Horizontal Compressive Forces Research Articles

РИФТОГЕНЕЗ В ОКРАИННЫХ МОРЯХ ЗАПАДНОЙ ПАЦИФИКИ

The paper presents a review of structural features of rifting during the formation of Cenozoic basins of marginal seas of the Western Pacific. The assumption that rifting always starts with a passive stage and is only occasionally interrupted by episodes of active rifting is fully confirmed by examples from the studied basins. Rifting occurred under a compressive regime with NE and NNE-directed shortening that resulted in the formation of either a chain of pull-apart basins or rifts scattered between major strike-slip faults in the great part of the South China Sea. The NE and NNE directions of horizontal compressive force are apparently related to the convective currents in the asthenospheric mantle that have extended from the spreading ridge of the Indian Ocean and have carried plate fragments deformed to some extent during their transportation. The north-northeastward drift of the Indian, Australian, and Eurasian plates associated with these mantle currents does not reveal any connection with subduction from adjacent Paleo-Pacific plates, which continue to move in the NW direction up to the present time. Thus, formation of the marginal basins of the Western Pacific were unaffected by subduction from the Pacific Ocean side and they can be regarded as back-arc basins only due to their geographical locations.

Biaxial Loading of Brazilian Disc Samples

The Brazilian test method has been widely used over the past 50 years to determine the indirect tensile strength of brittle materials, such as rocks and concrete. While the behaviour of Brazilian samples under conventional indirect tensile testing has been the subject of much research, the biaxial loading of a Brazilian sample under both horizontal and vertical compressive forces has rarely been studied. This study investigates the ultimate strength, crack propagation speed, fracture mode, and damage evolution in sandstone Brazilain samples using high-speed photography methods integrated with a newly designed hybrid biaxial-impact testing facility. The result shows that the samples fail under lower tensile stress than their Indirect Tensile Strength based on the principle of superposition.

The detection of forces acting on conveyor rollers of a laboratory device simulating the vertical section of a Sandwich Belt Conveyor

Using certain belt conveyors of a special design, it is possible to transport bulk material at angles of transport inclination significantly exceeding the permissible angle of inclination for traditional design. A special design of such a belt conveyor, described in this paper, uses the so-called cover belt to transport bulk materials at inclination angles from 20 to 90 degrees. By applying its weight or employing pressure, it exerts pressure on the surface of the layer of the transported material that is spread on the surface of the carrying belt. This paper aims to describe the laboratory measurement methodology and to detect the magnitude of the horizontal compressive forces of the transported bulk material acting on conveyor rollers (idlers). The article lists in the tables horizontal compressive forces obtained by measuring, which help to deduce the height of a bulk material column and its cross-section S2 [m2], defined by the width b [m] and height h [m] of the belts interspace. Knowledge of the magnitude of horizontal forces acting on conveyor rollers allows us to suitably design the stiffness of the compression coil spring in the pressing mechanism. It eliminates the undesirable shifts of working surfaces from the conveyor belts (carrying and covering), which prevents the flow of the transported material grains over the edges of the conveyor belts. One of the possible designs for the pressure mechanism is presented in this article. The article shows the values of the difference between the ratio of the measured compressive forces against the value of theoretically calculated stresses for different heights of the bulk material column and different heights of the interspace. For the maize material, the largest difference in the ratio of the measured compressive forces to the value of the theoretically calculated stresses was 78.1% and for river gravel, it was 71.6%.

