformity-bounded megasequence is recognized in the platform sequences of the two basins of the southeastern Iberian Plate: the Betic Basin, occupying the southern passive margin of the plate, and the Iberian Basin, occupying an intraplate position. A more stable palaeogeographical feature, the Albacete Domain, which developed on the rigid basement of the Hercynian Massif of the Central Meseta and was emergent during this time, separated the two basins. The palaeogeographical distribution of the depositional megasequence was controlled by complex tectonic restructuring produced by a regional event which caused the Iberian Plate to tilt towards the NNW during the late Cenomanian. In the Iberian Basin, a rapid transgression of the Atlantic induced the development of a widespread carbonate ramp. In the Betics, a great transgression also took place, but was confined to the most subsided zones, generated by listric fault movements. From this time until the end of the Cenomanian, differential subsidence of the basins was tectonically controlled, being positive in the Iberian Basin and probably negative in the Betics. Finally, the latest Cenomanian to earliest Coniacian interval represents a long period of tectonic stability, during which the whole area remained emergent, resulting in very low rates of sedimentation and the development of soils.

Coupled modes in a waveguide with a range-dependent rigid basement

Previous coupled mode approaches to acoustic propagation in ocean waveguides either (1) apply strictly to the case where interfaces other than the (free) ocean surface are range-independent, or else (2) they satisfy the boundary conditions at such interfaces approximately. In this paper coupled mode theory is extended to give a solution that exactly satisfies the boundary condition on a range-varying rigid basement. This is achieved by transforming the coordinate system so that at each range the basement coincides with a coordinate surface, and the partial separation of variables is construed with respect to the coordinate that is normal to that surface. Related work by other researchers is discussed, and the numerical implementation of the theory is described. Results for basements exhibiting both upsloping and downsloping regions are presented, and the exchange of radiated power between the modes is investigated for these cases.

Experiments on basin inversion above reactivated domino faults

Results are presented from scaled sand box experiments which investigate the structural inversion of half-graben basins generated by the contractional reactivation of extensional domino fault systems. The overburden was deformed above a series of rigid basement fault blocks that were extended in a domino manner. The models were constructed from either (1) sand with mica interlayers (anisotropic behaviour), (2) pure sand (isotropic behaviour), or (3) a sand-clay mixture (behaviour with increased competence). During the extensional phase of deformation, sand or alternating sand/mica layers were deposited in teh half-grabens associated with individual domino faults to simulate a syn-extensional stratigraphic sequence together with a component of overall subsidence. After termination of the extensional phase, sand layers were added to represent a post-rift passive subsidence stratigraphy. Subsequent contraction of the experimental configuration resulted in progressive reactivation of the domino fault system which was accompanied by back-rotation of both the footwall and hanging wall in each of the individual fault blocks. This produced null points along the profile section of the fault planes which separated net contraction above from net extension below. Continued shortening of the system led to further contraction being accommodated along footwall shortcut faults. It is suggested that these shortcuts developed in response to (1) the accommodation of space problems generated by increased bed length in the graben acquired during extension and (2) by more efficient low angle fault trajectories in keeping with Coulomb failure during horizontal to subhorizontal compression during inversion. In the anisotropic model, folding in the footwall partially suppressed the initiation of footwall shortcut faults. Footwall shortcut faulting did not occur in experiments where the dips of the original extensional faults were decreased before the inversion. In all inversion experiments the contractional deformation associated with the shortening of the half-graben was concentrated in the footwall to each domino basin-bounding fault. A control experiment in which an undeformed sand pack was deposited above an extended series of basement fault blocks revealed that the early extensional architecture formed in the other experiments had a profound influence on the resultant inversion geometries. The results show clear similarities to interpreted examples of natural basin inversion structures associated with reactivated domino fault systems, as well as to inferred strike-slip induced flower structures.

The interaction between oblique and layer-parallel shear in high-strain zones: Observations and experiments

The Scandinavian Caledonides consist of a rigid basement, a weak décollement zone, and a series of crystalline nappes. The basement is cut by oblique, west-dipping extensional shear zones, associated with west-verging folds in the overlying décollement zone. The oblique shear zones may, theoretically, have formed after, prior to, or during post-contractional, top-to-the-west shear movements. Physical modelling of this geometrical/mechanical situation shows that each of the three deformation histories results in a characteristic structure. If the oblique, extensional shear zones postdate the layer-parallel (décollement) shear, a normal-type, oblique shear zone structure occurs. If the layer-parallel shear occurs after the development of oblique shear zones, the resulting structure is characterized by a low-strain region above the oblique shear zone. Some folds typically develop above this “protected” region. However, simultaneous oblique and layer-parallel shear provides abundant asymmetric folds with gently dipping axial planes and sub-horizontal, shear zone-parallel fold axes in the region above the oblique shear zone. A neutral surface may be encountered above, and parallel to the oblique shear zone, separating a lower, thin zone of layer-parallel extension from the main, contractional region. The differences between the three types can be used to interpret field observations of this type of structures. The large-scale, post-contractional structures in the Scandinavian Caledonides are consistent with layer-parallel shear related to back-movement of nappes, synchronous with oblique shear zones in the basement. Similar geometries are found as meso- and micro-scale structures in ductile high-strain zones, where oblique shear bands or shear zones affect more competent layers within the tectonites. Also, these structures compare well with those produced experimentally by simultaneous layer-parallel and oblique shear, and are consistent with the general assumption that oblique shear bands form contemporaneous with the general shear in shear zones.