The assessment of possible deformations in the fault zone

One of the most relevant aspects for engineering geology regarding faults is the characterization of the changes induced by a fault and the assessment of possible fault movements. This zone cannot be clearly and well observed in field studies. However, it plays a negative role in the construction and operation of structures. In this zone, unfavorable phenomena, such as rock collapses, displacements, etc., may occur. The purpose of the present study is to estimate the size of fault changes, to reveal the peculiarities of stress distribution within the fault influence zone and to characterize free surface movements. The site of the Rogun HPP was chosen as the object of the study. Rogun HPP is located on Vakhsh River (Tajikistan) in a gorge of 400 - 500 m deep. The site of the dam, which will be about 350 m high, is located in a single tectonic block bounded by sub-parallel Ionakhsh and Gulizindan Regional Faults. The complex of Jurassic-Cretaceous sediments in this block lies monoclinically. According to the results of complex inclinometer-deformographic observations at the Rogun HPP site, the faults are tectonically active, with vertical displacements in 1-3 mm per year. In the scheme of numerical simulation, the width of the Ionakhsh Fault together with the crushing zone was taken as 20 m. For all rocks it was accepted: Young modulus – 3000 MPa; Poisson’s ratio - 0.3; density - 2.6 g/cm3. Horizontal compressive tectonic force was given as 12 MPa. The simulation showed that the dynamic influence zone is limited to 200 m on the side of the lying wing and 70 m on the side of the hanging wing, and the concentration zones are located at the mouth and in the middle part of the fault on the side of the lying wing and between the hanging side of the fault and the bottom of the valley. The analysis of the movements of eight points of the surface showed that the direction of movement corresponds to observed in nature. The total amplitude of vertical displacements of two points at the valley is 0.51 m. The values of the resulting displacements on the different sides of the fault are 0.22 and 0.49 m. According to geological data, the width of tectonic breccia zone of the Ionakhsh Fault is 120 m. However, according to numerical simulation the influence zone is about 270 m. Thus, the simulation showed that the size of real fault influence zone is more than twice as large as that measured in nature.

A new equilibrium solution for masonry spiral stairs

This paper considers stone or brick staircases that appear to cantilever from an exterior wall as they spiral upward. A new equilibrium solution is proposed that does not rely on cantilevering from the wall. Unlike previously proposed membrane solutions, the proposed new analytical solution, called Linear Arch Static Analysis (LASA), is based on a 1D no-tension filament, or a set of such 1D filaments (that -by following Rankine’s definition- we call space linear arches), and is therefore particularly efficient. Each space linear arch represents a compressive element that spirals down the staircase, simulating a compressive force that is transferred through and between each stair. In addition, the equilibrium of each filament requires the contribution of horizontal uniaxial compressive forces which are provided by the exterior wall support and transmitted horizontally through each stair. The lines of action of such uniaxial compressive stresses are entirely contained inside the masonry. The new solution is applied to both circular and elliptical stairs to demonstrate the new equilibrium solutions, but also to quantify and highlight the magnitude of resulting forces and the efficiency of the solution. The effect of geometric variations on internal forces are also explored to evaluate the efficiency and feasibility of various potential stair geometries.

Stress and deformation mechanisms at a subduction zone: insights from 2-D thermomechanical numerical modelling

SUMMARY Numerous processes such as metamorphic reactions, fluid and melt transfer and earthquakes occur at a subducting zone, but are still incompletely understood. These processes are affected, or even controlled, by the magnitude and distribution of stress and deformation mechanism. To eventually understand subduction zone processes, we quantify here stresses and deformation mechanisms in and around a subducting lithosphere, surrounded by asthenosphere and overlain by an overriding plate. We use 2-D thermomechanical numerical simulations based on the finite difference and marker-in-cell method and consider a 3200 km wide and 660 km deep numerical domain with a resolution of 1 km by 1 km. We apply a combined visco-elasto-plastic deformation behaviour using a linear combination of diffusion creep, dislocation creep and Peierls creep for the viscous deformation. We consider two end-member subduction scenarios: forced and free subduction. In the forced scenario, horizontal velocities are applied to the lateral boundaries of the plates during the entire simulation. In the free scenario, we set the horizontal boundary velocities to zero once the subducted slab is long enough to generate a slab pull force large enough to maintain subduction without horizontal boundary velocities. A slab pull of at least 1.8 TN m–1 is required to continue subduction in the free scenario. We also quantify along-profile variations of gravitational potential energy (GPE). We evaluate the contributions of topography and density variations to GPE variations across a subduction system. The GPE variations indicate large-scale horizontal compressive forces around the trench region and extension forces on both sides of the trench region. Corresponding vertically averaged differential stresses are between 120 and 170 MPa. Furthermore, we calculate the distribution of the dominant deformation mechanisms. Elastoplastic deformation is the dominant mechanism in the upper region of the lithosphere and subducting slab (from ca. 5 to 60 km depth from the top of the slab). Viscous deformation dominates in the lower region of the lithosphere and in the asthenosphere. Considering elasticity in the calculations has an important impact on the magnitude and distribution of deviatoric stress; hence, simulations with increased shear modulus, in order to reduce elasticity, exhibit considerably different stress fields. Limiting absolute stress magnitudes by decreasing the internal friction angle causes slab detachment so that slab pull cannot be transmitted anymore to the horizontal lithosphere. Applying different boundary conditions shows that forced subduction simulations are stronger affected by the applied boundary conditions than free subduction simulations. We also compare our modelled topography and gravity anomaly with natural data of seafloor bathymetry and free-air gravity anomalies across the Mariana trench. Elasticity and deviatoric stress magnitudes of several hundreds of MPa are required to best fit the natural data. This agreement suggests that the modelled flexural behaviour and density field are compatible with natural data. Moreover, we discuss potential applications of our results to the depth of faulting in a subducting plate and to the generation of petit-spot volcanoes.

The influence of hip mobility and quadriceps fatigue on sagittal spinal posture and muscle activation in rugby scrum performance

Rugby Union scrumming puts the spine under a high degree of loading. The aim of the current study was to determine how sagittal hip range of motion and quadriceps fatigue influence force output, spinal posture, and activation of the trunk and quadriceps muscles in rugby scrumming. Measures of sagittal hip flexion/extension range of motion were collected from 16 male varsity and club first XV level participants. Sagittal spine motion (electromagnetic motion capture), trunk and quadriceps activation (electromyography), and applied horizontal compressive force (force plate) were measured during individual machine scrumming. Participants performed a 5-trial scrum block involving 5 s of contact with 1–2 min recovery between each trial. They then performed a fatiguing protocol (wall sit to failure) and immediately returned to the scrum machine to perform another five trials. Though there was no significant influence of fatigue on the horizontal compressive force applied during contact (P = .83), there was a 52% increase in cervical flexion (P < .001), as well as decreased (18–23% lower) abdominal and erector spinae muscle activation (P < .05). Furthermore, quadriceps activation decreased (12–25% lower) over the course of the initial scrum block but increased (13–15% higher) in the post fatigue block (P < .05). Although athletes were able to maintain force output following the fatiguing wall sit task, there is the potential that they may be at an increased risk of spinal injury due to the combination of increased flexion and high-applied compressive force; a combination which has been shown to increase the likelihood of intervertebral disc herniation.

Critically Stressed Areas of Earth’s Crust as Medium for Man-caused Hazards

Despite advances in rockburst studies, suddenness of major geodynamic events is reported in a number of cases. Phenomenological tectonophysical model is suggested to explain some geodynamics phenomena. Prof. Petukhov I.M. suggested a concept: the Earth crust's critical stress condition is developed due to horizontal compressive forces and entrains rock strata from the sub-surface to a certain depth. The conditions that induced earthquake in 2013 at Bachat coal field in south west Kuzbass are considered in terms of critical stress developed in the top layer of the Earth crust. Estimates show that the size of the critical stress zone, produced presumably by interaction of huge (over 100 km) crustal blocks is at least 10km. Whereas critical stress zone is located in the top part of Earth's crust, mining operations in the pit including blast operations was making a direct impact on this area. Shallow occurrence of critical stress area and its size can provide insight into why mining works brought about induced earthquake with hypocenter at the depth of several kilometers. The conclusion has been made that regional areas of critical stress within rock massif developed as a result of crustal blocks interaction create hazard medium for mining.

Fault modeling of the 2012 Wutai, Taiwan earthquake and its tectonic implications

The Mw 5.9 Wutai earthquake of 26 February 2012 occurred at a depth of 26km in southern Taiwan, where the rupture is not related to any known geologic structures. To illustrate the rupture source of the mainshock, we employ an elastic half-space model and GPS coseismic displacements to invert for optimal fault geometry and coseismic slip distribution. With observations of both coseismic horizontal and vertical displacements less than 10mm, our preferred fault model strikes 312° and dips 30° to the northeast and exhibits a reverse slip of 28–112mm and left-lateral slip of 9–45mm. Estimated geodetic moment of the Wutai earthquake is 1.3×1018N-m, equivalent to an Mw 6.0 earthquake. The Wutai epicentral area is characterized by a NE–SW compression as evidenced by the slaty cleavage orientations and the interpretation of stress tensor inversion of earthquake focal mechanisms. Using the stress drops of the Wutai and the nearby 2010 Mw 6.4 Jiashian earthquakes, we obtain a lower bound of ~0.002 for the coefficient of friction on the fault. On the other hand, studying the crustal thickness contrast in southern Taiwan provides an upper bound of the average horizontal compressive force of 1.67×1012N/m transmitted through the Taiwan mountain belt and gives an estimate of the maximum friction coefficient for 0.03. The deviation of an order of magnitude difference between the upper and lower bounds for the coefficient of friction suggests that other fault systems may support substantial differential stress in the lithosphere as well.