Pliocene and Pleistocene stratigraphy and tectonics, Ohara Depression and Wakarara Range, North Island, New Zealand

Abstract The Ohara Depression ‐ Wakarara region is situated at the arcward margin of the forearc basin near Hawke's Bay on the North Island of New Zealand. Deformation of late Pliocene and Pleistocene sediments in approximately the last 2.5 Ma is a product of oblique convergence of the Australian and Pacific plates. The region lies directly to the east of Mesozoic greywacke of the axial ranges of the North Island, and deformation reflects the interaction of this rigid backstop with the inboard part of the forearc. Deformation has resulted in the differential movement of several relatively rigid basement blocks. A veneer of late Neogene and Quaternary sediments on these blocks records the timing and style of deformation. An important observation derived from the Quaternary tectonic record is that strain is partitioned between two northeast‐trending domains that parallel the margin. The western domain that abuts the axial ranges is a domain of strike‐slip faulting. Directly east of the strike‐slip domain, a...

Structure of the Sinu-San Jacinto fold belt — an active accretionary prism in northern Colombia

The Sinu and San Jacinto fold belts in northern Colombia from a wedge of sediments up to 12 km thick which has been accreted to the South American margin throughout the Cenozoic. The wedge is characterized by a very low topographic slope ( <-2°) and abundant mud volcanism. Many folds have a core of mobile overpressured mud. Active shortening at a rate of about 1 to 2 cm yr −1 is the result of convergence between the Caribbean plate and the northwest margin of South America. Aeromagnetic and gravity anomalies suggest a buttress of rigid basement rock dipping toward the toe of the wedge. A seismic reflector dipping gently (2°) toward the back of the wedge is interpreted as the basal décollement. According to the theory of critical taper for a noncohesive wedge of Coulomb material, fluid pressure ratios should approach lithostatic values (0.97) within the wedge and on the basal décollement. The predicted high fluid pressures within the wedge are consistent with the mud diapirism. The low slip rate and subduction of large quantities of young, high-porosity sediments explain the absence of large interplate thrust earthquakes.

Interaction between structures in soft strata: antiplane motion

An analytical-numerical method is presented to solve a problem of dynamic structure-soil-structure interaction in a soft stratum overlaying a rockbed. A two-dimensional system is formulated, which is formed by two shear buildings placed on rigid rectangular footings embedded in a linear-viscoelastic layer with a rigid rock basement subjected to antiplane SH motion. The model included several parameters typical of realistic problems. In order to obtain the solution the soil region is divided into three subdomains: two subdomains that contain the foundations are of near-field, and the other one that contains the rest of the stratum is of far-field. The solution is constructed with wave function expansions using multiple images, and calculated when solving the boundary conditions in the least squares sense. The analysis is made through the frequency domain in two stages: first the transfer function of the system is found, and then the motion at any structure level is evaluated by means of Fourier synthesis. To illustrate the effects of the structure-soil-structure interaction, a characteristic layered site idealized as an equivalent stratum is assumed, and the influence of some parameters as the separation between structures, foundation depth and structure period are studied. Transfer functions are computed, as well as time series and response spectra by setting the bedrock motion equal to the EW acceleration time history recorded at Tacubaya station in Mexico City during the great 1985 Michoacan earthquake.

Extension in compressional orogenic belts: The eastern Alps

Extension in the eastern Alps encompasses the following: (1) Extension accompanying simple shear crustal stacking. Subhorizontal stretching occurs along flat-lying foliations and regionally consistent stretching lineations with associated high strains. On a local scale, stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust-sheet imbrication. The overall geometric effect is crustal thickening. (2) Classical extensional tectonics. Extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge. Extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks. The overall geometric effect is crustal thinning. Extension is generally oriented subparallel to the strike of the orogen.

Relationships between the kineamatics of the top of a layer and the state of stress within it due to block motion at its bottom (in connection with the interpretation of recent movements)

The equilibrium of an infinite linearly viscous layer overlying a rigid basement which is separated into blocks by a narrow plane slit is considered. The state of stress in the layer is caused by slow translational motions of the basement blocks. This spatial problem is shown to separate into problems of plane and anti-plane strain; their solutions are constructed in analytic form. The theoretical study was accompanied by modelling experiments. The results are applied to a kinematic interpretation of recent movements in sediments.