The role of extensibility in the birth of a ruck in a rug

Everyday experience suggests that a ‘ruck’ forms when the two ends of a heavy carpet or rug are brought closer together. Classical analysis, however, shows that the horizontal compressive force needed to create such a ruck should be infinite. We show that this apparent paradox is due to the assumption of inextensibility of the rug. By accounting for a finite extensibility, we show that rucks appear with a finite, non-zero end-shortening and confirm our theoretical results with simple experiments. Finally, we note that the appropriate measure of extensibility, the stretchability, is in this case not determined purely by geometry, but incorporates the mechanics of the sheet.

Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera

The North American Cordillera is a classic example of accretionary orogen, consisting of multiple oceanic terranes attached to the western margin of Laurentia during the Mesozoic times. Although the Cordillera is linear for most parts, terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen, which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed. This paper discusses mechanisms, kinematics, and timing of initiation of one of these major curvatures, the Blue Mountains Province in northeastern Oregon. Here magmatic fabric patterns and anisotropy of magnetic susceptibility in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous. First is terrane-oblique ~NE-SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at ~140 Ma. Deformation subsequently switched to pure shear-dominated ~NNE-SSW shortening associated with crustal thickening, caused by continued impingement of the amalgamated Blue Mountains superterrane into a presumed westward concave reentrant in the continental margin at ~135–128 Ma. Upon impingement (at ~126 Ma), the northern portion of the superterrane became “locked,” leading to reorientation of the principal shortening direction to ~NNW-SSE while its still deformable southern portion rotated clockwise about a vertical axis. We thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries.

Visual observation of effect of tilting tool on forging action during FSW of aluminium sheet

Using a backing plate with prepared grooves of various lengths, the distribution features of forging result and the effect of tool tilt angle on forging action were investigated by visualisation. The height of the profile of Al extruded into the grooves increased with increasing tool tilt angle, showing that the forging effect can be enhanced by tilting tool. It was primarily attributed to the increase in actual forging force, because the measured downward forging forces at 0 and 3° tilt angles were 500 and 1500 N respectively. Moreover, the forging force was found to be composed of static component and dynamic component, and the latter resulted from the horizontal compressive force of moving worktable. Although the width of forging affected zone (FAZ) can reach the area covered by shoulder, the distribution of forging result was not uniform. So, small pin and considerable tool tilt angle are beneficial to eliminating void.

Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions

Relative SV and SH wave speeds are generally attributed to radial seismic anisotropy which can be used as the indicator of crust/mantle deformation styles. Surface wave data were initially collected from events of magnitude Ms ⩾ 5.0 and shallow or moderate focal depth occurring between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1525 source-station Rayleigh waveforms and 1464 Love wave trains were earlier analysed by multiple filtering to obtain Love- and Rayleigh wave group velocity curves in the broad period range 10–105 s. We have performed tomographic inversion to obtain period-dependent group velocity and further shear wave velocity at 2° × 2°-sized grid-cells of a mesh covering the model region, after averaging azimuthal effects. Horizontally and vertically varying shear-wave velocities are observed, but the models of isotropic seismic velocity in the crust and upper mantle cannot fit simultaneously the inverted group-velocity dispersion curves due to the discrepancy in the transmission velocities of Love and Rayleigh waves, whose likely origin is the existence of radial anisotropy in the continental crust and topmost mantle. The strength of radial anisotropy computed from the Love–Rayleigh discrepancy and its spatial extent beneath the Qinghai-Tibet Plateau are shown as maps of percentage anisotropy at various depths down to 170 km and cross-sections along five profiles of reference. Areas in which radial anisotropy is in excess of ∼ 6% are found in the crust and upper mantle underlying most of the plateau, and even up to 10% in some places. The strength and spatial configuration of radial anisotropy seem to indicate the existence of a regime of horizontal compressive forces in the frame of the convergent Himalayan–Tibetan orogen, the laterally variation of the lithospheric rheology and the differential movement as regards the compressive driving forces.

DESIGNING COMPOSITE STEEL BEAMS WITH PRECAST CONCRETE HOLLOW-CORE SLABS.

This paper presents a design procedure for steel beams acting compositely with proprietary precast hollow-core slabs. It also contains selected underpinning theoretical analyses that illustrate the rationale behind the design formulae and recommendations. Numerical results for the transfer of horizontal compressive forces through the numerous interfaces and through the load–slip characteristics of the shear studs are presented, i.e. the interfaces between the precast slab and in situ concrete infill, and the in situ infill and steel beam. A method for designing composite beams, including the effective breadth of the precast slab, the degree of shear interaction and prediction of the moment capacity, is given. Load against span graphs for composite beams are provided for use in initial design. A worked example based on the procedures illustrates their application in practice.

Designing composite steel beams with precast concrete hollow-core slabs

This paper presents a design procedure for steel beams acting compositely with proprietary precast hollow-core slabs. It also contains selected underpinning theoretical analyses that illustrate the rationale behind the design formulae and recommendations. Numerical results for the transfer of horizontal compressive forces through the numerous interfaces and through the load–slip characteristics of the shear studs are presented, i.e. the interfaces between the precast slab and in situ concrete infill, and the in situ infill and steel beam. A method for designing composite beams, including the effective breadth of the precast slab, the degree of shear interaction and prediction of the moment capacity, is given. Load against span graphs for composite beams are provided for use in initial design. A worked example based on the procedures illustrates their application in practice.

Seismicity and tectonics of the Bay of Bengal: Evidence for intraplate deformation of the northern Indian plate

Seismicity and fault plane solutions of several earthquakes in the Bay of Bengal have been studied to focus attention on the nature of intraplate deformation in the northern Indian plate. A widely dispersed diffuse seismicity provides evidence that the intraplate region of the entire Bay of Bengal and peninsular India of the Indian plate are seismically active of which the Gangetic delta and the eastern flank of the Ninety East Ridge show somewhat higher seismicity in comparison to the thickly populated epicenters at the eastern plate boundary. Predominance of thrust faulting earthquake mechanisms with consistent north-dipping fault planes and more or less NS pressure axes imply that NS compression is more prevalent in the intraplate region of the Bay of Bengal which is analogous to the Himalayan arc. This intraplate seismicity and NS compressive deformation is presumed to be the result of collisional resistance to the north moving Indian plate with the Eurasian plate at the Himalayas where subduction of the Indian lithosphere in mantle has stopped recently. As a result, to compensate the horizontal compressive force by ridge-push at the Carlsberg Ridge, the intraplate region of the northern Indian plate in continuation to the Central Indian Ocean Basin no longer remains passive due to a strong repulsive force at the collisonal plate boundary Himalaya that causes stress transmission towards south and makes the intraplate region seismically active.

An inverse method for calculating basement geometries in foreland basins

Flexural plate inversion uses a computer simulation program to model the characteristic patterns of subsidence, deformation, uplift and erosion of basement under sediment load and compressive thrusting. In this modelling it is assumed that lithosphere is an elastic material, which will deform when a force is applied and will return to its original shape when the force is removed. The program is effective in a given geological setting such as a foreland basin formed by compressive thrusting. The program is an inverse model which employs both non-linear least squares and linear search techniques to determine the best values of flexural parameters. Therefore, a user can set a dynamic range for the sizes of input parameters, based on geological and geophysical data, to understand the influence of these parameter changes on the evolution of a foreland basin. In this system, input data include a profile of the present-day basement. The parameters which determine the deformation of the basement consist of flexural rigidity ( D), horizontal compressive force ( P), bending moment ( M 0), applied initial load ( V 0), the dip angle (θ) and the “added length” ( L) of the basement. The outputs are a profile of the deformed basement configuration, which matches the present day basement and also a section of unloaded basin which represents the basement configuration prior to any sediment or thrust loading in the section. The results of synthetic data tests and real data from the Po Basin, Italy show that this system can be used to assess the evolutionary processes affecting a basin under compressive thrusting.