A Wrench Fault/Thrust-Fold Clay Model Study and Comparison with the Bonanza-Zeisman Area of the Eastern Big Horn Basin

The kinernatics and geometry of some of the basement-involved structures observed in the central Rocky Mountain foreland province have been investigated by means of a series of simple clay models. In these models, a relatively rigid Precambrian crystalline basement is simulated with plasticine oil-base clay (OBC) and the overlying, relatively ductile sedimentary section IS simulated with colored layers of a salt, cornstarch, and water mixture (SCW). In the model described here, a vertical cut and two perpendicular cuts dipping 30" were made in a block of OBC and lubricated. Horizontal layers of uncut SCW were laid over the cut block of OBC Horizontal compression was applied parallel to the vertical cut so that shortening along one side was greater than along the other side. Shortening was manifest in the differential development of perpendicular thrust-folds which accompanied left-lateral s11p along the buried vertical fault plane. In the overlying, unfaulted SCW layers, a plunge was created on both fold trends, and the anticlinal hinges above OBC thrust locations were deflected sinistrally across the buried wrench fault zone. There are some basic similantries, but also important differences, in the geometry of the deformed clay model and the structure observed in the Bonanza-Zeisman area of the eastern Big Horn basin of Wyoming. A significant component of left slip along a postulated northeast-trending wrench fault zone buried in the basement of the Bonanza-Zeisinan area 1s inferred from the northeastern alignment of sinistral deflections and abrupt terminations of thrust-fold structures which trend perpendicular to the zone Although the numbers of thrust-fold structures developed along either side of the postulated Bonanza-Zeisman wrench fault zone are not equal as in the clay model, the total horizontal shortening measured along either side of the fault line approximately balances. The inferred left-lateral slip on the postulated wrench fault zone 1s thought to be the result of Laramide reactivation of a pre-existing Pre-Cambrian fracture or shear zone

Relationships between the kinematics of the top of a layer and the state of stress within it due to block motion at its base-connections with interpretation of recent movements

Abstract The equilibrium of an infinite linearly viscous layer overlying a rigid basement which is separated into blocks by a narrow plane slit is considered. The state of stress in the layer is caused by slow translational motions of the basement blocks. This spatial problem is shown to separate into problems of plane and anti-plane strain; their solutions are constructed in analytic form. The theoretical study was accompanied by modelling experiments. The results are applied to a kinematic interpretation of recent movements in sediments.

MORPHOLOGICAL AND STRUCTURAL MODEL OF MEXICAN VOLCANIC BELT

Con base en datos geológicos, geomorfológicos y estructurales se reconocieron las principales unidades que constituyen el Cinturón Volcánico Mexicano (CVM). Estas unidades se formaron duran tensiónales desde el Oligoceno, y muchas de las estructuras vinculadas con ellas siguen todavía activas. Dentro del CVM se reconocieron tres sectores principales. El sector occidental, activado en el Plioceno; presenta un sistema de grábenes en dirección NW-SE, y está asociado a la apertura del Golfo de California. El sector central está representado por un sistema de bloques fallados y basculados en dirección WSW-ESE que rodean a una depresión central. En el sector oriental, las estructuras distensivas están representadas esencialmente por un sistema de fallas en dirección N-S. Estas fallas están relacionadas con los grandes estrato-volcánes CVM. El CVM atraviesa las antiguas estructuras de bloques fallados N-S y NNE-SSW, que pertenecen a la provincia Basin and Range. En los sectores central y occidental del CVM se reconocieron lineamientos tensiónales reactivados, pertenecientes a esa provincia. A lo largo del margen meridional del CVM, todas las estructuras distensivas mencionadas se interrumpen, desarrollándose un amplio colapso en dirección E-W, coincidente con el valle del Río Balsas.

A possible model for evaluating the topographic and mechanical effects of subducted oceanic plate with irregular surface

A simple model is proposed to evaluate the possible mechanical and topographic effects that might be caused when an oceanic plate with irregular surface is subducted beneath a continental plate. The model focuses on the effects of such features as oceanic ridges, seamounts, plateaus and other bathymetric highs. The model approximates a continental plate as a two-dimensional incompressible Newtonian viscous fluid with uniform thickness, and a subducting oceanic plate as a rigid basement slipping beneath the viscous fluid with a constant velocity. In this model, bathymetric highs on the oceanic plate are approximated by topographic irregularities of the rigid basement. Based on the fundamental solutions given by Budd (1970) which were applied originally to the analysis of glacial movements, the shear stress near the basement and surface profile of the overriding medium are calculated, for instance, for a model whose basement profile is represented simply as exp( −ax 2) where a is the geometrical constant representing the degree of regional slope, and x the horizontal distance from the axis of the ridge. Assuming reasonable values for viscosity (10 23 poise), density (3 g/cm 3), thickness of the viscous medium (30 km), elevation of the top of the ridge from the surroundings (1 km), geometrical constant of the ridge shape (10 −2 km −2), and slip velocity (10 cm/a), the resultant surface profile was inferred to be asymmetrical shape with its highest elevation of 1360 m and the lowest of −620 m, while the magnitude of the shear stress near the base showed a symmetrical distribution with the maximum of 12.7 kbar. The results from these calculations do not only allow us to make quantitative estimates of the geological consequences of the subduction of bathymetric highs beneath continental plates but also give possible explanation for some of the uneven seismic activities found around present subduction zones.