The effect of vertical crustal fractures on the rifting process

In the past, rifts have mainly been identified in terms of sediment troughs. They account for many of the elongate gravity lows distributed in a coherent rectilinear manner over the continent. Other gravity lows can be attributed to granites intruding rift compartments, and some gravity highs can be attributed to basic volcanics in compartments. The total number of rifts which can be thus inferred from gravity and magnetics is very large, and suggests rifting is pervasive over the whole continent and controlled by a systematically distributed "Cardinal" system of ancient vertical crustal fractures.The extensional concept of rifting is based on a finite number of rifts, all of which have "failed" to split the continent. When a far greater number of rifts is recognised, it becomes difficult to accept that all these rifts have "failed" to reach full opening by extensional processes. In view of the known horizontal compressive forces acting in the crust, it is more probable that rifting is caused by compression. The compartmentalization of rifts, clearly observed in gravity data, also implies compression.Closely spaced rectilinear dyke systems in shield areas may also represent the pervasive "Cardinal" fracture system. In general, this system of orthogonal fractures poses problems for the detatchment rifting concept which assumes that transfer faults are formed at the time a rift forms, whereas they in all probability predate the rift, and owe their existence to a fundamental process operating when continental crust first formed.Two types of compressive rift models are discussed. One is associated with shear couples between widely spaced parallel fractures. The other is based on the concept of a crust cut by closely spaced fractures in which compression is propagated along a network of linked blocks. In both cases the development of basement ridges is a key issue.

Teleseismic travel-time anomalies and deep crustral structure in central Australia

SUMMARY Teleseismic travel times have been recorded along two lines of instruments traversing the Amadeus Basin and southern Arunta Block of central Australia for the purpose of examining the deep crustal and upper-mantle structure underlying these features. Pronounced differential travel times, approaching 1.5 s, and with major azimuthal variation, have been recorded. Most of the useful observations originated from events to the north (Japan-Marianas) and to the east (Fiji-Tonga) at epicentral distances of 40-65d. A few observations of southerly azimuth events (Macquarie Ridge and South Sandwich Islands) were also recorded. The inferred structure beneath the basin and block exhibits major lateral variations, the essential aspects of which are: (1) a zone of relatively high-velocity material dipping steeply northwards such that the continuation of its southern edge to the surface coincides approximately with a mylonitic shear zone known as the Redbank Deformed Zone; (2) a steeply dipping zone of relatively low-velocity material immediately to the south of this shear zone and partly underlying the high-velocity zone. If the travel times are interpreted in terms of an undulating Moho, variations in Moho depth of 20 km occur over horizontal distances of less than 50 km with relatively shallow Moho depths occurring immediately to the north of the Redbank Deformed Zone. Such an interpretation is consistent with the gravity anomalies observed across the geological structures. The inferred structure is out of hydrostatic equilibrium and departures from local isostasy approach 150 MPa. These stress-differences must have persisted during the 300 Ma following the Alice Springs Orogeny that led to the present crustal structure. Evidence, in the form of the surface topography and seismicity, suggests that some stress relaxation has occurred and still is occurring but not at the shear zone itself. At the time of the orogeny the structure may have been close to isostatic equilibrium, but as erosion took place complete local rebound did not occur, either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time.

Crustal structure and evolution of the central Australian basins

Summary The basins of central Australia formed over a period of 600 Ma or more from late Proterozoic to Carboniferous time. These basins appear to overlie crust of normal thickness whereas the crust of the intervening arches has been uplifted and reduced in thickness. The Moho, in consequence, varies in depth by up to ± 10 km about its mean depth, over horizontal distances of 50 km or less. The stresses associated with this lower crustal structure are of the order of a few kbars and extend throughout the crust. These stresses have existed since at least late Proterozoic time, and because the Moho undulations appear to follow the near surface deformations, this rules out the existence of a mechanically weak lower crust. The region is out of isostatic equilibrium and this state must be maintained by either a mechanically strong lithosphere of more than 150 km thickness or by horizontal compressive stresses, of the order of a few kbars. The former possibility is ruled out by the available mantle flow laws and geotherms proposed for the central Australian shield. In the latter case the horizontal compressive forces have also played the key role in shaping the basins and crustal structure. The horizontal compression must act down to at least the Moho and must characterize the dominant state of stress since late Proterozoic time.